Kinetic/mechanistic aspects of radical polymerization: Homogeneous and heterogeneous systems
Yusuke Sugihara
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy
2014 March
Centre of Advanced Macromolecular Design (CAMD) School of Chemical Engineering The University of New South Wales (UNSW)
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Abstract The objective of this Thesis is to develop new contributions to the fundamental knowledge in the area of kinetics and mechanism of radical polymerization. Polymer science, the study of large molecules, is one of the most important subjects, which has led to the development and production of numerous of our commodities and household items such as plastics, fibres, elastic materials, paints, adhesives, and even electronic applications. The popularity and importance of polymer products has all through the ages been a sufficient impetus to improve various polymerization techniques, not only conventional bulk or solution homogeneous systems, but also various specific conditions such as heterogeneous systems, polymerization under microwave (MW) irradiation, as well as controlled/living radical polymerization. Regardless of the specific techniques, the understanding of kinetics and mechanism is of prime concern.
Despite the far-reaching achievements thus far, the subject of radical polymerization is of course far from complete. Radical polymerization itself has been advancing and new techniques and new interests are continuously emerging. In this Thesis, the on-going argument of the possible influence of MW irradiation on the kinetics of radical polymerization was experimentally investigated precisely with the model monomer of styrene. The kinetics of emulsion polymerization of styrene, one of the most precisely studied heterogeneous systems was studied to investigate the practical limit of the particle size in which the standard kinetic concept of ’zero-one theory’ is valid. A theoretical study is described for the kinetics of nitroxide-mediated polymerization (NMP) of styrene in under heterogeneous conditions of miniemulsion polymerization, where the influence of the particle size ’compartmentalization’ and ingredient partition are successfully combined for the first time. Finally, the elemental reactions of chain transfer to solvent were investigated for the conventional radical polymerization and NMP for the combination of N-isopropylacrylamide (NIPAM) monomer and dimetylformamide (DMF) solvent, not only in order to evidence the chain transfer to solvent reaction for this particular case, but also to gain a general understanding on how such side reactions influence the polymerization process.
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Acknowledgement My thanks is first to my supervisor Professor Per Zetterlund. He provided the opportunity for this PhD study. Our acquaintance has been since when we were in Japan, and my story in Australia and Ireland was with his support. He is the only one person to be able to know my true history who I was and who I am and why, as such his words have been important supplement for me to find my personal legend. Since now is the time we see the answer of our effort in this period as what I becomes of. Hardships do lie ahead of my future, and my challenge is to return the favour as I reach make it to a truth. I thank Professor Tom Davis for the foundation of CAMD. CAMD is the group developed by him and in this place I met many nice and diligent scientists and friends. Thanks to Dr Michael Whittaker and Dr Istvan Jacenyik for the management of our group and for the favour to my repeated requests. I appreciate Dr. Fawaz Aldabbagh, Ireland my work were also under the supervision of him in NUIG. It was my first place I accumulated the common sense and requirement of Chemistry. It was the only time of less than 2 years, but perhaps my best life was there in Ireland. For the project, thanks to Dr Orla Gibbons and Dr Liz Donovan for their patient instruction, and thanks to Padraig O’Connor for his support and assistance in Ireland. My thanks go also A/Professor Brian Hawkett and Professor Sébastien Perrier for my works with KCPC. Here I add thanks to Dr Stuart Thickett, now he is in CAMD. For me it has a special meaning and since long ago they has been my direction and guide board as a novice of polymer chemist. It is my luck I can be acquainted with them and given their wisdom. I thank Dr Hank De Bruyn. My dilatometry work really is indebted to his patience. All the time I asked him help and all the time he fixed! I also thank Duc Nguyen and Binh Pham for their matured and experienced advice for emulsion polymerization, and also thank Eh Hau Pan for his kind help in the experiment there. Importantly, all my life in Sydney and Galway has been supported by really a lot of very nice and exciting friends. I must apologize that I cannot express my heartfelt gratitude to everyone individually, and also I cannot thanks enough. You gave me the real smile and happiness on my life, otherwise my life was dead and in the depth of despair. You really gave me lots of memories. I must challenge my life so that I can see you again in smile… I have other people I must express my gratitude about some works which is not yet published. I strive to make it for sure, and give back them as a certain establishment. Also this PhD thesis is not directly concerned, but at this time I am completing my PhD, I would like to express my sincere gratitude and respect for Professor Robert Gilbert and Professor Shigeru Yamago. My life as a novice of polymer chemist started by following their great jobs on emulsion polymerization and organotellurium-mediated living radical polymerization, and since then my effort has been to adore them in my heart up to now. All the time, it was/is difficult, I am still poor and deficient. However, I wish someday I return the great favour. Finally, I have the most important gratitude for my family.
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Table of Contents
Chapter 1 Introduction 1
1.1 Overview 2
1.2 Aims and outline 5
1.3 References 6
Chapter 2 Literature Review 8
2.1 Radical Polymerization 9
2.1.1 Chain Initiation 10
2.1.2 Chain Propagation 13
2.1.3 Chain Termination 14
2.1.4 Chain Transfer 15
2.1.5 Polymerization Rate 17
2.1.6 Steady-State Analysis 18
2.2 Controlled/Living Radical Polymerization 19
2.2.1 Reversible Deactivation 20
2.2.2 Class of Reversible Deactivation Mechanism 24
2.2.3 Kinetic Consideration 26
2.3 Radical Polymerization in Heterogeneous Systems 30
2.3.1 Events in Heterogeneous Polymerization 31
2.3.2 Reaction Intervals in Emulsion Polymerization 34
2.3.3 Polymerization Rate in Heterogeneous System 35
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2.3.4 Compartmentalization 38
2.3.5 Class of Heterogeneous Polymerization 47
2.4 References 53
Chapter 3 Assessment of the Influence of Microwave 68 Irradiation on Conventional and Controlled/Living Radical Polymerization of Styrene
3.1 Abstract 69
3.2 Introduction 70
3.3 Experimental Section 71
Materials 71
Conventional Radical Polymerization and RAFT Polymerization of Styrene 72
Characterization 72
3.4 Results and Discussion 73
Conventional Radical Polymerization: Oil bath vs Microwave 73
Conventional Radical Polymerization: High Microwave Power (with Air 79 Cooling)
Conventional Radical Polymerization: High Microwave Power (Without Air 81 Cooling)
RAFT Polymerization: High Microwave Power (without Air Cooling) 81
Microwave-Induced Radical Generation? 87
Azo-initiator effectiveness to the whole kinetics 87
3.5 Conclusions 89
3.6 References 89
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Chapter 4 Validity Limits for the Zero-One Approximation 92 in Styrene Emulsion Polymerization
4.1 Abstract 93
4.2 Introduction 94
Smith-Ewart Theory for the Kinetics of Emulsion Polymerization 94
Establishment of Zero-One Approximation 95
Development of Zero-One Kinetics of Emulsion Polymerization of Styrene 97
Interest of the Validity of Zero-One Approximation on Large Particles 99
4.2 Experimental Section 100
Materials 100
Hydrodynamic Chromatography (HDC) 101
Synthesis of Seed Latex of Polystyrene 101
Interval III Seeded Emulsion Polymerization of Styrene 102
Dilatometry for Conversion Reading 103
Gravimetry for Conversion Reading 104
4.3 Results and Discussion 104
Interval III Seeded Emulsion Polymerization 104
The Constancy of 휌 and 푘 111
Change in 푐 during Interval III Emulsion Polymerization 113
Dependency of Zero-One Validity on 휌 and 푐 113
Estimation of the Maximum Particle Size for Zero-One Validity 114
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4.4 Conclusions 117
4.5 References 117
Chapter 5 Synergistic Effects of Compartmentalization and 121 Nitroxide Exit/Entry in Nitroxide-Mediated Radical Polymerization in Dispersed System
5.1 Abstract 122
5.2 Introduction 123
5.3 Model Development 125
Homogeneous System 125
Heterogeneous System 126
Model for Exit/Entry of Nitroxide 128
5.4 Results and Discussion 130
5.5 Conclusions 138
5.6 References 138
Chapter 6 Chain Transfer to Solvent in the Radical 144 Polymerization of N-Isopropylacrylamide
6.1 Abstract 145
6.2 Introduction 146
6.3 Experimental Section 147
Materials 147
Measurements 148
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General Polymerization Details 149
Conventional Radical Polymerization 149
Chain Transfer to Solvent (Mayo Plot) 149
Nitroxide-Mediated Polymerizations 150
Thermal Polymerization in the Absence of Initiator and Nitroxide 150
6.4 Results and Discussion 151
Limiting Molecular Weight 151
Conventional Radical Polymerization in DMF 151
Nitroxide-Mediated Radical Polymerization 152
Chain Transfer to Solvent/Monomer 156
Estimation of 퐶tr,S via Mayo Plot 161
Number of New Chains 162
Molecular Weight Distribution 164
Effect of Poly(acrylate) Macroinitiator 164
Spontaneous Initiation 165
Comparison with Literature 167
6.5 Conclusion 168
6.6 References 169
Chapter 7 Conclusions and Future Perspectives 175
Appendix 181
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List of Figures
Figure 2.1. Simulated propagating radical concentration ( [P] ; 10% 46 conversion) for TEMPO-mediated radical polymerizations of styrene ([PS-TEMPO]0 = 0.02 M) with () and without () thermal initiation at 125 C for different particle diameters (d). The horizontal lines show [P] in the corresponding bulk systems. Reprinted with permission from ref [130]. Copyright 2006 American Chemical Society. Figure 3.1. Conversion vs time plots for conventional radical polymerization 75 of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1) in oil bath at 60 () and 100 C () and under microwave irradiation using “Dynamic Mode” set at 60 () and 100 C () without air cooling and using "Fixed Power Mode" set at 300 W with air cooling (40 psi) (). Figure 3.2. Temperature (a) and microwave irradiation power (b) vs time for 76 conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1 ) under microwave irradiation using “Dynamic Mode” set at 60 C without air cooling for 2 (A), 4 (B), 8 (C), 16 (D), and 18 (E) h.
Figure 3.3. Temperature (a) and microwave irradiation power (b) vs time for 77 conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1 ) under microwave irradiation using “Dynamic Mode” set at 60 C without air cooling for 4 (A), 8 (B), 12 (C), and 16 (D) h.
Figure 3.4 Temperature (a) and microwave irradiation power (b) vs time for 78 conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1 ) under microwave irradiation using “Fixed Power Mode” set at 25 W without air cooling for 4 (A), 8 (B), 12 (C), and 16 (D) h.
Figure 3.5. Temperature (a) and microwave irradiation power (b) vs time for 80 conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1 ) under microwave irradiation using “Fixed Power Mode” set at 300 W with air cooling at 40 psi for 1 (A), 3 (B), 8 (C), and 12 (D) h.
Figure 3.6. Conversion vs time plots for conventional radical polymerization 83 ([St]0/[AIBN]0 = 2000/1 ) () and RAFT polymerization ([St]0/[CPDB]0/[AIBN]0 = 2000/4/1 ) () of styrene under microwave irradiation using “Fixed Power Mode” set at 200 W
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without air cooling, and RAFT polymerization in oil bath at 140 C ().
Figure 3.7. Temperature (a) and microwave irradiation power (b) vs time for 84 conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1 ) under microwave irradiation using “Fixed Power Mode” set at 200 W without air cooling for 10 (A), 30 (B), and 60 (C) min.
Figure 3.8. Temperature (a) and microwave irradiation power (b) vs time for 85 RAFT polymerization of styrene ([St]0/[CPDB]0/[AIBN]0 = 2000/4/1) using “Fixed Power Mode” set at 200 W without air cooling for 20 (A), 37 (B), 40 (C), and 60 (D, E) min.
Figure 3.9. Mw/Mn (top) and Mn (bottom) vs conversion for RAFT 86 polymerization of styrene ([St]0/[CPDB]0/[AIBN]0 = 2000/4/1) using “Fixed Power Mode” set at 200 W without air cooling.
Figure 3.10. Temperature (a) and microwave irradiation power (b) vs time for 88 conventional radical polymerization of styrene without initiator using “Fixed Power Mode” set at 300 W with air cooling (40 psi).
Figure 4.1. Conversion vs time plots for the Interval III seeded emulsion 108 polymerization of styrene with the seed of rs = 167 nm and [KPS]0 = 9.5 × 10-4 (blue), 4.7 × 10-4 (red) and 9.4 × 10-5 M (black).
Figure 4.2. 푛̅ plots to conversion for the Interval III seeded emulsion 108 polymerization of styrene with the seed of rs = 167 nm and [KPS]0 = 9.5 × 10-4 (blue), 4.7 × 10-4 (red) and 9.4 × 10-5 M (black).
Figure 4.3. Conversion vs time plots for the stage III seeded emulsion 109 polymerization of styrene with the seed of rs = 196 nm and [KPS]0 = 9.4 × 10-5 (red) and 9.3 × 10-6 M (black).
Figure 4.4. 푛̅ plots to conversion for the Interval III seeded emulsion 109 polymerization of styrene with the seed of rs = 196 nm and [KPS]0 = 9.4 × 10-5 (red) and 9.3 × 10-6 M (black).
Figure 4.5. Conversion vs time plot for the stage III seeded emulsion 110 polymerization of styrene with the seed of rs = 274 nm and no initiator addition.
Figure 4.6. 푛̅ plot to conversion for the Interval III seeded emulsion 110 polymerization of styrene with the seed of rs = 274 nm and no initiator addition.
Figure 4.7. wp vs rsb for the Interval III seeded emulsion polymerization of 116 styrene with the various sized seed. The curves are calculated on the assumption that cb is constant at certain to rsb.
Figure 5.1. Simulated conversion-time data for NMP of styrene using St- 131 TEMPO initiator at 125 C (a) without and (b) with exit/entry of TEMPO at various particle diameters ([St]0/[St-TEMPO]0 = 8.71/0.02). Broken lines denote simulated bulk NMP.
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Figure 5.2. Simulated conversion-time data for NMP of styrene using St- 132 TEMPO initiator at 125 C without (blue broken-dotted lines) and with (black solid lines) exit and entry of TEMPO at particle diameters of 10 (left), 40 (centre) and 70 nm (right) ([St]0/[St- TEMPO]0 = 8.71/0.02). The red broken lines denote simulated bulk NMP.
Figure 5.3. Simulated values of number fraction (relative to the initial amount) 132 of alkoxyamine as a function of conversion for NMP of styrene using St-TEMPO initiator at 125 C without (blue broken-dotted lines) and with (black solid lines) exit and entry of TEMPO at 10 (left), 40 (centre) and 70 nm (right) ([St]0/[St-TEMPO]0 = 8.71/ 0.02). The red broken lines denote simulated bulk NMP.
Figure 5.4. Simulated number of propagation events per activation- 134 deactivation cycle for an individual chain () vs conversion for NMP of styrene using St-TEMPO initiator at 125 C (a) without and (b) with exit/entry of TEMPO at various particle diameters ([St]0/[St-TEMPO]0 = 8.71/0.02). Broken lines denote simulated bulk NMP.
Figure 5.5. Simulated number of propagation events per activation- 136 deactivation cycle for an individual chain () vs conversion for NMP of styrene using St-TEMPO initiator at 125 C without (blue broken-dotted lines) and with (black solid lines) exit/entry of TEMPO at 10 (left), 40 (center) and 70 nm (right) ([St]0/[St- TEMPO]0 = 8.71/0.02). Red broken lines denote simulated bulk NMP.
Figure 6.1. MWDs for the conventional radical polymerization of NIPAM (2 152 M) in DMF initiated by TBP at 120 C for initiator concentrations of 0.41 (13%), 1.4 (33%) and 4.1 mM (22%), with monomer conversions as indicated.
Figure 6.2. Conversion vs. time data for NMP of NIPAM in DMF (2 M) using 153 Poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator at 120 C with [M]0/[MI]0 = 100 (○), 200 (□), and 300 (∆).
Figure 6.3. MWDs of poly(t-BA)-b-poly(NIPAM) and original poly(t-BA)- 154 SG1 for [M]0/[MI]0 = 100 (a), 200 (b), 300 (c) with NIPAM conversions as indicated.
Figure 6.4. Mw/Mn (top) and Mn (bottom) vs conversion plots for NMP of 155 NIPAM in DMF (2 M) at 120 C with poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator and [M]0/[MI]0 = 100 (○), 200 (□), and 300 (∆), and conventional radical polymerization of NIPAM (2 M) with various concentrations of TBP as initiator () in DMF at 120 C (each data point corresponding to a different [TBP]0). The full lines are theoretical Mn using Equation 4 and 7 with Ctr,S = 0.00065 (NMP) and 0.0008 (conventional radical polymerization).
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Figure 6.5. Mn vs conversion plots for NMP of NIPAM in DMF (2 M) at 157 120 C with poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator and [M]0/[MI]0 = 200. Solid lines are computed from Equation 3 with various Ctr,M.
Figure 6.6. Typical traces of Mn against conversion in conventional radical 160 polymerization (a) and controlled/living radical polymerization (b) with chain transfer to monomer (broken line) and chain transfer to solvent (solid line).
Figure 6.7. Mayo plot of NIPAM (2 M) in DMF initiated by TBP (0.41 mM) 161 at 120 C. The line is a best fit.
Figure 6.8. [chains]new vs conversion plots for NMP of NIPAM in DMF (2 M) 163 at 120 C using poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator and [M]0/[MI]0 = 200. Solid lines correspond to [chains]tr,M = xCtr,M[M]0 (see Equation 3).
Figure 6.9. [chains]new versus conversion plots for NMP of NIPAM in DMF (2 163 M) at 120 C using poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator with [M]0/[MI]0 = 100 (○), 200 (□) and 300 (∆). The line represents [chains]tr,S = -4 ln(1-x)-1Ctr,S[S]0 (see Equation 4) for Ctr,S = 6.5 × 10 .
Figure 6.10. Mw/Mn vs [chains]tot/[MI]0 for NMP of NIPAM in DMF (2 M) at 165 120 C using poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator with [M]0/[MI]0 = 100 (○), 200 (□), and 300 (∆).
Figure 6.11. First–order plot of spontaneous polymerization (in the absence of 166 initiator or nitroxide) of NIPAM (2 M) in DMF at 120 °C (the solid line is a best fit).
List of Schemes
Scheme 2.1. General description of radical polymerization components, 9 reproduced from ref [1].
Scheme 2.2. Dissociation of an initiator compound. 10
Scheme 2.3. Illustration of combination. 14
Scheme 2.4. Illustration of disproportionation. 15
Scheme 2.5. Termination as an entire reaction of combination and 15 disproportionation. Scheme 2.6. Degenerative transfer to polymer. 16
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Scheme 2.7 General description of reversible deactivation and activation. 21
Scheme 2.8. Illustration of chain growth for radical polymerization (a) and 23 controlled/living radical polymerization (b) against the real-time scale.
Scheme 2.9. Classification of mechanisms of reversible-deactivation, 24 reproduced from [58]. Scheme 2.10. General description of micellar nucleation. 32
Scheme 2.11. General description of homogeneous nucleation. 32
Scheme 2.12. General description of droplet nucleation. 33
Scheme 2.13. General description of three intervals in emulsion polymerization. 35
Scheme 2.14. General description of miniemulsion polymerization. 50
Scheme 2.15. General description of microemulsion polymerization. 51
Scheme 2.16. General description of dispersion polymerization. 52
Scheme 3. 1. Chemical structures of styrene, cyanoisopropyl dithiobenzoate 71 (CPDB) and azobisisobutyronitrile.
Scheme 4.1. Chemical structures of styrene, potassium persulphate (KPS), 100 ammonium persulphate (APS), sodium dodecyl sulphate (SDS) and sodium 1,4-dicyclohexyl sulphonatosuccinate (AMA80).
Scheme 5.1. Chemical structures of styrene and 2,2,6,6-tetramethyl-1-(1- 124 phenylethoxy)piperidine (St-TEMPO). Scheme 5.2. Nitroxide-mediated polymerization of styrene with TEMPO. 124
Scheme 6.1. Chemical structures of N-isopropylacrylamide (NIPAM), 148 dimethylformamide (DMF), tert-butyl peroxide (TBP), N-tert-butyl- N-[1-diethylphosphono(2,2-dimethylpropyl)]oxy (SG1) and poly(tert-butyl acrylate)-SG1 (Poly(t-BA)-SG1).
List of Tables
Table 4.1. The reaction condition of Interval III seeded emulsion 105 polymerization of styrene.
Table 4.2. Parameters for Interval III seeded emulsion polymerization of 105 styrene.
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Table 4.3. The 휌 and 푘 estimated, and the zero-one breakdown conversions 112 푥b and 푐b estimated at the conversions. Table 5.1. Rate Parameters Employed in the Simulations. 130
Table 6.1. Conventional radical polymerization of NIPAM with TBA in DMF 149 at 120 C.
Table 6.2. Conventional radical polymerization of NIPAM with TBP in DMF 150 at 120 C.
Table 6.3. Nitroxide-mediated radical polymerization of NIPAM with poly(t- 150 BA)-SG1 macroinitiator and free SG1 in DMF at 120 C.
Table 6.4. Conventional radical polymerization of NIPAM in absence of 150 thermal initiator in DMF at 120 C.
Table 6.5. Values of Ctr,S for radical polymerization of NIPAM in DMF at 120 168 C estimated in the present work.
Keys to Abbreviations and Symbols
AIBN Azobisisobutyronitrile
APS Ammonium persulphate
ATRP Atom transfer radical polymerization
BIRP Organobismuthine-mediated living radical polymerization
푡-BA tert-Butyl acrylate
CLRP Controlled/living radical polymerization
CMC Critical micelle concentration
CPDB Cyanoisopropyl dithiobenzoate
DMF N,N-dimethylformamide
DMSO Dimethyl sulfoxide
GPC Gel permeation chromatography
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HDC Hydrodynamic chromatography
ITP Iodine transfer polymerization
KPS Potassium persulphate
LCST Lower critical solution temperature
LiBr Lithium bromide
MCR Mid-chain radical
MI Macroinitiator
MMA Methyl methacrylate
MW Microwave
MWD Molecular weight distribution
NaHCO3 Sodium bicarbonate
NH4OH Ammonium hydroxide
NIPAM N-isopropylacrylamide
NMP Nitroxide-mediated polymerization
NMR Nuclear magnetic resonance
PRE Persistent radical effect
RAFT Reversible addition-fragmentation chain transfer
RCMP Reversible complexation mediated polymerization
RTCP Reversible transfer catalyzed polymerization
SARA ATRP Supplemental activator and reducing agent atom transfer radical polymerization
SBRP Organostibine-mediated living radical polymerization
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SDS Sodium dodecyl sulphate
SDBS Sodium dodecyl benzene sulfonate
SET-LRP Single-electron-transfer living radical polymerization
SG1 N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide
SRMP Stable radical mediated polymerization
St Styrene
TBP tert-Butyl peroxide
TEM Transmission electron microscopy
TEMPO 2,2,6,6-Tetramethylpiperidine 1-oxyl
TERP Organotellurium-mediated living radical polymerization
THF Tetrahydrofran
퐴 Pre-exponential factor
푐 Pseudo-first-order rate coefficient for bimolecular termination
퐶ex Chain exchange constant
퐶퐹 Contract factor
퐶p Monomer concentration in a particle
D Deactivator for reversible deactivation
퐷 Diffusion coefficient
퐷푃n Number average degree of polymerization
푑 Particle diameter
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푑m Density of monomer
푑n Number average particle diameter
푑org Density of organic phase
푑p Density of polymer
푑w Density of water
퐸a Activation energy
푓 Initiator efficiency
훤 Partition coefficient
퐻 Height of meniscus on the capillary
Int Intermediated inactive entity on reversible deactivation
I Initiator
I Initiator radical
Number of propagating radical for a particle for 2-dimension Smith- 푖 Ewart equation
Mean number of propagating radical for a particle for 2-dimension 푖 ̅ Smith-Ewart equation
Number of deactivator for a particle for 2-dimension Smith-Ewart 푗 equation
Mean number of deactivator for a particle for 2-dimension Smith-Ewart 푗 ̅ equation
푗crit Critical oligomer degree of polymerization for precipitation
푘 Pseudo-first-order rate coefficient for radical exit
푘act First order activation rate coefficient
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푘ads Adsorption rate coefficient to a particle
푘ct,X Second order chain transfer to an arbitrary reactant X rate coefficient
푘ct,M Second order chain transfer to monomer rate coefficient
푘ct,S Second order chain transfer to solvent rate coefficient
푘d First order dissociation rate coefficient
푘dM Frist order exit rate coefficient
푘deact Second order deactivation rate coefficient
푘entry First order entry rate coefficient
푘ex Second order chain exchange rate coefficient
푘i Second order initiation rate coefficient
푘i,th Third order thermal initiation of styrene rate coefficient
푘p Second order propagation rate coefficient
1 푘p Second order propagation rate coefficient for initiating radical
푘ri Second order re-initiation rate coefficient
푘t Second order bimolecular termination rate coefficient
푘tc Second order combination rate coefficient
푘td Second order disproportionation rate coefficient
M Monomer
푀 Molecular weight of monomer
푚M Mass of monomer
푀n Number-average molecular weight
푀w Weight-average molecular weight
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푛 Number of propagating radical for a particle
푛̅ Mean value of the number of propagating radical for a particle
푛̅ss Mean value of the number of propagating radical for a particle in the steady-state under zero-one approximation
푛M Moles of initial monomer per unit volume of the dispersion medium
푁A The Avogadro’s number
푁c Number concentration of particle in the dispersion medium
푁cr Number of particle to the entire system
푁(푖,푗) Population of particles which contain 푖 propagating radical and 푗 deactivator for 2-dimention Smith-Ewart equation
푁(푛) Population of particles which contain 푛 propagating radical
m 푁(푛) Population of particles which contain n monomeric radical
p Population of particles which contain n polymeric radical 푁(푛)
P Propagating polymeric radical
Pdead Dead polymer
PX Dormant species
(PXP) Intermediated radical for degenerative transfer
푅 The universal gas constant
휌 Pseudo-first-order rate coefficient for radical entry
휌re Pseudo-first-order rate coefficient for exited radical entry
푅act Rate of activation
푅ct,x Rate of chain transfer to the arbitrary reactant X
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푅d Rate of dissociation
푅deact Rate of deactivation
푅i Rate of initiation
푅initiation Rate of apparent initiation
푅p Rate of propagation
푅polymn. Rate of polymerization
푅ri Rate of re-initiation
푅t Rate of termination
푅tc Rate of combination
푅td Rate of disproportionation
S. C. Solids content of polymer latex
푇(휏1/2 =10ℎ) 10 hours half-life temperature
푉 Contraction volume of monomer
푣s Monomer swollen particle volume
푉w Volume of the dispersion medium
푤p Weight fraction of a particle
X Arbitrary entity
X Stable free radical of atom or group for reversible deactivation
푥 Fractional conversion of polymerization
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Yusuke Sugihara Chapter 1
Chapter 1
Introduction
1
Yusuke Sugihara Chapter 1
1.1 Overview At the present time of the early 21st century, our modern and comfortable lives are made possible due to the development of science and technology. Among that, polymer science, the study of large molecules, is one of the most important subjects, which has led to the development and production of numerous of our commodities and household items such as plastics, fibres, elastic materials, paints, adhesives, and even electronic applications.
The beginning of the use of polymeric materials dates back to many centuries ago, when people discovered and used plenty of natural polymers unconsciously as they are polymers, like natural fibres of wool, silk from animals, and cotton and natural rubbers from trees. In the late 19th century, some artificial polymers were first synthesized. In the early 20th century, Staudinger[1] successfully established the strict definition of ’polymer’, then the century was the time the great development had taken place to the understanding of the mechanism of polymerizations, the establishment of various polymerization methods, and the invention of an enormous number of types of new polymeric products. In combination with the dramatic development of the petrochemical industry, which gives rise to raw materials for most artificial polymers, the polymer industry has become a world giant market.
Radical polymerization[2-4] is probably the most important and general-purpose polymerization technique especially in the practical industrial field. The mechanism is based on the free radical activity of carbon active centres inducing elemental reactions, leading to only few restrictions on the selection of monomers, provided that they have reactive vinyl groups. Moreover, as such, it provides the preferable feature of robust tolerance to the surrounding environments of the reaction, coexisting with impurities in the system. Radical polymerization can be conducted in the presence of trace amounts of oxygen, and is compatible with water, both of which typically inhibit organic reactions. This robustness, generality and less-demanding nature of radical polymerization, render the technique the first option for many cases of polymer synthesis, against the counterparts of ionic or coordination polymerization chemistry.
The advantage associated with the high reactivity of carbon-centred radicals, however, also leads to the drawback in the ability to control the polymerization process, and as such radical polymerizations have had a critical deficiency, and fallen behind the ’living’ 2
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anionic polymerizations in regard of ’fine polymer synthesis’.[5] However, the emergence of the new concept of ’living’ radical polymerization in 1980, sparked by Otsu,[6, 7] and the significant development of the quality of ’control’, pioneered by the nitroxide-mediated polymerization (NMP) in 1986 by Rizzardo and co-workers[8] and later in 1993 by Georges and coworkers,[9] has overcome this issue to a great extent, and enabled radical polymerizations to control the polymerization process. It is now possible to synthesize polymer by free radical means with predetermined number average molecular weight (Mn), narrow molecular weight distribution (MWD), and one has the ability to design the chain-initiating location of polymerizations, as well as perform multi-step chain extensions (block copolymer), etc. This family of polymerization techniques is now referred to as ’controlled/living radical polymerization’ (CLRP).[10]
As is usual in any other chemistry fields, radical polymerizations are generally carried out in a ’homogeneous’ environment like bulk or solution polymerizations. However, the robust tolerance to the environment provides radical polymerizations with yet another polymerization environment of ’heterogeneous’ systems.[4] In a heterogeneous system, the polymerization proceeds in finely dispersed polymer particles in the dispersion medium (of mainly water), then the result of the polymerization process is a so called polymer latex. This fundamental difference in the process from homogeneous systems has several advantages and benefits in the polymerization mechanism and practical handling. For example, heterogeneous systems often have higher polymerization rates, and the continuous aqueous phase serves the additional role of effectively cooling the system, which can be an important advantage in case of exothermic polymerizations. Also the final form as a polymer latex has direct usages like paints and adhesives. Applicability to heterogeneous systems increases the range of practical superiority and utility of radical polymerizations.
The current industrial use of radical polymerization has of course been achieved by the persistent and great efforts of scientists. However, further understanding is always required to achieve improved control of polymerization processes, both for conventional radical polymerizations or CLRPs, and whether homogeneous or heterogeneous systems. The understanding of radical polymerization pioneered by Staudinger,[1] Taylor,[11, 12] Semenov,[13] Flory[14, 15] and many early giants have established profound
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knowledge. The field of CLRP has been the focus of very significant worldwide research efforts over the past two decades and now most CLRP techniques are mechanistically and kinetically well understood.[10, 16] However, new CLRP systems continue to be developed to this day. Radical polymerization in heterogeneous system is very complex. While the required considerations and experimental difficulty dramatically increase, significant understanding of the mechanism and kinetics have been established especially in the study of ’emulsion polymerization’.[4, 17]
Despite the far-reaching achievements thus far, the subject of radical polymerization is of course far from complete. Radical polymerization itself has been advancing and new techniques and new interests are continuously emerging. For example, the recent application of microwave (MW) irradiation[18] as an alternative heating device to radical polymerizations revealed it sometimes may affect the kinetics and mechanism, providing dramatically increased polymerization rates. Moreover, the proper understanding of the combination of the two complicated subjects of CLRP and heterogeneous system is all too complex which has yet to be significantly developed.[19- 21] In all cases, the understanding of the mechanism and chemical kinetics lie at the core of the development of these subjects, and in each stage of the field, there are plenty of room for further development to make radical polymerization a more efficient synthetic tool, ultimately resulting in more useful and high quality polymeric products.
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1.2 Aims and outline The main aim of this Thesis is to develop new fundamental knowledge in the area of kinetics and mechanism of radical polymerization, not only pertaining to simple bulk or solution homogeneous conditions of conventional radical polymerizations, but also various specific conditions such as heterogeneous systems, polymerization under microwave (MW) irradiation, as well as application of CLRP. To this end, kinetic-based standard introductions of radical polymerization, CLRP, and heterogeneous systems are provided in Chapter 2. Chapter 3 verifies the practical influence of MW irradiation on the kinetics of conventional radical polymerization and CLRP (reversible addition- fragmentation chain transfer (RAFT) polymerization) of styrene. Chapter 4 concerns the kinetics of emulsion polymerization of styrene, one of the well-established heterogeneous systems, and investigates the practical limit of the particle size in which the standard kinetic concept of ’zero-one theory’ is valid. In Chapter 5, a theoretical study is described for the kinetics of NMP of styrene under heterogeneous conditions of miniemulsion polymerization, where the influence of the particle size (’compartmentalization’) and ingredient partition are successfully combined for the first time. In Chapter 6, making use of NMP, the practical influence of the elemental reaction of chain-transfer to solvent is evidenced in the case of radical polymerization and CLRP of N-isopropylacrylamide (NIPAM) in dimethylformamide (DMF), by the excellent agreement between experimental results and theoretical predictions.
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1.3 References 1. Ringsdorf, H., Hermann Staudinger and the Future of Polymer Research Jubilees—Beloved Occasions for Cultural Piety. Angewandte Chemie International Edition, 2004. 43: p. 1064-1076. 2. Moad, G. and D.H. Solomon, The Chemistry of Radical Polymerization. 2006: Elsevier. 3. Odian, G., Principles of Polymerization. 2004: Wiley. 4. Gilbert, R.G., Emulsion Polymerization: A Mechanistic Approach. 1995: Academic Press. 5. Szwarc, M., `Living' Polymers. Nature, 1956. 178: p. 1168-1169. 6. Otsu, T. and M. Yoshida, Role of Initiator-Transfer Agent-Terminator (Iniferter) in Radical Polymerizations: Polymer Design by Organic Disulfides as Iniferters. Die Makromolekulare Chemie, Rapid Communications, 1982. 3: p. 127-132. 7. Matyjaszewski, K., A Commentary on “Role of Initiator-Transfer Agent- Terminator (Iniferter) in Radical Polymerizations: Polymer Design by Organic Disulfides as Iniferters” by T. Otsu, M. Yoshida (Macromol. Rapid Commun. 1982, 3, 127–132). Macromol. Rapid Commun., 2005. 26: p. 135-142. 8. Solomon, D.H., E. Rizzardo, and P. Cacioli, Polymerization Processes and Polymers Produced Thereby, 1986: U. S. Patent 4. 9. Georges, M.K., et al., Narrow Molecular Weight Resins by a Free-Radical Polymerization Process. Macromolecules, 1993. 26: p. 2987-2988. 10. Braunecker, W.A. and K. Matyjaszewski, Controlled/Living Radical Polymerization: Features, Developments, and Perspectives. Prog. Polym. Sci., 2007. 32: p. 93-146. 11. Taylor, H.S. and W.H. Jones, The Thermal Decomposition of Metal Alkyls in Hydrogen—Ethylene Mixtures. J. Am. Chem. Soc., 1930. 52: p. 1111-1121. 12. Starkweather, H.W. and G.B. Taylor, The Kinetics of the Polymerization of Vinyl Acetate. J. Am. Chem. Soc., 1930. 52: p. 4708-4714. 13. Semenov, N.N., I.A.I. Frenkelʹ, and I.A.R. Shmīdt-Chernysheva, Chemical Kinetics and Chain Reactions. 1935: The Clarendon press. 14. Flory, P.J., Principles of Polymer Chemistry: Paul J. Flory. 1953: Cornell University.
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15. Flory, P.J., The Mechanism of Vinyl Polymerizations1. J. Am. Chem. Soc., 1937. 59: p. 241-253. 16. Goto, A. and T. Fukuda, Kinetics of Living Radical Polymerization. Prog. Polym. Sci., 2004. 29: p. 329-385. 17. Thickett, S.C. and R.G. Gilbert, Emulsion Polymerization: State of the Art in Kinetics and Mechanisms. Polymer, 2007. 48: p. 6965-6991. 18. Kempe, K., C.R. Becer, and U.S. Schubert, Microwave-Assisted Polymerizations: Recent Status and Future Perspectives. Macromolecules, 2011. 44: p. 5825-5842. 19. Zetterlund, P.B., Y. Kagawa, and M. Okubo, Controlled/Living Radical Polymerization in Dispersed Systems. Chem. Rev. (Washington, DC, U. S.), 2008. 108: p. 3747-3794. 20. Monteiro, M.J. and M.F. Cunningham, Polymer Nanoparticles Via Living Radical Polymerization in Aqueous Dispersions: Design and Applications. Macromolecules, 2012. 45: p. 4939-4957. 21. Charleux, B., M.J. Monteiro, and H. Heuts, Living Radical Polymerisation in Emulsion and Miniemulsion, in Chemistry and Technology of Emulsion Polymerisation. 2013, John Wiley & Sons Ltd. p. 105-143.
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Chapter 2
Literature Review
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2.1 Radical polymerization Radical polymerizations[1-3] proceed and produce polymeric products as a result of several simultaneously occurring radical reactions among intra- and inter-radical chains. From the viewpoint of each polymer chain, the lifetime as a ’living’ free radical starts by chain initiation, which is followed by repeating chain-growing events of chain propagation, and finally stopped by chain-stopping events of chain termination or chain transfer, ending in a ’dead’ polymer chain or two chains. The lifetime of a living radical chain is short (on the time scale of 1 s) relative to the timescale of the entire polymerization which typically lasts several hours. During polymerization, chains are thus continuously generated and terminated (loss of radical activity), each chain spends only approx. 1 s as a radical. The general description of radical polymerization, originally proposed by Flory, is described in Scheme 2.1.[1-5]
Chain initiation
Chain propagation
Chain termination
Chain transfer
Scheme 2. 1. General description of radical polymerization components, reproduced from ref [1].
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2.1.1 Chain Initiation Chain initiation[3, 6] consists of two separate reaction steps,[7] first generating a primary radical I (radical directly derived from the radical source), and then reaction st with a monomer, producing an initiating radical (1 -unit propagating radical) P1 . The ambiguous expression for the first step generating a primary radical from a ’radical source’ in Scheme 2.1 is a reflection of the fact that one can make use of various radical sources with different mechanisms.
Chain Initiation by Initiator Compound
In general, a thermal initiator compound is employed in the polymerization system as the radical source denoted I, where azo-compounds leading to carbon-centered radicals, and peroxides to oxygen-centred radicals are the common options. In the case the first event is a simple homolytic fission of I,[8]
Scheme 2. 2. Dissociation of an initiator compound.
As the lifetime of a radical is very short, and if a high number of radicals are generated rapidly at one time, excessive bimolecular termination reactions occur (chain stopping reactions) rather than proper propagating. Thus, this dissociation needs to be occurring permanently and at a suitable rate over the entire polymerization period. This is a first concern with regards to the proper selection of the initiator compound and reaction temperature.[9] As with other elemental reactions, the rate of dissociation can be calculated with the rate coefficient of dissociation 푘d and initiator concentration [I]. The
10 hours half-life temperature 푇(휏1/2 =10ℎ) is an alternative criteria for the estimation of the mild dissociation.[10] 푇(휏1/2 =10ℎ) is calculated by the equation:
퐸 ln 2 푇 = a ln ( ) (1) (휏1⁄2=10ℎ) 푅 3.6 ∙ 104퐴
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where 퐸a is the activation energy, 푅 the universal gas constant, and 퐴 the pre- exponential factor for the reaction. For the application to various temperatures and solvent demands, many types of azo and peroxide initiator compounds are commercially available which have different 푘d and functional groups but hardly change the radical reactivity itself.
Dissociation is a unimolecular reaction producing two paired I . Those I s are both reactive with each other and have to diffuse away from one another to be identified to be eligible I entities for the reaction, otherwise they have a chance to undergo germinate recombination, leading back to the original I or some other inert entity, a process which is referred to as ’cage effect’.[11] The practical survivability of I, including all side reactions to loss of initiator radical, like the cage effect, non-radical decomposition, is defined as the initiator efficiency 푓.[12]
푑[I] = 2푓푅 = 2푓푘 [I] (2) 푑푡 d d
Practically, the subsequent initiation reaction of I with M takes place much faster than the dissociation, and as such the two initiation processes can be considered as one apparent process, and the apparent initiation rate, 푅initiation, can be approximated with the dissociation rate as,
푑[I] 푅 ≈ = 2푓푘 [I] (3) initiation 푑푡 d
The concept of 푓 is convenient, but the diffusion process to overcome the cage effect is significantly influenced by the environment. Different solvents, reaction temperatures, and even monomer conversion (influencing the viscosity) give different values of 푓.[6, 13-20] Thus 푓 is only investigated empirically for a specific condition, and the careless referring to literatures such that the same initiator compounds and monomer are employed but in different environment, may lead to poor estimation.
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Other Radical Sources It is well known that monomers themselves can be reactive to give birth to free radical entities at high temperatures. The thermal initiation of monomer is especially well studied for styrene, and the rate coefficient of thermal initiation of monomer 푘i.th is available.[21, 22] Typically, at less than 100 C the influence of thermal initiation of monomer is small and insufficient to be utilized as the primary radical source. In other words, at such mild temperatures for many general radical polymerizations, the influence of thermal initiation of monomer is negligible with regards to kinetics.
Light is also a well-known and useful source of stimulus for photoinitiator compounds, for chain initiation.[6, 23, 24] However, exact quantitative control may be difficult. More importantly, light is generated and irradiated from an external device, and the proper pathway must be prepared by the reaction place for the initiation to take place. Thus the turbidity of the system is the primary concern, and it can be effectively applied to clear or transparent systems. Generally, homogeneous systems like bulk and solution polymerizations are clear solution and fall under this possible application set. However, heterogeneous systems like emulsion polymerization are generally turbid, because the system is composed of dispersion of monomer droplets or polymer particles, and it would be difficult to apply this light use as the radical source,[25] except some particularly designed conditions of specifically minute droplets/particles like perfect microemulsion where all non-water-soluble monomer are located in small micelles (and the rest is perfectly dissolved in continuous phase) and the system forms thermodynamically stable seemingly homogeneous clear state introduced in Section 2.3.5 below.
γ-Radiolysis initiation is a very powerful technique for the study of kinetics of emulsion polymerization,[3, 26-31] γ-irradiation can penetrate the obstacle of opaque latex and create radicals uniformly in the system, and the insertion and removal of the reaction system in and out of the γ-source enables one to effectively switch on and off the radical source. The straightforward cut off of the initiation and the retardation of the rate of polymerization provides information in terms of reaction-stopping events (which is radical exit, in the study of zero-one kinetics).
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Microwave irradiation may also be a successful option for radical initiation process.[32, 33] The advantage of microwave irradiation is the strong penetrability through the environment, reaching the reactive entity.
2.1.2 Chain Propagation Propagation[3, 34] is the main reaction step of a chain polymerization, with repetitive reactions between a radical of the propagating chain and the vinyl group of monomer lasting until the chain experiences any chain-stopping event, ending in a dead chain, or two dead chains.
The reaction does not change the property of propagating radical except for the unit number (Pn to Pn+1, where the subscript n denotes the number of monomer units of the chain). Strictly, the chain-length changes the physical property of the chain and also chemical properties of the active radical centre. The diffusion of polymer chains is significantly influenced by the chain-length in a condensed system.[35] In the case of propagation, however, the reaction pair of the bimolecular reaction is a one-unit monomer, which has sufficient mobility to diffuse in the system. Therefore, propagation reaction is rather chemical-controlled than diffusion-controlled at low conversion. Contrary, if the reaction temperature is less than the possible glass transition temperature
푇g, the high mobility of monomer is also restricted at high conversion where the weight fraction 푤p reaches high, and propagation typically becomes diffusion controlled beyond 70% conversion in bulk.[36] While the chemical property is influenced by chain-length, the effective range only limits in the period of short chain-length, which does not last for long during the course of the living life of a polymer chain.[37] Thus, classical notation of the propagation rate coefficient 푘p which represents all chain- lengths (chain-length free) is of the value of long-chain limit.
푅p ≈ 푘p[P ][M] (4)
In this process, monomers are consumed and converted to the units of polymer chains. Thus regardless of conventional radical polymerizations or CLRPs, the reaction conversion and the entire polymerization rate are dominated by this propagating process (Sections 2.1.5 and 2.3.3).
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Therefore, when designing polymerization system, the values of 푘p and 푘d are the primary concern for any radical polymerization. Typically styrene has a relatively low
푘p, leading to low polymerization rate, while acrylates have quite high values of 푘p and faster polymerizations. Methacrylates exhibit 푘p values somewhat higher than styrene, but markedly lower than acrylates.[38]
2.1.3 Chain Termination Termination[3, 39] is generally major chain-stopping event and terminates the radical activity of a propagating chain, creating a ‘dead’ chain. Mechanistically, termination is further subdivided into two independent reactions of combination and disproportionation.
Combination is a simple coupling of two radicals, forming a covalent bond, resulting in a long dead chain which is the summation of two active polymeric chains in terms of the chain-length. Thus, exclusive termination by combination halves the total chain number. The reaction profile of this reaction is the same as simple fission of initiation by an initiator compounds, which do not have significant activation barrier for the reaction.[8] Thus, combination is rather diffusion controlled and the major termination for many monomers.
Scheme 2. 3. Illustration of combination.
Disproportionation is also a bimolecular reaction of free radical entities, where the reaction is not simple creation of a covalent bond, but hydrogen atom abstraction. Thus, the result of disproportionation is two dead chains, both of which keep the original chain- length, and yet the chemical structure of the end parts are not the same. This chemical reaction has a certain activation energy boundary, and thus tends to take place less than combination in general.[3]
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Scheme 2. 4. Illustration of disproportionation.
While the final form and the chain number of the products are different, combination and disproportionation are the same reactions in terms of the reactants pair being two propagating radicals. Therefore it is possible to apply the sum rule principle to the rate coefficients 푘t = 푘td + 푘tc, and it is convenient and general treatment to use the sum of both as a descriptive one reaction as simply “bimolecular termination” (Scheme 2.5).
Scheme 2. 5. Termination as an entire reaction of combination and disproportionation.
As is the bimolecular reaction between two long propagating chains, both combination and disproportion are diffusion controlled. From the beginning of the course of polymerization, the reactions are significantly influenced by the fractional conversion (the viscosity of the polymerization environment).[40] Especially, in the case of controlled/living radical polymerization in which chain growth occurs simultaneously for each polymeric chains, bimolecular terminations takes place quite rapidly (i.e. high
푘t) in the early period of low conversion and also low chain-length.[41, 42]
2.1.4 Chain Transfer Chain transfer reactions[3, 43] are also chain-stopping events, involving a propagating radical and any other entity as the reactants (propagating radical can be a candidate too,
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where the reactive point is on any other parts but the radical on it), thus X in Scheme 2.1 denotes any eligible entities such as monomer, polymer, initiator, solvent, surfactant, and so on. This reaction does not change the number of propagating radicals, and the product radical in the right hand side may also operate new initiation with a monomer (re-initiation). In this case, this is the reaction only exchanging the radical property, which is then termed “degenerative” transfer.
Generally, chain transfer is not a desirable process as an unpredicted chain-stopping reaction. For some particular purposes, typically to decrease the molecular weight and/or introduce chain end functionality, special chain-transfer agents are employed, in which chain-transfer is the primary chain-stopping event rather than termination.
Addition-fragmentation chain transfer agent (AFCTA) is a special class of chain transfer agent,[44, 45] which has a reactive double bond and weak single bond. For AFCTA, chain transfer reaction is two steps process of addition of the propagating radical to the double bond and 훽-session on the weak single bond producing another reactive radical. Because of its clear mechanism, the AFCTA is easy to design the reactivity by modifying the functional group to suit particular monomers. If the property between X and P is even identical between both sides of the chemical equation (Scheme 2.1), the reaction is “reversible” not changing any property except for the unit numbers of the propagating chains, which leads to the concept of reversible addition-fragmentation chain transfer (RAFT),[44, 46, 47] a representative of CLRP (Section 2.2.2)
Scheme 2. 6. Degenerative transfer to polymer.
Because of its unique kinetics based on ‘compartmentalization’ (Section 2.3.4), emulsion polymerization, one of the most general and important heterogeneous polymerizations, is known in which chain transfer to monomer is the dominating chain stopping reaction (bimolecular termination is significantly reduced).[3, 28] As such, emulsion polymerization enables to synthesize high molecular weight polymer product. In the condition in which bimolecular termination is effectively reduced, emulsion polymerization is also the preferable option to design and control the molecular weight more precisely by adding chain transfer agent.[3] 16
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2.1.5 Polymerization Rate The polymerization rate[3, 34] is obtained by tracing the mass conversion of monomer to polymer. Fundamentally, the fractional conversion, denoted 푥, is defined as the consumption of the monomer mass against the total mass of the entire system along the reaction time, and the essential description is,
푚Mc 푚M 푥 = = 1 − ( ) (5) 푚M0 푚M0
where, 푚M denotes the remaining mass of monomer, 푚M0 the initial mass of monomer,
푚Mc the consumed mass of monomer of the entire system. The detectable quantities and ways to measaure 푥 depend on the analytical techniques and instruments. Thus the rate 푑푥 of fractional conversion, is, 푑푡
푑푥 d 푚 푑 푚 = ( Mc ) = − ( M ) (6) 푑푡 dt 푚M0 푑푡 푚M0
Under homogeneous conditions, the mass change of monomer of the entire system is 푚 [M] 푑푥 proportional to that of concentration, M = . Thus for the kinetic analysis, and 푚M0 [M]0 푑푡 can be redescribed as,
푑푥 1 푑[M] 푅p 푘p[P ][M] = (− ) = = (7) 푑푡 [M]0 푑푡 [M]0 [M]0 where the equality of the first and second-hand-side of the equation holds in the approximation that the chain propagation is the dominant reaction for the monomer consumption. Thus, under homogeneous conditions, the absolute polymerization rate of 푑푥 the entire system 푅 = [M] is just identical to the propagation rate, polymn. 0 푑푡
푅polymn. = 푅p = 푘p[P ][M] (8)
This association of the detectable quantity of the fractional conversion 푥 to the elemental reaction of the propagation is of the primary importance to any kinetic discussion. 17
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However, this association does not always hold. For example, under heterogeneous conditions, this association holds only in particular conditions, and many times it is required to apply alternative treatments, described later in Section 2.3.3.[3]
2.1.6 Steady-State Analysis The general kinetic analysis of radical polymerizations stands on a steady-state assumption.[34] In the case of conventional radical polymerization, in the steady state
푑[P] = 0 holds, while elemental reactions occur. Under ideal conditions where chain 푑t transfer reactions are negligible, P is generated by initiation and consumed by termination, and it follows that 푅i = 푅t and therefore [P ] can be expressed as:
1 푅 2 [P] = ( 푖) (9) 푘t
If initiation is with common initiator compounds and 푅i ≈ 푅initiation holds (Section 2.1.1), [P] can be specified as,
1 2푓푘d 2 1 [P] = ( ) [I]2 (10) 푘t
At the time, the rate of propagation leads to,
1 푑[M] 푅푖 2 (11) 푅p = − = 푘p ( ) [M] 푑푡 푘t
[M] also the natural logarithm plot of ln 0 is, [M]
1 [M]0 1 푅i 2 (12) ln = ln (1 − ) = 푘p ( ) 푡 [M] 푥 푘t
[M] This is the first order plot in terms of [M]. In polymer chemistry, ln 0 is a common [M] alternative to a conversion-time plot, as this form not only shows the polymerization rate 18
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but also involves the characteristics of the way the polymerization proceeds. In this case, a linear first-order plot means that the product 푘p[P ] is constant (which means that [P ] is constant, assuming 푘p is constant (typically the case unless very high conversion in bulk)). Clearly whether conventional radical polymerization or CLRP, this first order trend only implies that polymerization proceeds in the steady-state or perhaps in a (pseudo)-equilibrium involving [P] . It is not possible to distinguish between conventional radical polymerization and CLRP based on a first-order plot. The frequent confusion in the literature that a linear first-order plot is seen as an evidence of successful CLRP is thus erroneous. Moreover, it is important to realize that first-order plot analysis is only applicable when the identification of total mass and molar concentration of reactants and products (discussed above) holds - various heterogeneous conditions may require additional considerations (Section 2.3.3).[3]
2.2 Controlled/Living Radical Polymerization The advantage of high reactivity of the ’free radical’, however, leads to an inability to control the polymerization process. As such, in terms of ’fine polymer synthesis’, radical polymerizations have fallen behind the advancing excellent invention of ’living’ anionic polymerization, which eliminates the termination process and the polymer chains are literally immortal.[48]
The concept of reversible deactivation for the application to radical polymerization has been first reported in the early 1980s by Otsu and coworkers, who proposed the work as ’iniferter’, short for initiator-transfer agent-terminator.[49-52] In the presence of an iniferter compound, the radical polymerization observed the characteristics of ’living’ with 푀n increasing linearly, but it lacked sufficient ’controllability’ and resulted in quite broad MWDs.
The significant development of this concept for both sufficient ’living’ and ’controlled’ abilities to the radical polymerization process was rediscovered in 1993 by the pioneering work of Georges et al. of what we now call nitroxide-mediated polymerization (NMP), using nitroxide as reversible combination-dissociation agent for the propagating radical at certain high temperatures[53], which was based on their early work on CSIRO.[54] After then, the concept of atom transfer radical polymerization 19
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(ATRP) was reported by the two independent groups of Sawamoto and coworkers[55] and Matyjaszewski and coworkers in 1995,[56] and reversible addition-fragmentation chain transfer (RAFT) polymerization by Moad, Rizzardo, Thang and coworkers in 1998.[46] These techniques have been established and systematized as controlled/living radical polymerization (CLRP),[57, 58] as a novel and next generation technique for the ’fine polymer synthesis’ with radical polymerization, which enables the precise control and design of the polymerization process; predicting the number-average molecular weight (푀n) , narrowing the molecular weight distribution (MWD), pinpointing the initiating location, synthesizing multi-step chain extensions (block copolymerization), etc, without spoiling the advantageous characteristics of radical polymerization (robustness, monomer versatility etc).
2.2.1 Reversible Deactivation The reason for failure of ’controllability’ of conventional radical polymerization are briefly found as: 1. Propagating radical species have high reactivity and termination and irreversible chain transfer reactions cannot be eliminated. 2. The chain stopping event is provided only as termination and irreversible chain transfer. 3. The lifetime of a polymer chain (the time from initiation of a given chain until loss of radical) is very short, typically on the order of 1 second.
Here, 1 is the identity of radical polymerization, and hardly modifiable. Because of 2, a polymer chain once initiated can only exist as first ’living (active) P’ and subsequent ’dead (inactive) Pdead’ state and the chronological sequence of the two states is one-way and irreversible. Then as 3, at the end of the entire polymerization process lasting several hours, all chains are dead and the duration of polymerization is sustained by consecutive initiations of new radicals. Therefore, while radical-radical (termination) reactions have been taking place during the entire polymerization, those reactions only occur between chains initiated at the same generation (at the same time). This situation is schematized in (a) of Scheme 2.8.
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As a radical polymerization, the concept of CLRP does anything but eliminate termination and irreversible chain transfer (1 holds), but employs an additive, a so-called deactivator, D in the system which causes a non-mortal chain deactivation with P forming a temporary inactive entity, Int. This temporary inactive entity Int with some incentive can be activated back to the original P and D. Therefore this system exhibits reversible-deactivation (Scheme 2.7). [57-59]
Scheme 2. 7. General description of reversible deactivation and activation.
In this reversible-deactivation, a propagating radical is provided with one other possible state of non-living and non-dead, a ’dormant’ state PX, where X means an atom or group transferred from D (D involves X site) or D itself, and the temporary inactive state Int is identified as PX. (in the case of the reversible-deactivation by degenerative chain transfer, D is equivalent to PX, so that Int is the other state of (PXP), as explained below).
If the mortal nature of a P holds and P itself has a very short living lifetime of a few seconds, from a viewpoint of a chain, the other possible option as PX is long-lasting, and consequently provides a high chance that the chain is living (P or PX) at the end of the entire polymerization process. This gives radical polymerization an apparent ’living’ characteristic. Also, from another viewpoint of all chains in the system, this reversible- deactivation process fulfils the role of exchanging a radical active centre and a dormant cap at the chain-end, then makes the growth of each chain close to equal in real time, which results in narrowing of the MWD of the polymer product. At any given instant, the majority of chains are in the dormant form, and consequently 푀n can be predetermined from the ratio of initial [PX]0 and [M]0 and the fractional conversion 푥. Thus this also gives ’molecular-weight controllability’ to the radical polymerization. The higher the rate of exchange between active and dormant states (relative to other radical reactions, mainly propagation), the more uniform is the chain-growth between different chains (i.e. chains grow at similar rates) and the more effective is the
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‘controllability’. This reversible deactivation can be effected by various mechanisms, and it does not necessarily affect the rate of polymerization.
The concept of CLRP may be understood by use of the following analogy: The lifetimes of individual people are different and have a broad distribution (corresponding to MWD), and they are very short relative to the entire period of humanity (corresponding to the entire polymerization). Somebody belonging to a certain generation is unable to see his offspring appearing centuries later, and any interactions with other humans (corresponding to radical reactions) only occur with others in the same generation. On the contrary, the progress of CLRP is like some science fiction stories, in which a person can prolong his/her living life to future centuries by spending most of life in a cold sleep (corresponding to dormant state), and yet he does anything but deny the natural aging (corresponding to propagating) and mortality (corresponding to chain-stopping events) as a human being, while he is off the sleep (corresponding to a living state). Moreover, if the evolution of the entire human race itself is artificially controlled and designed from just the beginning to the very end, where the total number of the race is predetermined
(corresponding to [PX]0) and all beings start their lives exactly at the origin, occasionally repeating such cold sleep (corresponding to reversible deactivation), the average of the age of all beings are somewhat related to the real time elapsing, (푀n is a function of time or 푥) and all beings have more or less the similar aging at a certain point of the real time (corresponding to narrow MWD), then, the practical dead is to be prolonged on the base of real time axis. In the active state, all humans may interact with each other, but a person can have such experiences in any different era. This is the sense of CLRP.
As such, the ’living’ characteristics in CLRP should not be thought of as perfect immortality but just by a life-sustaining treatment, and may be considered ’intermittent- cryonic’ radical polymerization. The usage of the same term ’living’ with living anionic polymerization which conceptually eliminates the termination, may be confusing.[60] Also the term ’controlled’ is ambiguous and widespread without specifying the target for control. IUPAC has addressed this issue and currently recommends the term ’reversible-deactivation radical polymerization’ which is to the point of the mechanism, though the term ’controlled/living radical polymerization’ is generally accepted as a proper term specifying this type of radical polymerization implicitly.[59]
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(a) Conventional radical polymerization
[P]
0 Time of the course of polymerization t
(b) Controlled/living radical polymerization
[P]
0 Time of the course of polymerization t
Polymer chains at the time t (a) (b)
States of a chain
Living Dormant Dead
Scheme 2. 8. Illustration of chain growth for radical polymerization (a) and controlled/living radical polymerization (b) against the real-time scale.
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2.2.2 Class of Reversible Deactivation Mechanism CLRP is still a relatively young subject and has been proliferating, also new techniques have been emerging. Thus the completion of perfect classification may be a difficult issue for various levels of reasons. In part it may be just the matter of confusion in terminology where other authors coin their own terms, or in part some techniques may have some parts in common mechanistically and schematically but the commonalities do not necessarily follow the same kinetic trend, or vice versa.[59]
The most general description, which simply identifies CLRP from conventional radical polymerization and embraces all CLRP techniques, would be the mechanism of reversible-deactivation in Scheme 2.7.
From a schematic viewpoint, CLRP may be classified into three classes as (a) reversible homolytic combination-dissociation, (b) reversible catalyzed atom (group) transfer, or (c) degenerative chain transfer in Scheme 2.9.[58]
(a) Reversible homolytic combination-coupling
(b) Reversible catalyzed atom (or group) transfer
(c) Degenerative chain transfer
Scheme 2. 9. Classification of mechanisms of reversible-deactivation, reproduced from [58]. 24
Yusuke Sugihara Chapter 2
Here, group (a) is the mechanism of reversible homolytic combination-dissociation, whereby P undergoes homolytic coupling with a stable radical X , and activation occurs by the unimolecular reaction of homolytic cleavage of PX back to the original P and X. The deactivator of stable radical X reacts only with P (as such the stable free radical is also called persistent (free) radical, and the mechanism to establish the reversible deactivation by this type of reversible coupling-cleavage with stable free radical is also called persistent radical effect (PRE) coined by Fisher.[61-64]. This type of CLRP is termed as the generic designation of stable radical mediated polymerization (SRMP)[57, 59] and is schematically identified as the general scheme of reversible deactivation (Scheme 2.7). Nitroxide-mediated polymerization (NMP),[44, 53, 65-67] organoheteroatom-mediated living radical polymerization[68, 69] (including organotellurium (TERP),[70-73] organostibine (SBRP)[74], and organobismuthine- mediated living radical polymerization (BIRP)[75]) fall into this category. Because of the necessity of some sort of stimuli for the homolytic cleavage of PX, the reaction typically requires high temperature.
Group (b) requires additives of catalyst in the system to establish the reversible- deactivation (the dormant species PX needs the catalyst to undergo activation). The deactivation and activation occurs exchanging an atom or group (X is an atom or group) with corresponding catalyst. While this mechanism is not achieved by stable free radical, X-catalyst can play the role as the analogy, so this mechanism is also regarded as PRE- type CLRP.[57, 76] This mechanism is most famous for atom transfer radical polymerization (ATRP),[56, 57, 77, 78] (here including single-electron-transfer living radical polymerization (SET-LRP), [79-81] which is also considered supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP), [82- 84]). Reversible transfer catalyzed polymerization (RTCP),[85-88] and reversible complexation mediated polymerization (RCMP),[89] are slimier class of CLRP techniques which require some catalyst added in the system, though the transferred entities are different.
Group (c) depicts reversible-deactivation by degenerative transfer of a group or an atom. Reversible addition fragmentation chain transfer (RAFT) polymerization,[44, 46, 47] iodine transfer polymerization (ITP),[90] TERP,[71, 72, 91] SBRP,[74, 92] and
25
Yusuke Sugihara Chapter 2
BIRP[75] belong to this group. Degenerative chain transfer is the reaction whereby a radical reacts with a chain transfer agent to generate a new radical and new chain transfer agent, both of which have the same activity as the original species. Thus different from (a) and (b), in the reversible-deactivation by degenerative chain transfer, the dormant species PX works as deactivator of P, then the intermediate state is a radical entity (PXP) or transition state.
The net effect of the chain transfer is simply transfer of the active radical centre between polymer chains, and it follows that the apparent rate constant of chain exchange 푘ex[44, 47, 93] is defined as (Scheme 2.9):
푘act′ 푘ex = 푘deact (13) 푘act + 푘act′
The effectiveness of reversible chain transfer relative to propagation is expressed with the chain exchange constant 퐶ex[44, 47, 93] by,
푅ex 푘ex 퐶ex = = (14) 푅p 푘p
2.2.3 Kinetic Considerations When the reversible-deactivation process is employed in the system, the species in the systems are active chains P, the deactivator additive D, and the intermediate Int, based on the general description of Scheme 2.7. Thus the general description of the rates are:[58]
푑[P ] = 푘 [Int] − 푘 [P][D] + 푅 − 푘 [P]2 (15) 푑푡 act deact i t
푑[D] = 푘 [Int] − 푘 [P][D] (16) 푑푡 act deact
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Yusuke Sugihara Chapter 2
푑[Int] = 푘 [P][D] − 푘 [Int] (17) 푑푡 deact act
Due to the nature of ’reversible’ deactivation, the system does reach (pseudo)-equilibrium at some point.
푘deact[P ]푒[D]푒 = 푘act[Int]푒 (18)
Here, 퐾 = 푘act/푘daect is determined as an equilibrium constant of reversible- deactivation,
[P][D] 퐾 = (19) [Int]
Reversible deactivation is intrinsic to all CLRP techniques, so that the kinetics fundamentally follows the Equations 15-17 and differ from that of conventional radical polymerization. However the practical kinetic characteristics of each technique are not unique, but depend on the impact of the activation rate, as follows.
Large K Equilibrium If the system has a very large 퐾, say, the activation occurs very rapidly, the equilibrium leans to the direction of increasing [P][D] and decreasing [Int]. And if 퐾 is large enough, it is practically approximated as [Int] ≈ 0, while the reversible deactivation keeps occurring effectively to exchange the radical centre between propagating chains. In this limit, the net of deactivation and activation does not change any quantities, and 푑[D] 푑[Int] the term of 푘 [Int] − 푘 [P][D] is to be 0. As a result, = 0 and = 0. act deact 푑푡 푑푡 푑[P] Also the only remaining equation becomes identified as that of conventional radical 푑푡 polymerization.
In general, CLRPs by degenerative transfer (standard RAFT except the case of dithioesters or trithiocarbonates for non-conjugated monomers,[44, 47, 93, 94] TERP,[71, 72, 91] SBRP,[74, 92] BIRP,[75] and ITP[90]) fall into this kinetic system.[58]
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Small K Equilibrium On the contrary, if 퐾 is small, meaning the activation is not very effective, the equilibrium leans to the direction of increasing [Int] and decreasing [P][D]. In this case, at the equilibrium sufficient Int exists in the system and the rate equations (Equations 15-17) must be solved as they are, unless the system reach the steady-state.[58]
In general, reactions based on reversible homolytic combination-dissociation (represented by NMP) and catalyzed atom (group) transfer (represented by ATRP), that is, the group of PRE-type CLRP system,[61-64] follow this kinetic system.
For this condition, Goto and Fukuda,[58, 95] and Fisher [64] have solved the rate equations systematically depending on the initial condition of the reactants as follows.
(1) Steady-state system
In the limit of the steady state and sufficiently high [Int]0 , the terms 푘act[Int] − 푘deact[P ][D] = 0, and [P ] can be expressed as for conventional radical polymerization:
1 푅 2 [P] = ( 푖) . (20) 푘t
Then, from this and the equilibrium equation (the equilibrium is necessary condition of the steady-state), [D] is solved as:
1 푘푡 2 (21) [D] = 퐾[Int]0 ( ) 푅i
[M] Therefore, the plot of ln 0, [M]
1 [M]0 1 푅i 2 (22) ln = ln (1 − ) = 푘p ( ) 푡 [M] 푥 푘t is first-order to the time, and actually it is obvious as the definition of steady-state. The practical concern is the time to reach steady-state. 28
Yusuke Sugihara Chapter 2
(2) Power-law system The concentrations until reaching steady-state must be solved directly from the rate equations. In the condition of 푅i = 0, the pseudo-equilibrium, and sufficiently high
[Int]0 constant to the time, first [D] is solved as:
1 2 2 3 (23) [D] = (3푘t퐾 [Int]0푡 + [D]0)3
Thus, [P] is solved as:
[ ] 퐾 Int 0 [P ] = 1 (24) 2 2 3 3 (3푘t퐾 [Int]0푡 + [D]0)
Therefore, if the system does not employ the deactivator [D]0 = 0,
1 퐾[Int] 3 [P] = ( 0) (25) 3푘t푡
[M] Then, the ln 0 plot, [M]
1 3 [M]0 3 퐾[Int]0 2 (26) ln = 푘p ( ) 푡3 [M] 2 3푘t turns the function of t to the power of 2/3.
Contrary, if the system employs sufficient amount of [D]0,
퐾[Int] [P] = 0 (27) [D]0
[M] thus, the ln 0 plot, [M]
[M] 푘p퐾[Int]0 ln 0 = ( ) 푡 (28) [M] [D]0
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Yusuke Sugihara Chapter 2
is the function of first order with regard to 푡. This means, in this condition without reaching the steady-state, because of the equilibrium with high [D], practically the system behaves as first-order with respect to monomer. Thus this first order by equilibrium must be distinguished from that by that of steady-state.
2.3 Radical Polymerization in Heterogeneous Systems The chemical tolerance to the impurities for the reaction and relatively less demanding to the environment are the distinctive advantages of radical polymerizations, which enable them to be executed under various heterogeneous systems.[2, 3, 96, 97] Heterogeneous condition means some ingredients for the reaction are not soluble in the dispersion medium (continuous phase), but in radical polymerization it is typically referred to for the relationship between the reaction location of polymer particles and the dispersion medium, generally water.
The influences of heterogeneous condition are not only the additional consideration of the direct chemistry between reactive radicals and the dispersion medium. Heterogeneous system means there are different ‘phases’: dispersed phase (mainly reaction location) and continuous phase (dispersion medium), and ‘surface’ between them, then surface and colloidal chemistry and physics, migration and partitioning of reaction ingredients, interaction of particles, and influences of the size, number and the solids content of the particles to the dispersion medium etc are all to be properly considered.[3, 28]
Also, the partitioning of the monomer not only to the reaction location needs alternative methodology for the polymerization rate treatment (Section 2.4.3). Moreover, the form as a dispersion where the reaction location consists of a tremendous amount of small independent particles requires another consideration for the way how to relate the kinetic rules which essentially assume and expect homogeneous conditions. Those kinetic rules whether conventional radical polymerization or CLRP are essentially applicable to each individual particle which is just a local element for the entire dispersion system, but the target of the analysis is the entire system. This issue is ’compartmentalization’ (Section 2.3.4).
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Yusuke Sugihara Chapter 2
Thus, it is learnt in no time that the heterogeneous system is too complex to analyse it all at once. Then the only way for us to develop the understanding of radical polymerizations in it is to dissert it into little controllable pieces and study them separately.
2.3.1 Events in Heterogeneous Polymerization In the heterogeneous radical polymerization process, the main reaction location is a dispersion phase of polymer particles and the radical polymerization following in the last chapters take place in each particles independently. However, not only polymer particles but also monomer droplets and micelles possibly coexist as other dispersed phases in the system. Therefore, radical polymerization in heterogeneous systems is to be considered as composite processes of not only radical polymerization itself but also other physical events as follows.[3, 98]
Particle Formation In a heterogeneous system, the main reaction location of polymer particles must be prepared as a dispersion in the system. In one way, preliminary prepared particles can be employed as seed (in the case, the methodology is termed ’seeded’ system) in the system. However, the standard approach is ’in situ’ system, and the particles are formed by nucleation processes during the course of the polymerization. Particle nucleation has a variety of options depending on the type of nucleus. Each schematic image is inserted in following items below in Schemes 2.10-12, while the general description of heterogeneous polymerization process as a whole is described in Scheme 2.13.
(1) Micellar nucleation Surfactant (or emulsifier, or stabilizer) is surface-active molecule and a requisite of heterogeneous polymerizations. The primary purpose is to locate on the surface of monomer droplets or polymer particles and decrease their surface tension, so that those droplets and particles can keep the form of minute dispersion and huge interfacial area. Moreover, when the excess amount of surfactants in dispersion medium reaches its
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Yusuke Sugihara Chapter 2
critical micelle concentration CMC they start to form micelles. While surfactant micelle is a dynamic entity and the surfactants are going out and coming in constantly, they are in equilibrium as a whole and swell with monomers or other ingredients. Thus, micellar nucleation is ignited by the entry event of primary or oligomer radicals from the continuous phase in typical emulsion polymerization, and it may be touched by internal initiation in the micelle itself and subsequent propagations if the initiator stays in the micelle in some microemulsion polymerization.
Nucleation (1)
Micelle Polymer particle (1) Internal initiation and propagations (2) External initiation, propagations and entry
Scheme 2. 10. General description of micellar nucleation.
(2) Homogeneous nucleation Homogeneous nucleation takes place by propagating radicals themselves when radicals (typically low degree of polymerization of oligomer-radical) grow enough to precipitate by reaching its critical oligomer degree of polymerization for precipitation, jcrit,[99-101] or form micelles with their sufficient surface activity, before captured by micelles, monomer droplets, or polymer particles.
Initiation and propagations Nucleation
Polymer particle
Chain number reaches the jcret
Scheme 2. 11. General description of homogeneous nucleation.
(3) Droplet nucleation If primary or oligomer radicals are captured by monomer droplets or monomer droplets have radical source in themselves, nucleation occurs in the monomer droplets. In droplet nucleation, the monomer droplets have sizes and its distribution, and they are directly reflected by the particle size and distribution. 32
Yusuke Sugihara Chapter 2
Nucleation
(1)
Monomer droplet Polymer particle (1) Internal initiation and propagations (2) External initiation, propagations and entry
Scheme 2. 12. General description of droplet nucleation.
Each class of heterogeneous polymerization system has its primary nucleation process. Typically coincidence of other side nucleation processes causes disproportion in particle size and decreasing the quality of the final polymer latex, so that proper tuning of the reaction conditions to avoid undesirable side nucleation is important for the reaction design.
Reactant Diffusion Unless nucleation occurs as droplet nucleation, reactants for the radical polymerization must be properly diffused to the reaction location of polymer particles, from the other dispersed phase or the dispersion medium. The polymer particles keep propagation and consuming monomer. Thus, even if the solubility of monomer is very low in the dispersion medium, the equilibrium of monomer with other dispersed phase works for the monomer to diffuse to the particles, in which monomer droplets only act monomer reservoirs. On the contrary, oligomer and polymer are typically non soluble in dispersion medium then the diffusion of them from polymer particles back to monomer droplets do not happen, and this irreversibility is the reason for each polymerization techniques to be able to design and control the particle size, depending on the nucleation process.
However, this diffusion process has often times difficulty in the application to CLRP to heterogeneous system. For the successful CLRP, the additives for CLRP must be located to the reaction location to participate in the reaction, and otherwise those additives in other places cannot play the role. As such, as mild diffusion as monomer is not sufficient. Moreover, the necessity of the diffusion process is completely mismatch if the CLRP additives are macromolecules non-soluble in the dispersion medium. This is the main reason of the difficulty of CLRP application to various heterogeneous polymerization techniques, unless it has droplet nucleation or homogeneous nucleation.
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2.3.2 Reaction Intervals in Emulsion Polymerization As a result of the physical events of particle formation and reactant diffusion as with radical polymerization itself, the period of heterogeneous polymerizations can be classified into three representative intervals each of which has its unique kinetic characteristics.[2, 3, 28, 98, 102-105] This classification is established in the development of the kinetics of ’emulsion polymerization’, one of the most general purpose heterogeneous techniques, and generally referred for discussing its kinetics. However, the process classification based on the physical conditions of the system which clearly describes the kinetic behaviour, is also useful and applicable even to refer to and understand the mechanism of other heterogeneous techniques.
(1) Interval I (in emulsion polymerization) Interval I in emulsion polymerization is defined as the period during which the particle nucleation take places. Thus this period technically occurs in ’ab initio’ heterogeneous polymerization, not ’seeded’ system. Because of the consecutive particle nucleation, the number of particles in the system continues to increase. The nucleation ceases when the nuclei give out or decrease so that nucleation can hardly occur. The entire polymerization rate typically gets to increase because of increase of the number of particles.
(2) Interval II (in emulsion polymerization) Interval II in emulsion polymerization is the period during which new particle formation does not occur but monomer droplets exist as monomer reservoirs. In the period the number of particles does not change, unless undesirable coagulation or side nucleation occur. From the viewpoint of reaction location of polymer particle, the consecutive supply of monomer from monomer droplets can keep [M] constant, despite its consumption by propagation, then it leads to the constant polymerization rate behaviour for both a particle and the entire system.
(3) Interval III (in emulsion polymerization) Interval III in emulsion polymerization is defined as the period when all monomer droplets and other outer monomer sources are exhausted, then the rest of monomers in the system only exist in polymer particles. Therefore, in the period, each particle behaves
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Yusuke Sugihara Chapter 2
like closed homogeneous system, then the rate of polymerization retards with the decrease of [M].
Interval I Interval II Interval III
Monomer droplet
Micelle Surfactant Polymer particle
Scheme 2. 13. General description of three intervals in emulsion polymerization.
2.3.3 Polymerization Rate in Heterogeneous System As mentioned above in 2.1.5, the fractional conversion of polymerization 푥, and the rate 푑푥 of conversion, that is, the fractional polymerization rate of the entire system , are 푑푡 essentially based on the mass conversion of the monomer to the polymer in the reaction system.
In heterogeneous system, the reaction location of each particle itself follows the kinetic system of homogeneous system. However, the monomer possibly exists not only in the polymer particles, but also in other phases such as monomer swollen micelles, monomer droplets, and dispersion medium. As such, the mass change of the monomer of the entire system is not directly proportional to that of concentration in the reaction space, and the 푑푥 entire 푥 and are not directly related with the reaction rate in the particle. Thus, the 푑푡 framework of the entire polymerization rate differs from that of heterogeneous system, based on the same root. It is generally divided into two methodology based on mass conversion and concentration conversion, as follows.[3, 28]
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Yusuke Sugihara Chapter 2
Polymerization Rate Based on Mass Conversion In the case of the heterogeneous system in which the monomer is partitioned not only in the polymer particles, the entire polymerization rate is built from the rate of entire mass 푑푚 consumption M, 푑푡
푑푚 푘p[M] 푛̅ 푀푁cr M = − (29) 푑푡 푁A where 푛̅ is the mean value of the number of propagating radical for a particle, 푀 the molecular weight of monomer, 푁cr the number of particle to the entire system, 푁A the Avogadro number. And the rate of fractional conversion is,
푑푥 1 푑푚 1 푘p[M]푛̅ 푀푁cr = (− M) = ( ) (30) 푑푡 푚M0 dt 푚M0 푁A
This is redescribed with the modification of 푁cr and 푚M0 to the value of concentration per the volume of dispersion medium 푉w, (this subscript w shorts for ’water’ as the primary option of continuous phase is water)
푑푥 푘p[M] 푛̅ 푁c = (31) 푑푡 푛M0푁A
where 푛M0 = 푚M0/(푀 푉w) is the moles of initial monomer per unit volume of continuous phase, and 푁c the number concentration of the particle (dispersed phase of reaction location) to the continuous phase. Here 푛M0 may seem something odd expression as physically it can mean the concentration of entire monomer (staying in any of dispersed phases not only polymer particles but also monomer droplets or even micelles) to a continuous phase, and from the derivation it is clear that this is the value just for convenience to process this equation coming from 푚M0 the initial entire mass 푑푥 which arises for normalization to the concept of . 푑푡 The importance of this form is that it can properly consider the existence of phases which contain monomer (monomer droplets, monomer-swollen micelles and even monomer- dissolved continuous phase) other than reaction location of polymer particles to 36
Yusuke Sugihara Chapter 2
associate the entire polymerization rate, and practically many heterogeneous polymerization system follows this manner, except for Interval III of emulsion polymerization or ideal miniemulsion polymerization.
Polymerization Rate Based on Concentration On the contrary, in the ideal condition of the heterogeneous system that there are no phases containing monomer other than the reaction locus of the polymer particles, it is possible to directly associate the entire mass of monomer in the system 푚M with the mean value of the concentration of monomer for the entire reaction location of polymer particle [M]. In this case, the entire polymerization rate equation derived in Section 2.1.5 for homogeneous system can be applied to the those heterogeneous system as,
푑푥 1 푑[M] 푅p 푘p[P ][M] = (− ) = = (32) 푑푡 [M]0 푑푡 [M]0 [M]0
This condition is realized in Interval III of emulsion polymerization or ideal miniemulsion polymerization. Actually, most kinetic discussion of miniemulsion polymerization is with this consideration that the existence and influence of the monomer in other phases than polymer particles to the kinetics should be successfully neglected.
It is noted that this classification of the entire polymerization rate whether based on 푚M in Equation 31 or [M] in Equation 32 in this section is independent of and compatible with the consideration of compartmentalization in the next section. This entire polymerization rate is very the concept directly stick to ’monomer’ consumption fundamentally equated in the Equation 6, and this section is the description of how to associate it with kinetic parameters in each cases. On the contrary, the reactant concentration and elemental reaction rates for a particle and those mean value for the entire polymer particles are influenced by the form of ’dispersion’ and the ’particle size’, that is, compartmentalization. How to derive the mean value of 푛̅ and [P] is dealt with in the next section and depending on the condition of monomer location those value is to be applied to the Equation 31 or Equation 32 to associate the entire polymerization rate.
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Yusuke Sugihara Chapter 2
2.3.4 Compartmentalization In heterogeneous polymerizations, the main locus of reaction is composed of an enormous amounts of polymer particles, which are seen as “nanoreactors”. This is an inherent nature when the reaction space is dispersed, and has a most important impact on establishing the specific kinetics of emulsion polymerization, which differs from that of the corresponding homogeneous system. This concept is called ’compartmentalization’ for the reaction rates.[3, 28]
Smith-Ewart Theory for the Kinetics of Emulsion Polymerization
The current theoretical system of the kinetics of emulsion polymerization, which is based on the compartmentalization of the propagating radicals into discrete polymerization locations of polymer particles in the dispersion medium, was first conceptually and qualitatively described by Harkins,[105] and then expressed mathematically by Smith and Ewart.[106] In the theory, the populations of particles containing 푛 radicals is given by the Smith-Ewart equation:
푑 푁(푛) = 휌[푁(푛−1) − 푁(푛)] + 푘[(푛 + 1)푁(푛+1) − 푛푁(푛)] 푑푡 (33)
+ 푐[(푛 + 2)(푛 + 1)푁(푛+2) − 푛(푛 − 1)푁(푛)] where 푛 is 0 and integers which specifies the number of propagating radicals in a single particle, 휌 is the pseudo-first-order rate coefficient for entry from the aqueous phase, 푘 is the pseudo-first order rate coefficient for radical exit of a single radical from a particle, and 푐 is the pseudo-first-order rate coefficient for bimolecular termination in a single particle.
This 푁(n) is a probability distribution of particle state with respect to 푛, and the average number of radicals for a single particle (also for the entire population of particles) is treated as the mean value of
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Yusuke Sugihara Chapter 2
∑ 푁(푛) = 1 (34) 푛
푛̅ = ∑ 푛푁(푛) (35) 푛
In emulsion polymerization of styrene, the rate of polymerization is described as a function of 푛̅ regardless of Interval I ~ III, so that the kinetics can be discussed based on the solution for the Smith-Ewart equation (Equation 33) for each 푁(n).
In equation 33, the parameters are 휌, 푘, and 푐. In the original work, Smith and Ewart established three possible kinetics cases:
Case 1: Fast desorption of propagating radical (large 푘)
푛̅ ≪ 0.5 (36)
Case 2: No desorption and fast termination (No 푘 and large 푐)
푛̅ = 0.5 (37)
Case 3: Fast desorption and slow termination (large 푘 and small 푐)
푛̅ ≫ 1.0 (38)
However, the rate coefficients 휌, 푘 and 푐 in Equation 33 are phenomenological values which are functions of various practical parameters, and it is very difficult to directly solve the equations without considering numerous assumptions. As such, the kinetics of emulsion polymerization was first believed to mainly follow 푛̅ = 0.5 (Smith-Ewart case 2) and subsequently 푛̅ ≫ 1 (case 3).[106, 107]
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The Smith-Ewart equation case 3, 푛̅ ≫ 1, is familiar as ‘pseudo-bulk’ limit. Pseudo-bulk limit means the particular condition of heterogeneous system in which the reaction kinetics is not influenced at least by the compartmentalization, and is the same as the counterpart of homogeneous environment, unless other factors like reactants partitioning among other phases are in effect.[3, 28]
Establishment of Zero-one Approximation
The direct resolution of these complicated kinetics was achieved in the early 1980s, when Hawkett and coworkers developed the practically applicable concept of zero-one approximation under the Smith-Ewart theory.[108, 109] This work is summarized as follows. The reduced bimolecular termination rate coefficient 푐 is the function of the termination rate coefficient 푘t and the monomer swollen particle volume 푣s:
푘 푐 = t (39) 푁A푣s
−1 where 푁A is the Avogadro’s number. This equation states that 푐 is a function of 푣s and that 푐 increases with decreasing particle size. Thus, for sufficiently small particle sizes, the system would produce the condition of 휌 ≪ 푐, where the radical entry in the particle which contains a radical leads to instantaneous termination, and the entry rate practically approximates the termination rate. In other words, the radical number in a particle is like binary digits toggled by the periodic entry events, and always takes on only 0 or 1 and hardly exceeds 1. Thus, under the zero-one limit, the Smith-Ewart equations for 푁(n) can be reduced to the simple matter of only two equations of 푁(0) and 푁(1) with approximating 푐 to 휌 as,
푑 푁 = 휌(푁 − 푁 ) + 푘푁 (40) 푑푡 (0) (1) (0) (1)
푑 푁 = 휌(푁 + 푁 ) − 푘푁 (41) 푑푡 (1) (0) (1) (1)
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Yusuke Sugihara Chapter 2
And, in zero-one limit, 푛̅ can also be reduced to the simple form of:
푛̅ = ∑ 푛푁(푛) = 푁(1) (42)
Therefore, the simultaneous rate equations (Equations 40 and 41) are resolved in terms of 푁(1) = 푛̅:
휌 휌 푛̅ = 푁 = + (푁 (푡 = 0) − ) 푒−2(휌+푘)푡 (43) (1) 2휌 + 푘 (1) 2휌 + 푘
(and 푁(0) = 1 − 푁(1) ). Also in the limit of 푡 → ∞, it approaches the asymptote,
휌 푛̅ = 푁 = (44) ss (1),ss 2휌 + 푘
The solution of Equations 43 and 44 state that at the zero-one limit,
1. 푛̅ possibly takes at most 0.5, and 푛̅ = 0.5 is attained in its steady-state 푛̅ss when 휌 ≫ 푘. This is when Smith-Ewart case 2 holds. 2. if 푘 is significant relative to 휌, 푛̅ becomes less than 0.5 (Smith-Ewart case 1) even in the steady-state. Thus, whether the system is Smith-Ewart case 1 or 2 in the steady- state depends on 푘.
This was the clear description that the particle growth of emulsion polymerization could take both Smith-Ewart case 1 and 2 as the result of zero-one limit.
The theoretical development of zero-one approximation by Hawkett and co- workers[108] had a very good accordance with the experiment. The meticulous kinetic experiments of seeded emulsion polymerization of styrene with rigorously determined particle size and number, and strict conversion reading by dilatometry demonstrated the long-time steady-state characteristics as expected in Equation 44 in which 푛̅ss was accurately 0.5 and decreased with the decrease of 휌 (as the function of the initiator concentration, which relatively increases the significance of 푘). This accuracy of the experiments enabled the direct measurement of unambiguous values of the 휌 and k by
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the ‘slope-and-intersect’ method from the linearity of conversion-time plot at the zero- one steady-state.
Development of Zero-One Kinetics of Emulsion Polymerization of Styrene
Since this first achievement of the theoretical and experimental methodology, the understanding of emulsion polymerization kinetics has been up-to-date developed.[3, 28] The significant effort and interest in this field was to develop a full description of the phenomenological rate coefficients 휌 and 푘 from the elemental events on the microscopic level.[3, 28, 110-112] In particular, the approach to directly treat the phenomenological values requires specific assumptions on the treatment of the fate of the exited radical such as re-entry and aqueous phase termination, and it should have a critical impact on the detailed description of emulsion polymerization kinetics. Considering the reality on the microscopic level, radical exit for a typical hydrophobic monomer of styrene, can only occur if the propagating radical is a monomeric entity M,[112] as the solubility of hydrocarbon molecules is significantly dependent on the molecular volume and the dimeric radical is less soluble by several orders of magnitude.[113] Therefore, detailed studies were conducted with the evolution model of the zero-one reduced Smith-Ewart equations (Equations 40 and 41) to distinguish m particle population which has a monomeric radical 푁(1) and that which has a polymeric p radicals 푁(1) and consider possible elemental events with the consideration of reactions in the continuous phase.
푑 푁 = 휌(푁p + 푁m − 푁 ) + 푘 푁m (45) 푑푡 (0) (1) (1) (0) dM (1)
푑 푁m = 휌 푁 − 휌푁m + 푘 푁m + 푘 퐶 푁p − 푘1퐶 푁m (46) 푑푡 (1) re (0) (1) dM (1) tr,M p (1) p p (1)
푑 푁p = 휌푁 − 휌푁p − 푘 퐶 푁p + 푘1퐶 푁m (47) 푑푡 (1) (0) (1) tr,M p (1) p p (1)
where 휌re is the pseudo-first-order rate coefficient for re-entry of an exited radical, 푘dM 1 is the exit rate coefficient, 푘tr,M the chain transfer coefficient to monomer and 푘p the 42
Yusuke Sugihara Chapter 2
propagating rate coefficient of initiating radical. This 푘dM is different from the phenomenological exit value of 푘 and derived from the microscopic event of equilibrium of radical adsorption and desorption between a particle and the dispersion medium of water.[3, 28, 114]
Processing of the detailed model was too complicated to resolve directly, but two sub- limits of zero-one kinetics were derived depending on specifying the major options for the fate of the exited radical.[3, 28, 111, 112] In the limit where the exited radicals completely follows aqueous phase termination (Limit 1):
푑 푛̅ = 휌(1 − 2푛̅) − 푘푛̅ (48) 푑푡
On the contrary, in the limit where the exited radicals completely undergoes re-entry into particles (Limit 2):
푑 푛̅ = 휌(1 + 2푛̅) − 2푘푛̅2 (49) 푑푡
Recent meticulous experiments with γ -irradiation[27, 31] (Section 2.1.1) and amphiphilic block copolymers[27, 31, 115-117] thanks to the development of reversible addition-fragmentation chain transfer (RAFT) polymerization enabled the synthesis of well-defined amphiphilic block copolymers and their application as electrosteric surfactants for various heterogeneous polymerization techniques,[31, 115-128] have demonstrated that, while there still remain the mystery and argument about the real kinetics of radical exit and the reason, the experimental results were consistent with first order kinetics at least phenomenologically.
Compartmentalization in CLRP
The effects of compartmentalization on the kinetics appear on the compartmentalized entities and on the reaction rates which involve those entities. For the development of compartmentalized kinetics of emulsion polymerization in conventional radical polymerization, one compartmentalized reactant P consideration is sufficient to establish an approximate picture of the full situation (strictly, P is further subdivided
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into monomeric radical M produced by chain transfer to monomer reaction, and the other).
Application of the Smith-Ewart theory to CLRP in order to explain the kinetics based on compartmentalization was established by Butte and coworkers,[129] and followed by various researchers.[130-145] CLRP is the technique to control chain-growth by employing additional reactants, deactivator D and corresponding intermediate entity Int in the system (Section 2.2). Therefore, it leads to an increase in the number of rate equations that must be considered (Section 2.2.1). From a mechanistic viewpoint, reversible homolytic combination-dissociation type and reversible catalyzed atom (or group) transfer type CLRP like NMP and ATRP produce both a P and a D from Int by an activation event. Thus, under conditions where P is compartmentalized, it is likely that D would be compartmentalized too. Thus the effects compartmentalization are to be considered for both entities P and D.
This is based on the same Smith-Ewart theory as that developed for emulsion polymerization for conventional radical polymerization described above. However, while the focus point in the study of emulsion polymerization is the reactant diffusion (entry 휌 and exit 푘 of the propagating radical P ) between a particle (the dispersed phase) and the dispersion medium (continuous phase), most studies in CLRP has limited the condition as an ideal miniemulsion system which completely eliminates reactant diffusion, treating the situation as a closed system, mainly because of the difficulty of model and algorithm development. As such, the original Smith-Ewart equation (Equation 33) was modified into a 2-dimension model in terms of the two reactants of P and D, for example considered activation, deactivation, termination and thermal initiation of monomer,[131]
푑 푁 = 푁 푣 푘 [PX]{푁 − 푁 } 푑푡 (푖,푗) A s act (푖−1,푗−1) (푖,푗) [ ]3 + 0.5푘i.th M {푁(푖−2,푗) − 푁(푖,푗)} (50) −1 + (푁A푣푠) 푘deact{(푖 + 1)(푗 + 1)푁(푖,푗) − 푖 푗푁(푖,푗)} −1 + (푁A푣s) 푘t{(푖 + 2)(푖 + 1) − 푖(푖 − 1)푁(푖,푗)}
44
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where 푖 and 푗 is 0 and integers which specify the number of P and D, in a single particle respectively.
This 푁(푖,푗) is a probability distribution of particle state with respect to 푛,
∑ 푁(푖,푗) = 1 (51) 푖,푗 and the average number of radicals for a single particle (also for the entire population of particles) is treated as the mean value of
푖̅ = ∑ 푖푁(푖,푗) (52) 푖,푗
푗̅ = ∑ 푗푁(푖,푗) (53) 푖,푗
In miniemulsion polymerization with NMP and ATRP (and following PRE-type techniques), the rate of polymerization is described as a function of 푖,̅ so that the kinetics can be discussed by giving the solution for the modified Smith-Ewart equation (Equation
50) for each 푁(푖,푗).
Because of the increase in the compartmentalized entities not only P but also D, the modified Smith-Ewart equation is more complicated to solve directly. However, all studies of the numerical analysis observe the same trend that NMP and ATRP in miniemulsion polymerization in the absent of reactant diffusion are influenced by compartmentalization, and it works to decrease the entire polymerization rate, increase the controllability and livingness. The smaller the particles, the more significant the polymerization rate retardation and the better the quality of the CLRP.
Zetterlund and Okubo[131] discussed the reason of the particular kinetics and found the influence of compartmentalization on [P] shown in Figure 2.1: (1) In the small particle 3 [P ] has a relationship of the proportion to 푑 (trivially proportion to 푣s), (2) it has a threshold at some particle size depending on the initial condition above which kinetics in miniemulsion polymerization exceeds that of corresponding bulk condition, (3) there
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is a certain maximum particle size at which the [P] has a maximum value, and (4) above the maximum the [P] decreases and reaches constant to the corresponding bulk condition.
Also, as the nature of reversible-deactivation, NMP and ATRP (PRE-typed CLRP) have a certain equilibrium constant 퐾 = 푘act/푘deact in the homogeneous system, depending on the condition. Zetterlund and Okubo[131] discussed that for the miniemulsion polymerization under compartmentalization, it was not the case to follow the homogeneous system: compartmentalization works on 퐾 which decreased significantly to the level of orders of several magnitude and it would be the reason to increase the controllability and the livingness.
Current studies not only based on Smith-Ewart theory,[131-146] but also Monte Carlo modelling[147-150], follow this paradigm.
Figure 2. 1. Simulated propagating radical concentration ([P]; ퟏퟎ% conversion) for
TEMPO-mediated radical polymerizations of styrene ([PS-TEMPO]0 = 0.02 M) with () and without () thermal initiation at 125 C for different particle diameters (d). The horizontal lines show [P] in the corresponding bulk systems. Reprinted with permission from ref [131]. Copyright 2006 American Chemical Society.
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2.3.5 Classes of Heterogeneous Polymerization Radical polymerization in heterogeneous system is the process such that the final form of polymer product is polymer latex in the dispersion medium. Thus, the reaction involves not only radical polymerization itself but also other events, in which particle formation is the most conspicuous process. It takes place with various nucleation manners, which results in the formation of different size of polymer particles ranging from 10 nm to over 1 µm. Therefore, depending on the purpose there are variety of heterogeneous polymerization systems available. [2, 3, 97]
The general descriptions of various heterogeneous polymerization systems follow in the rest of this section and it may be noted as a common confusing point that the specific name of each heterogeneous polymerization techniques are termed by quite customary manner, and the compound terms of ’X + polymerization’ does not necessarily follow that the polymerization takes place in the ’X’ condition. (e.g. monomer emulsion is defined as dispersion of monomer droplets in the dispersion medium, but emulsion polymerization does not take place in the monomer droplets, but polymer particles derived by nucleation of micelles).[3]
Emulsion Polymerization Among all heterogeneous polymerization systems, emulsion polymerization is the most common and industrially important technique.[2, 3, 28, 102-104] The subject of heterogeneous polymerization was first motivated by the impetus to produce synthetic rubber products around in World War I, and while really the first achievement was by suspension polymerization in the current definition, over the history of heterogeneous polymerization the centre of theoretical and practical development has been with emulsion polymerization.[2, 3]
The general emulsion polymerization is conducted with monomer, surfactant, and water- soluble initiator, and water as the dispersion medium. Surfactant is employed more than the CMC and forms micelles which are the size of more or less 10 nm in diameter, and monomer is sheared into micron scale of droplets by a mild agitation such as a magnetic stirring. Then, the initial condition of before the polymerization is two dispersed phases of monomer droplets and monomer swollen micelles in the dispersion medium of water.
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Although it depends on the initial ratio of the ingredients, typically the number of micelles in water is much higher than that of monomer droplets by several orders of magnitude, so that radicals produced by water-soluble initiator in water phase do hardly encounter the monomer droplets but only are captured by micelles. As such, particle formation in emulsion polymerization is typically occurring via miceller nucleation. The progress of the emulsion polymerization really follows the Interval I to III description in the previous Section 2.3.2, and the final form of the reaction is polymer latex of 50-300 nm in diameter.[2, 3, 28]
From the handling viewpoint, the preparation of emulsion polymerization is quite simple and what is need is just to add all ingredients with mild agitation, without special mechanistic devices, high energy, nor thermodynamic and physical treatment. This non- demanding manner for the system preparation makes emulsion polymerization most convenient and general-purpose heterogeneous technique. Another important advantage of emulsion polymerization is that the detailed and significant understanding of the mechanisms and kinetics of heterogeneous polymerization has been achieved in this emulsion polymerization. Particular kinetics by compartmentalization and Smith-Ewart equation has been developed and evidenced in emulsion polymerization to the level that the theory and empirical results are in good agreement.[3, 28] While emulsion polymerization is one of the most complicated heterogeneous polymerization techniques, it is the most matured and certain technique.
However, the mechanism of emulsion polymerization which requires the ingredient diffusion into the reaction locus of polymer particles, which can cause critical problems in CLRP applications. If it is employed in the reaction system, the CLRP agent cannot participate in the reaction, unless it is properly diffused into polymer particles from the original location of monomer droplets. As a result, the polymerization does not achieve effective controllability and livingness.[151, 152] For application of CLRP, one excellent solution has been achieved by Ferguson and coworkers,[118, 119] who used CLRP agent possessing hydrophilic oligomeric anchor so that particle nucleation takes place as homogeneous self-assembly of the propagating radicals with active CLRP end, consequently all CLRP agent can be involved in the reaction location of polymer particle at the end of nucleation process. This approach has been implemented with RAFT, ATRP, and NMP.[153] This system enables CLRP to be implemented into emulsion
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polymerization system without any pre-treatment. Actually, while it customary and schematically falls into emulsion polymerization, the mechanism of self-assembly nucleation has an analogy with homogeneous nucleation in dispersion polymerization (discussed below), different from micellar nucleation of normal emulsion polymerization. Regardless of its successful application to CLRP, fundamentally the final form is a latex of amphiphilic block copolymer of first hydrophilic anchor and main hydrophobic anchor, not the homopolymer. This technique also nowadays aims at the synthesis of various non-spherical morphologic polymer particles.[153] However the application of CLRP in the true emulsion polymerization system as a means to produce general polymer latex is still challenging.
Miniemulsion Polymerization
Miniemulsion polymerization is the heterogeneous polymerization system in which the initial system condition is monomer miniemulsion, and also nucleation process takes place in each miniemulsion as droplet nucleation.[154-156] Miniemulsion is a particular case of emulsion where the oil-droplets as dispersion have the diameters in the range from approximately 50 nm to 1 µm, therefore the final latex of miniemulsion polymerization has the same range of particle size. Also different from microemulsion, miniemulsion is thermodynamically unstable and not spontaneously formable,[157] so the preparation of such smaller droplets requires high-energy input emulsification devises, such as high-shearing stirring and high pressure homogenizers.
Because of the difference in the nucleation process, miniemulsion polymerization has a quite opposite strategic feature for the ingredients from emulsion polymerization. In this process, miceller nucleation is to be avoided as an undesirable side nucleation so that surfactant is required only to successfully stabilize the miniemulsion and employed by less than the CMC. Also oil-soluble initiator is more preferred for the droplet nucleation than water-soluble initiator, though water-soluble initiator is also available unless micelles exist inducing micellar nucleation in the dispersion medium.
Conceptually, this droplet-nucleation-based polymerization system can eliminate the necessity of ingredients diffusion and all monomer droplets works the role of reaction location not monomer reservoir. As such, the process is considered ’full-time Interval
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III’ equivalent of emulsion polymerization (Scheme 2.14), and typically the polymerization rate can be discussed with the [M] based on Equation 32.
Before Polymerization (Miniemulsion)
Monomer droplet Polymer particle
Scheme 2. 14. General description of miniemulsion polymerization.
This simplicity of the same kinetic consideration as homogeneous system, ideally which can discuss with the same equation as homogeneous system is one advantage of miniemulsion polymerization. Also the no-necessity of ingredients diffusion is a very good match with the application of any of CLRP techniques, and practically the main stream of CLRP development in heterogeneous system have been in this miniemulsion polymerization system.[151, 152, 154, 156, 158-163] However, the necessity of external high-energetic shearing device for the emulsification seriously limits the executable scale of the reaction only to laboratory level, and this is the critical drawback of miniemulsion polymerization. In other words, the further development of CLRP in heterogeneous polymerization as a general purpose technique for the industrial level, is to establish successful alternative which does not demand external devises for the commencement.
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Microemulsion Polymerization
Microemulsion polymerization is the heterogeneous polymerization system in which the initial system condition is monomer microemulsion.[164-166] Microemulsion is a special case of emulsion consisting of water immiscible oil, water and a huge amount of surfactant, in which all oil is completely diffused in a large number of micelles, then the system is thermodynamically stable and seemingly one phase. Therefore, microemulsion polymerization is considered one-side limit of emulsion polymerization where the number of micelle is large enough to swell with all monomer perfectly and no monomer droplets exist in the system. As with emulsion polymerization, the particle formation takes place as micellar nucleation, but the number of nuclei is so huge that consecutive particle formation would not cease by later phase of polymerization, the huge number of monomer swollen micelles do work as monomer reservoir, unless the nucleation is ignited in them. As a whole the system is seen as ’full-time Interval I’ or ’Interval I to III’ (skipping Interval II) systems of emulsion polymerization. During the process the particle number keeps increasing, also the number is extremely higher than corresponding emulsion polymerization. It results in relatively small particle size (the diameter is in 10-30 nm or even larger the subsequent coagulum nucleation follows).
Before Polymerization (Microemulsion)
Micelle Surfactant Polymer particle
Scheme 2. 15. General description of microemulsion polymerization.
Different from emulsion polymerization, the initial condition of thermodynamically stable microemulsion and the existence of CLRP agent in the nucleation location of monomer swollen micelles enables microemulsion polymerization to apply CLRP
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techniques, and there are various practical reports of the successful application.[146, 167-170] The fact of practical application of CLRP while the system still requires monomer and CLRP agent diffusion is to be discussed to develop the understanding of the real mechanism of microemulsion polymerization.
Dispersion Polymerization
Dispersion polymerization is a heterogeneous polymerization which starts in homogeneous condition with a monomer, surfactant, and initiator all dissolved in the dispersion medium.[99, 100] While the monomer can be soluble in the dispersion medium, more than certain chain-length of polymer chains cannot (the limit length is
퐽crit). Thus, the propagating chains undergo homogeneous nucleation, forming polymer particles, as they reach 퐽crit.[101] After then, polymer particles can swell with monomer and initiator and becomes main reaction location, and the continuous phase rather works as the monomer reservoir, then the final form of the polymerization is polymer latex.
Before Polymerization (Homogeneous Solution)
Surfactant Polymer particle
Scheme 2. 16. General description of dispersion polymerization.
The nucleation process of dispersion polymerization is complex.[171, 172] Also there are particular types of combinations of dispersion medium and surfactant to apply to each monomer, so that it may be difficult to find the universal knowledge for the entire frame of dispersion polymerization and also the synthesis of particular characteristics (such as size and morphology) of polymer latex of particular monomer can be successful based on rather than empirical knowledge than theory. But as typical characteristics, 52
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dispersion polymerization tends to produce larger polymer particle over micron scale than other heterogeneous techniques, it may be because polymer particles not only grow by monomer swelling but also undergo consecutive coalescence nucleation, as with its smaller amount of surfactant.
Mechanistically, precipitation polymerization is a similar heterogeneous polymerization with dispersion polymerization.[173] It usually starts from a homogeneous condition, where monomer, surfactant and initiator are dissolved in the dispersion medium, and undergoes homogeneous nucleation as the propagating chains reach 퐽crit. However in this system, polymer cannot swell monomer, so that even after the completion of the nucleation the main reaction location is still in the dispersion medium and polymer particle grows by consecutive precipitations of the growing chains on the surface of particles.
2.4 References 1. Moad, G. and D.H. Solomon, The Chemistry of Radical Polymerization. 2006: Elsevier. 2. Odian, G., Principles of Polymerization. 2004: Wiley. 3. Gilbert, R.G., Emulsion Polymerization: A Mechanistic Approach. 1995: Academic Press. 4. Flory, P.J., Principles of Polymer Chemistry: Paul J. Flory. 1953: Cornell University. 5. Flory, P.J., The Mechanism of Vinyl Polymerizations1. J. Am. Chem. Soc., 1937. 59: p. 241-253. 6. Moad, G. and D.H. Solomon, 3 - Initiation, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 49- 166. 7. Solomon, D.H. and G. Moad, Initiation. The Reactions of Primary Radicals. Makromolekulare Chemie. Macromolecular Symposia, 1987. 10-11: p. 109-125. 8. Gilbert, R.G. and S.C. Smith, Theory of Unimolecular and Recombination Reactions. 1990: Blackwell Scientific Publications. 9. Bandlish, B.K., et al., Substituent Effects in Radical Reactions. Iii. Thermolysis of Substituted Phenylazomethanes, 3,5-Diphenyl-1-Pyrazolines, and Azopropanes. J. Am. Chem. Soc., 1975. 97: p. 5856-5862. 53
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10. Suzuki, T., et al., Measurement of Tetrad Configurations in Poly(Methyl Acrylate) by 300 Mhz Pmr Spectroscopy. Journal of Polymer Science: Polymer Letters Edition, 1974. 12: p. 635-640. 11. Noyes, R.M., Effects of Diffusion Rates on Chemical Kinetics. Prog. React. Kinet. Mech., 1961. 1: p. 129-160. 12. Bartoň, J. and E. Borsig, Complexes in Free-Radical Polymerization. 1988: Elsevier. 13. Fukuda, T., Y.D. Ma, and H. Inagaki, Free-Radical Copolymerization. 3. Determination of Rate Constants of Propagation and Termination for Styrene/Methyl Methacrylate System. A Critical Test of Terminal-Model Kinetics. Macromolecules, 1985. 18: p. 17-26. 14. Russell, G.T., D.H. Napper, and R.G. Gilbert, Initiator Efficiencies in High- Conversion Bulk Polymerizations. Macromolecules, 1988. 21: p. 2141-2148. 15. O'Driscoll, K.F. and J. Huang, The Rate of Copolymerization of Styrene and Methylmethacrylate—I. Low Conversion Kinetics. Eur. Polym. J., 1989. 25: p. 629-633. 16. Achilias, D.S. and C. Kiparissides, Development of a General Mathematical Framework for Modeling Diffusion-Controlled Free-Radical Polymerization Reactions. Macromolecules, 1992. 25: p. 3739-3750. 17. Buback, M., et al., Initiator Efficiencies in 2,2′-Azoisobutyronitrile-Initiated Free-Radical Polymerizations of Styrene. Macromol. Chem. Phys., 1994. 195: p. 2117-2140. 18. Nakamura, H., et al., Kinetic and Epr Studies on Radical Polymerization. Radical Polymerization of Di-2[2-(2-Methoxyethoxy)Ethoxy]Ethyl Itaconate. Colloid Polym. Sci., 1995. 273: p. 122-129. 19. Subramanian, K., K. Shanmugananda Murthy, and K. Kishore, Kinetics of Poly(Styrene Peroxide) Initiated Photopolymerization of Methyl Methacrylate. Polymer, 1997. 38: p. 527-533. 20. Moad, G. and D.H. Solomon, 1 - Introduction, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 1- 9. 21. Mayo, F.R., The Dimerization of Styrene. J. Am. Chem. Soc., 1968. 90: p. 1289- 1295.
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22. Hui, A.W. and A.E. Hamielec, Thermal Polymerization of Styrene at High Conversions and Temperatures. An Experimental Study. J. Appl. Polym. Sci., 1972. 16: p. 749-769. 23. Fouassier, J.P., Photoinitiated Polymerisation: Theory and Applications. 1998: Rapra Technology Limited. 24. Thijs, L., S.N. Gupta, and D.C. Neckers, Photochemistry of Perester Initiators. The Journal of Organic Chemistry, 1979. 44: p. 4123-4128. 25. Hoijemberg, P.A., et al., Radical Photopolymerization in Miniemulsions. Fundamental Investigations and Technical Development. Macromolecules, 2011. 44: p. 8727-8738. 26. Lansdowne, S.W., et al., Relaxation Studies of the Seeded Emulsion Polymerization of Styrene Initiated by [Gamma]-Radiolysis. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1980. 76: p. 1344-1355. 27. Konkolewicz, D., H. de Bruyn, and B.S. Hawkett, Effect of Stabilizer Functionality on the Kinetics of Emulsion Polymerization in Hairy Particles. Macromolecules, 2011. 44: p. 8744-8754. 28. Thickett, S.C. and R.G. Gilbert, Emulsion Polymerization: State of the Art in Kinetics and Mechanisms. Polymer, 2007. 48: p. 6965-6991. 29. Morrison, B.R., et al., Free Radical Exit in Emulsion Polymerization. Ii. Model Discrimination Via Experiment. Journal of Polymer Science Part A: Polymer Chemistry, 1994. 32: p. 631-649. 30. Lacik, I., et al., Desorbed Free Radicals in Emulsion Polymerizations: Effect of Aqueous-Phase Spin Trap. Macromolecules, 1992. 25: p. 4065-4072. 31. Thickett, S.C. and R.G. Gilbert, Rate-Controlling Events for Radical Exit in Electrosterically Stabilized Emulsion Polymerization Systems. Macromolecules, 2006. 39: p. 2081-2091. 32. Hernández-Ortiz, J.C., et al., Modeling of Polymerization Kinetics and Molecular Weight Development in the Microwave-Activated Raft Polymerization of Styrene. Macromol. React. Eng., 2010. 4: p. 210-221. 33. Zetterlund, P.B. and S.b. Perrier, Raft Polymerization under Microwave Irradiation: Toward Mechanistic Understanding. Macromolecules, 2011. 44: p. 1340-1346.
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34. Moad, G. and D.H. Solomon, 4 - Propagation, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 167-232. 35. Tonge, M.P., et al., E.S.R. Measurements of the Propagation Rate Coefficient for Styrene Free Radical Polymerisation. Polymer, 1998. 39: p. 2305-2313. 36. Zetterlund, P.B., S. Yamauchi, and B. Yamada, High-Temperature Propagation and Termination Kinetics of Styrene to High Conversion Investigated by Electron Paramagnetic Resonance Spectroscopy. Macromol. Chem. Phys., 2004. 205: p. 778-785. 37. Heuts, J.P.A. and G.T. Russell, The Nature of the Chain-Length Dependence of the Propagation Rate Coefficient and Its Effect on the Kinetics of Free-Radical Polymerization. 1. Small-Molecule Studies. Eur. Polym. J., 2006. 42: p. 3-20. 38. Beuermann, S. and M. Buback, Rate Coefficients of Free-Radical Polymerization Deduced from Pulsed Laser Experiments. Prog. Polym. Sci., 2002. 27: p. 191-254. 39. Moad, G. and D.H. Solomon, 5 - Termination, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 233-278. 40. Barner-Kowollik, C. and G.T. Russell, Chain-Length-Dependent Termination in Radical Polymerization: Subtle Revolution in Tackling a Long-Standing Challenge. Prog. Polym. Sci., 2009. 34: p. 1211-1259. 41. Johnston-Hall, G. and M.J. Monteiro, Bimolecular Radical Termination: New Perspectives and Insights. Journal of Polymer Science Part A: Polymer Chemistry, 2008. 46: p. 3155-3173. 42. Barth, J., et al., Chain-Length-Dependent Termination in N-Butyl Methacrylate and Tert-Butyl Methacrylate Bulk Homopolymerizations Studied Via Sp-Plp-Esr. Macromolecules, 2009. 42: p. 481-488. 43. Moad, G. and D.H. Solomon, 6 - Chain Transfer, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 279-331. 44. Rizzardo, E. and D.H. Solomon, On the Origins of Nitroxide Mediated Polymerization (Nmp) and Reversible Addition–Fragmentation Chain Transfer (Raft)*. Aust. J. Chem., 2012. 65: p. 945-969.
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45. Moad, G., E. Rizzardo, and S.H. Thang, Radical Addition–Fragmentation Chemistry in Polymer Synthesis. Polymer, 2008. 49: p. 1079-1131. 46. Chiefari, J., et al., Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The Raft Process. Macromolecules, 1998. 31: p. 5559-5562. 47. Moad, G., E. Rizzardo, and S.H. Thang, Living Radical Polymerization by the Raft Process – a Third Update. Aust. J. Chem., 2012. 65: p. 985-1076. 48. Szwarc, M., `Living' Polymers. Nature, 1956. 178: p. 1168-1169. 49. Otsu, T. and M. Yoshida, Role of Initiator-Transfer Agent-Terminator (Iniferter) in Radical Polymerizations: Polymer Design by Organic Disulfides as Iniferters. Die Makromolekulare Chemie, Rapid Communications, 1982. 3: p. 127-132. 50. Otsu, T., M. Yoshida, and T. Tazaki, A Model for Living Radical Polymerization. Die Makromolekulare Chemie, Rapid Communications, 1982. 3: p. 133-140. 51. Otsu, T., Iniferter Concept and Living Radical Polymerization. Journal of Polymer Science Part A: Polymer Chemistry, 2000. 38: p. 2121-2136. 52. Matyjaszewski, K., A Commentary on “Role of Initiator-Transfer Agent- Terminator (Iniferter) in Radical Polymerizations: Polymer Design by Organic Disulfides as Iniferters” by T. Otsu, M. Yoshida (Macromol. Rapid Commun. 1982, 3, 127–132). Macromol. Rapid Commun., 2005. 26: p. 135-142. 53. Georges, M.K., et al., Narrow Molecular Weight Resins by a Free-Radical Polymerization Process. Macromolecules, 1993. 26: p. 2987-2988. 54. Solomon, D.H., E. Rizzardo, and P. Cacioli, Polymerization Processes and Polymers Produced Thereby, 1986: U. S. Patent 4. 55. Kato, M., et al., Polymerization of Methyl Methacrylate with the Carbon Tetrachloride/Dichlorotris- (Triphenylphosphine)Ruthenium(Ii)/Methylaluminum Bis(2,6-Di-Tert- Butylphenoxide) Initiating System: Possibility of Living Radical Polymerization. Macromolecules, 1995. 28: p. 1721-1723. 56. Wang, J.-S. and K. Matyjaszewski, Controlled/"Living" Radical Polymerization. Atom Transfer Radical Polymerization in the Presence of Transition-Metal Complexes. J. Am. Chem. Soc., 1995. 117: p. 5614-5615.
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57. Braunecker, W.A. and K. Matyjaszewski, Controlled/Living Radical Polymerization: Features, Developments, and Perspectives. Prog. Polym. Sci., 2007. 32: p. 93-146. 58. Goto, A. and T. Fukuda, Kinetics of Living Radical Polymerization. Prog. Polym. Sci., 2004. 29: p. 329-385. 59. Jenkins, A.D., R.G. Jones, and G. Moad, Terminology for Reversible- Deactivation Radical Polymerization Previously Called "Controlled" Radical or "Living" Radical Polymerization (Iupac Recommendations 2010). Pure Appl. Chem., 2010. 82: p. 483-491. 60. Darling, T.R., et al., Living Polymerization: Rationale for Uniform Terminology. Journal of Polymer Science Part A: Polymer Chemistry, 2000. 38: p. 1706-1708. 61. Fischer, H., The Persistent Radical Effect in “Living” Radical Polymerization. Macromolecules, 1997. 30: p. 5666-5672. 62. Bertin, D., et al., Kinetic Subtleties of Nitroxide Mediated Polymerization. Chem. Soc. Rev., 2011. 40: p. 2189-2198. 63. Tang, W., T. Fukuda, and K. Matyjaszewski, Reevaluation of Persistent Radical Effect in Nmp. Macromolecules, 2006. 39: p. 4332-4337. 64. Fischer, H., The Persistent Radical Effect: A Principle for Selective Radical Reactions and Living Radical Polymerizations. Chem. Rev. (Washington, DC, U. S.), 2001. 101: p. 3581-3610. 65. Solomon, D.H., Genesis of the Csiro Polymer Group and the Discovery and Significance of Nitroxide-Mediated Living Radical Polymerization. Journal of Polymer Science Part A: Polymer Chemistry, 2005. 43: p. 5748-5764. 66. Hawker, C.J., A.W. Bosman, and E. Harth, New Polymer Synthesis by Nitroxide Mediated Living Radical Polymerizations. Chem. Rev. (Washington, DC, U. S.), 2001. 101: p. 3661-3688. 67. Nicolas, J., et al., Nitroxide-Mediated Polymerization. Prog. Polym. Sci., 2013. 38: p. 63-235. 68. Yamago, S., Development of Organotellurium-Mediated and Organostibine- Mediated Living Radical Polymerization Reactions. Journal of Polymer Science Part A: Polymer Chemistry, 2006. 44: p. 1-12. 69. Yamago, S., Precision Polymer Synthesis by Degenerative Transfer Controlled/Living Radical Polymerization Using Organotellurium,
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Organostibine, and Organobismuthine Chain-Transfer Agents. Chem. Rev. (Washington, DC, U. S.), 2009. 109: p. 5051-5068. 70. Yamago, S., et al., Synthetic and Theoretical Studies on Group-Transfer Imidoylation of Organotellurium Compounds. Remarkable Reactivity of Isonitriles in Comparison with Carbon Monoxide in Radical-Mediated Reactions. J. Am. Chem. Soc., 2001. 123: p. 3697-3705. 71. Goto, A., et al., Mechanism-Based Invention of High-Speed Living Radical Polymerization Using Organotellurium Compounds and Azo-Initiators. J. Am. Chem. Soc., 2003. 125: p. 8720-8721. 72. Kwak, Y., et al., A Systematic Study on Activation Processes in Organotellurium- Mediated Living Radical Polymerizations of Styrene, Methyl Methacrylate, Methyl Acrylate, and Vinyl Acetate. Macromolecules, 2006. 39: p. 4671-4679. 73. Yamago, S., H. Miyazoe, and J.-i. Yoshida, Reversible Generation of Glycosyl Radicals from Telluroglycosides under Photochemical and Thermal Conditions. Tetrahedron Lett., 1999. 40: p. 2339-2342. 74. Yamago, S., et al., Highly Versatile Organostibine Mediators for Living Radical Polymerization. J. Am. Chem. Soc., 2004. 126: p. 13908-13909. 75. Yamago, S., et al., Highly Controlled Living Radical Polymerization through Dual Activation of Organobismuthines. Angewandte Chemie International Edition, 2007. 46: p. 1304-1306. 76. Tang, W., N.V. Tsarevsky, and K. Matyjaszewski, Determination of Equilibrium Constants for Atom Transfer Radical Polymerization. J. Am. Chem. Soc., 2006. 128: p. 1598-1604. 77. Matyjaszewski, K. and J. Xia, Atom Transfer Radical Polymerization. Chem. Rev. (Washington, DC, U. S.), 2001. 101: p. 2921-2990. 78. Matyjaszewski, K., Atom Transfer Radical Polymerization: From Mechanisms to Applications. Isr. J. Chem., 2012. 52: p. 206-220. 79. Percec, V., et al., Ultrafast Synthesis of Ultrahigh Molar Mass Polymers by Metal-Catalyzed Living Radical Polymerization of Acrylates, Methacrylates, and Vinyl Chloride Mediated by Set at 25 °C. J. Am. Chem. Soc., 2006. 128: p. 14156-14165.
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80. Rosen, B.M. and V. Percec, Single-Electron Transfer and Single-Electron Transfer Degenerative Chain Transfer Living Radical Polymerization. Chem. Rev. (Washington, DC, U. S.), 2009. 109: p. 5069-5119. 81. Nguyen, N.H., et al., A Comparative Study of the Set-Lrp of Oligo(Ethylene Oxide) Methyl Ether Acrylate in Dmso and in H2o. Polymer Chemistry, 2013. 4: p. 144-155. 82. Zhang, Y., et al., Copper-Mediated Crp of Methyl Acrylate in the Presence of Metallic Copper: Effect of Ligand Structure on Reaction Kinetics. Macromolecules, 2011. 45: p. 78-86. 83. Cordeiro, R.A., et al., Synthesis of Well-Defined Poly(2-(Dimethylamino)Ethyl Methacrylate) under Mild Conditions and Its Co-Polymers with Cholesterol and Peg Using Fe(0)/Cu(Ii) Based Sara Atrp. Polymer Chemistry, 2013. 4: p. 3088- 3097. 84. Zhong, M., et al., Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. Kinetic Simulation. Macromolecules, 2013. 46: p. 3816-3827. 85. Goto, A., et al., Germanium- and Tin-Catalyzed Living Radical Polymerizations of Styrene and Methacrylates. Macromol. Symp., 2007. 248: p. 126-131. 86. Goto, A., et al., Living Radical Polymerizations with Germanium, Tin, and Phosphorus Catalysts − Reversible Chain Transfer Catalyzed Polymerizations (Rtcps). J. Am. Chem. Soc., 2007. 129: p. 13347-13354. 87. Goto, A., et al., Reversible Chain Transfer Catalyzed Polymerizations (Rtcps) of Styrene and Methyl Methacrylate with Phosphorus Catalysts. Macromol. Symp., 2008. 261: p. 18-22. 88. Goto, A., Y. Tsujii, and T. Fukuda, Reversible Chain Transfer Catalyzed Polymerization (Rtcp): A New Class of Living Radical Polymerization. Polymer, 2008. 49: p. 5177-5185. 89. Goto, A., et al., Reversible Complexation Mediated Living Radical Polymerization (Rcmp) Using Organic Catalysts. Macromolecules, 2011. 44: p. 8709-8715. 90. Oka, M. and M. Tatemoto, Vinylidene Fluoride — Hexafluoropropylene Copolymer Having Terminal Iodines, in Contemporary Topics in Polymer
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Science, W. Bailey and T. Tsuruta, Editors. 1984, Springer New York. p. 763- 777. 91. Kayahara, E., et al., Optimization of Organotellurium Transfer Agents for Highly Controlled Living Radical Polymerization. Macromolecules, 2008. 41: p. 527- 529. 92. Kwak, Y.W., et al., Mechanism and Kinetics of Organostibine-Mediated Living Radical Polymerization of Styrene. Zeitschrift Fur Physikalische Chemie- International Journal of Research in Physical Chemistry & Chemical Physics, 2005. 219: p. 283-293. 93. Moad, G. and D.H. Solomon, 9 - Living Radical Polymerization, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 451-585. 94. Barner-Kowollik, C., et al., Mechanism and Kinetics of Dithiobenzoate- Mediated Raft Polymerization. I. The Current Situation. Journal of Polymer Science Part A: Polymer Chemistry, 2006. 44: p. 5809-5831. 95. Fukuda, T., Fundamental Kinetic Aspects of Living Radical Polymerization and the Use of Gel Permeation Chromatography to Shed Light on Them. Journal of Polymer Science Part A: Polymer Chemistry, 2004. 42: p. 4743-4755. 96. Urban, D. and K. Takamura, Polymer Dispersions and Their Industrial Applications. 2002: Wiley. 97. Lovell, P.A. and M.S. El-Aasser, Emulsion Polymerization and Emulsion Polymers. 1997: John Wiley & Sons Ltd. 98. Slomkowski, S., et al., Terminology of Polymers and Polymerization Processes in Dispersed Systems (Iupac Recommendations 2011). Pure Appl. Chem., 2011. 83: p. 2229-2259. 99. Barrett, K.E.J., Dispersion Polymerisation in Organic Media. British Polymer Journal, 1973. 5: p. 259-271. 100. Cawse, J.L., Dispersion Polymerization, in Emulsion Polymerization and Emulsion Polymers. 1997, John Wiley & Sons Ltd. p. 743-459. 101. Smeets, N.M.B., R.A. Hutchinson, and T.F.L. McKenna, Determination of the Critical Chain Length of Oligomers in Dispersion Polymerization. ACS Macro Letters, 2011. 1: p. 171-174.
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102. Chern, C.S., Emulsion Polymerization Mechanisms and Kinetics. Prog. Polym. Sci., 2006. 31: p. 443-486. 103. Nomura, M., H. Tobita, and K. Suzuki, Emulsion Polymerization: Kinetic and Mechanistic Aspects, in Polymer Particles, M. Okubo, Editor. 2005, Springer Berlin Heidelberg. p. 1-128. 104. Fitch, R.M.L., Polymer Colloids: A Comprehensive Introduction. 1997: Academic Press. 105. Harkins, W.D., A General Theory of the Mechanism of Emulsion Polymerization1. J. Am. Chem. Soc., 1947. 69: p. 1428-1444. 106. Smith, W.V. and R.H. Ewart, Kinetics of Emulsion Polymerization. J. Chem. Phys., 1948. 16: p. 592-599. 107. Smith, W.V., The Kinetics of Styrene Emulsion Polymerization. J. Am. Chem. Soc., 1948. 70: p. 3695-3702. 108. Hawkett, B.S., D.H. Napper, and R.G. Gilbert, Seeded Emulsion Polymerization of Styrene. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1980. 76: p. 1323-1343. 109. Hawkett, B.S., D.H. Napper, and R.G. Gilbert, Analysis of Interval Iii Kinetic Data for Emulsion Polymerizations. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1981. 77: p. 2395- 2404. 110. Maxwell, I.A., et al., Entry of Free Radicals into Latex Particles in Emulsion Polymerization. Macromolecules, 1991. 24: p. 1629-1640. 111. Casey, B.S., B.R. Morrison, and R.G. Gilbert, The Role of Aqueous-Phase Kinetics in Emulsion Polymerizations. Prog. Polym. Sci., 1993. 18: p. 1041-1096. 112. Casey, B.S., et al., Free Radical Exit in Emulsion Polymerization. I. Theoretical Model. Journal of Polymer Science Part A: Polymer Chemistry, 1994. 32: p. 605- 630. 113. McAuliffe, C., Solubility in Water of Paraffin, Cycloparaffin, Olefin, Acetylene, Cycloolefin, and Aromatic Hydrocarbons1. The Journal of Physical Chemistry, 1966. 70: p. 1267-1275. 114. Ugelstad, J. and F.K. Hansen, Kinetics and Mechanism of Emulsion Polymerization. Rubber Chem. Technol., 1976. 49: p. 536-609.
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115. Thickett, S.C. and R.G. Gilbert, Mechanism of Radical Entry in Electrosterically Stabilized Emulsion Polymerization Systems. Macromolecules, 2006. 39: p. 6495-6504. 116. Thickett, S.C. and R.G. Gilbert, Transfer to “Monomer” in Styrene Free-Radical Polymerization. Macromolecules, 2008. 41: p. 4528-4530. 117. Thickett, S.C., B. Morrison, and R.G. Gilbert, Particle Size Distributions in Electrosterically Stabilized Emulsion Polymerization Systems: Testing the “Mid- Chain-Radical” Hypothesis. Macromolecules, 2008. 41: p. 3521-3529. 118. Ferguson, C.J., et al., Ab Initio Emulsion Polymerization by Raft-Controlled Self- Assembly§. Macromolecules, 2005. 38: p. 2191-2204. 119. Ferguson, C.J., et al., Effective Ab Initio Emulsion Polymerization under Raft Control. Macromolecules, 2002. 35: p. 9243-9245. 120. Sprong, E., et al., Molecular Watchmaking: Ab Initio Emulsion Polymerization by Raft-Controlled Self-Assembly. Macromol. Symp., 2005. 231: p. 84-93. 121. Urbani, C.N. and M.J. Monteiro, Raft-Mediated Emulsion Polymerization of Styrene in Water Using a Reactive Polymer Nanoreactor. Aust. J. Chem., 2009. 62: p. 1528-1532. 122. Manguian, M., M. Save, and B. Charleux, Batch Emulsion Polymerization of Styrene Stabilized by a Hydrophilic Macro-Raft Agent. Macromol. Rapid Commun., 2006. 27: p. 399-404. 123. Save, M., et al., Synthesis by Raft of Amphiphilic Block and Comblike Cationic Copolymers and Their Use in Emulsion Polymerization for the Electrosteric Stabilization of Latexes. Macromolecules, 2004. 38: p. 280-289. 124. Pham, B.T.T., et al., Miniemulsion Polymerization with Arrested Ostwald Ripening Stabilized by Amphiphilic Raft Copolymers. Macromolecules, 2010. 43: p. 7950-7957. 125. Chaduc, I., et al., Amphiphilic Block Copolymers from a Direct and One-Pot Raft Synthesis in Water. Macromol. Rapid Commun., 2011. 32: p. 1270-1276. 126. Boisse, S., et al., Amphiphilic Block Copolymer Nano-Fibers Via Raft-Mediated Polymerization in Aqueous Dispersed System. Chem. Commun. (Cambridge, U. K.), 2010. 46: p. 1950-1952.
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127. Thickett, S.C., M. Gaborieau, and R.G. Gilbert, Extended Mechanistic Description of Particle Growth in Electrosterically Stabilized Emulsion Polymerization Systems. Macromolecules, 2007. 40: p. 4710-4720. 128. Thickett, S.C. and R.G. Gilbert, Midchain Transfer to Polymer in Styrene−Poly(Butyl Acrylate) Systems: Direct Evidence of Retardative Effects. Macromolecules, 2005. 38: p. 9894-9896. 129. Butté, A., G. Storti, and M. Morbidelli, Pseudo-Living Polymerization of Styrene in Miniemulsion. DECHEMA Monographs, 1998. 134: p. 497-507. 130. Charleux, B., Theoretical Aspects of Controlled Radical Polymerization in a Dispersed Medium. Macromolecules, 2000. 33: p. 5358-5365. 131. Zetterlund, P.B. and M. Okubo, Compartmentalization in Nitroxide-Mediated Radical Polymerization in Dispersed Systems. Macromolecules, 2006. 39: p. 8959-8967. 132. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization in Nanoreactors: Can Dilution or Increased Nitroxide Concentration Provide Benefits Similar to Compartmentalization? Aust. J. Chem., 2010. 63: p. 1195-1200. 133. Zetterlund, P.B., Controlled/Living Radical Polymerization in Nanoreactors: Compartmentalization Effects. Polymer Chemistry, 2011. 2: p. 534-549. 134. Simms, R.W. and M.F. Cunningham, Compartmentalization of Reverse Atom Transfer Radical Polymerization in Miniemulsion. Macromolecules, 2008. 41: p. 5148-5155. 135. Kagawa, Y., et al., Compartmentalization in Atom Transfer Radical Polymerization (Atrp) in Dispersed Systems. Macromol. Theory Simul., 2006. 15: p. 608-613. 136. Zetterlund, P.B. and M. Okubo, Compartmentalization in Tempo-Mediated Radical Polymerization in Dispersed Systems: Effects of Macroinitiator Concentration. Macromol. Theory Simul., 2007. 16: p. 221-226. 137. Zetterlund, P.B., Y. Kagawa, and M. Okubo, Compartmentalization in Atom Transfer Radical Polymerization of Styrene in Dispersed Systems: Effects of Target Molecular Weight and Halide End Group†. Macromolecules, 2009. 42: p. 2488-2496. 138. Zetterlund, P.B., J. Wakamatsu, and M. Okubo, Nitroxide-Mediated Radical Polymerization of Styrene in Aqueous Microemulsion: Initiator Efficiency,
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Compartmentalization, and Nitroxide Phase Transfer. Macromolecules, 2009. 42: p. 6944-6952. 139. Zetterlund, P.B. and M. Okubo, Compartmentalization in Nmp in Dispersed Systems: Relative Contributions of Confined Space Effect and Segregation Effect Depending on Nitroxide Type. Macromol. Theory Simul., 2009. 18: p. 277-286. 140. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization in Dispersed Systems: Compartmentalization and Nitroxide Partitioning. Macromol. Theory Simul., 2010. 19: p. 11-23. 141. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization in Nanoreactors: Factors Influencing Compartmentalization Effects on Bimolecular Termination. Polymer, 2010. 51: p. 6168-6173. 142. Thomson, M.E. and M.F. Cunningham, Compartmentalization Effects on the Rate of Polymerization and the Degree of Control in Atrp Aqueous Dispersed Phase Polymerization. Macromolecules, 2010. 43: p. 2772-2779. 143. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization of Butyl Acrylate Using Tempo: Improvement of Control Exploiting Nanoreactors? Macromol. React. Eng., 2010. 4: p. 663-671. 144. Zetterlund, P.B., Compartmentalization Effects on Bimolecular Termination in Atom Transfer Radical Polymerization in Nanoreactors. Macromol. Theory Simul., 2011. 20: p. 660-666. 145. Bentein, L., et al., Kinetic Modeling of Miniemulsion Nitroxide Mediated Polymerization of Styrene: Effect of Particle Diameter and Nitroxide Partitioning up to High Conversion. Polymer, 2012. 53: p. 681-693. 146. Wakamatsu, J., et al., Nitroxide-Mediated Radical Polymerization in Microemulsion. Macromol. Rapid Commun., 2007. 28: p. 2346-2353. 147. Tobita, H., Kinetics of Stable Free Radical Mediated Polymerization inside Submicron Particles. Macromol. Theory Simul., 2007. 16: p. 810-823. 148. Tobita, H., Kinetics of Controlled/Living Radical Polymerization in Emulsified Systems. Macromol. Symp., 2008. 261: p. 36-45. 149. Tobita, H. and F. Yanase, Monte Carlo Simulation of Controlled/Living Radical Polymerization in Emulsified Systems. Macromol. Theory Simul., 2007. 16: p. 476-488.
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150. Tobita, H., Effects of Fluctuation and Segregation in the Rate Acceleration of Atrp Miniemulsion Polymerization. Macromol. Theory Simul., 2011. 20: p. 179- 190. 151. Cunningham, M.F., Controlled/Living Radical Polymerization in Aqueous Dispersed Systems. Prog. Polym. Sci., 2008. 33: p. 365-398. 152. Zetterlund, P.B., Y. Kagawa, and M. Okubo, Controlled/Living Radical Polymerization in Dispersed Systems. Chem. Rev. (Washington, DC, U. S.), 2008. 108: p. 3747-3794. 153. Charleux, B., et al., Polymerization-Induced Self-Assembly: From Soluble Macromolecules to Block Copolymer Nano-Objects in One Step. Macromolecules, 2012. 45: p. 6753-6765. 154. Asua, J.M., Miniemulsion Polymerization. Prog. Polym. Sci., 2002. 27: p. 1283- 1346. 155. Antonietti, M. and K. Landfester, Polyreactions in Miniemulsions. Prog. Polym. Sci., 2002. 27: p. 689-757. 156. Schork, F.J., et al., Miniemulsion Polymerization, in Polymer Particles, M. Okubo, Editor. 2005, Springer Berlin Heidelberg. p. 129-255. 157. Capek, I., Degradation of Kinetically-Stable O/W Emulsions. Adv. Colloid Interface Sci., 2004. 107: p. 125-155. 158. Qiu, J., B. Charleux, and K. Matyjaszewski, Controlled/Living Radical Polymerization in Aqueous Media: Homogeneous and Heterogeneous Systems. Prog. Polym. Sci., 2001. 26: p. 2083-2134. 159. Cunningham, M.F., Recent Progress in Nitroxide-Mediated Polymerizations in Miniemulsion. C. R. Chim., 2003. 6: p. 1351-1374. 160. Monteiro, M.J. and B. Charleux, Living Radical Polymerisation in Emulsion and Miniemulsion, in Chemistry and Technology of Emulsion Polymerisation. 2007, Blackwell Publishing Ltd. p. 111-139. 161. Min, K. and K. Matyjaszewski, Atom Transfer Radical Polymerization in Aqueous Dispersed Media. Central European Journal of Chemistry, 2009. 7: p. 657-674. 162. Monteiro, M.J. and M.F. Cunningham, Polymer Nanoparticles Via Living Radical Polymerization in Aqueous Dispersions: Design and Applications. Macromolecules, 2012. 45: p. 4939-4957.
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163. Charleux, B., M.J. Monteiro, and H. Heuts, Living Radical Polymerisation in Emulsion and Miniemulsion, in Chemistry and Technology of Emulsion Polymerisation. 2013, John Wiley & Sons Ltd. p. 105-143. 164. Candau, F., Microemulsion Polymerization, in Polymeric Dispersions: Principles and Applications, J. Asua, Editor. 1997, Springer Netherlands. p. 127- 140. 165. Chow, P. and L. Gan, Microemulsion Polymerizations and Reactions, in Polymer Particles, M. Okubo, Editor. 2005, Springer Berlin Heidelberg. p. 257-298. 166. O'Donnell, J. and E.W. Kaler, Microstructure, Kinetics, and Transport in Oil-in- Water Microemulsion Polymerizations. Macromol. Rapid Commun., 2007. 28: p. 1445-1454. 167. O'Donnell, J.M., Reversible Addition-Fragmentation Chain Transfer Polymerization in Microemulsion. Chem. Soc. Rev., 2012. 41: p. 3061-3076. 168. Thomson, M.E., et al., High Solids Nitroxide-Mediated Microemulsion Polymerization of Mma with a Small Amount of Styrene and Synthesis of (Mma- Co-St)-Block-(Bma-Co-St) Polymers. Macromolecules, 2011. 44: p. 1460-1470. 169. Thomson, M.E., et al., Particle Nucleation in High Solids Nitroxide Mediated Emulsion Polymerization of N-Butyl Acrylate with a Difunctional Alkoxyamine Initiator. Polymer Chemistry, 2013. 4: p. 1803-1814. 170. Min, K., H. Gao, and K. Matyjaszewski, Development of an Ab Initio Emulsion Atom Transfer Radical Polymerization: From Microemulsion to Emulsion. J. Am. Chem. Soc., 2006. 128: p. 10521-10526. 171. Song, Z., et al., Mechanism of Seeded Dispersion Polymerization of Methyl Methacrylate Using Submicron Polystyrene Seed Particles. J. Appl. Polym. Sci., 2011. 122: p. 203-209. 172. Song, Z., et al., Tracking the Fate of Seed Particles in Dispersion Polymerization: Preparation and Application of Fluorescent Polymer Particles. J. Appl. Polym. Sci., 2013. 127: p. 2635-2640. 173. Hamilton, C.J. and B.J. Tighe, 20 - Polymerization in Aqueous Solution, in Comprehensive Polymer Science and Supplements, A. Editor-in-Chief: Sir Geoffrey, Editor. 1989, Pergamon: Amsterdam. p. 261-271.
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Chapter 3
Assessment of the Influence of Microwave Irradiation on Conventional and Controlled/Living Radical Polymerization of Styrene
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3.1 Abstract Conventional and controlled/living radical polymerization of styrene under microwave irradiation has been investigated by carefully conducted experiments with regards to temperature and microwave power conditions. Comparison with the corresponding polymerizations using oil-bath heating has revealed no significant effect of microwave irradiation on the polymerization rate under conditions where it is ensured that the temperature of the polymerization mixture is the same regardless of heating mode. The rate enhancement often seen under microwave irradiation is, at least for the monomer styrene, most likely associated with an increase in temperature caused by the microwave irradiation. This study also exposes a number of experimental parameters that should be carefully considered in regards to control of temperature and power of the microwave reactor when conducting any chemical reaction under microwave irradiation.
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3.2 Introduction The use of microwave irradiation as an alternative means of heating during chemical reactions has received increasing attention over the past decade.[1-4] There is no doubt that in many instances microwave heating ultimately results in higher reaction rates than conventional (oil-bath) heating. However, it remains to be established with certainty whether this increase in reaction rate is a genuine effect of the microwave irradiation, or whether it is merely caused by the temperature being elevated beyond the set/displayed reaction temperature.[5]
Recent years have seen a number of papers describing radical polymerization processes under microwave irradiation,[4, 6-22] including controlled/living radical polymerization
(CLRP).[23, 24] In many instances, the polymerization rate (푅p) is significantly elevated under microwave irradiation compared to when using conventional heating.[4, 14-18] In the case of CLRP, inspection of published data shows that the use of microwave irradiation in the most common CLRP techniques (nitroxide-mediated radical polymerization (NMP), transition metal mediated living radical polymerization (e.g. ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization) has led to inconsistent results, although rate acceleration and good control over the molecular weight distribution has been observed in many cases.[15, 17, 19] In the specific case of RAFT polymerization, several teams have examined the influence of microwave irradiation,[6-12, 20, 21] and enhanced polymerization rates while maintaining satisfactory control/livingness have been reported for styrene,[6, 8, 20, 21] methyl methacrylate (MMA),[6, 7] methyl acrylate,[6, 8] N-isopropyl acrylamide,[9, 10] N,N-dimethyl acrylamide,[10] diallyldimethylammonium chloride,[11] vinylcyclicsilazane,[12] as well as vinyl acetate[13] and block copolymers of vinyl acetate vinyl benzoate and vinyl pivalate.[13]
Based on modelling and simulations, and comparison with experimental data, the increase in 푅p during RAFT polymerization of styrene under microwave irradiation has been argued to be consistent with increases in both the propagation rate coefficient and the rate coefficient for radical addition to the RAFT end group,[25] as well as an additional initiation mechanism whereby radicals are generated from the monomer as a result of microwave irradiation.[21] However, Kwak et al.[26] recently showed by using kinetic considerations of 푅p and the polymer molecular weight that the increase in 푅p 70
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observed during conventional radical polymerization of MMA under microwave irradiation is mainly caused by a temperature increase. These authors also showed that both the decomposition rate coefficient of azobisisobutyronitrile (AIBN) and the monomer reactivity ratios for copolymerizations of styrene with butyl acrylate, MMA and 4-acetoxy styrene, respectively, were unaffected by the microwave irradiation.
The present chapter is concerned with a detailed study of the process of conventional and controlled/living (RAFT) radical polymerization of styrene under microwave irradiation. The focus has been on creating specific experimental conditions that facilitate direct, meaningful comparison with the corresponding oil-bath polymerizations, with particular attention being paid to the temperature and the magnitude of the microwave power applied. In agreement with the recent work by Kwak et al.,[26] the experimental data are consistent with the absence of a specific non- temperature related “microwave effect” on radical polymerization of styrene. This study has also unearthed a number of aspects of experimental nature that merit careful attention in regards to control of temperature and power when conduction any chemical reaction under microwave irradiation.
3.3 Experimental Section
Materials Styrene (St, Sigma-Aldrich, 99%) was purified by passing through an aluminium oxide column, azobisisobutylnitrile (AIBN, DuPont) was recrystallized from methanol, and cyanoisopropyl dithiobenzoate (CPDB) was prepared according to the literature[27] and purified by column chromatography with purity 98% as determined by 1H-NMR.
Styrene CPDB AIBN
Scheme 3. 1. Chemical structures of styrene, cyanoisopropyl dithiobenzoate (CPDB) and azobisisobutyronitrile.
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Conventional radical polymerization and RAFT polymerization of styrene
Typical RAFT polymerization: AIBN (0.0129 g, 0.0786 mmol) and CPDB (0.0695 g, 0.314 mmol) were mixed with St (16.36 g, 0.157 mol) to prepare a stock solution, which was subsequently distributed into glass tubes (specifically designed for the microwave reactor; 2 ml stock solution per glass tube) and sealed by special suba seals. Freeze-pump-thawing was carried out using needle techniques just before the polymerization.
Polymerizations were conducted under microwave irradiation employing a single mode microwave reactor (CEM Discover Microwave Synthesis system, with Intellivent Pressure Control system and Auto Sampler) equipped with an infrared temperature sensor. Two types of reaction modes were used: (i) “Dynamic Mode”, according to which the apparatus is programmed to adjust to a specific fixed temperature by adjustment of the microwave irradiation power, and (ii) “Fixed Power Mode”, whereby the irradiation power is fixed, without control of the reaction temperature. Temperature calibration of the infrared sensor was carried out just before each polymerization by heating DMSO to a temperature well above the polymerization temperature, and then monitoring the temperature during cooling with an alcohol thermometer. Calibrations were performed for the target temperature when using "Dynamic mode" and for preliminary measured temperatures for "Fixed Power Mode". Under conditions when stable temperatures were not obtained ("Fixed Power Mode" at high irradiation power setting without air cooling), calibration was carried out for 110 C.
Corresponding oil-bath heating polymerizations were carried out in closed glass tubes: exposed to several freeze-pump-thawing cycles, the tube with reaction mixture was closed by melting with flame under vacuum. The oil-bath (Memmert ONE 7-45) was equipped with the embedded thermometer and reaction tubes were fully immersed and shaken in fixed temperature environment during the course of the polymerization.
Characterization
Monomer conversions were measured by gravimetry. Number-average molecular weights (Mn) and polydispersities of the molecular weight distributions (푀w/푀n) were determined using gel permeation chromatography (GPC) with a Shimadzu modular 72
Yusuke Sugihara Chapter 3
system with THF as eluent at 40 C at a flow rate of 1.0 ml min-1 with injection volume of 40 l. The GPC was equipped with a DGU-12A solvent degasser, a LC-10AT pump, A CTO-10A column oven and an ECR 7515-A refractive index detector, and a Polymer Laboratories 5.0 m bead-size guard column (50 × 7.8 mm2) followed by four 300 × 7.8 mm2 linear Phenogel columns. The system was calibrated against polystyrene 6 −1 standards ranging from 500 to 10 g mol . 푀n values given in grams per mole (g mol−1).
3.4 Results and Discussion
Conventional Radical Polymerization: Oil bath vs Microwave.
There are two important interdependent experimental parameters of interest; temperature and microwave irradiation power. In general, one would strive to vary these two parameters independently to gain mechanistic understanding, but the problem is that changing one parameter often inadvertently leads to changes in the second parameter. It is well documented in the literature[1-4] that the microwave irradiation power has a major impact on the reaction temperature, thus making it difficult to elucidate the true influence of microwave irradiation on the polymerization process. In this study, a series of specific experiments were conducted with these issues in mind.
The influence of microwave irradiation on the conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = 2000/1) was investigated by comparing polymerizations under microwave irradiation at 60 and 100 C with the corresponding polymerizations using oil-bath heating. The resulting conversion-time data are plotted in Figure 3.1. These microwave-irradiated polymerizations were conducted in the so called “Dynamic Mode” (the microwave irradiation power is adjusted automatically to keep the temperature constant) without air cooling of the polymerization vessel to minimize inaccuracy in temperature readings by the infrared sensor, which is based on the surface temperature of the glass tube (polymerization vessel). Figures 3.2 and 3.3 show plots of the temperature and power vs time for target temperatures of 60 and 100 C, respectively. In both cases, there is an initial time period of approximately 30 – 60 min during which there were significant oscillations in both temperature and
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power. During this initial period, the power increased to levels as high as 150 W, much higher than the constant values of 8 and 25 W (for 60 and 100 C, respectively) that were eventually obtained after stabilization. During the same time period, the temperature increased by up to 10 C above the target temperature. However, with the exception of the initial oscillations, the “Dynamic Mode” approach resulted in constant temperatures at the pre-set levels with good reproducibility in terms of monomer conversion, temperature and power.
Inspection of the conversion-time plots (Figure 3.1) reveals that there are no significant differences between the oil bath- and the microwave data, i.e. the data suggest that there is no significant effect of microwave irradiation on the radical polymerization of styrene under these conditions. It is also apparent from the data that the initial instabilities in the conditions (temperature and microwave power) have no strong influence on the polymerizations, further reinforcing this initial conclusion.
Polymerizations were also conducted for the same system using the “Fixed Power Mode” (without air cooling), whereby the power is fixed and the temperature is not controlled (Figure 3.4). The target power was selected as 25 W based on the approximate power required to obtain a constant temperature of 100 C in the “Dynamic Mode” experiments (Figure 3.3b). Figure 3.4 shows the obtained temperature and power vs time, revealing good reproducibility. In this case, the system required 4 h to reach an approximately constant temperature, although there was a slow but steady decrease to approach 90 C. In other words, it is possible to obtain conditions similar to those in “Dynamic Mode” using the “Fixed Power Mode”. Therefore as expected, the conversion-time data in Figure 3.1 also shows the same trend, i.e. there is very little, if any, effect of the microwave irradiation on the polymerization.
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Conventional RP Oil-bath at 60 C Oil-bath at 100 C MW at 60 C MW at 100 C (Dynamic Mode (Dynamic Mode without air cooling) without air cooling) MW at 300 W MW at 25 W (Fixed Power Mode (Fixed Power Mode with air cooling) without air cooling)
100 90 80 70 60 50 40
30 (%) Conversion 20 10 0 0 2 4 6 8 10 12 14 16 18
Time (h)
Figure 3. 1. Conversion vs time plots for conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ) in oil bath at ퟔퟎ () and ퟏퟎퟎ C () and under microwave irradiation using “Dynamic Mode” set at ퟔퟎ () and ퟏퟎퟎ C () without air cooling and using "Fixed Power Mode" set at ퟑퟎퟎ W with air cooling (ퟒퟎ psi) ().
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(a) 80 70
C) 60 50 (A) (B) (C) (D) (E) 40
30 20
Temperature ( Temperature 10
0 (b) 160 140
120 100 80 60
(W) Power 40 (A) (B) (C) (D) (E) 20 0
0 2 4 6 8 10 12 14 16 18 20 Time (h)
Figure 3. 2. Temperature (a) and microwave irradiation power (b) vs time for conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ) under microwave irradiation using “Dynamic Mode” set at ퟔퟎ C without air cooling for ퟐ (A), ퟒ (B), ퟖ (C), ퟏퟔ (D), and ퟏퟖ (E) h.
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(a) 120
100
C)
80 60
40 (A) (B) (C) (D)
Temperature ( Temperature 20
0 (b) 160 140
120 100 80
60
Power (W) Power 40 (A) (B) (C) (D) 20
0 0 2 4 6 8 10 12 14 16 18 Time (h)
Figure 3. 3. Temperature (a) and microwave irradiation power (b) vs time for conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ) under microwave irradiation using “Dynamic Mode” set at ퟔퟎ C without air cooling for ퟒ (A), ퟖ (B), ퟏퟐ (C), and ퟏퟔ (D) h.
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(a) 120
100
C)
80
60
40 (A) (B) (C) (D)
( Temperature 20
0 (b) 160
140 120 100
80 60
Power (W) Power 40 (A) (B) (C) (D)
20 0 0 2 4 6 8 10 12 14 16 18 Time (h)
Figure 3. 4. Temperature (a) and microwave irradiation power (b) vs time for conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ) under microwave irradiation using “Fixed Power Mode” set at ퟐퟓ W without air cooling for ퟒ (A), ퟖ (B), ퟏퟐ (C), and ퟏퟔ (D) h.
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Conventional Radical Polymerization: High Microwave Power (with Air Cooling)
In the previous section, experiments were designed to enable comparison between microwave and oil bath polymerizations. However, relatively low microwave power (≤ 25 W) was required to reach the desired polymerization temperatures. One thus wonders whether a microwave effect on the polymerization may be significant at higher microwave power?
To this end, polymerizations were conducted in the “Fixed Power” mode at 300 W, and this time air cooling (40 psi) was employed to keep the temperature at a reasonable level with such a high irradiation power. Figure 3.1 shows the obtained conversion-time data of the conventional radical polymerization of styrene with [St]0/[AIBN]0 = 2000/1 (same recipe as above). As displayed in Figure 3.5a, the temperature remained approximately constant around 80 − 90 C after an initial period of 1 h of increasing temperature. However, the polymerization rate was similar to that observed at 60 C using oil bath or microwave irradiation at low power (without air cooling; Figure 3.1). Calibration of the infrared sensor (that measures the temperature) was conducted prior to each experiment as outlined in the Experimental section. Thus, it seems most likely that this discrepancy in polymerization rate for the conditions described originates in inaccurate temperature readings (overestimation of temperature) due to the air cooling. Notwithstanding the issue with the temperature measurement under air cooling, the results in Figure 3.1 support the above data that microwave irradiation has no significant influence on the polymerization process under these conditions, despite the very high microwave power of 300 W.
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(a) 100
80
C)
60
40 (A) (B) (C) (D)
Temperature ( Temperature 20
0
(b) 350 (A) (B) (C) (D) 300 250
200
150
(W) Power 100 50
0 0 2 4 6 8 10 12 14 16 Time (h)
Figure 3. 5. Temperature (a) and microwave irradiation power (b) vs time for conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ) under microwave irradiation using “Fixed Power Mode” set at ퟑퟎퟎ W with air cooling at ퟒퟎ psi for ퟏ (A), ퟑ (B), ퟖ (C), and ퟏퟐ (D) h.
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Conventional Radical Polymerization: High Microwave Power (Without Air Cooling)
If high microwave power is employed without air cooling, very high temperatures are reached. Figure 3.6 shows conversion-time data for conventional radical polymerization of styrene (same recipe as above) under microwave irradiation in the “Fixed Power Mode” of 200 W without air cooling, revealing how the conversion reached 100% in 1 h . Under these conditions, the temperature kept increasing throughout the polymerization, reaching temperatures well beyond 200 C (Figure 3.7a). The experimental reproducibility was poor, with very large temperature differences observed for runs A, B and C (identical runs with different polymerization times). Temperature calibration of the infrared sensor was carried out at 110 C for these experiments, and there is no reason to doubt the accuracy of the temperature measurements. Considering also the data in the preceding sections, the high polymerization rate in Figure 3.6 is ascribed to the high temperature rather than an effect of the microwave irradiation.
RAFT Polymerization: High Microwave Power (Without Air Cooling)
Experiments using “Fixed Power Mode” of 200 W without air cooling were also conducted for RAFT polymerization of styrene using [St]0/[CPDB]0/[AIBN]0 = 2000/4/1. Figure 3.6 shows that the reproducibility in the conversion-time data was poor, suggesting difficulties in controlling the polymerization conditions. Figure 3.8 shows the corresponding temperature and microwave power vs time, respectively, indeed revealing great variation in the temperature vs time profiles between experiments. As such, the poor conversion-time data come as no surprise (in fact, one can correlate the data “scatter” with the temperature variations; e.g. the 20 min polymerization with higher temperature reached higher conversion than the 35 min polymerization with lower temperature).
Similar to the case of conventional radical polymerization at high microwave power without air cooling (Figure 3.7), the temperature reached as high as 250 C, and close to 100% conversion was reached in only 1 h. The polymerization did not proceed in a satisfactory controlled/living manner as evidenced by high values of 푀w/푀n ( 1.4),
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although the 푀w/푀n values and the 푀n data are consistent with some control/livingness
(the 푀n values are not too dissimilar to the theoretical values (Mn,th)) (Figure 3.9). Presumably, the high temperature would lead to a very high rate of generation of radicals from AIBN decomposition as well as from spontaneous initiation of styrene, resulting in excessive bimolecular termination. Also, that high temperature would cause other side reactions that the back bighting of styrene radical polymerization has been attested in more than 250 C.[28] Thus, the poor controllability would evidence that the high polymerization rate be primarily attributed to that high temperature. It would appear that microwave irradiation does not exert any specific “positive” influence on the RAFT mechanism insofar as any such effect is unable to outweigh the problems encountered due to the very high temperature in this case.
Also, this irresponsible and uncontrollable temperature growth would be inevitable in that “radical” condition of extremely strong microwave irradiation power of 200 W without any cooling treatment. Throughout this chapter, other experiments also show temperature irreproducibility and uncontrollability somewhat until they reach their stable condition, and these 200 W experiments should be considered to have been carried out and completed in that “unstable” period.
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Conventional RP RAFT polymerization Under MW at 200 W MW at 200 W (Fixed Power Mode (Fixed Power Mode without air cooling) without air cooling) Oil bath at 140C
100
90 80 70 60 50
40 30
(%) Conversion 20 10 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Time (h)
Figure 3. 6. Conversion vs time plots for conventional radical polymerization
([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ ) () and RAFT polymerization ([St]0/[CPDB]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟒ/ퟏ) () of styrene under microwave irradiation using “Fixed Power Mode” set at ퟐퟎퟎ W without air cooling, and RAFT polymerization in oil bath at ퟏퟒퟎ C ().
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(a) 300 (C) 250 (B)
C)
200
150 (A) 100
Temperature ( Temperature 50
0
(b) 250 (A) (B) (C) 200
150
100
(W) Power 50
0 0 10 20 30 40 50 60 70 80 90 Time (min)
Figure 3. 7. Temperature (a) and microwave irradiation power (b) vs time for conventional radical polymerization of styrene initiated by AIBN ([St]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟏ) under microwave irradiation using “Fixed Power Mode” set at ퟐퟎퟎ W without air cooling for ퟏퟎ (A), ퟑퟎ (B), and ퟔퟎ (C) min.
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(a) 300
250 (D)
C)
200 (E)
150 (B)
100
Temperature ( Temperature 50 (A) (C)
0
(b) 250 (A) (B) (C) (D) (E) 200
150
100
Power (W) Power
50
0 0 10 20 30 40 50 60 70 80 90
Time (min)
Figure 3. 8. Temperature (a) and microwave irradiation power (b) vs time for RAFT polymerization of styrene ([St]0/[CPDB]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟒ/ퟏ ) using “Fixed Power
Mode” set at ퟐퟎퟎ W without air cooling for ퟐퟎ (A), ퟑퟕ (B), ퟒퟎ (C), and ퟔퟎ (D, E) min.
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1.7 1.6 1.5
n
M 1.4
/
w 1.3 M 1.2 1.1 1.0
5.0 MW at 200 W (Fixed Power Mode without air cooling) 4.0 Oil bath at 140C
(g/mol)
4 - 3.0
x 10 x
n 2.0
M
1.0
0 0 20 40 60 80 100 Conversion (%)
Figure 3. 9. Mw/Mn (top) and Mn (bottom) vs conversion for RAFT polymerization of styrene ([St]0/[CPDB]0/[AIBN]0 = ퟐퟎퟎퟎ/ퟒ/ퟏ) using “Fixed Power Mode” set at ퟐퟎퟎ W without air cooling.
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For comparison, RAFT polymerizations were also conducted in an oil bath at 140 C. The polymerization rate appeared lower than for the polymerization under microwave irradiation, although direct comparison is difficult due to the scatter of the data in the latter case. However, it is clear that also in this case, the control/livingness was poor, with 푀n and 푀w/푀n data being similar to the microwave case, lending further support to the notion that microwave irradiation in itself has little, if any, effect on the polymerization.
Microwave-Induced Radical Generation?
Based on modelling and simulations, it has previously been suggested that microwave irradiation can lead to generation of radicals.[21] In order to evaluate this possibility, polymerization of styrene (no initiator and no RAFT agent) were conducted in the “Fixed Power Mode” at 300 W with air cooling. Figure 3.10 shows the resulting temperature and power plots vs time, revealing that the temperature reached approximately 90 ℃ (recall, however, that this may be an underestimate due to inaccurate temperature measurement as discussed above). At the end of the experiment, gravimetry showed that no detectable polymer had been formed, thus strongly indicating that microwave irradiation does not induce formation of radicals to any significant extent.
Azo-initiator effectiveness to the whole kinetics Throughout this study, a fixed amount of AIBN initiator, a two thousandth of styrene
([St]0/[AIBM]0 = 2000/1), were employed except for the case study of microwave- induced radical generation (Figure 3.10). The aim is to execute all experiments in the constant initial condition, and the reason of AIBN addition is because proper thermal initiator is required for the RAFT polymerization at 60 C as the radical source. However, AIBN, known as a thermal initiator for mild temperatures, decomposes by 99.4% even in an hour at 100 C .[29] Therefore, the practical proceeding of polymerization must have been realized by spontaneous thermal initiation of styrene, which is generally effective over 100 C.
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Importantly, this ineffectiveness of AIBN in that high temperature experiments of over 100 C would not contradict any discussion and conclusion of this chapter. It would also paradoxically evidence the temperature overestimation of the probe under air cooling assumed above, and otherwise polymerization must have taken place at truly 90 C because of that spontaneous thermal initiation of styrene even to subtle extent regardless of whether microwave-induced radical generation should truly occur or not.
(a) 100
80
C)
60
40
Temperature ( Temperature 20
0 (b) 350
300 250
200 150
Power (W) Power 100 50
0 0 20 40 60 80 100 120 140 Time (min)
Figure 3. 10. Temperature (a) and microwave irradiation power (b) vs time for conventional radical polymerization of styrene without initiator using “Fixed Power Mode” set at ퟑퟎퟎ W with air cooling (ퟒퟎ psi).
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3.5 Conclusions
The effects (or lack thereof) of microwave irradiation on the conventional and controlled/living radical polymerization of styrene have been investigated with particular attention being paid to temperature conditions and the microwave irradiation power. Under carefully controlled reaction conditions, ensuring the temperature was the same under microwave irradiation and using oil-bath heating, the polymerization rates for conventional radical polymerization of styrene were not significantly different. As such, the experimental data are consistent with the absence of a specific non-temperature related “microwave effect” on radical polymerization of styrene under these conditions. This approach focusing the polymerization rates with kinetic background of styrene has established secure conviction on microwave effect interpretation, and further investigations reading other product properties such as the molecular weight and molecular weight distribution are also important and should give more convinced insight on the subject.
This study also highlights the importance of considering how power and temperature are controlled in a chemical reaction under microwave irradiation, and how small variations in the setup of the microwave reactor can dramatically affect the outcomes of a given chemical reaction.
3.6 References
1. de la Hoz, A., A. Diaz-Ortiz, and A. Moreno, Microwaves in Organic Synthesis. Thermal and Non-Thermal Microwave Effects. Chem. Soc. Rev., 2005. 34: p. 164-178. 2. Kappe, C.O., Microwave Dielectric Heating in Synthetic Organic Chemistry. Chem. Soc. Rev., 2008. 37: p. 1127-1139. 3. Polshettiwar, V. and R.S. Varma, Microwave-Assisted Organic Synthesis and Transformations Using Benign Reaction Media. Acc. Chem. Res., 2008. 41: p. 629-639. 4. Kempe, K., C.R. Becer, and U.S. Schubert, Microwave-Assisted Polymerizations: Recent Status and Future Perspectives. Macromolecules, 2011. 44: p. 5825-5842.
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5. Obermayer, D., B. Gutmann, and C.O. Kappe, Microwave Chemistry in Silicon Carbide Reaction Vials: Separating Thermal from Nonthermal Effects. Angew. Chem.-Int. Edit., 2009. 48: p. 8321-8324. 6. Brown, S.L., C.M. Rayner, and S. Perrier, Microwave-Accelerated Raft Polymerization of Polar Monomers. Macromolecular Rapid Communications, 2007. 28: p. 478-483. 7. Paulus, R.M., et al., High Temperature Initiator-Free Raft Polymerization of Methyl Methacrylate in a Microwave Reactor. Australian Journal of Chemistry, 2009. 62: p. 254-259. 8. Brown, S.L., et al., Ultra-Fast Microwave Enhanced... Chem. Commun., 2007: p. 2145-2147. 9. An, Z., et al., Facile Raft Precipitation Polymerization for the Microwave- Assisted Synthesis of Well-Defined, Double Hydrophilic Block Copolymers and Nanostructured Hydrogels. Journal of the American Chemical Society, 2007. 129: p. 14493-14499. 10. Roy, D., A. Ullah, and B.S. Sumerlin, Rapid Block Copolymer Synthesis by Microwave-Assisted Raft Polymerization. Macromolecules, 2009. 42: p. 7701- 7708. 11. Assem, Y., A. Greiner, and S. Agarwal, Microwave-Assisted Controlled Ring- Closing Cyclopolymerization of Diallyldimethylammonium Chloride Via the Raft Process. Macromolecular Rapid Communications, 2007. 28: p. 1923-1928. 12. Nguyen, C.T., et al., Microwave Assisted Synthesis of High Molecular Weight Polyvinylsilazane Via Raft Process. Polymer, 2009. 50: p. 5037-5041. 13. Roy, D. and B.S. Sumerlin, Block Copolymerization of Vinyl Ester Monomers Via Raft/Madix under Microwave Irradiation. Polymer, 2011. 52: p. 3038-3045. 14. Bogdal, D., et al., Microwave Assisted Synthesis, Crosslinking, and Processing of Polymeric Materials. Adv. Polym. Sci., 2003. 163: p. 193-263. 15. Hoogenboom, R. and U.S. Schubert, Microwave-Assisted Polymer Synthesis: Recent Developments in a Rapidly Expanding Field of Research. Macromolecular Rapid Communications, 2007. 28: p. 368-386. 16. Sinnwell, S. and H. Ritter, Recent Advances in Microwave-Assisted Polymer Synthesis. Australian Journal of Chemistry, 2007. 60: p. 729-743. 17. Bardts, M., N. Gonsior, and H. Ritter, Polymer Synthesis and Modification by Use of Microwaves. Macromol. Chem. Phys., 2008. 209: p. 25-31. 90
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18. Marestin, C. and R. Mercier, Microwave-Assisted Synthesis of Polymers in Aqueous Media RSC Green Chemistry Series, 2010. 7: p. 145-175. 19. Rigolini, J., et al., Microwave-Assisted Nitroxide-Mediated Polymerization for Water-Soluble Homopolymers and Block Copolymers in Homogeneous Aqueous Solution. J. Polym. Sci.; Part A: Polym. Chem., 2010. 48: p. 5775-5782. 20. Zhu, J., et al., Reversible Addition-Fragmentation Chain Transfer Polymerization of Styrene under Microwave Irradiation. Journal of Polymer Science, Part A: Polymer Chemistry, 2006. 44: p. 6810-6816. 21. Hernandez-Ortiz, J.C., et al., Modeling of Polymerization Kinetics and Molecular Weight Development in the Microwave-Activated Raft Polymerization of Styrene. Macromol. React. Eng., 2010. 4: p. 210-221. 22. Ebner, C., et al., One Decade of Microwave-Assisted Polymerizations: Quo Vadis? Macromolecular Rapid Communications, 2011. 32: p. 254-288. 23. Braunecker, W.A. and K. Matyjaszewski, Controlled/Living Radical Polymerization: Features, Developments, and Perspectives. Prog. Polym. Sci., 2007. 32: p. 93-146. 24. Zetterlund, P.B., Y. Kagawa, and M. Okubo, Controlled/Living Radical Polymerization in Dispersed Systems. Chem. Rev., 2008. 108: p. 3747-3794. 25. Zetterlund, P.B. and S. Perrier, Raft Polymerization under Microwave Irradiation: Toward Mechanistic Understanding. Macromolecules, 2011. 44: p. 1340-1346. 26. Kwak, Y., R.T. Mathers, and K. Matyjaszewski, Critical Evaluation of the Microwave Effect on Radical (Co)Polymerizations. Macromol. Rapid Commun, 2012. 33: p. 80-86. 27. Perrier, S., et al., Macromolecules, 2002. 35: p. 8300-8306. 28. Campbell, J.D., et al., High-Temperature Polymerization of Styrene: Mechanism Determination with Preparative Gel Permeation Chromatography, Matrix- Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry, and 13c Nuclear Magnetic Resonance. Journal of Applied Polymer Science, 2004. 94: p. 890-908. 29. Moad, G. and D.H. Solomon, 3 - Initiation, in The Chemistry of Radical Polymerization (Second Edition). 2005, Elsevier Science Ltd: Amsterdam. p. 49- 166.
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Chapter 4
Validity Limits for the Zero-One Approximation in Styrene Emulsion Polymerisation
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4.1 Abstract
Model experiments of interval III seeded emulsion polymerization of styrene were studied by dilatometry with various sizes of relatively large polystyrene seed latexes (the radius of swollen particles 푟s > 100 nm) to find the practical upper limit of the 푟s in which the emulsion polymerization undergoes zero-one approximation kinetics. Earlier theoretical works have suggested that the upper limit of 푟s for zero-one approximation in emulsion polymerization of styrene is less than 100 nm. However, the experimental results in the present work demonstrated the zero-one steady-state, at which the average number of propagating radical 푛̅ss = 0.5, for particles as large as 푟s = 274 nm, and the clear dependence of the zero-one approximation breakdown on the relationship between the entry rate coefficient 휌 and the reduced bimolecular rate coefficient 푐, independent of the particle size. Based on these results, it is expected that the conceptual upper limit of 푟s for the validity of zero-one approximation in this system can be extended to particles as large as 850 nm.
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4.2 Introduction
Because of its various advantageous features, such as the final form of polymer latex, the environmentally friendly dispersion medium of water, the easy handling and collection, and also its remarkable kinetics of high polymerization rate and high molecular weight of the polymer products, emulsion polymerization is one of the most industrially-important and general-purpose polymerization techniques.[1, 2] Heterogeneous systems are normally too complex to analyse all at once. Therefore, the current profound understanding of the kinetics of emulsion polymerization has been achieved by the persistent and great efforts of scientists with carefully designed kinetic experiments to dissect it into little controllable pieces and study them separately.[2-5] As such, the emulsion polymerization of styrene, for which various determined parameters are available, has been regarded as the ideal model system and studied most intensively.
Smith-Ewart Theory for the Kinetics of Emulsion Polymerization The current theoretical system of the kinetics of emulsion polymerization, which is based on the compartmentalization of the propagating radicals into discrete polymerization locations of polymer particles in the dispersion medium, was first conceptually and qualitatively described by Harkins,[6] and then expressed mathematically by Smith and Ewart.[7] In the theory, the populations of particles containing 푛 radicals is given by the Smith-Ewart equation:
푑 푁(푛) = 휌[푁(푛−1) − 푁(푛)] + 푘[(푛 + 1)푁(푛+1) − 푛푁(푛)] 푑푡 (1)
+ 푐[(푛 + 2)(푛 + 1)푁(푛+2) − 푛(푛 − 1)푁(푛)] where 푛 is 0 and integers which specifies the number of propagating radicals in a single particle, 휌 is the pseudo-first-order rate coefficient for entry from the aqueous phase, 푘 is the pseudo-first order rate coefficient for radical exit of a single radical from a particle, and 푐 is the pseudo-first-order rate coefficient for bimolecular termination in a single particle.
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This 푁(n) is a probability distribution of particle state with respect to 푛,
∑ 푁(푛) = 1 (2) 푛 and the average number of radicals for a single particle (also for the entire population of particles) is treated as the mean value of
푛̅ = ∑ 푛푁(푛) (3) 푛
In emulsion polymerization of styrene, the rate of polymerization is described as a function of 푛̅ regardless of Interval I ~ III, so that the kinetics can be discussed by giving the solution for the Smith-Ewart equation (Equation 1) for each 푁(n). However, the rate coefficients 휌, 푘 and 푐 in Equation 1 are phenomenological values which are functions of various practical parameters, and it is very difficult to directly resolve the equations without considering numerous assumptions. As such, the kinetics of emulsion polymerization was first believed to mainly follow 푛̅ = 0.5 (so called Smith-Ewart case 2) and subsequently 푛̅ ≫ 0.5 (case 3: pseudo-bulk limit).[7, 8]
Establishment of Zero-One Approximation The direct resolution for these complicated kinetics was achieved in the early 1980s, when Hawkett et al. developed the practically applicable concept of zero-one approximation under the Smith-Ewart theory.[9, 10] This work is summarized as follows. The reduced bimolecular termination rate coefficient 푐 is the function of termination rate ̅ coefficient 푘t and the monomer swollen particle volume 푣s:
푘̅ 푐 = 푡 (4) 푁A푣s
−1 where 푁A is the Avogadro’s number. This equation states that 푐 is a function of 푣s and that 푐 increases with decreasing particle size. Thus, for sufficiently small particle sizes, the system would produce the condition of 휌 ≪ 푐, where the radical entry in the particle 95
Yusuke Sugihara Chapter 4
which contains a radical leads to instantaneous termination, and the entry rate practically approximates the termination rate. In other words, the radical number in a particle is like binary digits toggled by the periodic entry events, and always takes on only 0 or 1 and hardly exceeds 1. Thus, under the zero-one limit, the Smith-Ewart equations for 푁(n) can be reduced to the simple matter of only two equations of 푁(0) and 푁(1) with approximating 푐 to 휌 as,
푑 푁 = 휌(푁 − 푁 ) + 푘푁 (5) 푑푡 (0) (1) (0) (1)
푑 푁 = 휌(푁 + 푁 ) − 푘푁 (6) 푑푡 (1) (0) (1) (1)
And, in zero-one limit, 푛̅ can also be reduced to the simple form of:
푛̅ = ∑ 푛푁(푛) = 푁(1) (7)
Therefore, the simultaneous rate equations (Equations 5 and 6) are resolved in terms of
푁(1) = 푛̅:
휌 휌 푛̅ = 푁 = + (푁 (푡 = 0) − ) 푒−2(휌+푘)푡 (8) (1) 2휌 + 푘 (1) 2휌 + 푘
(and 푁(0) = 1 − 푁(1) ). Also in the limit of 푡 → ∞, it approaches the asymptote,
휌 푛̅ = 푁 = (9) ss (1),ss 2휌 + 푘
The solution of Equations 8 and 9 state that at the zero-one limit,
1. 푛̅ possibly takes at most 0.5, and 푛̅ = 0.5 is attained in its steady-state 푛̅ss when 휌 ≫ 푘. This is when Smith-Ewart case 2 holds. 2. if 푘 is significant relative to 휌, 푛̅ becomes less than 0.5 (Smith-Ewart case 1) even in the steady-state. Thus, whether the system is Smith-Ewart case 1 or 2 in the steady- state depends on 푘. 96
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This was the clear description that the particle growth of emulsion polymerization could take both Smith-Ewart case 1 and 2 as the result of zero-one limit.
The original work of Hawkett and co-workers[9] was not only the theoretical development, but also the establishment of experimental methodology to analyse the kinetic parameters with other controllable parameters. The meticulous kinetic experiments of seeded emulsion polymerization of styrene with rigorously determined particle size and number, and strict conversion reading by dilatometry demonstrated the long-time steady-state characteristics as expected in Equation 9 in which 푛̅ss was accurately 0.5 and decreased with the decrease of 휌 (as the function of the initiator concentration, which relatively increases the significance of 푘). This accuracy of the experiments enabled the direct measurement of unambiguous values of the 휌 and k by the ‘slope-and-intersect’ method from the linearity of conversion-time plot at the zero- one steady-state for Interval II. It was also applied to Interval III kinetics,[10] and 휌 and 푘 were also obtained from another slope-and-intersect method using the linearity of − ln(1 − 푥) versus time plots (푥 is the fractional conversion) in the zero-one steady- state. Moreover, as Interval III emulsion polymerization is the system in which the
(average) monomer concentration in a particle 퐶p decreases and the weight fraction of a particle 푤p increases in proportion to 푥 , the breakdown of the zero-one limit was experimentally observed as the start of the value of 푛̅ gradually increasing from the long- time steady-state value of 푛̅ss. The observation of 푛̅ beyond the zero-one limit in the Interval III system also enabled the measurement of average (chain-length-dependence- ̅ free) termination rate coefficient 푘t (which also gives the value of 푐 in Equation 4) as a function of 푤p with the pseudo-steady-state treatment of the system at high-conversion ̅ where the time-varying of 푛̅ was gradual; the 푘t curve well fit with the literature value obtained in the corresponding the bulk condition[11] (it produces Equation 20 below).
Development of Zero-One Kinetics of Emulsion Polymerization of Styrene Since this first achievement of the theoretical and experimental methodology, the understanding of emulsion polymerization kinetics has been up-to-date developed.[2, 5] The significant effort and interest in this field was to develop a full description of the
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phenomenological rate coefficients 휌 and 푘 from the elemental events on the microscopic level.[2, 5, 12-14] In particular, the approach to directly treat the phenomenological values requires specific assumptions on the treatment of the fate of the exited radical such as re-entry and aqueous phase termination, and it should have a critical impact on the detailed description of emulsion polymerization kinetics. Considered the reality on the microscopic level, radical exit for a typical hydrophobic monomer as styrene, is practically capable only if the propagating radical is a monomeric entity M,[14] as the solubility of hydrocarbon molecules is significantly dependent on the molecular volume and the dimeric radical is less soluble by several orders of magnitude.[15] Therefore, detailed studies were conducted with the evolution model of the zero-one reduced Smith-Ewart equations (Equations 5 and 6) to distinguish monomeric radical M and the other polymeric radicals P and consider possible elemental events with the consideration of reactions in the continuous phase. Processing of the detailed model was too complicated to resolve directly, but widely acceptable two sub-limit of zero-one kinetics were obtained depending on specifying the major options for the fate of the exited radical.[2, 5, 13, 14] In the limit where all exited radicals undergo aqueous phase termination (Limit 1),:
푑 푛̅ = 휌(1 − 2푛̅) − 푘푛̅ (10) 푑푡
On the contrary, in the limit where the exited radicals all re-entry particles (Limit 2),
푑 푛̅ = 휌(1 + 2푛̅) − 2푘푛̅2 (11) 푑푡
For the investigation of the exact kinetics and to determine the rate coefficients, various powerful experimental tools were developed. γ -Radiolysis dilatometry is a very powerful technique for the study of kinetics of emulsion polymerization,[2, 5, 16-20] γ- irradiation can penetrate the obstacle of opaque latex and create radicals uniformly in the system, and the insertion and removal of the reaction system in and out of the γ- source enables one to effectively switch on and off the radical source. The straightforward cut off of the initiation and the retardation of the rate of polymerization provides information in terms of reaction-stopping events (which is radical exit, in the study of zero-one kinetics). 98
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The development of reversible addition-fragmentation chain transfer (RAFT) polymerization enabled the synthesis of well-defined amphiphilic block copolymers and their application as electrosteric surfactants for various heterogeneous polymerization techniques.[20-35]. The application of such well-defined electrosteric surfactants for the study of the kinetics of emulsion polymerization resulted in improved quality of the investigation of the kinetic parameters and the analysis of the chemistry for the exited radical and those steric chains on the particle surface.[5, 17, 20, 30-34]
Recent meticulous experiments with γ -irradiation[17, 20] and amphiphilic block copolymers[17, 20, 32-34] have demonstrated that, while there still remain the mystery and argument about the real kinetics of radical exit and the reason, the experimental results were consistent with first order kinetics at least phenomenologically. As such, the theoretical work in the original work[9, 10] are effective to reflect the experimental results of emulsion polymerization. Nonetheless, under conditions where 푘 is sufficiently small such that the influence of radical exit can be neglected, those worries regarding the treatment of 푘 will be properly eliminated.
Interest of the Validity of Zero-One Approximation to Large Particles According to the original work,[9, 10] there is another interest for emulsion polymerization kinetics with regard to the particle size limitation to the validity of the zero-one approximation. Zero-one kinetics can hold in the condition that the radical entry practically approximates the subsequent bimolecular termination 휌 ≪ 푐, thus this −1 is mainly the matter of relative comparison between 휌 and 푐. 푐 is the function of 푣s as Equation 4, whereas 휌 is the function of particle number and initiator concentration rather than the particle size, as radical entry is an external event of particles which regards particles as the object of the collision with a radical. As such it would be expected that the validity of the zero-one approximation has an upper limit in terms of particle size. Theoretical studies[2, 14, 36] have suggested that the zero-one approximation is valid only for particles with a radius of less than 90 nm[14] or even less than 70 nm.[36] However, from the original work by Hawkett et al. practical results and analysis of the zero-one breakdown suggest that the main impact to decrease 푐 to ̅ the breakdown is by 푘t which is a function of 푤p, and that the zero-one approximation would be conceptually valid for particle as large as 푟s = 850 nm (the theoretical 99
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development is in the discussion part). In the original work, experimental results were obtained in the range of 푟s < 100 nm. In this study, the methodology of the original work[9, 10] was adopted, and model experiments were carried out in interval III seeded emulsion polymerization of styrene using dilatometry with various sizes of relatively large polystyrene seed latexes (푟s > 100 nm) to see the practical validity of zero-one approximation in such large seed latex.
4.3 Experimental Section
Materials All water used in this work was high-purity deionized water (Milli-Q). Styrene, ammonium hydroxide (NH4OH), and sodium bicarbonate (NaHCO3) were purchased from Sigma Aldrich. Potassium persulphate (KPS), ammonium persulphate (APS) and sodium dodecyl sulphate (SDS) were purchased from Merck. AMA80 (sodium dihexyl sulfosuccinate and a few branched isomers thereof) was purchased from Cytec Industries Inc. All reagents were used as received, unless otherwise specified. Styrene was purified by passing the monomer through an inhibitor removal column (Sigma Aldrich) to remove inhibitor prior to use.
Styrene KPS APS
SDS AMA80
Scheme 4. 1. Chemical structures of styrene, potassium persulphate (KPS), ammonium persulphate (APS), sodium dodecyl sulphate (SDS) and sodium 1,4-dicyclohexyl sulphonatosuccinate (AMA80).
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Hydrodynamic Chromatography (HDC) Particle sizes were determined by HDC using a Polymer Laboratories Particle Size Distribution Analysis system. The system was calibrated with standards of diameters between 20-993 nm.
Synthesis of Seed Latex of Polystyrene Seed latex of 110 nm in radius
The small-sized seed in this study was prepared by ab initio emulsion polymerization. Preliminary styrene and Milli-Q water were separately deoxygenate by bubbling nitrogen and degassing under vacuum. The solution of AMA80 (6.7 g, 17 mmol) in Milli-Q water (613 g) was mixed with styrene (268 g, 2.57 mol). The emulsion was heated to 80 C with a septum sealing, then the solution of NaHCO3 (1.2 g, 14.2 mmol) and APS (1.1 g, 4.82 mmol) in Milli-Q water (10 g) was injected in the system. The reaction was carried out overnight with constant stirring. The resulting latex was passed over glass wool to remove any coagulum, and placed in a small pore dialysis membrane, and dialyzed for two weeks with twice daily water changes to minimize the residual surfactant. After dialysis the polystyrene seed latex was passed over glass wool, then the solids content = 18.1 % and (weight-average) particle radius = 111 nm were measured by gravimetry and HDC, respectively.
Seed latex of 129 nm in radius
Preparation was identical to the above, with the amounts adjusted as follows: AMA80 (6.7 g, 17 mmol) in Milli-Q water (613 g) was mixed with styrene (268 g, 2.57 mol).
The emulsion was heated to 80 C with a septum sealing, then the solution of NaHCO3 (1.2 g, 14.2 mmol) and APS (1.1 g, 4.82 mmol) in Milli-Q water (10 g) was added. Final solids content = 26.9%, particle radius = 129 nm.
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Seed latex of 180 nm in radius
The large-sized seed in this study was made by seeded emulsion polymerization of styrene with the of solids content = 26.9 % and radius = 126 nm. Preliminary styrene, seed latex, Milli-Q water were separately deoxygenated by bubbling nitrogen and degassing under vacuum. Styrene (65.2 g, 0.62 mol), seed latex (116.8 g), and SDS (2.08 g, 7.21 mmol) were charged in a round-bottom flask. The resulting mixture was stirred overnight to swell the seed latex effectively with styrene in a sealing manner. Milli-Q water (777 g) was charged in the flask, then KPS (23.5 mg, 8.69 × 10−2 mmol) in Milli-Q water (7 g) was charged in the system, and the reaction was carried at 50 C over 24 hours. The resulting latex was passed over glass wool to remove any coagulum, and placed in a small pore dialysis membrane, and dialyzed for a month, with twice daily water changes to minimize the residual surfactant. After dialysis the polystyrene seed latex was passed over glass wool, then the solids content = 8.71 % and (weight-average) particle radius = 181 nm were measured by gravimetry and HDC, respectively.
Interval III Seeded Emulsion Polymerization of Styrene A typical procedure of the reaction in dilatometer is as follows: Preliminary styrene, seed latex with SDS, and Milli-Q water were separately deoxygenated by bubbling high purity nitrogen and subsequent degassing under vacuum. Styrene (3.86 g, 37.0 mmol), seed latex (10.0 g), and SDS (0.129 g, 0.47 mmol) were charged in a dry dilatometer equipped with a stirring bar and a permanent-set capillary tube via syringe. The resulting mixture was stirred overnight to swell the seed latex effectively with styrene in a sealing manner. Milli-Q water (44 g) was charged in the dilatometer, the system was once heated up to 60C then partially degassed under vacuum to prevent the bubble from happening during the course of the reaction. Afterwards, at 50 C KPS (1.4 mg, 5.17 × 10−3 mmol) in Milli-Q water (3 g) was charged in the system, then the capillary was filled with water and capped with hexadecane to prevent evaporation. The meniscus height was monitored by an automated tracking device using a LED tracker to provide conversion-time data.
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Dilatometry for Conversion Reading Dilatometer is a novel and precise tool to the successive and real-time monomer conversion reading.[2, 5] The reactor in which emulsion polymerization proceeds is equipped with a long capillary tube. The main latex and liquid in the capillary constitute one body of fluid, and the contraction of the particle volume in the latex with the conversion from low density monomer to high density polymer results in the direct lowering of the height in the capillary. Thus the trace of the meniscus height with accurate initial system parameters gives the proper information of polymerization rate as the difference equations of the fractional conversion as follows:
−1 Given the pure densities of monomer ( 푑m = 0.878 g ml ) and polymer ( 푑p = 1.044 g ml−1) of styrene,[37] the contract factor (퐶퐹) of styrene is determined as:
−1 −1 퐶퐹 = 푑m − 푑p (12) and it provides the contraction ratio in the event of polymerization completion through 0 to 100% conversion (this time, 퐶퐹 = 0.181).
Contrary, the change of the height of the meniscus provides the rate of absolute contraction. Therefore the normalization of the absolute contraction rate to the total 푑푥 contraction (initial monomer volume × 퐶퐹) gives the rate of fractional conversion . 푑푡
푑푥 푑푉 1 푑퐻 휋푟2 = = (13) 푑푡 푑푡 푉0퐶퐹 푑푡 푉0퐶퐹
퐻휋푟2 푥 = (14) 푉0퐶퐹
where 푉 is the contraction volume of monomer (equals that of whole latex) and 푉0 the initial volume of monomer, 퐻 the height of the meniscus and 푟 the inner radius of the 푑푥 capillary. Practically is obtained as the approximation of difference equation. The 푑푡 purpose of practical rate reading is to calculate certain rate parameters, and in this case of Interval III emulsion polymerization the rate of fractional conversion is also related
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with the Equations 16 and 17 below and produces the target kinetic parameter of average particle number a particle 푛̅.
Gravimetry for Conversion Reading Gravimetry is a more general and alternative option for conversion reading to find the solids content of the product and the polymerization rate. This case of Interval III emulsion polymerization employs seed polystyrene particles as with other solid (non- volatile) ingredients, and the fractional conversion can be obtained by the calculation of practical mass of the polymerized polymer with the subtraction of any other possible ingredients and subsequent normalization by the initial monomer mass.
(푆표푙𝑖푑 푚푎푠푠 푎푡 푡ℎ푒 푡𝑖푚푒) − (𝑖푛𝑖푡𝑖푎푙 푠표푙𝑖푑 푚푎푠푠) 푥 = (15) (𝑖푛𝑖푡𝑖푎푙 푚표푛표푚푒푟 푚푎푠푠)
In this study, the use of gravimetry is relatively supportive and only to confirm that no polymerization has occurred during the swelling step. The difficulty of gravimetry is to take the latex such that all ingredients are equally distributed, and otherwise the value obtained be not accurate. Application of gravimetry to the latex which has caused coagulum does fail to read the proper value and leads to underestimation, unless the system as a whole (including latex and coagulum) is consumed for gravimetry.
4.4 Results and Discussion
Interval III Seeded Emulsion Polymerization For this study, three different sizes of seed latexes were synthesized for which the unswollen radii were measured by HDC as 111, 129 and 181 nm. Also monodispersity and no trace of secondary nucleation were confirmed by HDC and TEM.
Seeded emulsion polymerizations of styrene were designed as Interval III system (Table
1). In all cases, the initial 퐶p0 were 6 M (푤p0 = 0.33).[2, 38] Thus the radii of monomer swollen particle 푟s in the reaction were calculated as 167, 196 and 274 nm, respectively. The solids content of the reaction was prepared to be around 9.5 wt% for the systems of small and middle sized particle (푟푠 = 167 and 196, respectively), whereas 18.6 wt% for
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the systems of large particle (푟s = 274) with the intention to decrease 휌 by increasing
푁c (based on Equation 18). The seed latex was stabilized by an anionic surfactant of SDS. After the stirring over night at room temperature to swell the seed latex with monomer sufficiently, at 50 C KPS initiator solution was mixed into latex, capillary was filled with Milli-Q water by first half and hexadecane by another half, then the dilatometric polymerization was implemented. No polymerization had occurred during the preparation was confirmed by gravimetry.
Table 4. 1. The reaction condition of Interval III seeded emulsion polymerization of styrene. Parameter Value (styrene, 323 K) Reference −1 −1 푘p 258 M s [39] −3 퐶w 4.03 × 10 M [40]
퐶p 6 M [38] −1 푑m 0.878 g ml [37] −1 푑p 1.044 g ml [37] 푘̅ Equation 20 [10] t −6 −1 푘d(KPS) 1.18 × 10 s [41]
Table 4. 2. Parameters for Interval III seeded emulsion polymerization of styrene.
Entry rs [KPS]0 퐶p0 푁c 100% solids content (nm) (M) (M) (L-1) (wt %) 1 167 9.5 × 10−4 6.0 5.8 × 1015 9.6
2 167 4.7 × 10−4 6.0 5.9 × 1015 9.6
−5 15 3 167 9.4 × 10 6.0 5.7 × 10 9.4 4 196 9.4 × 10−5 6.0 3.6 × 1015 9.5
5 196 9.3 × 10−6 6.0 3.6 × 1015 9.4
15 6 274 0 6.0 2.9 × 10 18.6
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The conversion vs time plots for each Interval III seeded emulsion polymerization are shown in Figures 4.1, 4.3, and 4.5 for each particle size with various [KPS]0. After the reaction, all latexes were also processed by HDC and TEM, and it was confirmed that there was no trace of secondary nucleation. Therefore, the reactions were considered to have proceeded as ideal seeded polymerization systems, so that the equation for Interval III emulsion polymerization was possibly given as:
푑퐶p 푛̅ − = 푘p퐶p (16) 푑푡 푁A푣s
where 푘p is the propagation rate coefficient.[2, 10] From this equation, the instantaneous 푛̅ was obtained as:
푁A푣s 푑ln(1 − 푥) 푛̅ = − (17) 푘p 푑푡
For this calculation, styrene partition into the dispersion medium of water, in which styrene could dissolve only sparingly, was considered saturated always as 퐶w,sat = 4.03 × 10−3 M[2, 40] by the Gardon treatment.[42] However, it was actually 1500 times smaller than the initial 퐶p0 = 6 M and did hardly influence the kinetics within particles.
The 푛̅ vs conversion plots are shown in Figures 4.2, 4.4, and 4.6. Figure 4.2 demonstrates that for the seeded emulsion polymerization with 푟s = 167 nm, while the −4 highest initiator system [KPS]0 = 9.5 × 10 M was so fast and it exceeded 푛̅ = 0.5 −4 −5 most immediately, the systems of [KPS]0 = 4.7 × 10 and 9.4 × 10 M both showed the trend of horizontal period at close 푛̅ = 0.5 and subsequent creeping above around 푥 = 3.5 and 20% conversions, respectively. The trends of seeded emulsion polymerization of the particle 푟s = 196 nm are demonstrated in Figure 4.4, and both −5 −6 systems of [KPS]0 = 9.4 × 10 and 9.3 × 10 M showed the horizontal period at 푛̅ = 0.5 and the subsequent creeping above around 푥 = 7 and 18% , respectively. Figure 4.6 even demonstrates that for the seeded emulsion polymerization with seed particles as large as 푟s = 274 nm, the system has a 푛̅ = 0.5 horizontal period and subsequently 푛̅ begins to increase around 푥 = 15%. 106
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It is the theoretical and empirical conclusion deduced in Equation 9 that if 푛̅ experiences a horizontal period with 푛̅ ≤ 0.5 , it is valid that the system is under zero-one approximation and the horizontal trend of 푛̅ is of the zero-one steady-state, 푛̅ss.[2, 9, 10]
In the original study,[9, 10] the zero-one validity was confirmed for particles of 푟s < 100 nm, and mathematical derivations suggested the theoretical limit of zero-one validity was less than 90 nm[14] or even less than 70 nm.[36] Nevertheless, the experimental results in this study clearly demonstrate the validity of the zero-one approximation for particles 푟s > 100 nm, even for particles as large as 푟s = 274 nm. Also as is the nature of Interval III emulsion polymerization[10] the zero-one breakdown was observed at the point where 푛̅ crept above of 푛̅ss = 0.5. Interestingly, in the comparison between the trials of the same seed size but different [KPS]0 (Figures 4.2 and 4.3 for 푟s = 167 and 196 nm systems respectively), it was found that the system which employed lower [KPS]0 had a longer zero-one period and the zero-one breakdown points occurred at higher conversion (this will be discussed later in this Chapter).
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40
30
20
[KPS]0 (M) 9.5 x 10-4
Conversion(%) 10 4.7 x 10-4 9.4 x 10-5
0 0 2 4 6 8 10 12 Time (h) Figure 4. 1. Conversion vs time plots for the Interval III seeded emulsion polymerization
-4 -4 of styrene with the seed of rs = 167 nm and [KPS]0 = 9.5 × 10 (blue), 4.7 × 10 (red) and 9.4 × 10-5 M (black).
1.0
n 0.5
[KPS]0 (M) 9.5 x 10-4 4.7 x 10-4 9.4 x 10-5 0.0 0 5 10 15 20 25 Conversion (%)
Figure 4. 2. n plots to conversion for the Interval III seeded emulsion polymerization of
-4 -4 styrene with the seed of rs = 167 nm and [KPS]0 = 9.5 × 10 (blue), 4.7 × 10 (red) and 9.4 × 10-5 M (black).
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40
30
20
[KPS]0 (M) -5 Conversion(%) 10 9.4 x 10 9.3 x 10-6
0 0 4 8 12 16 20 24 Time (h)
Figure 4. 3. Conversion vs time plots for the stage III seeded emulsion polymerization of
-5 -6 styrene with the seed of rs = 196 nm and [KPS]0 = 9.4 × 10 (red) and 9.3 × 10 M (black).
1.0
n 0.5
[KPS]0 (M) 9.4 x 10-5 9.3 x 10-6
0.0 0 5 10 15 20 25 Conversion (%)
Figure 4. 4. n plots to conversion for the Interval III seeded emulsion polymerization of
-5 -6 styrene with the seed of rs = 196 nm and [KPS]0 = 9.4 × 10 (red) and 9.3 × 10 M (black).
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40
30
20
Conversion(%) [KPS] (M) 10 0 0
0 0 8 16 24 32 40 48 56 64 Time (h)
Figure 4. 5. Conversion vs time plot for the stage III seeded emulsion polymerization of styrene with the seed of rs = 274 nm and no initiator addition.
1.0
n 0.5
[KPS]0 (M) 0
0.0 0 5 10 15 20 25 Conversion (%)
Figure 4. 6. n plot to conversion for the Interval III seeded emulsion polymerization of styrene with the seed of rs = 274 nm and no initiator addition.
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The Constancy of 흆 and 풌 Provided persulphate was used as the initiator, Hawkett et al.[9] have reported an empirical approach to estimate 휌 as a function of 푁c,
1013 휌 = (1.3 × )(1 + 638[I]0.6) (18) 푁c
where 푁c is the number concentration of particle in the water and [I] denotes the concentration of thermal initiator compounds. This equation relies on the rate determining step for entry being the formation of a z-mer in the aqueous phase and therefore there being negligible dependence of 휌 on 푁c.
푘 as a function of 푟s,[9]
5.6 푘 = 2 (19) 푟s which is based on first-order loss consideration of exit.
The rate coefficients of 휌 and 푘 for this study were estimated using equations 18 and 19.
Thus, the difference of [KPS]0 in each experiment influenced 휌 . At 50 C, the decomposition of KPS is slow (the decomposition rate coefficient 푘d = 1.18 × 10−6 s−1),[41] and it remains more or less 95% after 12 hours which is the longest time considered in the discussion. The value of 휌 also depends on reactions in the continuous phase such as the propagation rate with which radicals grow to become z-mers that are able to undergo entry, and bimolecular termination. However, as discussed above, 퐶w,sat would be able to be considered constant in Interval III emulsion polymerization from Gardon’s work.[10, 42] Thus, [KPS] and 휌 were able to be treated as constants. Also, Equation 18 implies radical initiation occurs without thermal initiator compounds. While the exact initiation mechanism is not perfectly clear, the experiment of large seed latex
(푟s = 274 nm, Figures 4.5 and 4.6) was carried without initiator with the intention to minimize 휌, and polymerization proceeded to high conversion.
According to the contraction of the particle volume associated with monomer to polymer −3 conversion, (the density of styrene 푑m = 0.878 g cm and polystyrene 푑p = 111
Yusuke Sugihara Chapter 4
−3 1.044 g cm ),[37] 푟s and 푘 are functions of conversion. This contraction is the reason dilatometry can be used to measure the fractional conversion accurately. However, the practical contraction on the particle volume from the initial condition of 퐶p0 = 6 M
(푤p = 0.33) to the ideal 100% conversion in Interval III emulsion polymerization is 푣 only 12% (i.e. s(100%,conv.) = 0.88), and that on the particle radius is only 4% (i.e. 푣s0 푟 s(100%,conv.) = 0.96). Therefore, the contraction up to 20% fractional conversion in 푟s0 which is the target range of the discussion, is at most 1%. Consequently, during this period the 푘 would remain very close to constant.
Table 4.3 shows the 휌 and 푘 values estimated as constant values. In all cases, 푘 ≪ 휌, 휌 and also is close to 0.50. This estimation is in good accordance with the 2휌+푘 experimental results, and practically all steady-state were observed at close to 푛̅ss = 0.5. This would clarify that all experiments in this study were carried out under the condition where 푘 was relatively less important compared to 휌, so that the influence of 푘 would be negligible. This enables one to eliminate the formidable and ongoing issue of the exit treatment as to whether it is first order loss or second order, and as such would enable to give the more validity to Equations 8 and 9.
Table 4. 3. The 흆 and 풌 estimated, and the zero-one breakdown conversions 풙풃 and 풄풃 estimated at the conversions. 휌 휌 Entry rs 푘 푥b 푐b -1 -1 2휌 + 푘 -1 (nm) (s ) (s ) (%) (s ) 푐b 1 167 2.4 × 10−2 4.6 × 10−4 0.495 − − − 2 167 1.6 × 10−2 4.6 × 10−4 0.493 3.5 6.8 × 10−1 2.4 × 10−2 3 167 7.9 × 10−3 4.6 × 10−4 0.486 20 1.4 × 10−1 5.6 × 10−2
4 196 1.2 × 10−2 3.4 × 10−4 0.493 7 3.4 × 10−1 3.5 × 10−2 5 196 5.9 × 10−3 3.4 × 10−4 0.486 18 1.1 × 10−1 5.4 × 10−2
6 274 4.5 × 10−3 1.7 × 10−4 0.491 15 5.6 × 10−2 8.0 × 10−2
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Change in 풄 during Interval III Emulsion Polymerization ̅ The average termination rate coefficient for emulsion polymerization 푘t was estimated from the original work by Hawkett et al. as a function of 푤p:
̅ 2,1 푘t = 푘t0 exp(−19푤p ) (20)
where 푘t0 is the value of 푘t at 푤p = 0.[10] This estimation (i.e. Equation 20) was obtained as good fitting of experimental measurements of bulk polymerization of styrene for the low 푤p range,[11] and of Interval III emulsion polymerization of styrene for the high 푤p range[10]. Thus, while Equation 20 does not consider chain-length dependence, this equation is applicable as the average termination rate coefficient of this emulsion polymerization system. The reduced bimolecular rate coefficient 푐 is obtained with this ̅ 푘t in Equation 4. Different from the constancy for 휌 and 푘, the Equation 20 suggests that the 푐, which is the function of 푤p, should decrease significantly with the increase of the fractional conversion over the Interval III emulsion polymerization.
For all systems in this study with the seed latex of more than 100 nm in radius, the experimental results practically demonstrated certain kinetic development and several phases, starting from zero-one limit, zero-one steady-state, zero-one breakdown, ending in the subsequent steady 푛̅ increase (Figures 4.2, 4.4 and 4.6). Provided 휌 and 푘 are considered constant (푘 may even be negligible) during the course of an Interval III emulsion polymerization, the kinetic development would be rationally attributed to the changeable parameter of 푐(푤p) vs constant 휌.
Dependency of Zero-One Validity on 흆 and 풄 With regard to the validity of the zero-one approximation, the theory is based on the fundamental assumption that 휌 ≪ 푐. The experimental results practically showed good accordance with this assumption as it was observed that the lower 휌 system (by lowering
[KPS]0) could maintain the zero-one limit by lower 푐, as the breakdown occurred at relatively higher conversion (higher 푤p) which was clarified in comparison with systems of the same seed particles (Figures 4.4 and 4.6). It is important to mention that
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the relation of the zero-one validity on 휌 and 푐 is not apparent from the value of 푛̅. This is because 푛̅ is a mean value and essentially fails to keep the information about the frequency of entry. This mean value is, as it is, useful and valid to describe the kinetics of emulsion polymerization under the zero-one approximation, which is expressed in the Equations 8 and 9. In other words, the zero-one limit can be practically established no −2 matter what the value of 휌. However, 푛̅ss(휌 = 1.2 × 10 ) = 0.5 and 푛̅ss(휌 = 5.9 × 10−3) = 0.5 are strictly different from the viewpoint of a single particle. At the zero- one steady-state, if the frequency of entry 휌 is low and the time interval to the next entry is long, a relatively low c is still sufficient to establish the average 0.5 value. Consequently, this clarifies that the zero-one validity is practically dependent on 휌 and 푐, and zero-one validity established by low 휌 would be able to be sustained at high conversion and 푤p (i.e. a lower termination rate). Table 4.3 shows 푐 at the observed zero-one breakdown point 푐b, and 휌/푐b. The 휌/푐b values do not demonstrate a unique value.
Although it is clear that the parameter that changes with time (conversion) in Interval III emulsion polymerization is 푐(푤p), and the validity of the zero-one approximation is dependent on 휌 and 푐, it remains difficult or even impossible to determine the zero-one breakdown point as a function of 휌 and 푐. This difficulty is largely due to the ambiguity associated with the first premise of 휌 ≪ 푐. This statement is intuitively understandable, as it indicates the qualitative sense of direction that a sufficiently high bimolecular termination rate can be approximated with the relatively low entry rate, which is the rate determining step of a series of termination process in a single particle. However, it does not state anything quantitatively about what becomes of the system at the “threshold level” where 휌 and 푐 are sufficiently close to each other.
Estimation of the Maximum Particle Size for Zero-One Validity Given the fact that the zero-one validity is fundamentally dependent on the relationship of 휌 and 푐, it would be rational to assume that for a given value of 휌 the zero-one approximation will break down at approximately the same value of 푐b, irrespective of the particle size and monomer concentration that generates that value of 푐b. It is thus possible to estimate the particle size at which that value of 푐b would pertain at different
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concentrations of monomer within the particles. The swollen particle radius at the zero- one breakdown point 푟sb can be obtained by rearrangement of Equation 4,
1 ̅ 3 3푘t (21) 푟sb = ( ) 4휋푁A푐b
̅ This means according to the Equation 21 and substituting the 푘t value with the 푤p in
Equation 20, the 푟sb can be obtained as the function of 푤p for each 휌.
1 2,1 3 3푘t0 exp(−19푤p ) (22) 푟sb = ( ) 4휋푁A푐b
In other words, 푐b is a function of 휌, and also 푟sb is a function of 푤p and 푐b(휌).
Figure 4.7 demonstrates the 푟sb vs 푤p curves calculated using Equation 22 with the experimentally observed breakdown points from each experiments (Table 4.3). This figure includes two experimental plots and the corresponding calculated curves obtained in the original work by Hawkett et al.[10] with the small seed latex of 푟s = 65 and 80 −2 −3 −1 nm, for which the 휌 = 3.0 × 10 and 1.5 × 10 s and 푐b = 2.6 and 4.2 × 10−2 s−1, respectively.
Although the experimental plots were obtained with different sized seed particle, including the largest seed particles with 푟s = 274 nm, these curves line up perfectly in the unique order with respect to 휌 and 푐b, confirming the correctness of the notion that the lower the value of 휌, the lower is the value of 푐b. The higher the entry rate of radicals into a particle, the greater is the termination rate required within the particle to “keep up” with the entry rate to ensure that a significant number of particles do not contain two radicals at any given point in time.
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900 Stage II Stage III 800 -3 -1 -2 -1 =1.5x10 s , c b=4.2x10 s 700 -3 -1 -2 -1 =4.5x10 s , c b=5.6x10 s , 600 -3 -1 -1 -1 =5.9x10 s , c b=1.1x10 s 500 =7.9x10-3 s-1, c =1.4x10-1 s-1
(nm) b
sb 400 r -2 -1 -1 -1 =1.2x10 s , c b=3.4x10 s 300 -2 -1 -1 -1 =1.6x10 s , c b=6.8x10 s
200 -2 -1 -1 =3.0x10 s , c b=2.6 s 100
0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
w p
Figure 4. 7. wp vs rsb for the Interval III seeded emulsion polymerization of styrene with the various sized seed. The curves are calculated on the assumption that cb is constant at certain to rsb.
These curves predict that in the minimum case of 휌 = 1.5 × 10−3 s−1, conceptually the validity of zero-one approximation would be able to be found for particles as large as
푟s = 850 nm. In practice, the condition of 푤p < 0.33 (퐶p > 6.0) is an impossible imaginary range in the methodology of emulsion polymerization from the restriction of monomer saturation. However, it would be expected that such zero-one validity will be able to be practically observed in this large particles, in the event of the establishment of rigorously designed miniemulsion polymerization methodology.
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4.5 Conclusions
The studies by Hawkett et al.[9, 10] of the kinetics of emulsion polymerization to find the validity of zero-one approximation and its breakdown investigated under 푟s < 100 nm have been applied to much larger particles to investigate the practical validity limit. Interval III seeded emulsion polymerization of styrene with dilatometry at 50 °C demonstrated the zero-one steady-state at 푛̅ss = 0.5 and its breakdown for particles as large as 푟s = 274 nm, which is much larger than the maximum limit according to previously proposed theoretical approaches. By assuming that the validity of the zero- one approximation is attributed to the relationship between 휌 and 푐 in such a way so that at the breakdown point (i.e. the monomer conversion level at which the zero-one approximation is no longer valid), a given value of 휌 would correspond to a specific 푐b independent of particle size, it was possible to calculate the particle size at which zero- one breakdown would occur as a function of the fraction of polymer in the particles (monomer conversion). The results for different particle sizes and entry rates could be rationalized based on the notion that the higher the entry rate, the higher is the required termination rate to maintain zero-one conditions. As such, it is clear that whether zero- one conditions prevail or not depends not only on particle size and termination rate but also on entry rate, which has hitherto not been realized. Based on the above approach, the maximum limit of the validity of the zero-one approximation would be expanded conceptually to include particles as large as 푟s = 850 nm.
4.6 References
1. Urban, D. and K. Takamura, Polymer Dispersions and Their Industrial Applications. 2002: Wiley. 2. Gilbert, R.G., Emulsion Polymerization: A Mechanistic Approach. 1995: Academic Press. 3. Nomura, M., H. Tobita, and K. Suzuki, Emulsion Polymerization: Kinetic and Mechanistic Aspects, in Polymer Particles, M. Okubo, Editor. 2005, Springer Berlin Heidelberg. p. 1-128. 4. Chern, C.S., Emulsion Polymerization Mechanisms and Kinetics. Prog. Polym. Sci., 2006. 31: p. 443-486. 117
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5. Thickett, S.C. and R.G. Gilbert, Emulsion Polymerization: State of the Art in Kinetics and Mechanisms. Polymer, 2007. 48: p. 6965-6991. 6. Harkins, W.D., A General Theory of the Mechanism of Emulsion Polymerization1. J. Am. Chem. Soc., 1947. 69: p. 1428-1444. 7. Smith, W.V. and R.H. Ewart, Kinetics of Emulsion Polymerization. J. Chem. Phys., 1948. 16: p. 592-599. 8. Smith, W.V., The Kinetics of Styrene Emulsion Polymerization. J. Am. Chem. Soc., 1948. 70: p. 3695-3702. 9. Hawkett, B.S., D.H. Napper, and R.G. Gilbert, Seeded Emulsion Polymerization of Styrene. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1980. 76: p. 1323-1343. 10. Hawkett, B.S., D.H. Napper, and R.G. Gilbert, Analysis of Interval Iii Kinetic Data for Emulsion Polymerizations. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1981. 77: p. 2395- 2404. 11. Matheson, M.S., et al., Rate Constants in Free Radical Polymerization. Iii. Styrene1. J. Am. Chem. Soc., 1951. 73: p. 1700-1706. 12. Maxwell, I.A., et al., Entry of Free Radicals into Latex Particles in Emulsion Polymerization. Macromolecules, 1991. 24: p. 1629-1640. 13. Casey, B.S., B.R. Morrison, and R.G. Gilbert, The Role of Aqueous-Phase Kinetics in Emulsion Polymerizations. Prog. Polym. Sci., 1993. 18: p. 1041-1096. 14. Casey, B.S., et al., Free Radical Exit in Emulsion Polymerization. I. Theoretical Model. Journal of Polymer Science Part A: Polymer Chemistry, 1994. 32: p. 605- 630. 15. McAuliffe, C., Solubility in Water of Paraffin, Cycloparaffin, Olefin, Acetylene, Cycloolefin, and Aromatic Hydrocarbons1. The Journal of Physical Chemistry, 1966. 70: p. 1267-1275. 16. Lansdowne, S.W., et al., Relaxation Studies of the Seeded Emulsion Polymerization of Styrene Initiated by [Gamma]-Radiolysis. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1980. 76: p. 1344-1355. 17. Konkolewicz, D., H. de Bruyn, and B.S. Hawkett, Effect of Stabilizer Functionality on the Kinetics of Emulsion Polymerization in Hairy Particles. Macromolecules, 2011. 44: p. 8744-8754. 118
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18. Morrison, B.R., et al., Free Radical Exit in Emulsion Polymerization. Ii. Model Discrimination Via Experiment. Journal of Polymer Science Part A: Polymer Chemistry, 1994. 32: p. 631-649. 19. Lacik, I., et al., Desorbed Free Radicals in Emulsion Polymerizations: Effect of Aqueous-Phase Spin Trap. Macromolecules, 1992. 25: p. 4065-4072. 20. Thickett, S.C. and R.G. Gilbert, Rate-Controlling Events for Radical Exit in Electrosterically Stabilized Emulsion Polymerization Systems. Macromolecules, 2006. 39: p. 2081-2091. 21. Ferguson, C.J., et al., Ab Initio Emulsion Polymerization by Raft-Controlled Self- Assembly§. Macromolecules, 2005. 38: p. 2191-2204. 22. Ferguson, C.J., et al., Effective Ab Initio Emulsion Polymerization under Raft Control. Macromolecules, 2002. 35: p. 9243-9245. 23. Sprong, E., et al., Molecular Watchmaking: Ab Initio Emulsion Polymerization by Raft-Controlled Self-Assembly. Macromol. Symp., 2005. 231: p. 84-93. 24. Urbani, C.N. and M.J. Monteiro, Raft-Mediated Emulsion Polymerization of Styrene in Water Using a Reactive Polymer Nanoreactor. Aust. J. Chem., 2009. 62: p. 1528-1532. 25. Manguian, M., M. Save, and B. Charleux, Batch Emulsion Polymerization of Styrene Stabilized by a Hydrophilic Macro-Raft Agent. Macromol. Rapid Commun., 2006. 27: p. 399-404. 26. Save, M., et al., Synthesis by Raft of Amphiphilic Block and Comblike Cationic Copolymers and Their Use in Emulsion Polymerization for the Electrosteric Stabilization of Latexes. Macromolecules, 2004. 38: p. 280-289. 27. Pham, B.T.T., et al., Miniemulsion Polymerization with Arrested Ostwald Ripening Stabilized by Amphiphilic Raft Copolymers. Macromolecules, 2010. 43: p. 7950-7957. 28. Chaduc, I., et al., Amphiphilic Block Copolymers from a Direct and One-Pot Raft Synthesis in Water. Macromol. Rapid Commun., 2011. 32: p. 1270-1276. 29. Boisse, S., et al., Amphiphilic Block Copolymer Nano-Fibers Via Raft-Mediated Polymerization in Aqueous Dispersed System. Chem. Commun. (Cambridge, U. K.), 2010. 46: p. 1950-1952. 30. Thickett, S.C., M. Gaborieau, and R.G. Gilbert, Extended Mechanistic Description of Particle Growth in Electrosterically Stabilized Emulsion Polymerization Systems. Macromolecules, 2007. 40: p. 4710-4720. 119
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31. Thickett, S.C. and R.G. Gilbert, Midchain Transfer to Polymer in Styrene−Poly(Butyl Acrylate) Systems: Direct Evidence of Retardative Effects. Macromolecules, 2005. 38: p. 9894-9896. 32. Thickett, S.C. and R.G. Gilbert, Mechanism of Radical Entry in Electrosterically Stabilized Emulsion Polymerization Systems. Macromolecules, 2006. 39: p. 6495-6504. 33. Thickett, S.C. and R.G. Gilbert, Transfer to “Monomer” in Styrene Free-Radical Polymerization. Macromolecules, 2008. 41: p. 4528-4530. 34. Thickett, S.C., B. Morrison, and R.G. Gilbert, Particle Size Distributions in Electrosterically Stabilized Emulsion Polymerization Systems: Testing the “Mid- Chain-Radical” Hypothesis. Macromolecules, 2008. 41: p. 3521-3529. 35. Charleux, B., et al., Polymerization-Induced Self-Assembly: From Soluble Macromolecules to Block Copolymer Nano-Objects in One Step. Macromolecules, 2012. 45: p. 6753-6765. 36. Prescott, S.W., M.J. Ballard, and R.G. Gilbert, Average Termination Rate Coefficients in Emulsion Polymerization: Effect of Compartmentalization on Free-Radical Lifetimes. Journal of Polymer Science Part A: Polymer Chemistry, 2005. 43: p. 1076-1089. 37. Patnode, W. and W.J. Scheiber, The Density, Thermal Expansion, Vapor Pressure, and Refractive Index of Styrene, and the Density and Thermal Expansion of Polystyrene. J. Am. Chem. Soc., 1939. 61: p. 3449-3451. 38. Morton, M., S. Kaizerman, and M.W. Altier, Swelling of Latex Particles. Journal of Colloid Science, 1954. 9: p. 300-312. 39. Buback, M., et al., Critically Evaluated Rate Coefficients for Free-Radical Polymerization, 1. Propagation Rate Coefficient for Styrene. Macromol. Chem. Phys., 1995. 196: p. 3267-3280. 40. Lane, W.H., Determination of Solubility of Styrene in Water and of Water in Styrene. Industrial & Engineering Chemistry Analytical Edition, 1946. 18: p. 295-296. 41. Behrman, E.J. and J.O. Edwards, The Thermal Decomposition of Peroxodisulfate Ions. Rev. Inorg. Chem., 1980. 2: p. 179-206. 42. Gardon, J.L., Emulsion Polymerization. Vi. Concentration of Monomers in Latex Particles. Journal of Polymer Science Part A-1: Polymer Chemistry, 1968. 6: p. 2859-2879. 120
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Chapter 5
Synergistic Effects of Compartmentalization and Nitroxide Exit/Entry in Nitroxide-Mediated Radical Polymerization in Dispersed Systems
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5.1 Abstract Modeling and simulations of compartmentalization effects in tandem with nitroxide exit and entry have been performed for the nitroxide-mediated polymerization (NMP) of styrene in an aqueous dispersed system employing 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) at 125 C. It is demonstrated that even for a relatively water insoluble nitroxide like TEMPO, exit and entry can strongly influence the polymerization kinetics in submicron-size droplets/particles. In such systems, the polymerization is expected to proceed at a markedly higher rate than the corresponding bulk system at the expense of control/livingness. Depending on the deactivator water solubility, these findings will apply qualitatively to all controlled/living radical polymerization systems governed by the persistent radical effect (e.g. NMP and atom transfer radical polymerization (ATRP)).
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5.2 Introduction
Controlled/living radical polymerization (CLRP)[1] was initially developed for homogeneous polymerization systems, but the last decade has seen significant progress in (aqueous) dispersed systems.[2-4] There are a number of intrinsic characteristics of such dispersed systems, e.g. reactant partitioning and phase transfer events,[5-7] interface-[8-13] and compartmentalization effects,[14] and the polymerization may proceed quite differently compared to its homogeneous counterpart. Ultimately, one strives to understand and exploit these phenomena to further improve the efficiency of the process in terms of control of the molecular weight distribution (MWD) and the “livingness” (end-functionality).[15, 16]
Compartmentalization refers to confinement of reactants within discrete confined spaces (nanoreactors). A number of theoretical studies of compartmentalization effects in CLRP have been reported, using either modified Smith-Ewart equations[14, 15, 17-32] or Monte Carlo modeling.[33-40] In nitroxide-mediated polymerization (NMP)[15, 19, 22, 24-26, 30, 32, 39, 41] and atom transfer radical polymerization (ATRP),[16, 20, 23, 27, 29, 35, 39] both the propagating radicals and deactivator may be compartmentalized, and consequently not only bimolecular termination but also the deactivation reaction may be influenced by compartmentalization. However, the above studies all deal with the situation where the deactivator is unable to exit into the aqueous phase (or where the deactivator is not compartmentalized[18]). Although such systems undoubtedly exist (e.g. when using a polymeric nitroxide[8]), deactivator partitioning will occur to various extents in most systems. Bentein et al.[32] simulated N-tert-butyl-N-[1- diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1)-mediated radical polymerization of styrene in miniemulsion at 123 ºC with modified Smith-Ewart equations with nitroxide exit/entry modeled using a semi-empirical approach for a particle diameter of 70 nm. Not surprisingly, under such conditions (i.e. an NMP system with high equilibrium constant and “large” particles), no strong effects of nitroxide partitioning were observed due to the high number of SG1 species per particle (40).
In the present work, modeling and simulations of compartmentalization effects in tandem with nitroxide exit and entry have been performed for the NMP system St/2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO)/125 C. It is demonstrated that even
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for a relatively water insoluble nitroxide like TEMPO, exit and entry can strongly influence the polymerization kinetics in submicron-size droplets/particles.
Styrene St-TEMPO
Scheme 5. 1. Chemical structures of styrene and 2,2,6,6-tetramethyl-1-(1- phenylethoxy)piperidine (St-TEMPO).
kact +
kdeact
kt
kp
or
+
Scheme 5. 2. Nitroxide-mediated polymerization of styrene with TEMPO.
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5.3 Model Development
Homogeneous System Homogeneous NMP was modeled using the generally accepted approach based on Equations 1-5:
푑[M] = −푘 [P][M] − 1.5푘 [M]3 (1) 푑푡 p i,th
푑[P] = 푘 [PT] − 푘 [P][T] + 푘 [M]3 − 2푘 [P]2 (2) 푑푡 act deact i,th t
푑[T] = 푘 [PT] − 푘 [P][T] (3) 푑푡 act deact
푑[PT] = −푘 [PT] + 푘 [P][T] (4) 푑푡 act deact
푑[S] = 푘 [P]2 (5) 푑푡 t
where M denotes styrene, P propagating radical, PT alkoxyamine, T free TEMPO, S dead polymer, 푘p the propagation rate coefficient, 푘i,th the rate coefficient for thermal 3 initiation of M (on the basis of radical generation rate = 푘i,th[M] ;[42] three monomer molecules generate two radicals, with no “2” in front of 푘i,th in Equation 2, hence “1.5” in Equation 1), 푘act the first-order activation rate coefficient, 푘deact the deactivation rate coefficient, and 푘t the termination (combination) rate coefficient. All simulations were carried out with [M]0/[PT]0 = 8.71/0.02 for 125 C, and the values of all rate coefficients are listed in Table 5.1. All rate coefficients were assumed to be conversion- independent, and chain-length dependence of rate coefficients (mainly 푘t ) was not included in the model. The equations were implemented and solved using the software VisSim (version 8.0, Visual Solutions Inc.) employing numerical integration (Backward Euler integration algorithm). At time zero, a low molecular weight alkoxyamine is present
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(denoted St-TEMPO), with 푘act and 푘deact values corresponding to the polymeric polystyrene-TEMPO equivalent.
Heterogeneous System The model employed for heterogeneous NMP accounts for compartmentalization of both P and T as well as partitioning of T (exit and entry) between the dispersed phase and the continuous phase. This was achieved by use of two-dimensional modified Smith-
j Ewart equations,[17, 19] describing the number fraction of particles of Ni (particles containing 푖 P and 푗 T):
푑 푁 = 푁 푣 푘 [PT]{푁 − 푁 } 푑푡 (푖,푗) A s act (푖−1,푗−1) (푖,푗) 3 + 0.5푘i.th[M] {푁(푖−2,푗) − 푁(푖,푗)} + (푁 푣 )−1푘 {(푖 + 1)(푗 + 1)푁 − 푖 푗푁 } A s deact (푖+1,푗+1) (푖,푗) (6) −1 + (푁A푣푠) 푘t{(푖 + 2)(푖 + 1)푁(푖+2,푗) − 푖(푖 − 1)푁(푖,푗)} + 푁A푉푤푘entry[T ]{푁(푖,푗−1) − 푁(푖,푗)}
+ 푘dM{(푗 + 1)푁(푖,푗+1) − 푗푁(푖,푗)}
−1 −1 where 푘entry denotes the entry rate coefficient (s ), 푘dM the exit rate coefficient (s ),
푁A Avogadro number, 푣s the (monomer-swollen) particle volume, and 푉w the volume of the continuous phase. The concentrations without subscript refer to the overall concentrations of the dispersed phase, whereas the concentrations with subscript w refer to concentrations in the continuous phase. The rate coefficients 푘entry and 푘dM are explained below. It is assumed that T does not undergo chemical reactions in the continuous phase.
The overall concentrations of the compartmentalized entities P and T are given by:
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푖 [P ] = ∑ ∑ 푁(푖,푗) (7) 푁A푣s 푖 푗
푗 [T ] = ∑ ∑ 푁(푖,푗) (8) 푁A푣s 푖 푗
where the particle number distribution has been normalized to unity (∑푖 ∑푗 푁(푖,푗) = 1).
The individual concentrations of all reactants are computed based on Equations 9-14, accounting for effects of compartmentalization on P and T:
푑[M] = −푘 [P][M] − 1.5푘 [M]3 (9) 푑푡 p i,th
[ ] 푑 P 푘deact 3 = 푘act[PT] − 2 ∑ ∑ 푖푗푁(푖,푗) + 푘i,th[M] 푑푡 (푁A푣s) 푖 푗 (10) 푘t − 2 ∑ ∑ 푖(푖 − 1)푁(푖,푗) (푁A푣s) 푖 푗
[ ] 푑 T 푘deact 푉w = 푘act[PT] − 2 ∑ ∑ 푖푗푁(푖,푗) + 푘entry[T ]w ( ) − 푘dM[T ] (11) 푑푡 (푁A푣s) 푣s 푖 푗
푑[PT] 푘deact = −푘act[PT] + 2 ∑ ∑ 푖푗푁(푖,푗) (12) 푑푡 (푁A푣s) 푖 푗
푑[S] 푘t = 2 ∑ ∑ 푖(푖 − 1)푁(푖,푗) (13) 푑푡 (푁A푣s) 푖 푗
[ ] 푑 T w 푣s = 푁cr {푘dM[T ] ( ) − 푘entry[T ]w} (14) 푑푡 푉w
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where 푁cr is the total number of particles in the system (note that 푁cris not normalized to unity). Entry is defined as an event for a particle in the continuous phase, so it is converted 푉 to correspond to the dispersed phase by multiplication with ( w) in Equation 11. 푣s Conversely, exit is defined as an event of the dispersed phase, and the rate expression is 푣 therefore multiplied by ( d ) in Equation 14. All simulations were carried out with 푉w
[M]0/[PT]0 = 8.71/0.02 for 125 C, and the values of all rate coefficients are listed in Table 5.1. All rate coefficients were assumed to be conversion-independent, and chain- length dependence of rate coefficients (mainly 푘t) was not included in the model. The solids content (S. C.) was 20 wt% (i.e. S. C. = mass of organic phase/total mass = 0.2). The equations were implemented and solved using the software VisSim (version 8.0, Visual Solutions Inc.) employing numerical integration (Backward Euler integration
푁 algorithm). The number of particles per aqueous phase volume ( cr) is required to solve 푉w Equation 14. This quantity is obtained as follows:
푚 ( org ) 푁 푣 푑 S. C. 푑 cr s org w (15) = 푚 = 푉w ( w) (1 − S. C. )푣s푑org 푑w
where 푚 denotes mass, 푑 denotes density (푑org taken to be the density of styrene; 푑org = 3 0.91 g/cm ), the subscripts org and w denote organic and aqueous phase, 푑w and 푑org are the densities of water and the organic phase.
Model for Exit/Entry of Nitroxide Exit/entry of T was modeled by adopting the methodology developed for exit/entry of small radicals in emulsion polymerization based on conventional (not controlled/living) radical polymerization.[43] The rate coefficient for adsorption of T by a particle (푘ads; M−1s−1) is given by Equation 16 based on the Smoluchowski equation:
푘ads = 2휋퐷푑푁A (16)
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where 퐷 denotes the diffusion coefficient of T in the aqueous phase and 푑 is the particle diameter. The value of 퐷 for TEMPO in water at 125 ºC is not available, and 퐷 = 1.50 × 10−7 dm2 s−1 was employed in all simulations as a reasonable estimate based on literature values for 퐷 of small molecules in low viscosity solvents.[44] In previous work involving simulations of diffusion of TEMPO in the aqueous phase at 135 ºC, Ma et al.[5] used a similar estimate of 퐷 = 5 × 10−7 dm2 s−1. The entry rate coefficient −1 푘entry (s ) is related to 푘ads via Equation 17:
푘ads 2휋퐷푑 푘entry = = (17) 푁A푉w 푉w
As in the published modelling approach,[43] it is assumed that the average number of entry events per particle equals the average number of exit events per particle, i.e. 푘ads[T ]w = 푁A푣s푘dM[T ], thus allowing us to express 푘dM as:
12퐷 푘 = (18) dM 훤푑2
where the partition coefficient = [T ]/[T ]w = 98.8 as estimated experimentally for TEMPO for styrene/water at 135 ºC.[45] This approach ensures that the overall mass balance on nitroxide is satisfied (if not, nitroxide would be either “created” or “consumed” by this imbalance, which would cause an erroneous model output). However, the exit rate is not equal to the entry rate at a given point in time for a given particle type (“particle type” referring to (1,1), (0,2) etc), i.e. 푁A푉w푘entry[T ]w푁(푖,푗) ≠ 푘dM푗푁(푖,푗).
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Table 5. 1. Rate Parameters Employed in the Simulations. Rate parameter Value Reference
−1 −1 3 푘p (M s ) 2.32 × 10 [46] −1 −3 푘act (s ) 1.60 × 10 [47] −1 −1 7 푘deact (M s ) 7.60 × 10 [47, 48] −2 −1 a) −10 푘i,th (M s ) 1.70 × 10 [42] −2 −1 b) 8 푘t (M s ) 1.72 × 10 [49] 퐷 (dm2 s−1) 1.50 × 10−7 [44] (−)c) 98.8 [45]
a) 3 On the basis of the radical generation rate = 푘i,th[M] b) On the basis of the termination rate = 2푘 [P] t c) Experimental value at 135 C
5.4 Results and Discussion
All simulations correspond to the system St/TEMPO/ 125 ºC ([alkoxyamine]0 = 0.02 M). It should be noted that particle diameters below 50 nm correspond to microemulsion polymerizations, emulsion polymerizations at very low conversion, and miniemulsion systems with exceptionally small particles. Figure 5.1a shows simulations in the absence of exit/entry of TEMPO, revealing how the rate of polymerization (푅p) decreases with decreasing particle size in agreement with previous theoretical work.[19]
Now, accounting for exit/entry of TEMPO results in completely different trends – 푅p increases markedly with decreasing particle size (Figure 5.1b). The dramatic effect of
TEMPO partitioning on 푅p is further examined in Figure 5.2, which shows overlays of conversion-time data with and without exit/entry and the corresponding homogeneous (bulk) system for diameters 10, 40 and 70 nm. For the smallest diameter of 10 nm, there is an enormous difference in 푅p with and without exit/entry. The difference is less dramatic for 40 nm, and both systems exhibit 푅p similar to in bulk for 70 nm.
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(a) No exit/entry of TEMPO 16
14 Particle diameter 100, 90, 80, 70, 60, 50, 40, 30, 20 and 10 nm 12
10 8
6
(%) Conversion 4
2 0 0 10 20 30 40 50 60 Time (min)
(b) With exit/entry of TEMPO 16 14
12
10
8 6
Conversion (%) Conversion 4 Particle diameter 100, 90, 80, 70, 60, 50, 40, 2 30, 20 and 10 nm
0 0 10 20 30 40 50 60 Time (min)
Figure 5. 1. Simulated conversion-time data for NMP of styrene using St-TEMPO initiator at ퟏퟐퟓ C (a) without and (b) with exit/entry of TEMPO at various particle diameters
([St]0/[St-TEMPO]0 = ퟖ. ퟕퟏ/ퟎ. ퟎퟐ). Broken lines denote simulated bulk NMP.
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d = 10 nm d = 40 nm d = 70 nm 16 16 16
14 14 14
12 12 12
10 10 10 8 8 8 6 6 6
Conversion (%) Conversion (%) Conversion (%) Conversion 4 4 4
2 2 2 0 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (min) Time (min) Time (min)
Figure 5. 2. Simulated conversion-time data for NMP of styrene using St-TEMPO initiator at ퟏퟐퟓ C without (blue broken-dotted lines) and with (black solid lines) exit and entry of
TEMPO at particle diameters of ퟏퟎ (left), ퟒퟎ (centre) and ퟕퟎ nm (right) ([St]0/[St-
TEMPO]0 = ퟖ. ퟕퟏ/ퟎ. ퟎퟐ). The red broken lines denote simulated bulk NMP.
d = 10 nm d = 40 nm d = 70 nm
1.0000 1.0000 1.0000
0.9998 0.9998 0.9998
0.9996 0.9996 0.9996
0
0 0 0.9994 0.9994 0.9994
0.9992 0.9992 0.9992
[PT]/[PT] [PT]/[PT] [PT]/[PT] 0.9990 0.9990 0.9990
0.9988 0.9988 0.9988
0.9986 0.9986 0.9986 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 Conversion (%) Conversion (%) Conversion (%)
Figure 5. 3. Simulated values of number fraction (relative to the initial amount) of alkoxyamine as a function of conversion for NMP of styrene using St-TEMPO initiator at ퟏퟐퟓ C without (blue broken-dotted lines) and with (black solid lines) exit and entry of
TEMPO at ퟏퟎ (left), ퟒퟎ (centre) and ퟕퟎ nm (right) ([St]0/[St-TEMPO]0 = ퟖ. ퟕퟏ/ퟎ. ퟎퟐ). The red broken lines denote simulated bulk NMP.
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The “livingness” (end-group fidelity) can be assessed by examination of the ratio of alkoxyamine chain ends relative to the initial amount ([PT]/[PT]0). Figure 5.3 shows
[PT]/[PT]0 vs conversion with and without exit/entry and the corresponding bulk system for the particle diameters 10, 40 and 70 nm The livingness is higher for both dispersed systems than the bulk system, and this difference is more pronounced for smaller particles, and the livingness with exit/entry is lower than without exit/entry. The livingness is extremely high in all cases (in agreement with earlier theoretical work[14]) and the absolute differences in livingness between the different cases are very small, suggesting the practical relevance may be limited. It should also be mentioned that a significant fraction of the original alkoxyamine initiator would be remaining (i.e. not yet chain-extended) at 10% conversion (especially for small particles, as they exhibit higher
푅p).
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(a) No exit/entry of TEMPO 50 45
40 35
30 Particle diameter 25 100, 90, 80, 70, 60, 50, 40, 30, 20 and 10 nm 20
15 10 5 0 0 2 4 6 8 10
Conversion (%)
(b) With exit/entry of TEMPO 450 Particle diameter 400 100, 90, 80, 70, 60, 50, 40, 30, 20 and 10 nm 350
300
250 200
150 100 50
0 0 2 4 6 8 10 Conversion (%)
Figure 5. 4. Simulated number of propagation events per activation-deactivation cycle for an individual chain () vs conversion for NMP of styrene using St-TEMPO initiator at ퟏퟐퟓ
C (a) without and (b) with exit/entry of TEMPO at various particle diameters ([St]0/[St-
TEMPO]0 = ퟖ. ퟕퟏ/ퟎ. ퟎퟐ). Broken lines denote simulated bulk NMP.
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It is not possible to compute the MWD using the present model, but the influence of compartmentalization on the MWD can be examined by inspection of the number of monomer units added per chain per activation-deactivation cycle (), which is given by:[19]
푘p[M][P ] 푣 = −2 (19) (푁A푣s) 푘deact ∑푖 ∑푗 푖푗푁(푖,푗)
For a given molecular weight, the polydispersity (푀w/푀n) decreases with decreasing (all other parameters being constant). Figure 5.4 shows vs conversion with and without exit/entry, revealing how exit/entry dramatically alters the behaviour of the system. In the case of exit/entry, is larger than in bulk, and increases with decreasing particle size, i.e. the exact opposite to the behaviour without exit/entry. Figure 5.5 shows vs conversion for the systems with and without exit/entry as well as bulk for the particle diameters 10, 40 and 70 nm. There are only minor differences between the systems at 70 nm (particles too large for compartmentalization effects to be strong), whereas (no exit/entry) < (bulk) < (exit/entry) for 40 and 10 nm (for 10 nm, both dispersed systems give values off scale. The very high values of in the case of exit/entry for small particles can be explained by considering that an activation event in a particle that contains no propagating radicals and no nitroxide (which is the case for the vast majority of particles) results in a particle containing one propagating radical and one nitroxide. Now, exit/entry of nitroxide means that this nitroxide may exit, and nitroxide may also enter from the aqueous phase. The net result is that the time during which the propagating radical exists alone in the particle is prolonged, thus effectively reducing the deactivation rate and resulting in a high value of .
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d = 10 nm d = 40 nm d = 70 nm
50 50 50 45 45 45 With exit/entry of TEMPO 40 40 40 35 35 35
30 30 30
25 25 25 20 20 20 15 15 15
10 10 10 No exit/entry of TEMPO 5 5 5 0 0 0 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 Conversion (%) Conversion (%) Conversion (%)
Figure 5. 5. Simulated number of propagation events per activation-deactivation cycle for an individual chain () vs conversion for NMP of styrene using St-TEMPO initiator at ퟏퟐퟓ C without (blue broken-dotted lines) and with (black solid lines) exit/entry of TEMPO at
ퟏퟎ (left), ퟒퟎ (center) and ퟕퟎ nm (right) ([St]0/[St-TEMPO]0 = ퟖ. ퟕퟏ/ퟎ. ퟎퟐ). Red broken lines denote simulated bulk NMP.
The above results show that even for a relatively hydrophobic nitroxide such as TEMPO, it is essential to consider phase transfer events. Exit/entry of TEMPO has a major effect on the system – relative to the corresponding bulk system, 푅p increases, the livingness increases, but the MWD becomes broader. This is in sharp contrast to the situation when exit/entry of nitroxide is not considered, in which case compartmentalization leads to
(relative to bulk) lower 푅p, higher livingness and narrower MWD. From a mechanistic viewpoint, the results can be explained by considering that for a compartmentalized NMP system, the very vast majority of particles contain no propagating radicals and no nitroxide, and thus exit of TEMPO from a particle where an activation event has occurred results in a particle containing only a propagating radical. In such a particle, uncontrolled propagating will occur (unless a TEMPO species enters from the aqueous phase or another activation event occurs), thus accounting for the increase in 푅p and (partial) loss of control over the MWD (higher ; Figure 5.4). A detailed, complete 136
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explanation of the above results from a mechanistic perspective will be provided in the near future in a forthcoming full paper.[50]
A survey of the literature on TEMPO-mediated polymerization of styrene in dispersed systems reveals a mixed bag of results. The majority of the miniemulsion systems studied exhibit very similar behavior as in bulk.[51-57] The particle diameters in such systems generally tend to be in the range 80 – 200 nm , which is too large for compartmentalization effects to be significant. Moreover, particle size distributions in miniemulsions are often relatively broad,[52, 58, 59] which would reduce any effect of compartmentalization as the predominant loci of polymerization (in terms of mass of polymer formed) would be the larger particles. Maehata et al.[60] reported data consistent with compartmentalization effects without exit/entry for TEMPO-mediated miniemulsion polymerization of styrene at 135 ºC using the surfactant Dowfax 8390
( 푑n = 47 − 163 nm ), whereas TEMPO-mediated miniemulsion polymerization of styrene at 125 ºC using sodium dodecyl benzene sulfonate (SDBS) has been reported to proceed with higher 푅p than the corresponding bulk system.[61] Nakamura et al.[8, 13] found that 푅p increased and the control/livingness decreased with decreasing particle size for the TEMPO-mediated miniemulsion polymerization of styrene at 125 ºC using
SDBS (푑n = 70 − 170 nm). Other than compartmentalization effects, factors such as a rate enhancing effect of SDBS[61] as well as an interface effect (adsorption of nitroxide at the oil-water interface)[8, 13] have been put forward as possible explanations. TEMPO-mediated microemulsion polymerization of styrene at 125 ºC using a radical initiator (e.g. benzoyl peroxide) and free TEMPO has also been reported by Wakamatsu et al.,[41] revealing a dramatic decrease in 푅p relative to bulk. However, in light of the findings in the present study, this low 푅p may have originated in an excess of free TEMPO.[24] It is thus surmised that none of the above studies concern experimental conditions consistent with a situation where an increase in 푅p and (partial) loss of control/livingness would occur as a result of compartmentalization and exit/entry of TEMPO. For this to occur, the initiating system must be an alkoxyamine with no (or very little) free TEMPO (i.e. not a radical initiator/free TEMPO system) and sufficiently small particles.
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5.5 Conclusions
In summary, it has been shown by modeling and simulations that TEMPO-mediated polymerization of styrene in a dispersed system where compartmentalization effects are operative can be dramatically influenced by nitroxide exit/entry for sufficiently small particles. In such systems, the polymerization is expected to proceed at a markedly higher Rp than the corresponding bulk system at the expense of control/livingness. Depending on the deactivator water solubility, these findings will apply qualitatively to all CLRP systems governed by the persistent radical effect (e.g. NMP and atom transfer radical polymerization (ATRP)).
While this theoretical work finding a new aspect of NMP in heterogeneous system considering compartmentalization and reactant partitioning has its own right, truly the final goal is to meet and predict the practical results. Besides, the proper interpretation of the theoretical side still needs further strict and convinced discussion. Those developments are ongoing targets by authors.
5.5 References 1. Braunecker, W.A. and K. Matyjaszewski, Controlled/Living Radical Polymerization: Features, Developments, and Perspectives. Prog. Polym. Sci., 2007. 32: p. 93-146. 2. Zetterlund, P.B., Y. Kagawa, and M. Okubo, Controlled/Living Radical Polymerization in Dispersed Systems. Chem. Rev., 2008. 108: p. 3747-3794. 3. Cunningham, M.F., Controlled/Living Radical Polymerization in Aqeuous Dispersed Systems. Prog. Polym. Sci., 2008. 33: p. 365-398. 4. Zetterlund, P.B., F. Aldabbagh, and M. Okubo, Controlled/Living Heterogeneous Radical Polymerization in Supercritical Carbon Dioxide. J. Polym. Sci.; Part A: Polym. Chem., 2009. 47: p. 3711-3728. 5. Ma, J.W., et al., Interfacial Mass Transfer in Nitroxide-Mediated Miniemulsion Polymerization. Macromol. Theory Simul., 2002. 11: p. 953-960. 6. Zetterlund, P.B. and M. Okubo, Nitroxide-Mediated Radical Polymerization in Miniemulsion at Stationary State: Rationale for Independence of Polymerization
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Rate on Nitroxide Partitioning Using Oil-Phase Initiation. Macromol. Theory Simul., 2005. 14: p. 415-420. 7. Kagawa, Y., et al., Atrp in Miniemulsion: Partitioning Effects of Cu(I) and Cu(Ii) on Rp, Livingness, and Mwd. Macromolecules, 2007. 40: p. 3062-3069. 8. Zetterlund, P.B., T. Nakamura, and M. Okubo, Mechanistic Investigation of Particle Size Effects in Tempo-Mediated Radical Polymerization of Styrene in Aqueous Miniemulsion. Macromolecules, 2007. 40: p. 8663-8672. 9. Zetterlund, P.B., N. Alam, and M. Okubo, Effects of the Oil-Water Interface on Network Formation in Nanogel Synthesis Using Nitroxide-Mediated Radical Copolymerization of Styrene/Divinylbenzene in Miniemulsion. Polymer, 2009. 50: p. 5661-5667. 10. Zetterlund, P.B., et al., Nitroxide-Mediated Controlled/Living Free Radical Copolymerization of S and Dvb in Aqueous Miniemulsion. Macromol. Rapid Commun., 2005. 26: p. 955-960. 11. Saka, Y., P.B. Zetterlund, and M. Okubo, Gel Formation and Primary Chain Lengths in Nitroxide-Mediated Radical Copolymerization of Styrene and Divinylbenzene in Miniemulsion. Polymer, 2007. 48: p. 1229-1236. 12. Alam, M.N., P.B. Zetterlund, and M. Okubo, Network Formation in Nitroxide- Mediated Radical Copolymerization of Styrene and Divinylbenzene in Miniemulsion: Effect of Macroinitiator Hydrophilicity. Polymer, 2009. 50: p. 1632-1636. 13. Nakamura, T., P.B. Zetterlund, and M. Okubo, Particle Size Effects in Tempo- Mediated Radical Polymerization of Styrene in Aqueous Miniemulsion. Macromol. Rapid Commun., 2006. 27: p. 2014-2018. 14. Zetterlund, P.B., Controlled/Living Radical Polymerization in Nanoreactors: Compartmentalization Effects. Polym. Chem., 2011. 2: p. 534 - 549. 15. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization in Nanoreactors: Can Dilution or Increased Nitroxide Concentration Provide Benefits Similar to Compartmentalization? Aust. J. Chem., 2010. 63: p. 1195-1200. 16. Simms, R.W. and M.F. Cunningham, Compartmentalization of Reverse Atom Transfer Radical Polymerization in Miniemulsion. Macromolecules, 2008. 41: p. 5148-5155.
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17. Butte, A., G. Storti, and M. Morbidelli, Pseudo-Living Polymerization of Styrene in Miniemulsion. DECHEMA Monographs, 1998. 134: p. 497-507. 18. Charleux, B., Theoretical Aspects of Controlled Radical Polymerization in a Dispersed Medium. Macromolecules, 2000. 33: p. 5358-5365. 19. Zetterlund, P.B. and M. Okubo, Compartmentalization in Nitroxide-Mediated Radical Polymerization in Dispersed Systems. Macromolecules, 2006. 39: p. 8959-8967. 20. Kagawa, Y., et al., Compartmentalization in Atom Transfer Radical Polymerization (Atrp) in Dispersed Systems. Macromol. Theory Simul., 2006. 15: p. 608-613. 21. Luo, Y., et al., Effect of Reversible Addition-Fragmentation Transfer (Raft) Reactions on (Mini)Emulsion Polymerization Kinetics and Estimate of Raft Equilibrium Constant. Macromolecules, 2006. 39: p. 1328-1337. 22. Zetterlund, P.B. and M. Okubo, Compartmentalization in Tempo-Mediated Radical Polymerization in Dispersed Systems: Effects of Macroinitiator Concentration. Macromol. Theory Simul., 2007. 16: p. 221-226. 23. Zetterlund, P.B., Y. Kagawa, and M. Okubo, Compartmentalization in Atom Transfer Radical Polymerization of Styrene in Dispersed Systems: Effects of Target Molecular Weight and Halide End Group. Macromolecules, 2009. 42: p. 2488-2496. 24. Zetterlund, P.B., J. Wakamatsu, and M. Okubo, Nitroxide-Mediated Radical Polymerization of Styrene in Aqueous Microemulsion: Initiator Efficiency, Compartmentalization and Nitroxide Phase Transfer. Macromolecules, 2009. 42: p. 6944-6952. 25. Zetterlund, P.B. and M. Okubo, Compartmentalization in Nitroxide-Mediated Polymerization in Dispersed Systems: Relative Contributions of Confined Space Effect and Segregation Effect Depending on Nitroxide Type. Macromol. Theory Simul., 2009. 18: p. 277-286. 26. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization in Dispersed Systems: Compartmentalization and Nitroxide Partitioning. Macromol. Theory Simul., 2010. 19: p. 11-23.
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27. Zetterlund, P.B., Compartmentalization in Atom Transfer Radical Polymerization to High Conversion in Dispersed Systems: Effects of Diffusion- Controlled Reactions. Macromolecules, 2010. 43: p. 1387-1395. 28. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization in Nanoreactors: Factors Influencing Compartmentalization Effects on Bimolecular Termination. Polymer, 2010. 51: p. 6168-6173. 29. Thomson, M.E. and M.F. Cunningham, Compartmentalization Effects on the Rate of Polymerization and the Degree of Control in Atrp Aqueous Dispersed Phase Polymerization. Macromolecules, 2010. 43: p. 2772-2779. 30. Zetterlund, P.B., Nitroxide-Mediated Radical Polymerization of Butyl Acrylate Using Tempo: Improvement of Control Exploiting Nanoreactors? Macromolecular Reaction Engineering, 2010. 4: p. 663-671. 31. Zetterlund, P.B., Compartmentalization Effects on Bimolecular Termination in Atom Transfer Radical Polymerization in Nanoreactors. Macromolecular Theory and Simulations, 2011. 20: p. 660-666. 32. Bentein, L., et al., Kinetic Modeling of Nitroxide Mediated Polymerization of Styrene: Effect of Particle Diameter and Nitroxide Partitioning up to High Conversion. Polymer, 2012. 53: p. 681-693. 33. Tobita, H., Kinetics of Stable Free Radical Mediated Polymerization inside Submicron Particles. Macromol. Theory Simul., 2007. 16: p. 810-823. 34. Tobita, H. and F. Yanase, Monte Carlo Simulation of Controlled/Living Radical Polymerization in Emulsified Systems. Macromol. Theory Simul., 2007. 16: p. 476-488. 35. Tobita, H., Kinetics of Controlled/Living Radical Polymerization in Emulsified Systems. Macromol. Symp., 2008. 261: p. 36-45. 36. Tobita, H., Raft Miniemulsion Polymerization Kinetics, 1-Polymerization Rate. Macromol. Theory Simul., 2009. 18: p. 108-119. 37. Tobita, H., Raft Miniemulsion Polymerization Kinetics, 2-Molecular Weight Distribution. Macromolecular Theory and Simulations, 2009. 18: p. 120-126. 38. Tobita, H., Fundamentals of Raft Miniemulsion Polymerization Kinetics. Macromol. Theory and Simul., 2010. 288: p. 16-24.
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39. Tobita, H., Effects of Fluctuation and Segregation in the Rate Acceleration of Atrp Miniemulsion Polymerization. Macromolecular Theory and Simulations, 2011. 20: p. 179-190. 40. Tobita, H., Effects of Retardation and Variation of Monomer Concentration in Raft Miniemulsion Polymerization. Macromolecular Theory and Simulations, 2011. 20: p. 709-720. 41. Wakamatsu, J., et al., Nitroxide-Mediated Radical Polymerization in Microemulsion. Macromol. Rapid Commun., 2007. 28: p. 2346-2353. 42. Hui, A.W. and A.E. Hamielec, J. Appl. Polym. Sci., 1972. 16: p. 749-769. 43. Gilbert, R.G., Emulsion Polymerization: A Mechanistic Approach. 1995, London: Academic Press. 44. Wilke, C.R. and P. Chang, Correlation of Diffusion Coefficients in Dilute Solutions. Aiche Journal, 1955. 1: p. 264-270. 45. Ma, J.W., et al., Nitroxide Partitioning between Styrene and Water. J. Polym. Sci.; Part A: Polym. Chem., 2001. 39: p. 1081-1089. 46. Buback, M., et al., Macromol. Chem. Phys., 1995. 196: p. 3267-3280. 47. Goto, A., et al., Macromol. Rapid Commun., 1997. 18: p. 673-681. 48. Fukuda, T., et al., Macromolecules, 1996. 29: p. 6393-6398. 49. Buback, M., et al., Termination Kinetics of Styrene Free Radical Polymerization Studied by Time-Resolved Pulsed Laser Experiments. Macromol. Chem. Phys., 2000. 201: p. 464-469. 50. Sugihara, Y. and P.B. Zetterlund, Manuscript in preparation. 51. Prodpran, T., et al., Nitroxide-Mediated Living Free Radical Miniemulsion Polymerization of Styrene. Macromol. Symp., 2000. 155: p. 1-14. 52. Pan, G., et al., Macromolecules, 2001. 34: p. 481-488. 53. Keoshkerian, B., P.J. MacLeod, and M.K. Georges, Block Copolymer Synthesis by a Miniemulsion Stable Free Radical Polymerization Process. Macromolecules, 2001. 34: p. 3594-3599. 54. Pan, G., et al., Macromolecules, 2002. 35: p. 6915-6919. 55. Cunningham, M.F., et al., Nitroxide-Mediated Living Radical Polymerization in Miniemulsion. Macromol. Symp., 2002. 182: p. 273-282. 56. Cunningham, M.F., et al., Nitroxide-Mediated Styrene Miniemulsion Polymerization. Macromolecules, 2002. 35: p. 59-66.
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57. Cunningham, M., et al., Maximizing Polymer Livingness in Nitroxide-Mediated Miniemulsion Polymerizations. Polymer, 2005. 46: p. 1025-1032. 58. Asua, J.M., Miniemulsion Polymerization. Prog. Polym. Sci., 2002. 27: p. 1283. 59. Landfester, K., Polyreactions in Miniemulsions. Macromol. Rapid Commun., 2001. 22: p. 896. 60. Maehata, H., et al., Compartmentalization in Tempo-Mediated Styrene Miniemulsion Polymerization. Macromolecules, 2007. 40: p. 7126-7131. 61. Lin, M., J.C.C. Hsu, and M.F. Cunningham, Role of Sodium Dodecylbenzenesulfonate in 2,2,6,6-Tetramethyl-1-Piperidinyloxy-Mediated Styrene Miniemulsion Polymerization. J. Polym Sci.; Part A: Polym. Chem., 2006. 44: p. 5974-5986.
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Chapter 6
Chain Transfer to Solvent in the Radical Polymerization of N-Isopropylacrylamide
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6.1 Abstract Chain transfer to solvent has been investigated in the conventional radical polymerization and nitroxide-mediated polymerization (NMP) of N- isopropylacrylamide (NIPAM) in N,N-dimethylformamide (DMF) at 120 C. The extent of chain transfer to DMF can significantly impact the maximum attainable molecular weight in both systems. Based on a theoretical treatment, it has been shown that the same value of chain transfer to solvent constant, 퐶tr,S, in DMF at 120 C (within experimental error) can account for experimental molecular weight data for both conventional radical polymerization and NMP under conditions where chain transfer to solvent is a significant end-forming event. In NMP (and other controlled/living radical polymerization systems), chain transfer to solvent is manifested as the number-average molecular weight (푀n) going through a maximum value with increasing monomer conversion.
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6.2 Introduction Poly(N-isopropylacrylamide, NIPAM) is a well-known temperature-sensitive polymer with a lower critical solution temperature (LCST) in water of around 33 °C.[1-3] The proximity of the LCST to physiological temperature has led to intense research into biological/medical applications.[4, 5] There are numerous reports of homo- and copolymerizations of NIPAM under conventional (non-living) radical and controlled/living radical polymerization (CLRP) conditions. CLRP of NIPAM has been reported using nitroxide-mediated polymerization (NMP),[6-12] atom transfer radical polymerization (ATRP),[7, 13, 14] reversible addition-fragmentation chain transfer (RAFT) polymerization,[7, 15-25] organotellurium-mediated radical polymerization (TERP),[26] and single electron transfer living radical polymerization (SET-LRP).[27]
NIPAM is a solid at room temperature (푚p = 60 − 63 °C), and is most commonly polymerized in solution, using benzene,[9, 15, 28] alcohols,[13, 27, 29] 1,4-dioxane,[15, 16, 21, 22, 25] N,N-dimethylformamide (DMF),[6, 8, 10, 11, 18, 26] anisole[7, 14] or water[23, 24] as solvents.
Chain transfer to monomer or solvent can play an important role in radical polymerization under certain conditions. Ultimately, for a given set of conditions, chain transfer to monomer or solvent dictates an upper limit in accessible molecular weight, which may influence both a conventional radical polymerization and CLRP.[30] In CLRP, significant occurrence of such chain transfer events does not only influence the accessible molecular weight, but also compromises both control over the molecular weight distribution (MWD) and livingness (end-functionality). For example, it has been reported that chain transfer to solvent (DMF, anisole and p-xylene) in the NMP of tert- butyl acrylate caused the number-average molecular weight (푀n) to deviate downwards from the theoretical 푀n (푀n,th ) with increasing conversion, and even go through a maximum.[31] It has also been shown that chain transfer to monomer in NMP may cause a similar, but less pronounced, deviation from 푀n,th.[32] However, poor performance of a CLRP may be due to a number of reasons, and it can often be difficult to ascribe deviations from ideal controlled/living behaviour specifically to chain transfer events.
In the present contribution, chain transfer to solvent has been analyzed in detail in both the conventional radical polymerization and CLRP of NIPAM in DMF. As far as we are aware, chain transfer to solvent constants (퐶tr,S) for this important monomer have to date 146
Yusuke Sugihara Chapter 6
not been reported for any solvent. It is shown that significant chain transfer to solvent occurs, and that these chain transfer reactions preclude synthesis of high molecular weight poly(NIPAM) by NMP in DMF. Various analytical equations are employed to show that the same values of 퐶tr,S (within experimental error) can accurately describe the chain transfer events observed in both conventional radical polymerization and CLRP of NIPAM.
6.3 Experimental Section
Materials tert-Butyl acrylate (t-BA, Sigma Aldrich, 98%) and N-isopropylacrylamide (NIPAM, Sigma Aldrich, 97%) were purified by distillation under reduced pressure and recrystallization from 3:2 benzene:hexane, respectively. 2,2’-Azobisisobutyronitrile (AIBN, DuPont Chemical Solution Enterprise) was recrystallized twice from methanol, and tert-butyl peroxide (TBP, Sigma Aldrich, 98%) was used as received. N-tert-Butyl- N-[1-diethylphosphono(2,2-dimethylpropyl)]oxy (SG1) was prepared according to the literature [33], and purified by column chromatography with purity (96%) determined using 1H-NMR spectroscopy from the reaction of SG1 radical with pentafluorophenylhydrazine (Sigma Aldrich). Poly(t-BA)-SG1 macroinitiator (MI) with −1 −1 푀n = 3,000 g mol and 푀w/푀n = 1.17 g mol was prepared by precipitation NMP in supercritical carbon dioxide, according to our published procedure.[34] HPLC grade solvents were used throughout, and lithium bromide (LiBr) was used as received.
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NIPAM DMF AIBN tBPO
SG1 PtBA-SG1
Scheme 6. 1. Chemical structures of N-isopropylacrylamide (NIPAM), dimethylformamide (DMF), tert-butyl peroxide (TBP), N-tert-butyl-N-[1- diethylphosphono(2,2-dimethylpropyl)]oxy (SG1) and poly(tert-butyl acrylate)-SG1 (Poly(t-BA)-SG1).
Measurements
Number-average molecular weight (푀n) and polydispersity (푀w/푀n) were determined using a gel permeation chromatography (GPC) system consisting of a Viscotek DM 400 data manager, a Viscotek VE 3580 refractive-index detector, and two Viscotek Viscogel
GMHHR-M columns. Measurements were carried out at 60 C at a flow rate of 1.0 mL min-1 using HPLC-grade DMF containing 0.01 M LiBr as the eluent.[35, 36] The columns were calibrated using six poly(styrene, St) standards ( 푀n = 376 − −1 −1 2,570,000 g mol ) and 푀n is given in grams per mole (g mol ) throughout. The present GPC methodology for poly(NIPAM) has previously been adopted in NIPAM CLRP studies.[10, 17, 19, 26] To the best of our knowledge, Mark-Houwink-Sakurada (MHS) parameters are not available for poly(NIPAM)/DMF/LiBr/PSt. Work by
Ganachaud et al.[15] has indicated that the error in Mn values estimated by GPC analysis of poly(NIPAM) relative to polystyrene standards using THF as eluent is only approximately 5% on the average (depending on the molecular weight), and it is expected that the GPC error in the present work is of similar order. The GPC analysis is
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further complicated by the fact that the polymers prepared by NMP are block copolymers of t-BA and NIPAM.
1H NMR spectra were recorded using a Joel GXFT 400 MHz instrument equipped with a DEC AXP 300 computer workstation.
General Polymerization Details All reaction mixtures were added to Pyrex ampoules and subjected to several freeze- thaw degas cycles before sealing under vacuum. The ampoules were heated in an aluminium heating block at the designated temperature for various times. Polymerizations were stopped by placing ampoules in an ice-bath. GPC measurements were carried out on the non-precipitated reaction mixtures. Conversions were estimated using 1H NMR by comparing the integration of the polymer peak at 3.85 ppm
(CH(Me)2) with NIPAM monomer at 4.01 ppm (CH(Me)2).
Conventional Radical Polymerization Stock solutions containing 0.41, 1.37 and 4.1 mM TBP were made up in DMF. The obtained stock solution (4 mL) was added to NIPAM (0.91 g, 8.00 mmol) in a Pyrex ampoule. Evacuated ampoules were heated at 120 C for various times.
Table 6. 1. Conventional radical polymerization of NIPAM with TBA in DMF at ퟏퟐퟎ∘퐂. Entry NIPAM TBP DMF Temperature (mmol) (mmol) (ml) (C) CRP1 8.00 1.64 × 10−3 4 120 CRP2 8.00 5.48 × 10−3 4 120
CRP3 8.00 1.64 × 10−2 4 120
Chain Transfer to Solvent (Mayo Plot) DMF stock solution (4 mL) containing 0.41 mM of TBP was added to NIPAM (0.29 g, 2.56 mmol; 0.39 g, 3.45 mmol; 0.58 g, 5.12 mmol; and 1.17 g, 10.34 mmol) in a Pyrex ampoule. Evacuated ampoules were heated at 120 °C for various times. Conversions were less than 5% in all cases.
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Table 6. 2. Conventional radical polymerization of NIPAM with TBP in DMF at ퟏퟐퟎ∘퐂. Entry NIPAM TBP DMF Temperature (mmol) (mmol) (ml) (C) CRP4 2.56 1.64 × 10−3 4 120 CRP5 3.45 1.64 × 10−3 4 120 CRP6 10.34 1.64 × 10−3 4 120
Nitroxide-Mediated Polymerizations
The following polymerizations in DMF (4 mL) were carried out [NIPAM]0/
[poly(푡-BA)-SG1]0 = 100 (a), 200 (b), and 300 (c); (a) NIPAM (0.91 g, 8.00 mmol), poly(t-BA)-SG1 (0.24 g, 8.00 × 10−2 mmol), and SG1 (5.89 mg, 2.00 × 10−2 mmol); (b) NIPAM (0.91 g, 8.00 mmol), poly(t-BA)-SG1 (0.12 g, 4.00 × 10−2 mmol), and SG1 (2.94 mg, 1.00 × 10−2 mmol); (c) NIPAM (0.91 g, 8.00 mmol), poly(t-BA)-SG1 (0.080 g, 2.67 × 10−2 mmol), and SG1 (1.96 mg, 0.67 × 10−2 mmol).
Table 6. 3. Nitroxide-mediated polymerization of NIPAM with poly(t-BA)-SG1 macroinitiator and free SG1 in DMF at ퟏퟐퟎ∘퐂. Entry NIPAM poly(t-BA)-SG1 SG1 NIPAM:poly(t-BA)-SG1 DMF Temperature (mmol) (mmol) (mmol) (mole ratio) (ml) (C) NMP1 8.00 8.00 × 10−2 2.00 × 10−2 100 4 120 NMP2 8.00 4.00 × 10−2 1.00 × 10−2 200 4 120 NMP3 8.00 2.67 × 10−2 0.67 × 10−2 300 4 120
Thermal Polymerization in the Absence of Initiator and Nitroxide Evacuated ampoules containing NIPAM (0.91 g, 8.00 mmol) in DMF (4 mL) were heated at 120 °C for various times.
Table 6. 4. Conventional radical polymerization of NIPAM in absence of thermal initiator in DMF at ퟏퟐퟎ∘퐂. Entry NIPAM TBP DMF Temperature (mmol) (mmol) (ml) (C) CRP7 8.00 − 4 120
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6.4 Results and Discussion
Limiting Molecular Weight Under normal conditions of conventional (non-living) radical polymerization in the absence of a chain transfer agent, the main end-forming event is bimolecular termination. However, when the rate of initiation in a bulk polymerization is sufficiently low, the propagating radical concentration becomes so low that the bimolecular termination rate is reduced to the point that chain transfer to monomer becomes the main end-forming event.[30] The ratio 푘p/푘tr,M (= 1/퐶tr,M; 푘p is the propagation rate coefficient, 푘tr,M the rate coefficient for chain transfer to monomer) dictates the maximum attainable number-average degree of polymerization (퐷푃n) for a given monomer. In a solution polymerization, the chain transfer to monomer limit is only reached if the rate of chain transfer to solvent is negligible relative to the rate of chain transfer to monomer. If chain transfer to solvent is the main end-forming event, 퐷푃n = 푘p[M]/푘tr,S[S] ( = [M]/
퐶tr,S[S]) (푘tr,S is the rate coefficient for chain transfer to solvent, [M] and [S] are the monomer and solvent concentrations, respectively).
Conventional Radical Polymerization in DMF If one performs a series of radical polymerizations with decreasing initiator concentration, 퐷푃n will increase with decreasing initiator concentration until the maximum 퐷푃n is reached, corresponding to either the chain transfer to monomer or solvent limit. Polymerizations of NIPAM in DMF were carried out at 120 C (relevant to NMP) using three different low concentrations of the high temperature initiator TBP.
All MWDs are very similar (Figure 6.1; 푀n = 20,100 ([TBP]0 = 4.1 mM), 17,600 −1 ([TBP]0 = 1.4 mM), 21,100 ([TBP]0 = 0.41 mM), and 21,600 g mol ([TBP]0 = 0), consistent with the majority of propagating radicals being transformed to dead chains and new propagating radicals by chain transfer to monomer or solvent.
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1.2
1.0 13% 0.8 22% ) (a. u.) M 0.6 33%
(log (log 0.4
w 0.2
0 2.5 3.5 4.5 5.5 6.5 log M
Figure 6. 1. MWDs for the conventional radical polymerization of NIPAM (2 M) in DMF initiated by TBP at ퟏퟐퟎ C for initiator concentrations of ퟎ. ퟒퟏ (ퟏퟑ%), ퟏ. ퟒ (ퟑퟑ%) and ퟒ. ퟏ mM (ퟐퟐ%), with monomer conversions as indicated.
Nitroxide-Mediated Polymerization NMP can be a useful mechanistic tool in radical polymerization since ideally the number of chains is constant throughout the polymerization. Moreover, if one initiates the polymerization with a macroinitiator of relatively high molecular weight, chain transfer events to low molecular weight species like monomer and solvent are likely to be readily detected in the MWDs.[32] The concept of a maximum molecular weight imposed by chain transfer to monomer or solvent applies to both conventional radical polymerization and CLRP (incl. NMP).
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100 90 80 70
60
50
40
(%) Conversion 30
20
10
0 0 4 8 12 16 20 24 28 32 36
Time (hour)
Figure 6. 2. Conversion vs. time data for NMP of NIPAM in DMF (2 M) using Poly(t-BA)- SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator at ퟏퟐퟎ C with
[M]0/[MI]0 = ퟏퟎퟎ (○), ퟐퟎퟎ (□), and ퟑퟎퟎ (∆).
Figure 6.2 shows conversion vs. time data for NMP of NIPAM in DMF at 120 C initiated by three different concentrations of poly(t-BA)-SG1 (MI), revealing that 푅p is (within experimental error) independent of the macroinitiator concentration to high conversion. Such behaviour is observed if the polymerization proceeds in the stationary state with respect to the propagating radical concentration, e.g. styrene/TEMPO/ 125 C .[37, 38] A somewhat more special case where 푅p is independent of the macroinitiator concentration is when the rate of spontaneous radical initiation (from monomer or adventitious impurities) is close to zero in the presence of an excess amount of free nitroxide.[38, 39] Due to uncertainties in rate coefficients of the present system, we refrain from further speculation on this topic. The MWDs shift to higher molecular weights with increasing conversion, but significant low molecular weight tailing is visible (Figure 6.3), consistent with chain transfer to monomer or solvent. Figure 6.4 shows 푀n vs conversion, revealing how 푀n initially increases with conversion, but then reaches a maximum value and decreases at high conversion. The values of 푀w/푀n gradually increase with increasing conversion (Figure 6.4).
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1.2 (a) Poly(tBA)-SG1 Poly(tBA)-b-poly(NIPAM) 1.0 25% 0.8 57% ) (a. u.) M 0.6 92%
(log (log 0.4
w 0.2
0 2.5 3 3.5 4 4.5 5 5.5 log M
1.2 (b) Poly(tBA)-SG1 Poly(tBA)-b-poly(NIPAM) 1.0 22% 0.8 56%
) (a. ) (a. u.)
M 0.6 92%
(log (log 0.4
w 0.2
0 2.5 3 3.5 4 4.5 5 5.5 log M
1.2 (c) Poly(tBA)-SG1 Poly(tBA)-b-poly(NIPAM) 1.0 18% 0.8 75%
) (a. u.)
M 0.6 95%
(log (log 0.4
w 0.2
0 2.5 3 3.5 4 4.5 5 5.5 log M
Figure 6. 3. MWDs of poly(t-BA)-b-poly(NIPAM) and original poly(t-BA)-SG1 for
[M]0/[MI]0 = ퟏퟎퟎ (a), ퟐퟎퟎ (b), ퟑퟎퟎ (c) with NIPAM conversions as indicated.
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2.2
2.0
1.8
n
M 1.6 /
w
M 1.4
1.2
1.0
22
20
18
16
14
12
(kg/mol)
n
M 10
8
6
4
2
0 0 20 40 60 80 100 Conversion (%)
Figure 6. 4. Mw/Mn (top) and Mn (bottom) vs conversion plots for NMP of NIPAM in DMF (2 M) at ퟏퟐퟎ C with poly(t-BA)-SG1 as macroinitiator with ퟐퟓ mol% free SG1 relative to macroinitiator and [M]0/[MI]0 = ퟏퟎퟎ (○), ퟐퟎퟎ (□), and ퟑퟎퟎ (∆), and conventional radical polymerization of NIPAM (2 M) with various concentrations of TBP as initiator () in DMF at ퟏퟐퟎ C (each data point corresponding to a different [TBP]0). The full lines are theoretical Mn using Equation 4 and 7 with Ctr,S = ퟎ. ퟎퟎퟎퟔퟓ (NMP) and ퟎ. ퟎퟎퟎퟖ (conventional radical polymerization).
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Chain Transfer to Solvent/Monomer
The concept of instantaneous 퐷푃n is not applicable to CLRP since chains grow throughout the polymerization. The value of 퐷푃n as a function of conversion is equal to the concentration of reacted monomer plus the concentration of monomeric units in the macroinitiator, divided by the total number of chains. The latter equals the macroinitiator concentration plus the concentration of chains generated during the course of the polymerization. Thus, in the case of new chain generation by chain transfer, 퐷푃n is given as a function of [M] by:
훼[푀]0 + 퐷푃MI[MI]0 퐷푃n = (1) [MI]0 + [Chains]new
[M] ∑ 퐶 [X ] [Chains] = − ∫ ( 푖 tr,i i ) 푑[M] (2) new [M] [푀]0
where 훼 denotes monomer conversion, [Xi] is the concentration of a low molecular weight species to which chain transfer occurs (where i denotes monomer, solvent, chain transfer agent, etc.), and [MI] is the macroinitiator concentration. In the case of chain transfer to monomer only (where 퐶tr,M is the chain transfer to monomer constant), we obtain:
훼[푀]0 + 퐷푃MI[MI]0 퐷푃n = (3) [MI]0 + 푥퐶tr,M[M]0
In the case of chain transfer to solvent only, assuming also that [S] is constant, we can write:[40]
훼[푀]0 + 퐷푃MI[MI]0 퐷푃n = −1 (4) [MI]0 + ln(1 − 푥) 퐶tr,M[S]0
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14 Ctr,M = 0 Ctr,M = 0.00065
12 Ctr,M = 0.0065
10
8
(kg/mol) Ctr,M = 0.013
n 6
M 4 C = 0.065 2 tr,M
0 0 20 40 60 80 100 Conversion (%)
Figure 6. 5. Mn vs conversion plots for NMP of NIPAM in DMF (2 M) at ퟏퟐퟎ C with poly(t-BA)-SG1 as macroinitiator with ퟐퟓ mol% free SG1 relative to macroinitiator and
[M]0/[MI]0 = ퟐퟎퟎ. Solid lines are computed from Equation 3 with various Ctr,M.
Figure 6.5 shows 푀n vs conversion computed from Equation 3 for various values of
퐶tr,M for [M]0/[MI]0 = 200. It is immediately obvious that regardless of the value of
퐶tr,M, the agreement with the experimental data is not satisfactory. A maximum in 푀n vs conversion cannot be explained by chain transfer to monomer, regardless of the value of 퐶tr,M. Figure 6.4 shows experimental data of 푀n vs conversion for three different −4 [M]0/[MI]0 overlaid with predictions from Equation 4 using 퐶tr,S = 6.5 × 10 . The agreement between model and experiment is very good, and it is noted how Equation 4 correctly predicts the maximum observed experimentally. Moreover, the data sets corresponding to all three macroinitiator concentrations can be successfully fitted with the same value of 퐶tr,S. The maximum 푀n reached in the NMPs are all markedly lower than the maximum 푀n as dictated by the expression 푘p[M]0/푘tr,S[S]0 −1 −4 ( 27,000 g mol based on 퐶tr,S = 6.5 × 10 , not considering 푀n of the macroinitiator). The reason is that in NMP (and any CLRP), each chain transfer to solvent event generates a new living chain, thus reducing the number of chains over
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which the remaining monomer units are to be distributed (this is accounted for in Equation 4).
It has very recently been reported[12] that nitroxide-mediated (SG1) polymerization of NIPAM in supercritical carbon dioxide (as an inverse suspension polymerization, i.e. in the absence of solvent capable of undergoing chain transfer) proceeds to 푀n ≈ −1 12,100 g mol with 푀n vs conversion being a straight line close to 푀n,th , thus providing strong evidence that chain transfer to monomer is not occurring to any significant extent in the present study either.
Chain-stopping events specific to NMP in general include (polymeric) alkoxyamine decomposition[41] and hydrogen transfer from hydroxylamine (formed by hydrogen abstraction by nitroxide) to propagating radicals.[42] The rates of these reactions in the case of SG1 and NIPAM under the present conditions are not known. However, these reactions do not alter the number of chains, and therefore 푀n is not expected to be affected. The fact that very similar chain transfer to solvent constants were obtained for conventional radical polymerization and NMP in the present work suggest that any influence of such side reactions is very minor at most.
In the case of conventional radical polymerization, the instantaneous 퐷푃n is equal to
[M]/(∑퐶tr,i[Xi]) + 퐷푃n,0 (where 퐷푃n,0 is 퐷푃n in the absence of transfer) as given by the Mayo equation. The Mayo equation can be formulated as a function of [M], and the cumulative 퐷푃n at a given conversion (i.e. the overall 퐷푃n of the polymer formed between zero and a given conversion) is equal to:
[M] 1 푘p[M] 퐷푃 = ∫ ( ) 푑[M] (5) n [M] − [M] ∑ 푘 [X ] + (푘 + 푘 )[P] 0 [M]0 i tr,i i t td
where 푘t is the overall termination rate coefficient, 푘td is the rate coefficient for termination by disproportionation, and 푘tr,i is the rate coefficient for chain transfer to species Xi . Under the assumption that all end-forming events are chain transfer to monomer (Equation 6) or solvent (Equation 7; with the additional assumption of constant [S]), 퐷푃n can be expressed as functions of conversion as:
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1 퐷푃n = (6) 퐶tr,M
(2 − 푥)[M]0 퐷푃n = (7) 2[S]0퐶tr,S
Even if chain transfer to monomer or solvent is the predominant end-forming event, some fraction of propagating radicals will inevitably undergo bimolecular termination, which will cause deviation from 퐷푃n as given by Equations 6 and 7. Note that Equation
6 shows that the cumulative 퐷푃n is the same as the instantaneous 퐷푃n when chain transfer to monomer is the end-forming event. In the case of chain transfer to solvent being the end-forming event, however, Equation 7 reveals that the cumulative 퐷푃n decreases with conversion.
The conventional data at three different (low) initiator concentrations at 120 C in DMF
(from Figure 6.1) were plotted as 푀n vs conv. in Figure 6.4. Equation 7 was −4 subsequently fitted to these data, resulting in good agreement with 퐶tr,S = 8.0 × 10 .
This value of 퐶tr,S is very close to the value derived by fitting Equation 4 to the NMP −4 experiments (퐶tr,S = 6.5 × 10 ). The somewhat higher value of 퐶tr,S obtained in the case of conventional radical polymerization may be a result of bimolecular termination reactions between propagating radicals (which would cause propagating radicals to stop growing prior to reaching the chain transfer to solvent limit). Bimolecular termination would of course also occur to some minor extent in the NMP system, but the extent of such termination reactions in an NMP system without transfer reactions would obviously be much lower than in a conventional radical polymerization without chain transfer (in the latter case, all propagating radicals would undergo bimolecular termination). The data nicely illustrate how the same 퐶tr,S (within experimental error, and allowing for the “error” due to termination in the conventional radical polymerization) can be employed to (and also that 퐶tr,M cannot) rationalize the polymerization behaviour of NIPAM in DMF with regards to chain transfer to solvent both in controlled/living (NMP) and conventional radical polymerization.
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Yusuke Sugihara Chapter 6
6
5 (a) Conventional RP
4
.)
a.u 3 (
n
M 2
1
0 0 20 40 60 80 100 Conversion (%) 6
5 (b) CLRP
4 .)
a.u 3
(
n M 2
1
0 0 20 40 60 80 100 Conversion (%)
Figure 6. 6. Typical traces of Mn against conversion in conventional radical polymerization (a) and controlled/living radical polymerization (b) with chain transfer to monomer (broken line) and chain transfer to solvent (solid line).
The four different scenarios, i.e. chain transfer to monomer or solvent in conventional radical polymerization and NMP, are plotted in Figure 6.6. Conventional radical polymerization: In the case of chain transfer to monomer, theory predicts 퐷푃n to remain constant with conversion, whereas in the case of chain transfer to solvent, 퐷푃n decreases with conversion. CLRP: In the case of chain transfer to monomer, 퐷푃n gradually
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Yusuke Sugihara Chapter 6
deviates downward from the theoretical values (i.e. a straight line), whereas in the case of chain transfer to solvent, 퐷푃n goes through a maximum.
Estimation of 푪퐭퐫,퐒 via Mayo Plot
The value of Ctr,S in the conventional radical polymerization of NIPAM in DMF at 120 C was also estimated based on the classical Mayo treatment:
1 1 [S] = + 퐶tr,S (8) 퐷푃n 퐷푃n,0 [M]
where 퐷푃n,0 is 퐷푃n when [S] = 0. The slope of the straight line obtained by plotting −4 1/퐷푃n vs [S]/[M] (Figure 6.7) yields 퐶tr,S = 9.2 × 10 , which is in relatively good −4 −4 agreement with 퐶tr,S = 8.0 × 10 (Equation 7) and 6.5 × 10 (Equation 4) (Table 6.1).
0.020
0.016
0.012 n
DP 1/ 0.008
0.004
0 0 5 10 15 20 25
[S]0/[M]0
Figure 6. 7. Mayo plot of NIPAM (2 M) in DMF initiated by TBP (ퟎ. ퟒퟏ mM) at ퟏퟐퟎ C. The line is a best fit.
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Yusuke Sugihara Chapter 6
Number of New Chains Alternatively, the effects of chain transfer in NMP can be discussed based on the number of new chains generated as a function of conversion. Experimental values of the concentrations of new chains ([chains]new) can be readily calculated from Equation 9:
푀n,th [Chains]new = [MI]0 ( − 1) (9) 푀n
Figure 6.8 shows the experimental [chains]new (from Equation 9) as well as theoretical predictions based on chain transfer to monomer ( [chains]tr,M = 푥[M]0퐶tr,M ) for
[M]0/[MI]0 = 200, revealing that the theoretical prediction is in disagreement with the experimental data regardless of the value of 퐶tr,M. However, if we instead compute the concentration of new chains based on chain transfer to solvent ( [chains]tr,S = −1 ln(1 − 푥) 퐶tr,S[S]0; Figure 6.9), the data for all three macroinitiator concentrations are −4 in good agreement with theory for 퐶tr,S = 6.5 × 10 (the value obtained from fitting
푀n vs conversion). One single master curve is formed for all macroinitiator concentrations, because the number of new chains is a function of the total number of propagation steps and thus independent of [MI]0 (as apparent from the term −1 ln(1 − 푥) 퐶tr,S[S]0 in Equation 4).
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Yusuke Sugihara Chapter 6
0.025
Ctr,M = 0.065 Ctr,M = 0.013 0.020
(M) 0.015
new
0.010 C = 0.0065 chains] tr,M
[
0.005
Ctr,M = 0.00065 0.000 0 20 40 60 80 100
Conversion (%)
Figure 6. 8. [chains]new vs conversion plots for NMP of NIPAM in DMF (2 M) at ퟏퟐퟎ C using poly(t-BA)-SG1 as macroinitiator with 25 mol% free SG1 relative to macroinitiator and [M]0/[MI]0 = ퟐퟎퟎ. Solid lines correspond to [chains]tr,M = xCtr,M[M]0 (see Equation 3).
0.025
0.020
(M) 0.015
new
0.010
chains] [ 0.005
0.000 0 20 40 60 80 100 Conversion (%)
Figure 6. 9. [chains]new versus conversion plots for NMP of NIPAM in DMF (2 M) at ퟏퟐퟎ C using poly(t-BA)-SG1 as macroinitiator with ퟐퟓ mol% free SG1 relative to macroinitiator with [M]0/[MI]0 = ퟏퟎퟎ (○), ퟐퟎퟎ (□) and ퟑퟎퟎ (∆). The line represents
-1 -4 [chains]tr,S = ln(1-x) Ctr,S[S]0 (see Equation 4) for Ctr,S = 6.5 × 10 .
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Molecular Weight Distribution
A plot of 푀w/푀n vs [chains]tot/[MI]0 (i.e. the total number of chains relative to the initial number of chains as given by the initial macroinitiator concentration; Figure 6.10) for all three macroinitiator concentrations shows clearly how the generation of new chains is an important factor causing a gradual loss of control with increasing conversion. Interestingly, the data points for all three macroinitiator concentrations fall on the same master curve in this particular case. The effect of new chain generation on the MWD increases in magnitude with increasing 푀n,th (i.e. decreasing macroinitiator concentration, Figure 6.4). The NMP data in Figure 6.4 reveal that 푀n begins to deviate downwards from 푀n,th with increasing conversion, consistent with chain transfer to solvent becoming increasingly significant. The newly generated radicals would have the same probability of propagation as longer living radical chains, and the low MW tail observed in the MWDs would thus mainly consist of living chains from chain transfer to solvent shifting to higher MW with conversion (Figure 6.3). Consequently, even in the event of fairly significant chain transfer to solvent, the livingness may still be reasonable (especially considering the excess of free SG1 used in the present work).
Effect of Poly(acrylate) Macroinitiator Acrylate polymerization is complicated by the formation of mid-chain radicals (MCRs) that are formed by intra- (backbiting) or intermolecular chain transfer reactions.[43-48] It has been reported recently that the extent of branch formation via MCRs is significantly lower in CLRP than conventional radical polymerization.[49] However, it is conceivable that MCRs may form during NMP due to the presence of the poly(acrylate) macroinitiator, and subsequent fragmentation of MCR would result in an increase in the number of chains and thus a decrease in 푀n. However, if this occurred to any significant extent, the master curve of [chains]new vs conversion (Figure 6.9) would not be observed, and furthermore, it would not be possible to fit all data (NMP for three different macroinitiator concentrations and conventional radical polymerization) with one single value of 퐶tr,S (Figures 6.4 and 6.9). It can thus be concluded that MCR formation followed by fragmentation is not a significant mechanism with regards to generation of new chains.
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This also makes clear the point that chain transfer to solvent is radical reaction and in itself is not seriously impacted by the structure of initiator (macroinitiator). Thus this upward arcuate trend on 푀n due to chain transfer to solvent is general regardless of whether alkoxyamine or macroalkoxyamine (poly(t-BA)-SG1, in this study) is employed for NMP. Perhaps, the use of macroinitiator may not be desirable for the purpose of rigorous determination of kinetic parameters, as 푀n of macroinitiator is in itself of just mean value, and causes inevitable inaccuracy. However, on the other hand, macroinitiator is useful to visually grasp the role of chain transfer to solvent on 푀n, as it is technically possible to give 푀n lower than that of initial value in sufficiently high conversions.
2.0
1.8
n 1.6
M
/
w M 1.4
1.2
1.0 0 1.5 2.0 2.5 3.0 3.5 4.0
[chains]tot/[MI]0
Figure 6. 10. Mw/Mn vs [chains]tot/[MI]0 for NMP of NIPAM in DMF (2 M) at 120 C using poly(t-BA)-SG1 as macroinitiator with ퟐퟓ mol% free SG1 relative to macroinitiator with
[M]0/[MI]0 = ퟏퟎퟎ (○), ퟐퟎퟎ (□), and ퟑퟎퟎ (∆).
Spontaneous Initiation Spontaneous (thermal) initiation (i.e. no initiator present) has been previously reported in the polymerization of acrylates and acrylamides.[50-52] In order to estimate the extent of thermal generation of chains, polymerizations of NIPAM (2 M) in the absence of 165
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initiator and nitroxide were carried out in DMF at 120 C. The rate of thermal initiation
(푅i,th) can be estimated from the slope of a first-order plot (Figure 6.11) according to Equation 10:
0.5 푅i,th 푆푙표푝푒 = 푘p[P ] = 푘p ( ) (10) 2푘t
−6 0.5 0.5 0.5 Based on a slope of 3.19 × 10 and a literature value for 푘p/푘t = 0.24 M s in −10 −1 DMF at 65° C ,[53] one obtains 푅i,th = 1.8 × 10 s . Assuming that 푅i,th is independent of conversion, the concentration of chains generated by spontaneous initiation is equal to 푅i,th multiplied by the polymerization time (longest time = 36 h), which gives 2.3 × 10−5 M. Considering that the macroinitiator concentration is 6.7 × 10−3 M or higher, it can be safely concluded that the contribution of spontaneous initiation to the overall number of chains in the system is insignificant (especially 0.5 considering that the value of 푘p/푘t will be higher at 120 C than at 65 C).
0.30
0.25
0.20
/[M]) 0 0.15
ln([M] 0.10
0.05
0 0 5 10 15 20 25
Time (hour)
Figure 6. 11. First–order plot of spontaneous polymerization (in the absence of initiator or nitroxide) of NIPAM (2 M) in DMF at ퟏퟐퟎ °C (the solid line is a best fit).
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Comparison with Literature
Table 6.1 lists the values of 퐶tr,S estimated in this work based on Equation 4 (NMP) and
7 (conventional radical polymerization). To the best of our knowledge, 퐶tr,S for NIPAM have to date not been reported for any solvent. Min et al.[54] studied the conventional radical polymerization of NIPAM initiated by -radiation in a wide range of solvents, reporting strong solvent effects on the molecular weights obtained, and speculated that this may be caused by differences in the extents of chain transfer to solvent. Of the solvents investigated, the highest molecular weight was obtained for water and the lowest for THF (DMF not investigated). McCormick and co-workers[18] reported
RAFT of NIPAM in DMF at 25 C, obtaining close to linear 푀n vs conversion plots −1 with 푀n as high as 44,500 g mol with excellent control over the MWD. The absence of any apparent influence of chain transfer to DMF is most likely a result of the low polymerization temperature (퐶tr,S increases with temperature) as well as the higher ratio [monomer]/[solvent] in their work. Nitroxide-mediated stabilizer-free inverse suspension polymerization of NIPAM in supercritical CO2 has recently been reported.[12] In this system, 푀n did not deviate significantly from 푀n,th with increasing conversion, consistent with chain transfer to CO2 being negligible (as well as chain transfer to monomer), and thus supporting the present results.
Conventional radical polymerization of St (60 °C) and t-BA (115 °C) in DMF have been −4 −4 reported to proceed with 퐶tr,S = 4 × 10 [55] and 8.6 × 10 , respectively.[31] The
퐶tr,S values for these monomers are similar to 퐶tr,S for NIPAM/DMF observed in the present work. Downward deviations of 푀n from 푀n,th with increasing conversion
(푀n < 푀n,th) have also been reported for other CLRPs in DMF, including the SG1- mediated polymerizations of t-BA[31] and RAFT-mediated polymerizations of hydrophobic acrylamides,[56] consistent with chain transfer to solvent. Thus, chain transfer seems to be more significant to DMF than to other common polymerization solvents, possibly due to the greater stability of the DMF adduct radical. There are nevertheless scant reports of DMF generating radicals in small molecule reactions,[57] and DMF has continued to be widely used in the conventional radical polymerization and CLRP of NIPAM[6, 8, 10, 11, 18, 22, 26] and other monomers.[11, 52, 56, 58-61] The wide literature use of NIPAM/DMF probably stems from its good solvent properties; our preliminary studies showed poly(NIPAM) to be poorly soluble
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(precipitates to form heterogeneous mixtures in NMPs) in benzene, anisole and m-xylene under the polymerization conditions of the present paper.
Table 6. 5. Values of Ctr,S for radical polymerization of NIPAM in DMF at 120 C estimated in the present work. Ctr,S from Mayo plot Ctr,S from Equation (7) Ctr,S from Equation (4) (conv. rad. pol.) (conv. rad. pol.) (NMP)
9.2 × 10−4 8.0 × 10−4 6.5 × 10−4
6.5 Conclusion
Chain transfer to solvent can be a significant factor in limiting the maximum attainable molecular weight in both conventional radical polymerization and NMP of NIPAM.
Based on a theoretical treatment, it has been demonstrated that the same value of 퐶tr,S (within experimental error) can be invoked to quantitatively rationalize experimental molecular weight data both in conventional radical polymerization and NMP in DMF at 120 C under conditions where chain transfer to solvent is a significant end-forming event. The extent of chain transfer to solvent can have deleterious effects on both the control over the MWD (higher 푀w/푀n) and the maximum attainable molecular weight in NMP (which is normally carried out at elevated temperatures). Chain transfer to solvent in NMP (or any controlled/living radical polymerization technique) may lead to
푀n going through a maximum with increasing conversion. This is distinctly different from the case of chain transfer to monomer, in which case 푀n also deviates downward from 푀n,th, but never goes through a maximum regardless of the extent of chain transfer to monomer.
This work, the interpretation of a kinetic data of 푀n, provided the secure conviction for the occurrence of chain transfer to solvent in this particular combination of NIPAM and DMF, and as such it limits the discussion only to evidence the occurrence and not to identify the exact chemical reaction mechanism. It would be future perspective and the mechanism identification must be completed with further investigations such as rigorous end-group quantification with IR analysis.
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6.6 References
1. Heskins, M. and J.E. Guillet, Solution Properties of Poly(N- Isopropylacrylamide). 1968. 2: p. 1441-1455. 2. Schild, H.G., Poly(N-Isopropylacrylamide): Experiment, Theory and Application Prog. Polym. Sci., 1992. 17: p. 163-249. 3. Cho, E.C., J. Lee, and K. Cho, Role of Bound Water and Hydrophobic Interaction in Phase Transition of Poly(N-Isopropylacrylamide) Aqueous Solution. Macromolecules, 2003. 36: p. 9929-9934. 4. Kikuchi, A. and T. Okano, Nanostructured Designs of Biomedical Materials: Applications of Cell Sheet Engineering to Functional Regenerative Tissues and Organs. J. Control. Release, 2005. 101: p. 69-84. 5. Wei, H., et al., Thermo-Sensitive Polymeric Micelles Based on Poly(N- Isopropylacrylamide) as Drug Carriers. Prog. Polym. Sci., 2009. 34: p. 893-910. 6. Bosman, A.W., et al., A Modular Approach toward Functionalized Three- Dimensional Macromolecules: From Synthetic Concepts to Practical Applications. J. Am. Chem. Soc., 2003. 125: p. 715-728. 7. Savariar, E.N. and S. Thayumanavan, Controlled Polymerization of N- Isopropylacrylamide with an Activated Methacrylic Ester. J. Polym. Sci. Pol. Chem., 2004. 42: p. 6340-6345. 8. Kuroda, K. and T.M. Swager, Fluorescent Semiconducting Polymer Conjugates of Poly(N-Isopropylacrylamide) for Thermal Precipitation Assays. Macromolecules, 2004. 37: p. 716-724. 9. Schulte, T., et al., Nitroxide-Mediated Polymerization of N- Isopropylacrylamide: Electrospray Ionization Mass Spectrometry, Matrix- Assisted Laser Desorption Ionization Mass Spectrometry, and Multiple-Angle Laser Light Scattering Studies on Nitroxide-Terminated Poly-N- Isopropylacrylamides. Macromolecules, 2005. 38: p. 6833-6840. 10. Gibbons, O., et al., Nitroxide-Mediated Controlled Statistical Copolymerizations of N-Isopropylacrylamide with N-Tert-Butylacrylamide. J. Polym. Sci. Pol. Chem., 2006. 44: p. 6410-6418.
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11. Binder, W.H., et al., Telechelic Poly(N-Isopropylacrylamides) Via Nitroxide- Mediated Controlled Polymerization and "Click" Chemistry: Livingness and "Grafting-from" Methodology. Macromolecules, 2007. 40: p. 3097-3107. 12. O'Connor, P., P.B. Zetterlund, and F. Aldabbagh, Nitroxide-Mediated Stabilizer- Free Inverse Suspension Polymerization of N-Isopropylacrylamide in Supercritical Carbon Dioxide. Journal of Polymer Science Part A: Polymer Chemistry, 2011. 49: p. 1719-1723. 13. Xia, Y., et al., Thermal Response of Narrow-Disperse Poly(N- Isopropylacrylamide) Prepared by Atom Transfer Radical Polymerization. Macromolecules, 2005. 38: p. 5937-5943. 14. Rathfon, J.M. and G.N. Tew, Synthesis of Thermoresponsive Poly (N- Isopropylmethacrylamide) and Poly(Acrylic Acid) Block Copolymers Via Post- Functionalization of Poly(N-Methacryloxysuccinimide). Polymer, 2008. 49: p. 1761-1769. 15. Ganachaud, F., et al., Molecular Weight Characterization of Poly(N- Isopropylacrylamide) Prepared by Living Free-Radical Polymerization. Macromolecules, 2000. 33: p. 6738-6745. 16. Schilli, C., M.G. Lanzendorfer, and A.H.E. Muller, Benzyl and Cumyl Dithiocarbamates as Chain Transfer Agent in the Raft Polymerization of N- Isopropylacrylamide. In Situ Ft-Nir and Maldi-Tof Ms Investigation. Macromolecules, 2002. 35: p. 6819-6827. 17. Ray, B., et al., Synthesis of Isotactic Poly(N-Isopropylacrylamide) by Raft Polymerization in the Presence of Lewis Acid. Macromolecules, 2003. 36: p. 543-545. 18. Convertine, A.J., et al., Facile, Controlled, Room-Temperature Raft Polymerization of N-Isopropylacrylamide. Biomacromolecules, 2004. 5: p. 1177-1180. 19. Ray, B., et al., Raft Polymerization of N-Isopropylacrylamide in the Absence and Presence of Y(Otf)(3): Simultaneous Control of Molecular Weight and Tacticity. Macromolecules, 2004. 37: p. 1702-1710. 20. Schilli, C.M., et al., A New Double-Responsive Block Copolymer Synthesized Via Raft Polymerization: Poly(N-Isopropylacrylamide)-Block-Poly(Acrylic Acid). Macromolecules, 2004. 37: p. 7861-7866.
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21. Liu, B. and S. Perrier, Thermoresponsive Micelles from Well-Defined Block Copolymers Synthesized Via Reversible Addition-Fragmentation Chain Transfer Polymerization. J. Polym. Sci. Pol. Chem., 2005. 43: p. 3643-3654. 22. Carter, S., B. Hunt, and S. Rimmer, Highly Branched Poly(N- Isopropylacrylamide)S with Imidazole End Groups Prepared by Radical Polymerization in the Presence of a Styryl Monomer Containing a Dithioester Group. Macromolecules, 2005. 38: p. 4595-4603. 23. Smith, A.E., et al., "Schizophrenic" Self-Assembly of Block Copolymers Synthesized Via Aqueous Raft Polymerization: From Micelles to Vesicles. Macromolecules, 2010. 43: p. 1210-1217. 24. Millard, P.E., et al., Synthesis of Water-Soluble Homo- and Block-Copolymers by Raft Polymerization under Gamma-Irradiation in Aqueous Media. Polymer, 2010. 51: p. 4319-4328. 25. Nuopponen, M., J. Ojala, and H. Tenhu, Aggregation Behaviour of Well Defined Amphiphilic Diblock Copolymers with Poly (N-Isopropylacrylamide) and Hydrophobic Blocks. Polymer, 2004. 45: p. 3643-3650. 26. Yusa, S., et al., Thermo-Responsive Diblock Copolymers of Poly(N- Isopropylacrylamide) and Poly(N-Vinyl-2-Pyrroridone) Synthesized Via Organotellurium-Mediated Controlled Radical Polymerization (Terp). Macromolecules, 2007. 40: p. 5907-5915. 27. Nguyen, N.H., B.M. Rosen, and V. Percec, Set-Lrp of N,N-Dimethylacrylamide and of N-Isopropylacrylamide at 25 Degrees C in Protic and in Dipolar Aprotic Solvents. J. Polym. Sci. Pol. Chem., 2010. 48: p. 1752-1763. 28. Zhou, S.Q., et al., Light-Scattering Studies of Poly(N-Isopropylacrylamide) in Tetrahydrofuran and Aqueous Solution. Polymer, 1995. 36: p. 1341-1346. 29. Yu, T.L., et al., Solvents Effect on the Physical Properties of Semi-Dilute Poly(N- Isopropyl Acryl Amide) Solutions. Polymer, 2004. 45: p. 5579-5589. 30. Kukulj, D., T.P. Davis, and R.G. Gilbert, Chain Transfer to Monomer in the Free-Radical Polymerizations of Methyl Methacrylate, Styrene, and Alpha- Methylstyrene. Macromolecules, 1998. 31: p. 994-999. 31. Lessard, B., C. Tervo, and M. Maric, High-Molecular-Weight Poly(Tert-Butyl Acrylate) by Nitroxide-Mediated Polymerization: Effect of Chain Transfer to Solvent. Macromol. React. Eng., 2009. 3: p. 245-256.
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32. Zetterlund, P.B., et al., Nitroxide-Mediated Radical Polymerization of Styrene: Experimental Evidence of Chain Transfer to Monomer. Polymer, 2006. 47: p. 7900-7908. 33. Cuervo-Rodriguez, R., et al., Nitroxide-Mediated Free-Radical Copolymerization of Styrene with Butyl Acrylate. J. Polym. Sci. Pol. Chem., 2004. 42: p. 4168-4176. 34. O'Connor, P., P.B. Zetterlund, and F. Aldabbagh, Effect of Monomer Loading and Pressure on Particle Formation in Nitroxide-Mediated Precipitation Polymerization in Supercritical Carbon Dioxide. Macromolecules, 2010. 43: p. 914-919. 35. Ishizone, T. and M. Ito, Synthesis of Well-Defined Poly(N-Isopropylacrylamide) by the Anionic Polymerization of N-Methoxymethyl-N-Isopropylacrylamide. J. Polym. Sci. Pol. Chem., 2002. 40: p. 4328-4332. 36. Ito, M. and T. Ishizone, Living Anionic Polymerization of N-Methoxymethyl-N- Isopropylacrylamide: Synthesis of Well-Defined Poly (N-Isopropylacrylamide) Having Various Stereoregularity. J. Polym. Sci. Pol. Chem., 2006. 44: p. 4832- 4845. 37. Fukuda, T., et al., Mechanisms and Kinetics of Nitroxide-Controlled Free Radical Polymerization. Macromolecules, 1996. 29: p. 6393-6398. 38. Goto, A. and T. Fukuda, Kinetics of Living Radical Polymerization. Prog. Polym. Sci., 2004. 29: p. 329-385. 39. McHale, R., et al., Nitroxide-Mediated Radical Dispersion Polymerization of Styrene in Supercritical Carbon Dioxide Using a Poly(Dimethylsiloxane-B- Methyl Methacrylate) Stabilizer. Macromolecules, 2006. 39: p. 6853-6860. 40. Loiseau, J., et al., Synthesis and Characterization of Poly(Acrylic Acid) Produced by Raft Polymerization. Application as a Very Efficient Dispersant of Caco3, Kaolin, and Tio2. Macromolecules, 2003. 36: p. 3066-3077. 41. Goto, A., et al., Macromolecules, 2002. 35: p. 3520. 42. Gridnev, A.A., Macromolecules, 1997. 30: p. 7651. 43. Plessis, C., et al., A Decrease in Effective Acrylate Propagation Rate Constants Caused by Intramolecular Chain Transfer. Macromolecules, 1999. 33: p. 4-7.
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44. Plessis, C., et al., Seeded Semibatch Emulsion Polymerization of N-Butyl Acrylate. Kinetics and Structural Properties. Macromolecules, 2000. 33: p. 5041-5047. 45. Yamada, B., et al., Free Radical Polymerization of Cyclohexyl Acrylate Involving Interconversion between Propagating and Mid-Chain Radicals. Polymer, 2000. 41: p. 5611-5618. 46. Sato, E., et al., Influence of Mid-Chain Radicals on Acrylate Free Radical Polymerization: Effect of Ester Alkyl Group. Macromol. Chem. Phys., 2004. 205: p. 1829-1839. 47. Yamada, B., P.B. Zetterlund, and E. Sato, Utility of Propenyl Groups in Free Radical Polymerization: Effects of Steric Hindrance on Formation and Reaction Behavior as Versatile Intermediates. Prog. Polym. Sci., 2006. 31: p. 835-877. 48. Junkers, T. and C. Barner-Kowollik, The Role of Mid-Chain Radicals in Acrylate Free Radical Polymerization: Branching and Scission. J. Polym. Sci. Pol. Chem., 2008. 46: p. 7585-7605. 49. Ahmad, N.M., et al., Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of N-Butyl Acrylate. Macromol. Rapid Commun., 2009. 30: p. 2002-2021. 50. Ryan, J., et al., First Nitroxide-Mediated Controlled/Living Free Radical Polymerization in an Ionic Liquid. Macromol. Rapid Commun., 2004. 25: p. 930-934. 51. Moad, G. and D.H. Solomon, The Chemistry of Radical Polymerization. 2006, Oxford: Elsevier. p. 107. 52. Gibbons, O., et al., Nitroxide-Mediated Radical Polymerization of N-Tert- Butylacrylamide. Macromol. Chem. Phys., 2008. 209: p. 2434-2444. 53. Costioli, M.D., et al., Investigation of the Telomerization Kinetics of N- Isopropylacrylamide Using 3-Mercaptopropionic Hydrazide as Chain Transfer Agent. Macromolecules, 2005. 38: p. 3630-3637. 54. Min, Y., L. Jun, and H. Hongfei, Radiation Preparation of the Water-Soluble Temperature Sensitive Polymers in Organic Solvents. Radiat. Phys. Chem., 1995. 46: p. 855-858. 55. Ueda, A. and S. Nagai, Transfer Constants to Monomers, Polymers, Catalysts and Initiators, Solvents and Additives, and Sulfur Compounds in Free Radical
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Polymerization, in Polymer Handbook, J. Brandrup, E.H. Immergut, and E.A. Grulke, Editors. 1999, Wiley: New York. p. II/97. 56. de Lambert, B., et al., Raft Polymerization of Hydrophobic Acrylamide Derivatives. Polymer, 2005. 46: p. 623-637. 57. Opstad, C.L., et al., Formation of Dmso and Dmf Radicals with Minute Amounts of Base. Tetrahedron, 2009. 65: p. 7616-7619. 58. Gotz, H., et al., Synthesis of Lipo-Glycopolymer Amphiphiles by Nitroxide- Mediated Living Free-Radical Polymerization. J. Polym. Sci. Pol. Chem., 2002. 40: p. 3379-3391. 59. Xu, W.J., et al., Atom Transfer Radical Polymerization of Hexadecyl Acrylate Using Cuscn as the Catalyst. Macromol. Res., 2004. 12: p. 32-37. 60. Gonzalez, N., C. Elvira, and J.S. Roman, Novel Dual-Stimuli-Responsive Polymers Derived from Ethylpyrrolidine. Macromolecules, 2005. 38: p. 9298- 9303. 61. Eggenhuisen, T.M., et al., Libraries of Statistical Hydroxypropyl Acrylate Containing Copolymers with Lcst Properties Prepared by Nmp. Macromolecules, 2008. 41: p. 5132-5140.
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Chapter 7
Conclusions and Future Perspectives
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The main aim of this Thesis is to develop new fundamental knowledge in the area of kinetics and mechanism of radical polymerization. In the Chapters 3 to 6, various new contributions to this subject have been achieved not only pertaining to simple bulk or solution homogeneous conditions of conventional radical polymerizations, but also various specific conditions such as heterogeneous systems, polymerization under microwave (MW) irradiation, as well as application of CLRP.
Chapter 3 verified the practical influence of the microwave irradiation on the kinetics of conventional radical polymerization and RAFT polymerization of styrene. MW irradiation is a relatively new and hot field for the chemistry of radical polymerization, and currently there have been numerous contradicting experimental and theoretical works in part to claim that there must be a specific MW influence on the kinetics of the radical polymerization (to accelerate it compared to the corresponding traditional heating system), or in part to claim that there must not. The work in this Thesis is a contribution from the practical implementation of the experiments for the conventional radical polymerization and RAFT polymerization of styrene at various temperatures and MW irradiation powers. The gist of the work is that all experiments were carried out under the conditions in which the temperatures and MW powers were rigorously controlled and determined based on the consistent and careful calibration of the reaction temperatures for each implementation, which may seem to be a minimum requirement for any chemical experiments, but practically it was very difficult with the pioneering MW instrument. The experimental results clearly demonstrated that no matter what techniques of conventional and RAFT polymerizations, MW irradiation exerted no influence on the polymerization rate, and no influence on the kinetics of the radical polymerization of styrene. The present work has convincingly demonstrated that a number of earlier proposed theories are erroneous, such as that MW irradiation may have an influence on the rate coefficient of the radical polymerization of styrene, or MW irradiation may undergo a specific initiation reaction other than the normal thermal initiators. This achievement was not only to reveal the particular kinetics of the specific monomer styrene, but is also applicable to various other monomers. MW influence must be due to the specific chemical structure of each molecule, and as such any significant kinetic influence must depend on each monomer. There is an ongoing debate as to whether there ispractical influence on various monomers, and most likely some are
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affected by MW whereas others are not. Proper clarification can only be achieved by the step by step experiments with carefully determined and designed experimental conditions. It remains very important to investigate the practical MW influence for various common monomers for the further development of this field.
Chapter 4 is concerned with the kinetics of emulsion polymerization of styrene, which is one of the most established and rigorously studied heterogeneous systems, and investigated the practical limit of the particle size in which the specific kinetics under zero-one approximation was valid. Previous theoretical works predicted the threshold particle size of the specific kinetics to be less than 100 nm in radius, and although practical investigations have been conducted in such small particle ranges under 100 nm in radius, the experimental results in this Thesis clearly demonstrated that the particular kinetics of zero-one approximation of emulsion polymerization of styrene can be observed for particles as large as 274 nm in radius, much larger than previous predictions. Based on the underlying assumption that enabled this practical demonstration, it is also possible to conceptually predict that the practical limit can be extended to particles as large as 850 nm in radius, as long as the experimental conditions are properly and accurately prepared. It may be difficult to further develop and explain the strict chemistry, but it would be worthwhile to apply this methodology to monomers other than styrene. More importantly, one should apply this methodology to the investigation of the uncultivated field of the kinetics of CLRP in the heterogeneous systems. As one of the most mature and studied field among all heterogeneous polymerizations from both experimental and theoretical sides, it was my luck to be able to be involved with this project and become familiar with the experimental and theoretical philosophy as a person who is a “wannabe” scientist of radical polymerization.
In Chapter 5, a theoretical study was described for the kinetics of NMP of styrene under heterogeneous conditions of miniemulsion polymerization, where the influence of the particle size termed ’compartmentalization’ and reactant partitioning were successfully combined for the first time. This work is linked with Chapter 4, which deals with the experimental investigation of the kinetics of emulsion polymerization of conventional polymerization of styrene. Actually, both Chapters have their roots in the Smith-Ewart
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theory, but the standpoint as a subject is significantly different. As a discipline, the study of emulsion polymerization of styrene (Chapter 4) has a matured history and development over 60 years since before WW II, whereas the application into CLRP is at most around 20 years development and now under developing. The work described in this Thesis was a theoretical contribution to this field, and the contribution is very important for the modelling and algorithm, which had so far only been successfully performed in the closed condition to eliminate the consideration of practically surely elemental events of reactant exit and entry over 15 years, and the influence of consideration was really significant to completely change the direction to which the compartmentalization worked in the polymerization rate and the controllability and livingness of CLRP. Considering the general sense of chemistry and physics of reactants which must have a certain solubility in the dispersion medium of water, even sparingly, the achievement described in Chapter 5 is eligible as the standard and general answer to predict the kinetics of CLRP in any heterogeneous system. Also, as a contribution from the theoretical side, the goal is distinguished into the achievement of (1) the model and algorithm development and (2) its proper interpretation. (1) is the starting point of new development, but the importance is rather on (2). In the event of the achievement of further modelling and algorithm, the numerical processing thanks to our friend of computer would provide more profound and widespread information. Our task is to sincerely face up to the new descriptions, then at the stage not only new discovery on the new world but also more accomplished insight on the previous understanding would be able to be found. Pursuing the better and more accurate interpretation is very the development of the theoretical side. And it is also very important to invest our wisdom to develop the experimental methodology to investigate the nature of the heterogeneous polymerizations of CLRP eliminating or otherwise properly considering any practical cumbersome events such as droplets collapse, superswelling and Ostwald ripening, wide distribution of particle size, and so on, as high quality as the seeded emulsion polymerization of conventional radical polymerization that in Chapter 4 I took over and utilized as one of the most optimized methodologies established by the pioneering diligent scientists over 60 years.
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In Chapter 6, making use of NMP, the practical influence of the elemental reaction of chain transfer to solvent is evidenced in the case of radical polymerization and CLRP of NIPAM in DMF, by the excellent agreement between experimental results and theoretical predictions. Not only to describe the practical harmony between the theory and experiment, this work also developed and provided a new way to investigate and distinguish whether chain transfer to solvent occurs to a significant extent. Of course, the Mayo equation is one of the classics of polymer chemistry. However, the formulation of the degree of polymerization (leading to the numberaverage molecular weight) as function of the fractional conversion for conventional radical polymerization and CLRP are quite new, pioneered for RAFT and NMP as developed by others. CLRP is now one of the most common and important techniques to synthesize various stimuli responsible or amphiphilic block copolymers. As such, the promise if there is no chain starting events such as chain transfer to solvent or monomer must be the prime concern to be properly eliminated, otherwise any of the subsequent analysis of the physical characteristics may be erroneous. The work in this Thesis suggests that it is not suitable to conduct NIPAM polymerization in DMF at the high temperature of 120 C. Of course, this methodology can be applied to investigate other combinations of monomers and solvents, and temperatures, to investigate whether there is significant chain transfer to solvent, at least qualitatively. However, the present work has a certain limitation in accuracy because of the limitation of the analytical techniques. All experiments of conventional radical polymerization and NMP of NIPAM in DMF solvent at 120 C were carried out in the same manner, same methodology of conversion reading using NMR and molecular weight reading using GPC. As such, the relative comparison between those detected values could read the accurate and correct characteristics to indicate one direction. However, the GPC calibration was by polystyrene, and as such the discussion is sure to include certain inaccuracy if it really pursues to obtain perfectly correct absolute quantitative parameters of 퐶tr,S . Considering this fact, the difference between the detected 퐶tr,S values using the Mayo equation, the 푀n vs conversion equations of conventional radical polymerization and NMP cannot be further discussed, unless more accurate instrumental techniques are available such as MALDI-TOF to detect the absolute 푀n. The importance is to see the reason of such inaccuracy, and not to hold wrong discussion, while it is not due to misnomer of the theory and equations. As such, this study of chain transfer to solvent is expected to lead to interesting future works. First,
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one must focus on the chemistry of the reason of chain transfer in order to control and optimize the experimental conditions and to select the proper solvent. Given the excellent instrumental tools mentioned above, further investigations of the value of 퐶tr,S would also be worthwhile. Of course, investigations of the existence of chain transfer to solvent for various combinations of monomers and solvents is of great practical importance.
The thesis truly has tackled a wide variety of the steel-level topics in each chapter, interest on the practical influence of microwave irradiation on the kinetics of radical polymerization of styrene, the maximum particle size of the emulsion polymerization of styrene where the zero-one approximation kinetics is valid, the theoretical development to the understanding of the kinetics of NMP (and PRE type CLRP) of styrene under ideal miniemulsion polymerization in successfully reckoning compartmentalization and ingredients partitioning, and the evidence of an elemental kinetic event of chain transfer to solvent in the radical polymerization of NIPAM in DMF at that high temperature of 120 ∘C. However, all themes are wired in and fall under one rational set, the new contribution to the kinetics and mechanism understanding of radical polymerization, and as such have a striking relevancy to the review part of Chapter 2, the legend of radical polymerization since after it has appeared.
Throughout this Thesis, the most difficulty has been always in finding the validity, in that matter both practice and theory have the same characteristic that what they are now have been cultivated and developed with mighty long time efforts by enormous numbers of intelligent and sincere scientists with uncountable amount of their tries and errors. The challenge of my works has been to establish new development, but it truly equals to the challenge to understand the true estate of those pioneering giants. Knowing and following them is the only one way to properly pinpoint myself in the world of science. Polymer chemistry is complicated compound field of various core subjects, and much more is its application to heterogeneous systems. It technically means that all scientists of this field must be real expert of everything background, otherwise it is very easily to get lost in the flood of required considerations. As a result of this Ph.D. project, it only turned out that I am still clearly of ignorance, but it has given me lots of opportunities to be enlightened in the right direction, the loyalty to the science, not one own small benefits.
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Appendix Publications
Journal Papers:
1. Yusuke Sugihara, Padraig O’Connor, Per B. Zetterlund and Fawaz Aldabbagh, Chain Transfer to Solvent in the Radical Polymerization of N- Isopropylacrylamide, J. Polym. Sci. Part A: Polym. Chem., 2011, 49: p. 1719– 1723. 2. Yusuke Sugihara and Per B. Zetterlund, Synergistic Effects of Compartmentalization and Nitroxide Exit/Entry in Nitroxide-Mediated Radical Polymerization in Dispersed Systems, ACS Macro Lett, 2012. 1: p. 692−696. 3. Yusuke Sugihara, Mona Semsarilar, Sebastien Perrier and Per B. Zetterlund, Assessment of the influence of microwave irradiation on conventional and RAFT radical polymerization of styrene, Polym. Chem., 2012, 3: p. 2801–2806.
Conferences:
1. Poster Presentation: Assessment of the influence of microwave irradiation on conventional and RAFT radical polymerization of styrene, 2010 – 32nd Australia Polymer Symposium (APS) – Coffs Harbour. 2. Poster Presentation: RAFT Polymerization under Microwave Irradiation, 2011 – Chemeca 2011 – Sydney. 3. Poster Presentation: Synergistic Effects of Compartmentalization and Nitroxide Exit/Entry in Nitroxide-Mediated Radical Polymerization in Dispersed Systems, 2012 – Polymers in Dispersed Media (PDM) 2012 – Lyon, France.
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