ASPECTS OF CONTROLLED RADICAL POLYMERIZATION IN DISPERSED MEDIA
J.D. BIASUm THE UNIVERSITY OF NEW SOUTH WALES Thesis/Project Report Sheet Surname or Family Name : Biasutti First Name John Other Name/s: Daniel Abbreviation for degree as given by the university calendar: ME School : Chemical Engineering and Industrial Chemistry Faculty : Engineering Title Aspects of controlled radical polymerization in dispersed media
Several aspects of two different controlled radical polymerization techniques were examined in both single phase polymerization and in two phase polymerization methods.
Chain Transfer Constants (Cs) were measured for the Catalytic Chain Transfer (CCT) polymerization of 2- hydroxyethyl methacrylate (HEMA) at 40°C and 60°C. These were compared with the values for methyl methacrylate (MMA) and mixtures of MMA and ethanol. Chain transfer constants for HEMA at these temperatures were found to be 6.2x 10^ and 6. Ix 10^, only half that expected by theoretical comparison with MMA. The Cs values measured for mixtures of MMA and ethanol also showed deviations of about the same factor of two from those measured for MMA in bulk. Examination of the UV/Vis absorbance spectrum showed a shift in the peak absorbance from 444.5nm to 457.5nm. This difference in peak absorbance frequency was also observed between MMA and HEMA, suggesting that the electronic environment around the cobalt centre had changed, probably by exchange of the ligands associated with the cobalt complex.
Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization of MMA using 2-cyanoprop-2-yl dithiobenzoate (CPDB) was achieved at 60°C with AIBN initiator. Reaction rates were similar to those seen in the absence of the catalyst with very little retardation. Molecular weights were close to the theoretical values and polydispersities as low as 1.02 were observed. RAFT polymerization in suspension polymerization of MMA at 70°C was accomplished using CPDB RAFT agent and sodium poly methacrylate surfactant. Reaction rates were similar to those seen in bulk at the same concentrations of initiator and very little retardation was observed. Good control over molecular weight, high conversion and low polydispersity were observed. Average particle sizes ranged from 160|Lim to 550|j,m with particle size dependent on RAFT agent concentration. Chain extension of the polymer produced showed a high degree of viable end functionality.
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C/VIVID Centre for Advanced M a c ro m o 1 e c u I a r Design
Aspects of Controlled Radical Polymerization in Dispersed Media
John Daniel Biasutti
A dissertation submitted in fulfillment of the requirements for the award of
Master of Engineering Research
2006 U NS W 2 9 MAR 2007 LIBRARY Abstract Several aspects of two different controlled radical polymerization techniques were examined in both single phase polymerization and in two phase polymerization methods.
Chain Transfer Constants (Cs) were measured for the Catalytic Chain Transfer
(CCT) polymerization of 2-hydroxyethyl methacrylate (HEMA) at 40°C and 60°C.
These were compared with the values for methyl methacrylate (MMA) and mixtures of
MM A and ethanol. Chain transfer constants for HEMA at these temperatures were found to be 6.2x10^ and 6.1x10^, only half that expected by theoretical comparison with
MMA. The Cs values measured for mixtures of MMA and ethanol also showed deviations of about the same factor of two from those measured for MMA in bulk.
Examination of the UV/Vis absorbance spectrum showed a shift in the peak absorbance from 444.5nm to 457.5nm. This difference in peak absorbance frequency was also observed between MMA and HEMA, suggesting that the electronic environment around the cobalt centre had changed, probably by exchange of the ligands associated with the cobalt complex.
Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization of
MMA using 2-cyanoprop-2-yl dithiobenzoate (CPDB) was achieved at 60°C with
AIBN initiator. Reaction rates were similar to those seen in the absence of the RAFT
Agent with very little retardation. Molecular weights were close to the theoretical values and polydispersities as low as 1.02 were observed. RAFT polymerization in suspension polymerization of MMA at 70°C was accomplished using CPDB RAFT agent and sodium poly methacrylate surfactant. Reaction rates were similar to those seen in bulk at the same concentrations of initiator and very little retardation was observed. Good control over molecular weight, high conversion and low polydispersity were observed.
Average particle sizes ranged from 160)im to 550}im with particle size dependent on
RAFT agent concentration. Chain extension of the polymer produced showed a high degree of viable end functionality. Acknowledgements
There are many people whose presence has made the last few years a very rewarding time for me. It has been a privilege to work in an area as new and exciting as controlled radical polymerization, and to be one of its very early investigators. It will be interesting to see what revelations the future holds.
I would like to thank my supervisors: Dr. Frank Lucien, Dr. Johan P. A. Heuts and Prof Tom P. Davis. Without the ongoing faith and support of these marvelous people I would not have been able to accomplish what I have to date.
Thanks also to all those who helped with instrumental analysis techniques: Lachlan Yee, Philipp Vana and Evan Roberts for enduring all my questions about size exclusion chromatography; Luca Albertin for his help with flash chromatography; Martin Jesberger for his great knowledge of organic chemistry. Muriel Lansalot for her insights into dispersed phase polymerization and Henry Yin for his help with the pulsed laser polymerization apparatus.
Thanks to Nik Zwaneveld and Romily Webster for their unending labours in setting up the data storage and backup systems for the group. Antonio Pantalone for his sense of humor and Tony Granville for stepping into the breach after Antonio had departed.
I would also like to thank my family and friends, who supported me throughout the period of my candidature. Without their ongoing encouragement and trust I would not have been able to accomplish a tenth of what I have.
Finally I would like to thank my girlfriend Sarah for her help and support, especially during the writing up stage.
ni List of Publications
Heuts, Johan P. A.; Roberts, G. Evan; Biasutti, John D.. Catalytic chain transfer polymerization: an overview. Australian Journal of Chemistry (2002), 55(6 & 7), 381-398.
John D. Biasutti, G. Evan Roberts, Frank P. Lucien, Johan P. A Heuts. Monomer and Solvent Effects in the Catalytic Chain Transfer Polymerization of 2-hydroxyethyl methacrylate.26^^ Australian Polymer Symposium (2002) PA6.
Biasutti, John D.;Evan Roberts, G.; Lucien, Frank P.;Heuts, Johan P. A. Substituent effects in the catalytic chain transfer polymerization of 2-hydroxyethyl methacrylate. European Polymer Journal (2003), 39(3), 429-435
Biasutti, John D; Frank P. Lucien, Johan P.A. Heuts Suspension Polymerization of methyl methacrylate mediated by 2-cyanoprop-2-yl dithiobenzoate. Australian Colloid and Interface Science Symposium (2003), pi04.
Biasutti, John D.; Davis, Thomas P.; Lucien, Frank P.; Heuts, Johan P. A. Reversible addition-fragmentation chain transfer polymerization of methyl methacrylate in suspension. Journal of Polymer Science, Part A: Polymer Chemistry (2005), 43(10), 2001-2012. Table of Contents
1 Introduction 2 Free Radical Polymerization 2.1 Introduction 2-1 2.1.1 Initiation 2-2 2.1.2 Propagation 2-2 2.1.3 Termination 2-3 2.1.4 Chain Transfer 2-4 2.2 Kinetics of Free Radical Polymerization Reactions 2- 7 2.2.1 The long chain assumption 2-7 2.2.2 Steady state approximation 2-9 2.2.3 Solutions to the rate equations 2-11 2.3 Measurement of Rate Coefficients 2-13 2.3.1 Initiator Decay Coefficients 2-13 2.3.2 Propagation rate coefficients 2-13 2.3.3 Termination rate coefficient 2-14 2.3.4 Transfer constants 2-15 2.3.5 Termination rate coefficients 2-19 2.3.6 Summary 2-21 2.4 Controlled/Living Polymerization 2-22 2.4.1 Features of Living Polymerization 2-22 2.4.2 Types of Controlled Polymerization 2-23 2.4.3 Reversible Addition Fragmentation Chain Transfer Polymerization 2-40 2.5 Structures Accessible by Controlled Polymerization 2-47 2.5.1 Block Copolymers 2-47 2.5.2 Gradient Copolymers 2-47 2.5.3 Graft Copolymers 2-47 2.5.4 Star Polymers/Copolymers 2-48 2.6 Conclusions 2-49 2.7 References 2-50 3 Polymerization in Two Phase Systems 3.1 Introduction 3-1 3.2 Suspension Polymerization 3-2 3.3 Emulsion Polymerization 3-4
3.4 Miniemulsion Polymerization 3-5
3.5 Precipitation Polymerization 3-6
3.6 Controlled Polymerization in Dispersed Systems 3-6
3.7 Summary 3-8
3.8 References 3-9
4 Substituent Effects on the Catalytic Chain Transfer Polymerization of 2- Hydroxyethyl Methacrylate
4.1 Introduction 4-1
4.2 Experimental 4-3 4.2.1 Materials 4-3 4.2.2 Synthesis of COBF 4-3 4.2.3 General Polymerization Procedure 4-4 4.2.4 Viscometry 4-5 4.2.5 UV/Vis Spectrometry 4-5 4.2.6 General Molecular Weight Characterization 4-7 4.2.7 Molecular Weight Analysis of PHEMA Samples 4-8
4.3 Results and Discussion 4-11 4.3.1 Chain transfer constants 4-11 4.3.2 Effect of the hydroxyl functionality 4-16
4.4 Conclusions 4-18
4.5 References 4-19
5 Reversible Addition Fragmentation Polymerization in Aqueous Suspension
5.1 Introduction 5-1
5.2 Experimental Section 5-2 5.2.1 Materials 5-2 5.2.2 Bulk Polymerization 5-2 5.2.3 Suspension Polymerization 5-4 5.2.4 Block Copolymer Synthesis 5-4 5.2.5 Molecular Weight Analysis 5-6 5.2.6 Particle Size Distributions 5-6
5.3 Results and Discussion 5-7 5.3.1 Bulk RAFT Polymerizations S-1 5.3.2 RAFT Polymerization in Suspension 5-11 5.3.3 Particle Size Distributions 5-15 5.3.4 Chain Extension Experiments 5-18 5.4 Conclusions 5-20
5.5 References 5-21
6 Conclusions
Vll List of Figures
Figure 2-1 : Decomposition of AIBNto produce cyanoisopropyl radicals and the effect of solvent cage combination of these radicals, as well as the first initiation step in the polymerization of methyl methacrylate. 2-5
Figure 2-2 : Propagation of poly(methyl methacrylate) radicals. 2-5
Figure 2-3 : Termination by (a) combination and (b) disproportionation. 2-6
Figure 2-4 : Chain transfer mechanism involving poly (methyl methacrylate) radicals and carbon tetrabromide. 2-6
Figure 2-5 Radical Scheme for free radical polymerization (including chain transfer to monomer and solvent). 2-8
Figure 2-6: Reaction Rate equations for free radical polymerizations in the absence of inhibitors or chain transfer agents. 2-9
Figure 2-7: Mayo plot for the determination of Chain Transfer Constants for pH-COBF in MMA. 2-16
Figure 2-8 : Stable free radical polymerization mechanism using TEMPO as and intermediate. 2-25
Figure 2-9: Early catalytic chain transfer (CCT) agents - the cobalt porphyrins. The later developed of the cobalt glyoxime catalysts, COBF andpH-COBF. 2-28
Figure 2-10 : Chemical structures of the active centers of several important biological proteins: Vitamin B12, Chlorophyll, and Haemoglobin. 2-33
Figure 2-11 : Mechanism of Catalytic Chain Transfer Polymerization of Methacrylates. 2-35
Figure 2-12: Cobalt-carbon bond formation in the CCTP of acrylate and styrene monomers. 2-37
Figure 2-13: Addition Fragmentation Mechanism for oligomers of methacrylates derived from CCT. 2-41
Figure 2-14 : General formula for the dithiobenzoate group of RAFT agents.
I. 2-cyanoprop-2-yl dithiobenzoate. 11. Cumyl phenyldithioacetate. 2-42
Figure 2-15 : Kinetic Scheme for RAFT polymerization 2-46
Figure 2-16 : SEM Image of macroporous film of poly(styrene-co-acrylic acid) 2-49
Vlll Figure 4 1: Typical molecular weight distribution and its first derivative of PHEMA produced by a pulsed laser polymerization at 20 °C. [AIBN] = 1x10-3 mol L-1, pulsing ft-equency = 20 Hz. 4-9
Figure 4 2: Theoretical molecular weight (from PLP experiment) versus those obtained against styrene standards. 4-10
Figure 4 3 : Mayo and CLD plots for the COBF-mediated free-radical
polymerization ofHEMA at 60°C. 4-12
Figure 4 4 : UV/Vis absorbance spectra of COBF in various solutions at 60 °C 4-17
Figure 5-1: Synthesis of RAFT agent 2-cyanoprop-2-yl dithiobenzoate. 5-3
Figure 5-2.-Reactor design and cross section of reactor 5-5
Figure 5-3 .-Molecularof methyl methacrylate weight evolutions mediated of AIBN-initiated by CPDB at 60bulk °C. polymerizations 5-10
Figure 5-4: Typical evolution of the molecular weight distributions in the bulk RAFT polymerization of MM A mediated by CPDB. 5-10
Figure 5-5: First-order kinetic plots of the AIBN-initiated radical polymerization of MMA at 70 °C mediated by CPDB in bulk and in suspension. 5-12
Figure 5-6: Molecular weight and polydispersity index evolutions of the AIBN- initiated radical polymerization of MMA at 70 °C mediated by CPDB in bulk and in suspension.. 5-13
Figure 5-7: First-order kinetic plots of the AIBN initiated suspension polymerization of MMA at 70 T mediated by CPDB. 5-13
Figure 5-8: Typical molecular weight distributions for a CPDB-mediated suspension polymerization at 70 °C. 5-14
Figure 5-9 : Molecular weight and polydispersity index results for the CPDB-mediated suspension polymerization of MMA at 70°C. Conditions as in Figure 5, solid lines are the theoretical predictions based on [MMA]0/[CPDB] 0. 5-14 Figure 5-10: Microscope images of particles produced via RAFT suspension polymerization of MMA at 70 °C using [CPDB] of 53 mM 5-16
Figure 5-11. Microscope images of particles produced via RAFT suspension polymerization of MMA at 70 °C using [CPDB] of 37 mM. 5-17
Figure 5-12 : Microscope images of particles produced via RAFT suspension polymerization of MMA at 70 °C using [CPDB] of 19 mM. 5-17
Figure 5-13 : Molecular weight distributions for the chain extension of a PMMA suspension polymer (Mn = 20.9U 103 g/mol') with HEM A at successive reaction times. 5-19 ix List of Tables
Table 2-1 : Chain transfer constants of cobalt derivatives with a range of monomers 2-30
Table 2-2 : Variation of Cs.kp. r\ with monomer type 2-37
Table 2-3 : Raft agents used for controlled radical polymerization 2-44
Table 4-1 : Drain times for HEMA and Water through and Ostwald viscometer (size A), times are in minutes and seconds 4-6
Table 4-2 : Drain times and viscosities of HEMA, Water and MM A. 4-7
Table 4-3: Molecular weight characteristics ofpolymers produced for the chain transfer constant determination of COBF in the bulk free- radical polymerization of HEMA at 40 and 60°C.a 4-13
Table 4-4: Summary of chain transfer constants of HEMA and MMAa at 40 and 60 °C measured via the Mayo and CLD methods 4-14
Table 4-5: Summary of kinetic and viscosity data of the COBF-mediated free- radical polymerization of HEMA and MMA at 40 and 60 °C 4-15
Table 4-6: Chain transfer constants of COBF in the free-radical polymerization
of MMA at 60 °C with varying ethanol concentrations 4-16
Table 4-7 : Summary of absorbance maxima of COBF in various solutions 4-17
Table 5 1: Experimental Results of AIBN-Initiated Bulk Methyl Methacrylate Polymerizations at 60 °C mediated by CPDB. 5-8
Table 5 2 : Particle size results for the AIBN-initiated suspension polymerization of methyl methacrylate at 70 °C mediated by CPDB. 5-16
Table 5 3 : Reaction conditions and summary of the results of the chain extension ofpoly (methyl methacrylate) by RAFT suspension polymerization 5-18 1 Introduction
Since the first world war the use of plastics over more traditional materials has expanded until they are almost ubiquitous in there abundance. Plastics have become one of the most versatile building materials ever developed by man: bakelite; vulcanised rubber; nitrocellulose; polyolefms; polyesters; and polyurethanes all featured hugely in the development of modem material and in our day to day lives. However the use of plastics has not been limited to applications as structural materials, new developments in polymer chemistry have led to uses as adhesives, surfactants and more recently as medicinal compounds. Free radical polymerization in particular has been the focus of much study due to its versatility, robust synthesis techniques and very large range of functionalised monomers available.
Free radical polymerization has been an important technological area for seventy years, as a synthetic process it has enabled the production of materials that have enriched the lives of millions on a daily basis. The very utility of free radical polymerization has led to the situation in which millions of tonnes of polymer products are produced even though the reaction mechanism is poorly understood. For example poly styrene was in commercial production well before the foundations of free radical
1-1 polymerization were laid down by Mayo and Walling in 1940-1945. Indeed even after these foundations the processes were poorly understood, most probably due to the time
consuming methods of measuring molecular weights of macromolecules available at the time. It has been said that the margins of error were so wide that it was impossible to compare rate coefficients measured in different laboratories. The advent of HPLC techniques for molecular weight determination has alleviated this problem somewhat, to such an extent that the lUPAC working group on free radical polymerization has been able to systematically review data on rate coefficients of a large number of reactions.
In the mid 1980's the focus shifted to living polymerization in the form of cationic, anionic and group transfer polymerization in the attempt to exploit the greater control over molecular architecture offered by these techniques. High density polyethylene and polypropylene are products that have been commercialised from this research effort. However the intrinsic limitations of the ionic polymerization, in particular its intolerance of functional groups and impurities have proved very difficult to overcome. Thus free radical polymerization has remained dominant as the method of choice for bulk and specialty polymer synthesis. In the late 1980's the seeds were laid for the further development of free radical polymerization. Catalytic Chain Transfer polymerization (CCT) was discovered by Enikopyan and Gridnev in the USSR and later developed in a number of companies, including ICI and Dupont, in order to produce a large range of functional oligomers for the production of block and graft copolymers. CCT was later used by Wayland for the control of molecular weight during the polymerization of acrylates. The use of iniferters was pioneered by Otsu in Japan and allowed the production of block copolymers, although the early iniferters such as dithiuram still produced polymers with broad molecular weight distributions.
The 1990's were the time of great development of free radical polymerization in the research field. New techniques for the control over conventional free radical polymerization began to be developed both in academia and in industry. Nitroxide
1-2 mediated polymerization (NMP) was developed from the work of Solomon, Rizzardo and Moad on efficient trapping of carbon centred radicals which later developed into the reversible termination processes required for living free radical polymerization. Atom
Transfer Radical Polymerization (ATRP) was developed by Sawamoto and Matjasewski following on from developments in transition metal initiated redox processes and inhibition with transition metal compounds. Reversible Addition Fragmentation chain
Transfer (RAFT) polymerization was developed by Chiefari and Rizzardo following on from developments in thiol mediated chain transfer polymerisation and the addition fragmentation polymerization of methacrylate oligomers formed by CCT.
These new techniques allow the controlled polymerization of a very large range of monomers allowing the synthesis of produced polymers with narrow molecular weight distributions and functional endgroups. The new century has seen a new shift in the focus of both industrial and academic research away from the discovery of new techniques and back to the application of these techniques. More and more effort has gone into the development of living free radical polymerization in dispersed systems.
CCT, NMP,ATRP and RAFT have all been attempted in emulsion, miniemulsion, dispersion and suspension polymerization, with varying degrees of success. The applications of these technologies has required a rethink of the processes behind free radical polymerization in dispersed media as molecular weight effects on the kinetics and particle morphology are very different from the well understood industrial processed.
This study will focus on two methods of controlled radical polymerization:
Catalytic Chain Transfer Polymerization and Reversible Addition Fragmentation Chain
Transfer Polymerization. In particular the effect of the dispersed phase on reversible addition fragmentation polymerization will be investigated. The most notable variables will be the concentration of control agent and the effect of reaction conditions.
1-3 Chapter two gives a brief background summary of free radical polymerization methods, kinetics and mechanism, followed by a description of the more recent developments in controlled free radical polymerization. In particular the chapter will describe the current state of research into the fields of catalytic chain transfer polymerization (CCT) and reversible addition fragmentation chain transfer polymerization (RAFT).
Chapter three is a description of the most important methods of dispersed phase polymerization and a summary of the progress in adapting these techniques to the new chemistry of controlled free radical polymerization.
Chapter four examines the effect of hydroxy groups on the catalj^ic chain transfer polymerization of methacrylate esters, in particular the effect of a substituent hydroxyl group on the monomer and the effect of a hydroxyl group on a solvent. The chain transfer constant of 2-hydroxyethyl acrylate was measured at under a number of different conditions and compared to that for un-substituted methacrylate esters. The effect of the hydroxy group was evaluated by measuring the chain transfer constant of methyl methacrylate in the presence of ethanol. The UV/Vis absorbance spectrum of the catalyst was also examined for solutions containing ethanol, methyl methacrylate and 2- hydroxyethyl methacrylate and it was possible to show changes in the UV spectrum that could be directly correlated to a drop in chain transfer activity.
Chapter five investigates the effect of molecular weight reduction on the reaction kinetics and morphology of polymers produced by suspension polymerization.
Initial investigations detail the selection of the molecular weight control agent and the testing of the catalyst in bulk at 60°C. Further investigations look at the kinetics of
RAFT polymerization in suspension at 70°C and compare these reactions to those in bulk. The living nature of the system is tested by successfully chain extending macro
1-4 raft agents with methyl methacrylate, styrene and 2-hydroxyethyl methacrylate. The particle size and morphology of the particles was examined using microscopy and image analysis .
Chapter six summarizes the major conclusions of this work and draws attention to future work in the area of controlled free radical polymerization in dispersed media.
1-5 2 Free Radical Polymerization
2.7 Introduction
Free radical polymerization is a technique for the polymerization of unsaturated hydrocarbons developed shortly after the in the late 1930's. It involves the attack of a free radical on a vinyl bond resulting in a chain reaction leading to polymeric molecules.
The basic processes involved in free radical polymerization are initiation, propagation and termination. Other reactions that can take place are chain transfer to monomer, solvent or chain transfer agent and transfer to polymer. Transfer reactions are very important in determining the final properties of the material formed by the polymerization rates. Properties of the polymer such as hardness, density, ultimate tensile strength and melt viscosity are all determined by the molecular weight and also the chain structure of the polymer, which is ultimately determined by chain transfer events.
The following sections outline the basic free radical reactions involved in free radical polymerization, the means to accurately measure rate coefficients and also the effect of these reactions on the morphology and properties of the polymer. The influence of morphology on the final properties of the polymer is also considered.
2-1 Controlled free radical polymerization is then discussed in detail with a summary of the
methods, mechanisms and the products that have been detailed in the literature.
2.1.1 Initiation
The most common initiators used in both research and industrial applications are azobisisobutyronitrile (AIBN) and tert-butyl hydroperoxide. Initiation is usually achieved by homolytic cleavage of certain species, usually an azide or peroxide species, to yield a pair of free radicals illustrated for AIBN below in Figure 2-1). Alternatively, initiating species may be formed using thermal ultraviolet, plasma or ionizing radiation or via redox processes [1,2]. The reaction continues with the addition of the monomer unit to the free radical, creating a propagating chain with the initiator fragment on one end and a free radical at the other. This step is complicated by solvent cage effects, in which the free radicals can recombine to form species incapable of initiating polymerization, resulting in a yield of free radicals that is less than two. This process complicates the study of free radical polymerization kinetics as the initiator efficiency, f, is dependent on the solvent polarity, temperature and viscosity, all of which can change throughout the duration of the polymerization reaction.
There have been studies on the effect of solvents on free radical initiators.
Involving techniques such as free radical scavenging, However there is still a great deal of uncertainty as to the value of the initiator efficiency. [3]
2.1.2 Propagation
The free radical generated by the decomposition of the initiator goes on to react via free radical addition to the vinyl bond of the monomer. This reaction occurs at a rate proportional to the concentration of free radicals and also proportional to the monomer
2-2 concentration. The structure of the propagating radical during the free radical polymerization of methyl methacrylate is shown in Figure 2-2. The determination of the propagation rate coefficient, kp, has been one of the major activities of researchers in the last few years and propagation rates for a large number of species are now available [4].
There is evidence to suggest that kp is chain length dependent [5,6], solvent dependent [7] and also subject to pressure effects [8]. Propagation rate coefficients generally decrease with increasing chain length and in the presence of solvent. In contrast, kp increases with increasing pressure. These observations have been explained as a result of variations in the local monomer concentration around the growing chain end.
2.1.3 Termination
Once a propagating chain has been established the polymer continues to react with monomer species until some form of termination occurs. There are two types of termination event: combination and disproportionation. Combination occurs as a result of the reaction of two propagating radicals, resulting in the formation of a polymer chain with an initiator fragment at each end. Disproportionation initially occurs in the same way as combination but the energy associated with the reaction of the two free radicals enables one polymer chain to abstract a hydrogen atom from the other, resulting in the formation of two polymer chains. Both of these chains contain the initiator fragment while one is terminated by a saturated monomer unit and the other contains a vinyl bond. Figure 2-3 shows the structures formed by combination and disproportionation during the free radical polymerization of methyl methacrylate.
There have been studies to determine the tendency of monomers to undergo termination by either combination or disproportionation [9]. The general conclusion is
2-3 that vinyl monomers with alpha methyl groups, such as methacrylates, tend to terminate predominantly by disproportionation while those without, including acrylates and stryenics, tend to terminate by combination.
2.1.4 Chain Transfer
The chain transfer reaction is very important to the industrial plastics and resins industry as it allows the synthesis of end functional polymers. These are very useful as feedstocks in the preparation of block copolymers, surfactants, curable coatings and surface active compounds.
The chain transfer process involves the abstraction of a hydrogen or halide species from a polymer, monomer, solvent or catalyst by the growing free radical. This results in the formation of a dead polymer chain and a small free radical capable of initiating polymerization. As an example the chain transfer reaction of methyl methacrylate to carbon tetrabromide is illustrated in Figure 2-4.
The chain transfer rate coefficient of a large number of species can be readily determined by measuring the kinetic chain length. A large data set has been collated for an extremely wide range of monomers, solvents and additives, many of which have been tabulated in the polymer handbook [10].
2-4 CH3 CH3 CH3 NC- N=:N- -CN 2 NC- + N, CH-, CHq CH,
CH. CH3 CH3 2 NC—• NC- -CN CH3 CH3 CH3
H,C CH3 CH CH, CH2 NC- -c- NC- + O CH. y=o CH. O o H^C CH.
Figure 2-1 : Decomposition of AIBN to produce cyanoisopropyl radicals and the effect of solvent cage combination of these radicals, as well as the first initiation step in the polymerization of methyl methacrylate
CN—^^ CN^ -O -O o =0 O O o O n+l Vn ^
Figure 2-2 : Propagation of poIy(methyl methacrylate) radicals
2-5 (a)
O Mc 0=( \j\r\j\Aj\f\j\f —rv/w-nyw^ \J\r\J\AJ\f\/\r— -orv/wv-to
=0 O^ -0 Q O o
(b)
CN— |y\/vwvw——0 V— Q ^
Q O \ / ^td
o CN— LfWWVUX/- 0=1 -'vwnruvj CN o
Figure 2-3 : Termination by (a) combination and (b) disproportionation.
CN- Br CN- -Br Br =0 -H Br- -O + Br- -Br 0 Br O Br O O,\ /n
Figure 2-4 : Chain transfer mechanism involving poly(methyl methacrylate) radicals and carbon tetrabromide.
2-6 2.2 Kinetics of free radical polymerization reactions
Free radical polymerization kinetics have historically been modeled by representing all of the basic reactions being first order with respect the free radical concentration and also first order with respect to monomer, solvent, chain transfer agents. The termination reaction being a bimolecular reaction is second order with respect to the concentration of the propagating radical. The complete series of reactions is listed in Figure 2-5 and the corresponding rate equations are listed in Figure 2-6.
Theoretically these reactions can be used to describe the polymerization reaction.
However as the number of equations required to be solved (and thus the number of rate coefficients required to be estimated) there are a number of assumptions that are required before solutions to these equations can be found.
2.2.1 The long chain assumption
The long chain assumption simplifies the equations for free radical reactions by assuming that the rate coefficient for each of the reactions is independent of chain length. This enables the use of a single average value to be used for the entire reaction scheme[ 11,12]. Note that this is very important for the case of the bi-molecular termination reaction as institution of chain length dependence greatly increases the calculation times required for simulations. The long chain assumption does not hold particularly well for chains of less than 50 monomer units and it is often possible to observe differences in the polymerization rate between reaction producing polymers with different molecular weights.[ 513,14]
2-7 kd I >2L (2-1
kd I- +M >Rr (2-2
k^ Rn- + M > Rn+r (2-3
k Rn- + Rm > ^Pn+m + (I'A. )(Pn + Pm)
krr.M Rn- + M >Rr+Pn (2-4
ktr.S Rn- + S > S- + Pn ( 2-5
kp.s S- + M > Ri- (2-6
Figure 2-5 Radical Scheme for free radical polymerization (including chain transfer to monomer and solvent)
2-8 (2-7
dR-. 00
i=i
dR-. 00
= kpR-._,M~ kpR-M-R-j - k,rRS (2-9
j=l
dP. 00 OO
-^ = (1- + R'._R'J+ KR'S ( 2-10
j-1 j-1
m ^ = (2-11
Figure 2-6: Reaction Rate equations for free radical polymerizations in the absence of inhibitors or chain transfer agents.
2.2.2 Steady State Approximation
The general theory on chain reactions involves making the assumption that the concentration of all radical intermediates remains constant throughout the reaction. This leads to the conclusion that the rate of initiation in a free radical polymerization is equal to the rate of termination and the resultant simplification of the rate equations.
kdfI = ktR^ (2-12
where R is the total radical concentration, kd is the rate coefficient for the initiator decomposition, / is the fractional efficiency of the initiator (the fraction of radicals that initiate polymerization) and / is the initiator concentration This leads directly to the conclusion that the radical concentration is related to the square root of the initiator concentration.
2-9 Considering that the propagation rate is directly proportional to both the overall radical concentration and the monomer concentration we can simplify the rate of monomer consumption to
from which we can see that the rate of monomer consumption is pseudo first order with respect to monomer. Thus from a single reaction we can evaluate a numerical value for the term kp but it is not possible to separate the effects of the propagation rate and termination rate from a single experiment. The early methods for the determination of the propagation rate coefficient involved the use of sequential photoinitiation via the so called rotating sector experiment [15,16]. Later work with laser pulses [17] culminated in the lUPAC benchmark experiment (PLP-SEC) [18] that allowed the systematic determination of rate coefficients for a large range of monomers[19,4].
Equation 2-14 is the basis of the so called first order plot. If we make the assumption that the radical concentration is constant (representing the constant p) we find
dM = -p.dt (2-15 M
The integration of this equation is trivial, giving us an exponential for the concentration of monomer versus time. Introducing the monomer conversion and setting to equal to zero simplifies the equations even further
2-10 \n§- = -W-to) (2-16
= (2-17
Thus in the situation where the radical concentration is constant the first order plot, that is, a plot of -ln(l-X) against time, is linear. This applies in the absence of tromsdorf (gel) effects [20], where the termination rate coefficient can be linked to the viscosity of the medium and in the absence of auto acceleration effects where the heat of reaction results in an increase in temperature of the reaction medium and thus an increase in the propagation rate coefficient.
2.2.3 Solutions to the rate equations
Applying both the steady state assumption and the long chain assumption allows the solution of the rate equations. Classically, these have been described by using the
Mayo equation[21], which is given below.
DP - k^JW "" ^""[Mf ^
Where DP is the degree of polymerization kd is the rate coefficient for initiator fragmentation (s'^) /is the initiator efficiency k^ is the propagafion rate coefficient (L mol'^ s'^) kt is the rate coefficient for termination (L mol'^ s'^) Cs is the chain transfer constant to solvent Cm is the chain transfer constant to monomer Ci is the chain transfer constant to initiator Cx is the chain transfer constant to chain transfer agent [M] is the monomer concentrafion (mol L"^) [I] is the initiator concentration (mol L"') Rp is the rate of polymerization (mol L'^) [X] is the concentration of chain transfer agent (mol L'^)
2-11 This equation can be simplified by including all of the terms for transfer to polymer, initiator and solvent in a term for the degree of polymerization in the absence of chain transfer agent.
(219 where DPo is the degree of polymerization with no added chain transfer agent at the same temperature, initiator and monomer concentration. Measurement of the molecular weights of polymers produced at several different concentrations of chain transfer agent allows the determination of the chain transfer constant as the slope of the plot of the reciprocal of the degree of polymerization against the ratio of chain transfer agent and monomer concentrations.
The same analysis can be done for systems with more than one polymerizable component. However, this results in much more complicated rate expressions and so only the solutions for the homopolymerization are presented here.
2-12 2,3 Measurement of Rate coefficients
2.3.1 Initiator decay coefficients
The rate of decay of the initiator is easily measured in solvents and monomer mixtures using spectrophotometry. Most of the initiators used for free radical polymerization absorb strongly in the ultraviolet spectrum and so measurement of the initiator decay has been well characterized for many years.
More problematic is the initiator efficiency as this is not only a function of the initiator species but also a function of the environment in which the initiator is dissolved.
Temperature, solvent polarity and solvent viscosity are all known to have an effect on the initiator efficiency. Experiments using radical scavenger systems have yielded estimates of the initiator efficiency in the range of 0.6 to 0.8. [22]
2.3.2 Propagation rate coefficients
Obtaining the propagation rate coefficient of a polymerization reaction is difficult due to the large number of processes going on at the same time. In particular, it is very difficult to separate the propagation rate coefficients from the termination rate coefficient and the fractional initiator efficiency. The lUPAC recommended method of obtaining propagation rate coefficients involves the pairing of pulsed laser polymerization (PLP) with size exclusion chromatography, a technique pioneered by
Olaj in the 1980's[17,23] and adopted by a large number of laboratories[24, 4].
Individual laser pulses both initiate and terminate chains such that the majority of chains have lengths which correlate to the number of propagation events between laser pulses.
The polymer produced by pulsed laser polymerization has a very distinctive molecular
2-13 weight distribution, with features that correspond to the degree of polymerization during successive dark times. The mode common method of relating these features to the
Minf = kp -M-td -mo (2-20 where Minf is the inflection point of the molecular weight distribution, td is the time between laser pulses mo is the molecular weight of the monomer
The lUPAC experiment requires consistency checks in that the propagation rate coefficient must be invariant with laser power, pulsing frequency and initiator and monomer concentration.
The greatest problem with obtaining further results today is the efficient determination of molecular weight, with gel permeation chromatography being accurate only to within 10% in the best case. Further attempts to link PLP to mass spectrometry to obtain unambiguous determination of the molecular weights of polymer species have been unsuccessful[25,26]
2.3.3 Termination rate coefficient
Historically the termination rate coefficient has been measured by using a technique such as the rotating sector or PLP-SEC experiment to first determine the propagation rate coefficient and then using this value to derive the termination rate coefficient from the rate of polymerization using equation ???? This provides the average termination rate over the chain lengths produced in the homopolymerization
More recently the use of single pulse PLP has enabled the determination of average chain length dependent termination rate coefficients [27] and the use of
2-14 controlled radical polymerization has enabled the determination of chain length specific termination rate coefficients.[28]
2.3.4 Transfer Constants
Consideration of the overall kinetic scheme for free radical polymerization allows the derivation of the kinetic chain length of polymers produced in free radical polymerization. In general, this is the rate of propagation divided by the rate of all other termination steps. rate of propagation kpMR- (221 rate of termination steps (l+l)
Inverting this equation and dividing through by the overall radical concentration gives us a linear relationship between the inverse of the degree of polymerization and the ratio of the transfer and propagation rate coefficients, which are known as the transfer constants. J__(l+X)
Making note of the fact that the first term on the right side of the equation should be independent of the concentration of chain transfer agent or solvent added to the system, we find that the relationship can be simplified by combining the first two terms into a single term that relates to the degree of polymerization in the absence of chain transfer agent This results in the simplified form of the Mayo equation as follows
— = — + r — ( 2-23 DP DPo 'M ^
Note that this relationship is an approximation and is not exact as the first term on the right includes terms for the average termination rate coefficient, the overall radical concentration and the propagation rate coefficient, all of which are known to vary with the chain length of the polymer produced. However, in cases where the radical concentration is low and the molecular weight in the absence of chain transfer is 2-15 very high, introducing this approximation results in neghgible error and allows determination of the chain transfer constants from the molecular weight of the polymer alone.
0.020 - 0.018- 0.016- 0.014- 0.012 - 0.010- 0.008 - • M n o M vv 0.006 - A A 0.004 - 0.002 - 0.000 -I 1 1 1 1 1 1 1 1 1 1 1 1 1 ^ 1 1 0.0 1.0x10"' 2.0x10"' 3.0x10"' 4.0x10"' 5.0x10"' 6.0x10"' 7.0x10"' 8.0x10"' [phCOBF] [MMA]
Figure 2-7: Mayo plot for the determination of Chain Transfer Constants for pH- COBF in MMA note that the measurements based on the number average molecular weight produces a slightly lower estimate of Cs than that based on M^ or Ahigh
2-16 The degree of polymerization is directly accessible by measuring the number
average molecular weight. There are several ways of determining the molecular weight
averages. The easiest and most popular by far being size exclusion chromatography
(SEC). The most direct measurement of the degree of polymerization {DP) is simply the
number average molecular weight divided by the molecular weight of the repeat
unit (Mo).
= S (2-24
However it has been noted in a large number of papers that the number average
molecular weight obtained from SEC is quite sensitive to baseline selection and
instrument noise. An alternative method in transfer dominated systems is to use the
mass average molecular weight (Af^) divided by the theoretical polydispersity as a better
indicator of number average molecular weight.
DP = ^ (2-25 ^^ 2Mo ^ ^ ^^
The Mayo equation (2-23), relates the average molecular weight of polymer produced in low conversion experiments. This works very well in most cases however it has been noted that the average molecular weight is very sensitive to error due to baseline selection or to artifacts in the GPC spectrum. It has been demonstrated that it is possible to relate the shape of the molecular weight distribution back to the chain transfer constant for polymerizations that are chain transfer dominated[29]. Using much the same argument as that provided in the derivation of the Mayo equation, this method involves an examination of the slope of the molecular weight distribution. The starting point for the examination is the population balance equations considering the fate of radicals of length i,
2-17 dR = Rate of generation of Ri - rate of consumption of Ri
(2-26
dR = kpMRi-i - kpMRi - ktrMMRi - kr,sSRr2R^J^\ Rj j=i
(2-27
1l (2-28 Ri-i kpRiM + ktrsSRi + Rj j=i
Applying the steady state approximation makes equation 2-27 equal to zero and
allows us to find an expression for the concentration of radicals of length i in terms of
radicals of length i-1. We can simplify this relationship somewhat by inverting the right
hand side and introducing chain transfer constants to monomer and chain transfer agent,
Cm and Cs
-/ / 2Ri Ri
A (2-29 Ri-i
Taking note of the fact that the right hand side of the equation above is very
close to unity we can approximate it as the negative exponential of the non unity terms
and then take the log of both sides.
Ri In Ri (2-30 \Ri-lJ CM + CsY} + V M kpM
Realising that the log of a quotient is the same as the difference of two logarithms and making the assumption that the distribution of dead polymer is the same
2-18 as that of the radical population leaves us with the slope of the log of the number
distribution on the left hand side.
u
fdlnp;" (dlnR^ A = (2-31 I di J L di J
and an expression containing the chain transfer constants and termination rate of the right hand side. Measuring the slope at the high molecular weight limit of the distribution eliminates any effect of the variation of kt with chain length. However, it does make the value of A much more sensitive to artifacts and baseline inconsistencies.
Previous studies have shown that in transfer dominated systems far more consistent results have been obtained by measuring the slope of the number distribution in the region of the peak molecular weight. This is due to the fact that the signal to noise ratio at this section of the distribution is much higher thus providing a more consistent estimate of A. Figure 2-7 shows a comparison of the CLD procedure and the Mayo plot, notice that although both procedures give a similar amount of scatter the CLD procedure and Mayo plot are most consistent when Mw is used to obtain an estimate of the degree of polymerization.
This method has been used by a number of authors to provide reliable estimates of chain transfer coefficients of solvents additives both by conventional polymerization[30] of monomers in the presence of polymeric chain transfer agents and by pulsed laser polymerization[31].
2.3.5 Termination rate coefficients
Measurement of the termination rate coefficient is very difficult as it requires estimating the concentration of free radicals in the system. It has been shown that it is
2-19 not possible to determine the value of the termination rate coefficient by means of classical rate analysis [32]. The measurement of the termination rate coefficient is often included with the measurement of the propagation rate coefficient. This is due to the fact that once the propagation rate is known, the termination rate can be found simply by measuring the rate of polymerization and making use of equation 2-14 allowing the determination of average termination rate coefficient as
kJ^ (2-32
A number of other techniques have been devised to measure the termination rate coefficient, including single pulse PLP experiments which measure propagation rate coefficients independently [33]. More recently Yin et al[34] measured chain length dependence of kt of methyl methacrylate (MMA) using dodecanethiol as a chain transfer agent. This study evaluated the effect of chain length dependent propagation on the studies involving kt.
2-20 2.3.6 Summary
The kinetics of free radical polymerization, although well described theoretically are still a subject of intense academic and industrial scrutiny and there are ongoing efforts to quantify rate coefficients for a whole range of monomer systems. The situation is complicated by the development of new techniques for molecular weight control that introduce further steps into the mechanism. These techniques and the efforts to characterize them will be dealt with in the next section.
2-21 2.4 Controlled/Living Polymerization
Much of the theoretical work on free radical polymerization was done more than
sixty years ago with the development of the terminal model[35], while newer models
such as the explicit and implicit penultimate models provide a tool for estimating the
reaction kinetics and copolymer composition[36,37]. More recently, developments in
molecular weight control techniques[3 8] have come close to rivalling the control of
molecular weight seen with anionic and cationic polymerization techniques while
preserving the robustness of free radical polymerization techniques.
The recently developed controlled/living polymerization techniques allow
control over the structure and sequence distribution of macromolecular species. There
has been some discussion on how to classify these reactions with the conclusion that
controlled radical polymerization should include all techniques that enable control over
molecular weight, end functionality or molecular architecture[3 9]. These techniques are,
in order of discovery, catalytic chain transfer polymerization, degenerative transfer polymerization, nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation polymerization (RAFT).
All of these techniques offer a degree of control over the molecular weight, sequence distribution and overall macromolecular structure.
2.4.1 Features of Living Polymerization
Ideal living polymerization involves the instantaneous initiation of all chain ends at the beginning of the reaction follow by propagation until all of the monomer is used up. Webster [40] proposed a definition of the requirement for living polymerization:
"ideally the molecular weight should increase linearly with conversion, the
2-22 polydispersity should be low and the chain ends should remain viable so that
polymerization will continue upon the subsequent addition of monomer." Until recently
the only systems that approached this ideal were cationic polymerization and anionic
polymerization[41,42]. Both of these systems suffer from problems associated with
being highly sensitive to water and other impurities and also require very demanding
reaction conditions (some reactions having to be conducted at well below zero degrees
centigrade). More recently, techniques of conducting polymerization under free radical
condition have made it possible to attain polymers with properties very close to those
produced by cationic and anionic polymerization systems. Not all of the techniques
mentioned above qualify as living free radical polymerization with only NMP, ATRP
and RAFT producing polymers of low polydispersity.
2.4.2 Types of Controlled Polymerization
Degenerative Chain Transfer
In the case where the product of a chain transfer agent is in itself a chain transfer agent it is possible to achieve living behaviour. Examples of this include iodide mediated polymerization and addition fragmentation polymerization.[43,44]. This group of reactions also includes reversible addition fragmentation polymerization and addition fragmentation polymerization, but these techniques are deemed important enough to have there own sections below.
Iniferters
The first developed living polymerization techniques were the iniferters, developed by Otsu who also produced a recent review of the subject[45]. These additives act as initiator, chain transfer agent and terminators. Examples of iniferters include diphenyldisulfide[46,47] and benzyl N,N-diethyldithiocarbamate. Iniferters,
2-23 although providing control over molecular weight and allowing chain extension, do tend
to have low chain transfer coefficients, resulting in polymers with polydispersities
greater than 1.3.
Nitroxide mediated living polymerization
Nitroxide mediated living polymerization proceeds by a reversible termination
method, which has come to be known as the persistent radical effect [48] and was first
reported in the mid 1980's[49]. There has been a considerable amount of work
published in the field since then, particularly in the patent literature, much of which has
been recently reviewed[50]. The nitroxides used provide very stable radicals which do
not propagate or terminate radicals in solution. This causes a build up of the nitroxide
terminated intermediate and the formation of an equilibrium between propagating and
inactive chains resulting in a large proportion of "living" chains. As the system is still thermally initiated it requires a constant input of new radicals in order to maintain the reaction so the molecular weight distributions are broader than those expected from the ideal systems. The best known of catalyst for nitroxide mediated polymerization is
2,2,6,6-tetramethylpiperidinyl-N-oxyl or TEMPO as it has come to be known. TEMPO had been investigated for a number of years for its properties as a very efficient radical scavenger. Figure 2-8 shows the mechanism for stable free radical polymerization of styrene in the presence of TEMPO.
2-24 o H3C. N^CHa H3Q PH3 HX •CH-: H.13Q CH3 0-N // \\ )-N BPO H3C CH r^^HgC bH
Figure 2-8 : Stable free radical polymerization mechanism using TEMPO as and intermediate
The rate of reaction of stable free radicals such as TEMPO with carbon centered free radicals has been found to be of the order of 10^ L mol"' s'^[51,52]. Simulation has confirmed that the reaction must be at least this order of magnitude to obtain polymers with low polydispersity. Fukuda and coworkers investigated the TEMPO mediated polymerization of styrene and estimated the equilibrium constant to be 2.1 x 10-'^ and that the presence of the nitroxide, apart from causing an initial slowing of the polymerization, did not reduce the rate of polymerization. [53]
Nitroxide mediated polymerization is best known for the polymerization of acrylate and styrenic systems at elevated temperature and also for the use in the synthesis of a number of block[54] and star[55] copolymers . However, to date, NMP has been applied to a considerably smaller range of monomers than other living free radical polymerization techniques. NMP has recently been reviewed by Hawker et aL[56].
2-25 Atom Transfer Radical Polymerization (ATRP)
Atom transfer polymerization is a technique for producing polymers of low
polydispersity by a free radical mechanism. It was first reported almost simultaneously
by Sawamoto[57] in Japan and Matyjaszewski[58] in the United States. The technique
is derived from the synthetic chemistry technique of atom transfer radical addition. The
polymerization involves the insertion of a reversible termination step as depicted in the
reaction below.
MX2+R- ^->RX + MX
The reversible termination system, where the polymer is capped by the halide species (usually chloride, bromide or in the case of some early work, with iodide [59]), results in a suppressed radical concentration and a population of dormant polymer species. As the termination is reversible the system results in a population of radicals that grow simultaneously, producing polymers with low polydispersity and halide end functionality, allowing the creation of block structures. [60]
The reverse ATRP system makes use of a conventional radical initiator and the metal halide. Early in the reaction, the free radicals generated by the initiator react with the metal halide. This sets up the same equilibrium that is seen in the first case. The advantage of this system is that fewer and less refined reagents are necessary.
ATRP has been applied in the preparation of well defined polymers from a large range of differing feedstocks. To date acrylates and methacrylates, styrenics, acrylamides, vinylpyridines, acrylonitrile and methacrylic acids have been polymerized by ATRP. [60]
2-26 The major difficulty of most types of ATRP system is the degree of solubility of
the metal halide. As this species must be in solution in quite high concentration to
mediate the free radical polymerization, a ligand is required. There has been a large
amount of work published in this area, much of which has been reviewed recently by
Matyj aszewski[61 ].
Catalytic Chain Transfer Polymerization
Catalytic chain transfer polymerization, although not a living process, is often
grouped with the so-called living free radical processes as it provides a method of
controlling the chain length and architecture of polymers produced by free radical
processes. Catalytic Chain Transfer Polymerization involves the abstraction of a
hydrogen atom from a growing radical by a low spin cobalt chelate such as those seen
in Figure 2-9.
According to Gridnev[62] the discovery of CCT was made by Boris Smimov
and Alexander Martchenko when looking at the activity of cobalt porphyrins as
catalysts for the redox decomposition of peroxy initiators. It was found that although the polymerization reaction ran to completeness the polymer remained in the liquid phase
indicating the formation of low molecular weight species. Formulation of a mechanism
for CCT was formulated by Enikolopyan[63] which has been the accepted model to date.
Despite the model being able to explain the major interactions present in CCT there is as yet no unambiguous proof for a complete mechanism for the hydrogen abstraction from the radical species.
2-27 Figure 2-9: Early catalytic chain transfer (CCT) agents - the cobalt porphyrins. The later developed of the cobalt glyoxime catalysts, COBF and pH-COBF[64].
Gridnev claims[63] to have attempted the immobilization of porphyrin catalysts on alumina, silica and on macroporous cross-linked poly(methyl methacrylate) although he does not refer to any published material. The failure to observe any behaviour consistent with CCT was attributed to a lack of mobility of the catalyst which must diffuse to the radical site. Bel'govsky later put this on a more sound theoretical foundation.
Later work identified the cobaloximes as excellent chain transfer agents[65,66] . Little work was done using these compounds due to their instability in air. The
2-28 discovery of the BF2 bridged cobaloximes by Janowicz[67] made the further study of the CCT activity of this class of compounds possible and a large range of macromonomers have been prepared using various cobaloxime catalysts. Table 2-1 gives a summary of some of the monomers that have been polymerized with cobaloxime derivatives. More recently, a number of thiolate-type catalysts which change a nitrogen on the ligand for a sulfur have been found to be good CCT catalysts although their chain transfer activity is much lower than that of cobaloximes. [68]
2-29 Table 2-1 : Chain transfer constants of cobalt derivatives with a range of monomers
Monomer Solvent Catalyst Temperature Cs/10' Ref °C MMA Bulk COBF 60 24-40 69,70,71,72,73,74
Bulk phCOBF 60 18-24 75,76,72,77
Bulk Co(dmg)2 60 2.2-2.0 78
Bulk cycCOBF 60 13.7 77
Bulk CoP 60 3.6 79
Bulk CoTMHP 60 2.4 80
Toluene COBF 60 41-60 81
Butanone COBF 60 26.5 77
Methanol COBF 60 10.1 77
Ethanol COBF 60 16-25 82
SCCO2 ph-COBF 50 110 83
scCOz COTFPP 60 1.3 84
EMA Bulk COBF 60 27 77
n-BMA Bulk COBF 60 16-28 81,77
t-BMA Bulk COBF 60 14.0- 85 16.8
BzMA Bulk COBF 60 5.7-6.9 86
2-EHMA Bulk COBF 60 11.9 81
EHMA Bulk COBF 60 0.7 87
POEMA Bulk Ph-COBF 60 2.0 88
GlyMA H20/Etha COBF 80 1.0 89 nol
2-30 Monomer Solvent Catalyst Temperature Cs/lO' Ref °C HEMA Bulk COBF 60 0.6 82
HEMA HzO/Ethanol COBF 80 1.1 89
MAA HzO/Ethanol COBF 55 1.1 89
TRIS Toluene COBF 60 0.8-1.7 90
DMI Bulk COBF 60 7.3-9.5 81
STY Bulk COBF 60 0.35- 91,71,81,92,73,77 8.3
AMS Bulk COBF 60 89.3 91
PAA 10%in MMA COBF 60 138- 93 157
MA Bulk COBF 60 0.008- 86 0.022
BA Bulk COBF 60 0.7 80
AAM Acetic Acid CoPc 60 0.1 94
MMA Bulk COLS 95,96
MMA methyl methacrylate EMA ethyl methacrylate n-BMA n-butyl methacyrlate t-BMA t-butyl methacrylate bz-MA benzyl methacryate EHMA 2-ethylhydroxymethacyrlate
POEMA phenoxyethyl methacrylate GLYMA glycidyl methacrylate
HEMA hydroxyethyl methacrylate MAA methacrylic acid
TRIS 3-[tris(trimethylsilyoxy)sityl]-propyl methacrylate
DMI dimethyl Itaconate STY styrene
AMS alpha-methyl styrene PAA 2-Phenylallyl Alcohol
MA methyl acrylate AAM acrylamide
2-31 Structure and Properties of Cobalt CCT catalysts
Much of the early work on the chemistry of metal complexes was based upon the role of these species in biological systems. Many of the enzymes necessary for life are based around proteins containing metal chelates. These include chlorophyll, haemoglobin and vitamin B12., the chemical structures of which are shown in Figure
2-10. Study of the mechanisms of reaction of these species has always been difficult as the synthesis of these species is incredibly difficult or even impossible and purification from natural sources is very expensive. For this reason much of the investigation into the mechanism of operation of these systems was done by the use of model systems, the most common of which were the porphyrins and glyoximes[97]. The use of model compounds to elucidate the mechanisms of vitamin B12 reaction has been reviewed a number of times. [98]
There has been a large amount of work done on the structure and properties of various glyoxime species, mainly in the work of Bakac and Espenson [99] and a large number of cobalt glyoximes with different structures and substituents have been characterized by NMR, UVA^is and X-ray diffraction. The reactions of a number of cobalt species with free radicals as well as a range of oxidants have also been studied[100]. Many new organometallic species have been screened for their activity as catalysts for chain transfer in free radical polymerization[64].
2-32 NH,
CO2' CO2" C2H4
O C CH3 HC^ CH:
Figure 2-10 : Chemical structures of the active centers of several important biological proteins: Vitamin B12, Chlorophyll, and Haemoglobin.
2-33 Mechanism of chain transfer polymerization of low spin cobalt complexes.
To date there has been no mechanism that provides an unequivocal explanation of how the catalytic chain transfer reaction takes place[101] However, there has been a number of different mechanisms proposed[ 102]. The first is that which has been proposed by Enikolopyan[103,104] which involves the extraction of a hydrogen atom from the alpha carbon (where applicable, and from the polymer backbone where there is no methyl group) by the cobalt chelate followed by the almost instantaneous re- initiation of a new polymer chain by the transfer of the hydrogen atom to a monomer unit as depicted below.
Co(II)+ Rn« Co(III)-H + Pn (2-33 ktr
Co(III)-H + M Ri- + Co(II) (2-34 kfast
The mechanism is very similar to that of conventional chain transfer except that new polymer molecules are initiated by the hydrogen atom rather than by incorporation of the chain transfer agent into the polymer backbone,the end groups of which can be distinguished by both MALDI-TOF-MS and NMR[ 105,106]. If this mechanism is correct then the rate of re-initiation by the cobalt hydride must be extremely fast as the intermediate has never been directly observed. Other proposed mechanisms that avoid the formation of this intermediate, such as hydrogen elimination and direct monomer involvement in the reaction mechanism, have sound evidence against them.[107,108].
The mechanism for CCT results in no loss of radicals and as such should have little effect on the rate of polymerization. However, experimental results show significant reductions in conversion with increasing concentration of cobaloxime. This
2-34 has been explained by the increase in the termination rate coefficient seen at lower molecular weights[109]. This results in lower radical concentrations for those systems with higher concentrations of chain transfer agent and thus lower conversion. The effect is much more pronounced for those systems which produce very low molecular weight polymers (<10000 AMU).
It is widely held that the rate of transfer for cobaloxime species is diffusion controlled as the magnitude of the rate coefficient for transfer is very close to the diffusion limits and is of the same magnitude as the rate coefficient for termination, which is widely held to be diffusion controlled[110]. Evidence for this point of view includes the variation in chain transfer constants between the homologous series of methacrylates and the reduction in chain transfer constant seen in systems with very high viscosity.
Propagation
CH2
Re Initiation COoR
Transfer
CH3 CH3 HoC: H7C: / CO2R CO2R CO2R
Figure 2-11 : Mechanism of Catalytic Chain Transfer Polymerization of methacrylates
2-35 Table 2-2 shows the relationship between chain transfer rate coefficient and
monomer viscosity. This relationship can be represented by the equation below.
CgX^pX;; = constant (2-1
Evidence for a chemically controlled process includes the invariance of chain transfer constant at high viscosities and variation of the chain transfer constant in the presence of solvents [73].
Cobalt Carbon Bonding
While the work that has been done on methacrylic monomers has resulted in the formulation of a fairly simplistic mechanism, work done with acrylates and styrenic monomers has resulted in a very large variation in the chain transfer constants. Indeed, it is even possible for some types of monomers, such as methyl acrylate, to undergo living free radical polymerization in the presence of cobaloximes[ 111, 112]. The mechanism proposed for this reaction is the direct reaction of the cobaloxime with the free radical to form a stable species. This species forms an equilibrium with the propagating radical that results in the reversible termination reaction shown in Figure
2-12.
The strength of the bond between the polymer and the cobaloxime is very strong and the equilibrium illustrated in Figure 2-12 lies strongly on the side of the cobalt alkyl species. Evidence for this includes the extremely reduced Cs values for styrene and acrylate [113,114] and the ability to detect the cobalt terminated polymer using MALDI
[115]. Recently, a very thorough study combining experimental work with modelling of molecular weight distributions using PREDICI™ allowed evaluation of individual rate coefficients for the activation/deactivation of the cobalt catalyst.[l 16]
2-36 Table 2-2 : Variation of CsXA^pXwith monomer type
Monomer Catalyst CsXA:pX77 Reference CsxA:pX77 Ref xlO^ system xlO^ POEMA COPhBF 0.48 MMA 0.59 88 above
BzMA COBF 1.2 MMA 1.0 86
2-EHMA COBF 1.5 n-BMA 1.4 77
t-BMA COBF 0.67 n-BMA 0.95 85
HEMA COBF 0.43 MMA 1.0 82
DMI COBF 0.04 MMA 1.0 77
C0(II)+ Rn« => C0(III)-H + Pn ktr
Co(III)-H + M Ri* + Co(II)
kfast
C0(II)+ Rn* <-> C0(III)-Rn
Figure 2-12: Cobalt-carbon bond formation in the CCTP of acrylate and styrene
monomers.
According to equation 2-22 the degree of polymerization should decrease as the
chain transfer agent to monomer ratio increases during the course of a polymerization.
However, it has been consistently observed that the molecular weight distributions of polymers formed by CCT remain virtually unchanged during the course of a polymerization [117]. The simplest explanation proposed for this is the progressive
deactivation of the catalyst during the course of the polymerization. This explanation has the shortcoming that the deactivation of the catalyst must proceed at exactly the
same rate as the consumption of monomer and this is not likely to happen in all cases.
2-37 Other explanations, such as participation of monomer in the chain transfer process have
been proposed but in these cases the rate of chain transfer should be proportional to
cobalt concentration alone, which was found not to be the case.
The major difficulty in assessing the chain transfer coefficients is the very high
activity of the catalyst makes it necessary to use extremely low concentrations, in the
parts per million range, in order to obtain a broad enough range of molecular weights to
obtain a good Mayo plot. As the propagation rate is chain length dependent this is
reflected in the chain transfer constant. At these low concentrations the possibility of
catalyst poisoning by residual oxygen or by contaminants in the monomer, solvent or
initiator is very high. It has been shown, for example, that the apparent chain transfer
constant of COBF in styrene is dependent on the method of purification of the
monomer.[118]
The sensitivity of the catalyst system to oxygen has always been a problem for
mechanistic studies as many effects can be attributed to trace amounts of oxygen in
solution. The solubility of oxygen in many organic media has only recently been possible due to the sensitivity of many of the commercial oxygen sensors to organic
solvents. A newly developed technique using UV./Vis spectrophotometry has made it possible to measure the solubility of oxygen in a range of organic solvents[l 19]. In general, the concentration of oxygen is expected to be in the order of 1 x 10'^ M. Three orders of magnitude difference in the concentration of oxygen and catalyst in low conversion studies causes complete deactivation of the catalyst. Some work has been done on the oxidation of cobalt macrocycles by hydrogen peroxides by Bakac and
Espenson[100] who found that the cobalt(III) glyoximes are easily reduced to cobalt(II) by Fe^^ ions. Little work has been done on the effect of radicals on cobalt(III) species
2-38 although some early work suggests that free radicals can reduce cobalt(III) back to the active cobalt(II) oxidation state.
Industrial Uses of CCT
CCT has become one of the early controlled radical polymerization techniques mainly due to the ability to easily synthesize a large range of industrially important intermediates for the coatings industry. The most important of these is probably the creation of telechelic oligomers[ 120,121 ] for use as cross linkers for UV curable coatings. Other uses involve the formation of reactive surfactants and pigment dispersants.[122]
2-39 2.4.3 Reversible Addition Fragmentation Chain Transfer Polymerization
Reversible addition fragmentation chain transfer polymerization was developed
at CSIRO following studies on oligomers of certain methacrylates as addition
fragmentation agents[123]. The initial study aimed at producing branch and comb
polymers from oligomers similar to those produced by CCT. However due to the fact
that the radical produced is extremely hindered, it undergoes p-scission rather than polymerization to produce either the initial radical, or a radical with a fragment of the leaving group. Importantly the remaining fragment is similar to the initial oligomer and can react in the same manner resulting in a reversible reaction shown in Figure 2-13.
This reaction results in the reversible transfer of the radical and thus storage of growing chains and results in a process with living behaviour.
The efficiency of the process is very low as the rate of transfer to the oligomers is low[124] and the rate transfer between active and inactive polymer chains is much slower than the rate of propagation resulting in broadened molecular weight distributions. Use of compounds with higher chain transfer coefficients, namely the dithioester family made it possible to obtain living behaviour with low polydispersity with a large number of monomer types[125]. Figure 2-14 shows the general structure of the dithioester RAFT agents and also two examples of commonly used RAFT agents, cyanoisopropyl dithiobenzoate and cumyl phenyldiothioacetate[126]. The suitability of a particular RAFT agent for a particular monomer lies in the correct selection of the R and Z group. The Z group must provide a stable enough radical such that the transfer rate to the RAFT agent is at least as fast as the propagation reaction while the R group must be able to reinitiate polymerization.
2-40 Figure 2-13: Addition Fragmentation Mechanism for oligomers of methacrylates derived from CCT, (Note R is the ester substituent, COOCH3 in methacrylates )
2-41 -R
II
Figure 2-14 : General formula for the dithiobenzoate group of RAFT agents. I. 2- cyanoprop-2-yl dithiobenzoate. II. Cumyl phenyldithioacetate.
2-42 Influence of the Z and R groups
There has been much debate and study on the influence of the structure of the RAFT
agent on the polymerization kinetics. There are two major effects in play, one is the
ability of the R group to leave the intermediate radical and reinitiate polymerization and
the other is the degree of stability of the radical intermediate formed. The leaving group
R must be more likely than the monomeric or polymeric radical to leave the
intermediate, otherwise there is only storage of inactive chains as the intermediate
radical and no transfer. As the intermediate radical concentration never reaches the
limits of detection, it does not reach high enough concentration to mediate living
polymerization. In most cases, this means that the leaving group must be a higher order
radical than the propagating radical.
This brings in the complication that it is not possible to form block copolymers of
monomers that propagate via tertiary radicals with monomers which propagate via
secondary radicals unless the monomer that propagates via the tertiary radical is polymerized first. The Z group must sufficiently stabilise the radical centre to allow fast
transfer between the propagating radical and the leaving group. If the intermediate
radical is too stable, the system will show both inhibition and retardation, as the RAFT
agent will form a radical sink. The effect of the structure of the RAFT agent has been
recently reviewed in the thesis of John F. Quinn[127]. Table 2-3 below shows a
summary of the monomers that have been successfully shown to exhibit living polymerization with dithioester intermediates and the structure of the RAFT agents that were used to mediate the reaction.
2-43 Table 2-3 : Raft agents used for controlled radical polymerization
S^S-R Monomer
R = C(CH3)2Ph Z = Ph Methyl methacrylate [128,129] C(CH3)2CN SCH3 CH3 Styrene [128,129] R = C(CH3)2Ph Z = Ph C(CH3)2CN SCH3 CH(CH3)Ph CH3 CH2Ph pyrrole Methyl acrylate [128,129] R = C(CH3)2CN Z = Ph CH(CH3)Ph SCH3 CHsPh CH3 CH(CH3)C02Et pyrrole Acrylic acid [129] R = C(CH3)2CN Z = Ph CH(CH3)Ph N,N-dimethylacrylamide [129, R = C(CH3)2Ph Z = Ph 130] CH2Ph C(CH3)(CN)(CH2)2C02Na CH(CH3)C0N(CH3)2 CH2C0N(CH3)2 N,N-Dimethylaminoethyl R = C(CH3)(CN)(CH2)2C02H Z = Ph methacrylate [128] Vinyl acetate [129] R = CH2CN Z = OEt CH(CH3)C02Et N(Et)2 N-Vinylpyrrolidone [130] R=CH2CN Z = OEt Sodium 4-styrenesulfonate R = C(CH3)(CN)(CH2)2C02Na Z = Ph [128,131] C(CH3)(CN)(CH2)2C02H
2-44 Kinetics of RAFT polymerization
The kinetic scheme devised for RAFT polymerization involves a reversible transfer reaction between the free radical and the dithioester moeity. This allows the simultaneous growth of all polymer chains. The mechanism shown in Figure 2-14 does however show some deficiencies. The greatest of these is the failure of the model to account for the retardation of the polymerization caused by the presence of the RAFT agent. The cause of the retardation would have to involve some sort of radical sink. To date there are three possible explanations as to the cause of retardation:
(1) slow fragmentation of the radical intermediate[132,133] (2) Slow re-initiation by the leaving (R) group of the RAFT agent (3) termination of the radical intermediate to form three arm stars and reversible termination between RAFT intermediates.[134,135]
There is as yet no solid evidence for either the slow fragmentation model or the intermediate termination model mainly due to the lack of success in finding the intermediates that should be produced. In the slow fragmentation case the radical concentration should be high enough to measure by ESR spectroscopy while the three arm stars formed by intermediate termination should be detectable by NMR or
ESR[136]. Work is ongoing to identify the intermediates in this reaction using a range of advanced techniques. [137]
2-45 (I) Initiation Initiator 21'
Monomer Pr
S^S-R Pm-S^S-R Pm-S^S ^ (II) Chain Transfer P^ ^ Y T — "" Y ^ R- z z z (1) (2) (3)
(III) Reinitiation r. p.
S S^P (IV) Addition / Fragmentation p. - ^m-S^^ P^-S^S-Pn ^ ". p• Z Z z (4)
(V) Termination P/:, + Pfn n+m
Figure 2-15 : Kinetic Scheme for RAFT polymerization
2-46 2.5 Structures accessible by controlled polymerization
2.5.1 Block Copolymers
Block copolymers are the most common of the products produced by living radical polymerization[138]. They are produced by sequential addition of monomers to a reaction medium. Block copolymers have potential applications in high impact strength polymers[139,140], coatings[ 141,142,143], adhesives[141] detergents and inks[ 144 ]. Block copolymers generally exhibit superior performance to random copolymers as the separation of the different repeat units allovv^s for the formation of micro-domains in the structure of the crystalline material, leading to high impact strength. High tech applications of block copolymers include stabilisers for aqueous dispersions and compatibilizers for high pressure polymer synthesis and modification[145,146]. A large range of block copolymers have been produced by the new living radical polymerization techniques.
2.5.2 Gradient Copolymers
Gradient polymers are produced when a mixture of monomers with low cross propagation coefficients. This results in a system where one of the monomers is preferentially incorporated in the polymer. The polymers have properties that are similar to block copolymers but are accessible by a one pot synthesis. Gradient polymers have found applications in the cosmetics[ 147,148], pigments[ 149], lubricants[ 150] and synthetic rubber[151] industries. Gradient polymers have been synthesized by a large number of living radical techniques [152,153,154].
2-47 2.5.3 Graft Copolymers
Graft copolymers can be formed in a number of ways, the most common synthesis techniques being the arms-first and backbone-first techniques. The arms-first technique involves the synthesis of polymers that include a polymerizable vinyl bond known as macromonomers[155]. When these are incorporated into a polymer chain it results in a branched structure with the macromonomer contributing the branches and the monomer forming the backbone. The second method involves the use of a functionalized monomer that can be formed into an ATRP or RAFT agent such that polymers can be grown off the backbone. Other techniques involve the grafting of polymers to surfaces [156,157]
2.5.4 Star Polymers/Copolymers
Star copolymers are a special case of graft copolymers where the chains are all initiated from a small molecule such as a sugar[158], cyclodextrin[159,160] or low molecular weight polymers[161]. Star polymers have some useful properties, including interesting melt viscosities. It has been shown that when cast in a humid environment star polymers produce macroporous films from solvents such as dichloromethane and carbon disulphide in a humid environment. The structure of these films has been shown to vary with the molecular weight of the polymer, type of substituent and the amount of linear polymer present. An example of a macroporous film produced from a star polymer is shown in Figure 2-15.
2-48 040051 2KV X3.00K 9.90uin
Figure 2-16 : SEM Image of macroporous film of poly(styrene-co-acrylic acid) (Mn = 53500-27500 g mol"^). Cast from carbon disulphide (20mg ml"^) in a humid atmosphere[162]
2.6 Conclusions Recent advanced in the field of free radical polymer chemistry have made it possible to access structures that were previously only attainable via anionic and cationic polymerization. This will enable the production of unique structures for coatings, structural and biomedical applications that have previously been prohibitively expensive. While further study is required in order to provide greater understanding of the mechanisms of controlled free radical polymerization the focus of research has begun to turn away from the mechanisms and towards the applications of these new techniques.
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2-73 3 Polymerization in Two Phase Systems
3.1 Introduction
For the most part research into the kinetics of free radical polymerization has focused on single phase systems, while industrial synthesis of polymers tends take place in multi phase systems to take advantage of improved mixing transport and heat distribution made possible by heterogeneous polymerization systems[l]. In particular emulsion polymerization is used for production of latexes for use in the coatings and industry while suspension polymerization produces products more suited to molding or extrusion applications. The research into controlled radical polymerization has followed a course that is very similar, with most methods of controlled radical polymerization being well characterized in singe phase systems. This chapter will describe the most utilized methods of dispersed phase polymerization and then review the literature on attempts to perform controlled radical polymerization in dispersed media.
3-1 3,2 Suspension Polymerization
Polymerization in suspension involves the creation of a suspension of monomer
droplets in a suspension media (usually water). A free radical initiator in the dispersed
phase causes polymerization of the droplets to form polymer beads with a size range between 50Dm and 1000Dm[2]. The size of the particles is dependent on the size of the
droplets formed in the initial dispersion.
In the ideal suspension polymerization the initiator is soluble only in the organic phase and the monomer is not soluble in the continuous phase, this produces kinetics that exactly mimic those of bulk polymerization and a product the same characteristics.
In reality this model is inaccurate in that the initiator and monomer are often partially soluble in the continuous phase and the continuous phase has some solubility in the monomer. This leads to inclusion of a small concentration of suspension media in the final product and incomplete conversion of monomer dissolved in the continuous phase.
In some systems the monomer in the continuous phase re-dissolves in the organic phase, leading to slow polymerization late in the reaction. However in many cases the monomer dissolved in the continuous phase does not polymerize leading to slightly lower conversions that would be observed in bulk.[3]
The most thorough study on mass transfer effects was done by Ray[4]. This study found that, despite being thermodynamically favored, monomer did not transfer from the dispersed phase back to the monomer phase. It was concluded that the increase in the resistance to mass transfer in the monomer phase during the course of polymerization leads to the incomplete conversion as monomer is trapped in the continuous phase. The use of high concentrations of sodium chloride in the dispersed phase was shown to increase the conversion of monomer by reducing the solubility of
3-2 the monomer in water. Some water soluble monomers have been polymerized in
suspension by making use of perfluroalkanes[5] as novel suspension media.
The particles produced in suspension polymerization reactions can be directly correlated to the size of the monomer droplets present at the beginning of the reaction.
The droplet size of in these systems is a result of an equilibrium between droplet breakup in high shear regions and droplet coalescence in regions of low shear, a number of models have been developed by researchers both within and outside the field of polymer research. [6]
A review of the applicability of these models was carried out by Yuan in
1991 [7]. There are two models for liquid-liquid dispersions that are used for different situations. The first of these is that of liquid-liquid dispersions which form in the absence of any coalescence of particles, this model may be used at very low dispersed phase concentrations .land in the case of very high interfacial tension between the dispersed and continuous phase. More complex models involve dealing with the effects of increased dispersed phase concentration in that the equilibrium between droplet formation and droplet coalescence must be described. The two region model of
Tanaka[8] divides the vessel into two regions, one in the region of the impeller where very little droplet coalescence occurs, and the other consisting of the rest of the reactor volume where very little droplet break up occurs.
3-3 Until recently most modeling of dispersion has been done with non reacting
solutions. Theories developed from these experiments have been used to develop select
design criteria for suspension polymerization reactors. The advent of increased
computing power has led to the development of more complex models for suspension polymerization behaviour. In particular the efforts of Kalfas and Ray[9] have resulted in the development of models that can accurately predict both particle size distribution and monomer conversion. The best of these yet found is the "reaction diffusion model" which is defined by two sets of differential equations under the assumption that the monomer droplets are stagnant and the continuous phase is well mixed.
There is virtually no information in the open literature on the effect of stabilizers and reactor design on the amount of coagulum formed in suspension polymerization reactors. As it has only lately been possible to obtain very low molecular weight polymers at high conversion without resorting to very high concentrations of chain transfer agents little work has been done regarding these effects, most of which has been restricted to commercial research.
3,3 Emulsion Polymerization
Emulsion polymerization is a technique for polymerization where a poorly water soluble monomer is polymerized in an aqueous medium. The addition of monomer, surfactant such as sodium dodecyl sulfate and a water soluble initiator to the reaction medium result first in the formation of micelles and then the swelling of those micelles with monomer. Initiation of polymer chains takes place in the aqueous phase and results in the generation of oligomers in the aqueous phase. These species cannot grow past a few monomer units as they are even less soluble than the monomer and are quickly absorbed by the swollen micelle. The fate of these radicals depends on the stage of
3-4 polymerization and the concentration of radicals and a number of different models have
been proposed in order to model these reaction.
The models that have been most popular are the zero one system and the pseudo
bulk systems[10]. The zero one system occurs when micelles are small and reaction is
relatively low conversion. In this situation a new radical has two fates, it can either enter
a micelle that has no radicals, in which case the radical begins to propagate; or it can
enter a radical in which another radical is already propagating, which results in
instantaneous termination. In this system exactly half of the micelles will have a propagating radical and thus the rate of propagation will be proportional to the number
of micelles rather than to the radical concentration. Thus compartmentalization of radicals in the micellular phase allows for much higher polymerization rates than
achievable in bulk polymerization. In the pseudo bulk system particles are large enough that many radicals can exist in a particle, creating a system where the kinetics is more similar to that of bulk polymerization.
The particle size of latexes produced by emulsion polymerization range from 10 up to 500 nanometers, with the concentration of surfactant having the greatest influence on particle size.
3,4 Miniemulsion Polymerization
Miniemulsion polymerization involves recipes that are very similar to those used for emulsion polymerization, with most using the same surfactant/stabilizer systems with the addition of a co-surfactant, usually a hydrophobe such as hexadecanol. The reaction mixture is then homogenized by the use of high shear or by techniques such as high energy ultrasound. The presence of the hydrophobe stabilizes the miniemulsion by prevention of ostwald ripening and the surfactant prevents the droplets from combining.
3-5 Emulsions prepared in this manner can be stable for several weeks and can have droplet
sized ranging from nanometer scales right up to micron size. Attempts to produce
particles of a larger size results in the polymerization by emulsion polymerization and
the production of a bimodal particle size distribution. Miniemulsion polymerization has
been reviewed recently by Asua[ll] who notes the increasing interest in the technique
due to its applications in low viscosity latexes, controlled radical polymerization and
microencapsulation.
3.5 Precipitation Polymerization
Precipitation polymerization involves the polymerization of a soluble monomer in a solvent which the polymer is insoluble. The initiator is soluble and the reaction starts initially as a single phase. As the polymerization reaction continues the polymer becomes insoluble and drops out of solution as micro particles. The polymerization kinetics of precipitation polymerization are slightly slower than for solution polymerization due to the fact that there is an addition means of termination as the polymer falls out of solution, resulting in a great decrease in the local monomer concentration.
3.6 Controlled Polymerization in Dispersed Systems
The use of controlled radical polymerization in dispersed phases has been reviewed by Qui and Charleux[ 12 ]. While the theoretical aspects of controlled polymerization have been dealt with by Charleux[13]. The most significant amount of work has utilized emulsion and miniemulsion, mainly due suitability of these systems for laboratory scale investigation. Their work details some of challenges to be overcome before it is possible to employ free radical polymerization in dispersed systems.
3-6 These challenges include temperature requirements that are incompatible with aqueous systems, for example nitroxide mediated polymerization is usually performed at high temperatures (>110°C) in order to promote release of the radical by the stable complex[14] and a pressure vessel is required to perform polymerization under these conditions[15,16]. Partitioning of the active species in the aqueous phase especially with Atom Transfer Radical Polymerization although the correct choice of ligand can eliminate these problems[17]. The most promising candidate, RAFT runs into problems due to the slow transport of the RAFT agent through the aqueous phase, resulting in loss of control of molecular weight, broadened particle size distributions and the formation of a red layer on the surface of the reaction. Mini emulsion has been more successful with RAFT due to the avoidance of the transport step
Lately successes have been achieved with the RAFT emulsion polymerization of styrene utilizing both seeded emulsion polymerization[18], where the polymeric raft agent is incorporated in the seed and in ab initio emulsion polymerization[19] whereby a water soluble raft agent is utilized. Other successes have been achieved using
.dispersion polymerization[20] and minemulsion[21] for the production of functionalized microspheres. Catalytic Chain Transfer polymerization has been successfully carried out in emulsion[ 22 , 23 ] and miniemulsion[ 24 ] polymerization. High yields of macromonomers have been achieved with only slightly higher polydispersity than that seen in bulk. The catalyst suffers some degradation due to hydrolysis during the reaction and continuous addition of catalyst is required in order to maintain control over the molecular weight. ATRP has been shown to be effective in emulsion[ 25 ], miniemulsion[26] and suspension[27] polymerization although slightly broadened distributions have been obtained through loss of control due to solubility of the catalyst/ligand system in the aqueous phase. Degenerative chain transfer was one of the
3-7 first of the pesudoliving polymerization techniques developed in the late 1980's and as
such it was one of the first to be applied to dispersed systems. Degenerative chain
transfer has been successfully implemented in the miniemulsion polymerization of
styrene[28]. Nitroxide mediated polymerization has been achieved in emulsion[29]and miniemulsion[30] polymerization, however in many cases it is necessary to carry out the reaction under pressure due to the high temperature requirements for polymerization by this method.
3,7 Summary
The technology for controlled radical polymerization has been developed to a point where it is now possible to control molecular weight of the polymer is systems involving dispersed phases by a number of different methods. Interestingly enough suspension polymerization seems to have been less researched than other techniques, perhaps due to a focus on the production of micro and nanoparticles for the pharmaceutical industry.
3-8 3,8 References
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[2] Dowding, P.J. and B. Vincent, Suspension polymerization to form polymer beads.
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[3] Zhang, S.X. and H.W. Ray, Modeling and Experimental Studies of Aqueous
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[5] Zhu, D.W., Perfluorocarbon Fluids: Universal Suspension Polymerization Media.
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(c) Kiparissides, C., et al., Population Balance Modeling of Particulate Polymerization
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3-9 [8] Hosogai, K. and M. Tanaka, Study of suspension polymerization of styrene with a
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[9](a) Kalfas, G. and W.H. Ray, Modeling and experimental studies of aqueous
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(b) Kalfas, G., H. Yuan, and W.H. Ray, Modeling and experimental studies of aqueous suspension polymerization processes. 2. Experiments in batch reactors. Industrial &
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[10] Clay, P.A., D.I. Christie, and R.G. Gilbert, Rate coefficients controlling molecular weight distributions in emulsion polymerizations. Polymer Preprints (American
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[12] Qui, J., B. Charleux, and K. Matyjaszewski, Controlled/Living Polymerization in aqueous media: homogeneous and heterogeneous systems. Prog. Polym. Sci., 2001. 26: p. 2083-2134.
[13] Charleux, B., Theoretical Aspects of Controlled Radical Polymerization in a
Dispersed Medium. Macromolecules, 2000. 33(15): p. 5358-5365.
[14] Wannemacher, T., D. Braun, and R. Pfaendner, Novel copolymers via nitroxide mediated controlled free radical polymerization of vinyl chloride. Macromolecular
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[15] Georges, M.K., J.L. Lukkarila, and A.R. Szkurhan, TEMPO-Mediated n-Butyl
Acrylate Polymerizations. Macromolecules, 2004. 37(4): p. 1297-1303.
3-10 [16] MacLeod, P.J., et al., Stable free radical miniemulsion polymerization.
Macromolecular Symposia, 2000. 155(Emulsion Polymers): p. 31-38.
[17] (a)Gaynor, S.G., et al.. Controlled/living radical polymerization applied to water-
borne systems. Polymeric Materials Science and Engineering, 1999. 80: p. 536-537.
(b)Matyjaszewski, K., et al., ControlledA"living\" radical polymerization applied to
water-borne systems. Book of Abstracts, 217th ACS National Meeting, Anaheim,
Calif., March 21-25, 1999: p. PMSE-325.
(c)Jousset, S., et d\.,Atom Transfer Radical Polymerization of Methyl Methacrylate in
Water-Borne System. Macromolecules, 2001. 34(19): p. 6641-6648.
(d) Ali, M.M. and H.D.H. Stoever, Polymeric Capsules Prepared by in Situ Synthesis and Cross-Linking of Amphiphilic Copolymer by Atom Transfer Radical
Polymerization. Macromolecules, 2003. 36(6): p. 1793-1801.
(e) Bicak, N., et al., Utility of atom transfer radical polymerization for the preparation of poly (methyl methacrylate) beads in an aqueous suspension. Journal of Polymer
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[18] Prescott, S.W., et al.. Successful Use of RAFT Techniques in Seeded Emulsion
Polymerization of Styrene: Living Character, RAFT Agent Transport, and Rate of
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[19] Ferguson, C.J., et al.. Effective ab Initio Emulsion Polymerization under RAFT
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[20] Earner, L., et al.. Synthesis of core-shell poly(divinylbenzene) microspheres via reversible addition fragmentation chain transfer graft polymerization of styrene.
Journal of Polymer Science, Part A: Polymer Chemistry, 2004. 42(20): p. 5067-5076.
3-11 [21] Lansalot, M., T.P. Davis, and J.P.A. Heuts, RAFTminiemulsionpolymerization:
influence of the structure of the RAFT agent. Macromolecules, 2002. 35(20): p. 7582-
7591.
[22] Kukulj, D., et al., CCTin emulsion - copoly MMA and BMA. J. Polym. Sci., Polym.
Chem., 1997. 35: p. 859.
[23] Haddleton, D.M., et al., The effect of feed conditions in the emulsion catalytic chain
transfer polymerization of alkyl methacrylates. J. Polym. Sci., Part A: Polym. Chem.,
1999. 37(18): p. 3549-3557.
[24] Pierik, S.C.J., B. Smeets, and A.M. Van Herk, Catalytic Chain Transfer in a
Miniemulsion Copolymerization of Methyl Methacrylate and n-Butyl Acrylate.
Macromolecules, 2003. 36(25): p. 9271-9274.
[25](a) Chambard, G., P. De Man, and B. Klumperman, Atom transfer radical polymerisation in emulsion. Macromolecular Symposia, 2000. 150(Polymers in
Dispersed Media): p. 45-51.
(b) Qiu, J., et al.. Mechanistic Aspect of Reverse Atom Transfer Radical Polymerization
of n-Butyl Methacrylate in Aqueous Dispersed System. Macromolecules, 2000. 33(20):
p. 7310-7320.
[26] Min, K., et al., Synthesis of star block copolymers via sr&ni atrp in miniemulsion.
Polymer Preprints (American Chemical Society, Division of Polymer Chemistry), 2004.
45(2): p. 682-683.
[27] Ali, M.M. and H.D.H. Stoever, Mechanism of capsule formation by suspension
atom transfer radical polymerization. Polymer Preprints (American Chemical Society,
Division of Polymer Chemistry), 2002. 43(2): p. 59-60.
3-12 (b) Ali, M.M. and H.D.H. Stoever, Polymeric Capsules Prepared by in Situ Synthesis
and Cross-Linking of Amphiphilic Copolymer by Atom Transfer Radical
Polymerization. Macromolecules, 2003. 36(6): p. 1793-1801.
(c)Zhu, C., et al, Atom transfer radical suspension polymerization of methyl
methacrylate catalyzed by CuCl/bpy. Polymer, 2004. 45(4): p. 1141-1146.
[28](a) Butte, A., G. Storti, and M. Morbidelli, Miniemulsion Living Free Radical
Polymerization of Styrene. Macromolecules, 2000. 33(9): p. 3485-3487.
(b)Lansalot, M,, et al.. Controlled free-radical miniemulsion polymerization of styrene using degenerative transfer. Macromolecules, 1999. 32(22): p. 7354-7360.
[29] (a)Charleux, B., et al., Nitroxide-mediated controlled free-radical emulsion polymerization of styrene. Book of Abstracts, 218th ACS National Meeting, New
Orleans, Aug. 22-26, 1999: p. POLY-046.
(b)Marestin, C., et al., Nitroxide Mediated Living Radical Polymerization of Styrene in
Emulsion. Macromolecules, 1998. 31(12): p. 4041-4044.
[30] (a)Butte, A., G. Storti, and M. Morbidelli, Miniemulsion Living Free Radical
Polymerization of Styrene. Macromolecules, 2000. 33(9): p. 3485-3487.
(b) Lin, M., M.F. Cunningham, and B. Keoshkerian, Achieving high conversions in nitroxide-mediated living styrene miniemulsion polymerization. Macromolecular
Symposia, 2004. 206(Polymer Reaction Engineering V): p. 263-274.
(c) Georges, M.K., J.L. Lukkarila, and A.R. Szkurhan, TEMPO-Mediated n-Butyl
Acrylate Polymerizations. Macromolecules, 2004. 37(4): p. 1297-1303.
(d) Cunningham, M.F., M. Lin, and B. Keoshkerian, Optimizing nitroxide-mediated miniemulsion polymerization processes. JCT Research, 2004. 1(1): p. 33-39.
3-13 4 Substituent Effects in the Catalytic Chain Transfer Polymerization of 2-Hydroxyethyl Methacrylate
4J Introduction
In recent years there has been a growing demand for functionalized low molecular weight polymers in applications ranging from high solids coatings and UV curable resins[l,2] to biomedical applications such as contact lenses[3]. Many of these applications require the use of these low molecular weight polymers as prepolymers or macromonomers and as such they are used in further cross linking and/or polymerization reactions. Although there are various procedures for the synthesis of macromonomers[4], perhaps the simplest and most versatile technique to date is catalytic chain transfer polymerization[5], i.e., a free-radical polymerization technique mediated by low-spin Co(II) complexes. In this process, which was said to have been discovered in the late 1970s by Smimov and co-workers[5d], the Co(II) complexes act as very efficient catalysts for the chain transfer to monomer reaction and only ppm quantities of these catalysts are required to obtain significant molecular weight reductions.
4-1 The most efficient catalytic chain transfer to date appears in methacrylate polymerizations mediated by BF2-bridged cobaloximes such as bis[(difluoroboryl) dimethyl-glyoximato]cobah(II) (COBF, 1), which lead to macromonomers (2) of the type shown in Scheme 1. These methacrylate macromonomers are very versatile in that they can copolymerize with styrene and acrylate-like monomers to yield comb structures[6], they undergo addition-fragmentation chain transfer in copolymerizations with other methacrylates yielding a,co-telechelic polymers or block copolymers[7], and in the presence of divinyl compounds their copolymerization leads to star polymers.
Considering the versatility of the obtained macromonomers, it is not surprising that several studies have focused on the preparation of macromonomers of functional methacrylates.
2-Hydroxyethyl methacrylate (HEMA, 3) is one of the more important functional methacrylate monomers, because of its hydrophilic nature and the availability of the hydroxy group for isocyanate curing[8,l]. Several reports in the open literature exist where catalytic chain transfer has been used to synthesize and subsequently use HEMA macromonomers[9], but to date no detailed study of the kinetics of the process have been undertaken. It is the aim of this study to investigate the kinetics of the catalytic chain transfer polymerization of HEMA in more detail and to investigate any possible substituent effects.
4-2 4,2 Experim en tal
4.2.1 Materials
The bis(methanol) complex of COBF was prepared by the method of Bakac and
Espenson[10] as detailed in the following section; a single batch of catalyst, of which the purity was determined by measuring the chain transfer constant in bulk methyl methacrylate at 60°C (Cs = 34-10^), was used for all experiments. Methyl methacrylate
(MMA; Aldrich, 99%) was passed through a column of activated basic alumina
(ACROS, 50-200 |im) in order to remove the MEHQ inhibitor. HEMA (Aldrich, 95%) was passed through a column of basic alumina to remove the inhibitor, subsequently polymerized using AIBN under UV light to about 1% conversion, followed by vacuum distillation at 35°C and at a pressure of 0.3 Torr. The purity was assured by 'H NMR.
Ethanol (Absolute), analytical grade THF (Spectrosol, 99.99+%)) and hydroquinone
(Aldrich, 99%) were used without further purification. All solutions were thoroughly deoxygenated by purging with high purity nitrogen (BOC) for at least 1.5 hours prior to use. AIBN (DuPont) was recrystallized twice from methanol and used as initiator.
4.2.2 Synthesis of COBF
The bis(methanol) complex of COBF was prepared by modification of the method of Bakac and Espenson. 90mL of analytical grade methanol was deoxygenated by purging with nitrogen. 1.9g of dimethylglyoxime was placed in a lOOmL flask, purged with nitrogen and then placed under vacuum. 2g of cobalt acetate tetrahydrate was treated in the same manner. 50ml of methanol was transferred to the lOOmL flask with a syringe in order to dissolve dimethylglyoxime. 25 ml of methanol was transferred to the flask containing the cobalt acetate tetrahydrate. Both solutions were
4-3 agitated until clear solutions were obtained, although the dimethylglyoxime solution
was heated to 35°C in order to obtain complete dissolution. The cobalt solution was
then added to the dimethylglyoxime solution using a cannula and a dark red brown
solution immediately formed. This solution was left stirring at ambient temperature for
four hours before the brown liquid was removed from above the dark red precipitate.
The precipitate was washed three times with methanol to remove any remaining
reagents before being dried under vacuum. 50ml of deoxygenated diethyl ether was
added to the flask containing the dried cobaloxime and the solution was cooled to 0°C
using an ice bath. 10ml of borontrifluoride diethyl etherate was added to the solution
and the solution stirred overnight at room temperature. The solvent was removed with a
cannula and the red precipitate washed twice with deoxygenated water and twice with
deoxygenated methanol. The precipitate was dried under vacuum. Because of the
difficulty in analyzing the purity of the cobaloxime, the purity was assured by
measuring the chain transfer constant in pure methyl methacrylate. A value of 29 000
was obtained and this is within the range of values that have been reported in the
literature.
4.2.3 General Polymerization Procedure
Two stock solutions were prepared: (i) an initiator stock solution, and (ii) a
catalyst stock solution. The initiator solution was prepared by dissolution of
approximately -10 mg of AIBN in 70 mL of deoxygenated HEMA (50 mg in the case
of MMA or MMA/ethanol). The catalyst stock solution (ii) was prepared by dissolution
of approximately 1-3 mg of catalyst into 10 mL of solution (i) and subsequent twofold
dilution with solution (i) (in the case of the MMA polymerizations, a tenfold dilution was used). The reaction ampoules, specially modified for use with standard Schlenk equipment, were deoxygenated by repeated evacuation and subsequent purging with 4-4 nitrogen. Five reaction mixtures were then prepared in duplicate, each containing 4.0
mL of solution (i) and 0.10, 0.20, 0.30, 0.40, and 0.50 mL of catalyst solution (ii),
respectively. The reaction ampoules were then sealed and placed in a thermostated
water bath until -5% conversion was attained. Polymerization was stopped by precipitating the reaction mixture into a 1:1 mixture of THF and n-hexane containing a
small amount of hydroquinone (in the case of MMA it was sufficient to place the reaction mixtures into pans containing a few crystals of hydroquinone). The remaining monomer and solvent were removed by evaporation at ambient temperature and pressure followed by complete drying under vacuum.
4.2.4 Viscometry
Absolute monomer viscosities were measured using an Ostwald viscometer (size
A) immersed in a temperature-controlled water bath. The calibration was performed with water at both 40°C and 60°C in order to eliminate error due to thermal expansion of the instrument. The viscosity of HEM A was found to be 3.44 cP and 2.10 cP at 40°C and 60°C, respectively; in the same temperature range MMA has viscosities of 0.45 cP and 0.37 cP. Table 4-1 and 4-2 give the results of the experiments and comparison of the viscosity of the monomers.
4.2.5 UVA/is Spectrometry
UV/Vis spectra of COBF (V-IO'^ mol-L'^) in MMA, HEMA and MMA/ethanol mixtures (1,0.75,0.5 and 0.125 vol% MMA) were obtained at 60°C using a Carey 300
UV/Visible Spectrophotometer with a Peltier rack. Each of the spectra were obtained against a blank containing the same solution in the absence of COBF.
4-5 Table4-1 : Drain times for HEMA and Water through and Ostwald viscometer
(size A), times are in minutes and seconds
Water Water HEMA HEMA
40°C 60°C 40°C 60°C
3:37 2:38 18:05 11:24
3:35 2:38 18:02 11:23
3:36 2:38 18:02 11:21
3:36 2:38 18:02 11:21
18:02 11:23
18:03 11:24
18:05 11:23
18:06
4-6 Table 4-2 : Drain times and viscosities of HEMA, Water and MMA.
Substance Temperature (°C) Time (s) Viscosity (cP)^
Water 40 216 0.655 Water 60 158 0.466 HEMA 40 10841 3.445 HEMA 60 682 2.105
MMA 40 - 0.450
MMA 60 - 0.370 ^The viscosity of water were found in Perry's Chemical Engineers Handbook and that of MMA was found in Forster et al.[17c].The viscosity of HEMA was calculated from the drain times of HEMA and Water at equal temperatures in order to avoid the effect of thermal expansion of the instrument.
4.2.6 General Molecular Weight Characterization
Molecular weight distributions were determined by size exclusion chromatography using a Shimadzu LC-10 AT VP pump, a Shimadzu SIL-IOAD VP Autoinjector, a column set consisting of a Polymer Laboratories 5.0 mm bead-size guard column (50 x 7.5 mm) followed by three linear PL columns (10^ ,10"^ and 10^ A) in a column oven at 40°C in the case of THF eluent (for PMMA samples) and 60°C in the case of dimethyl acetamide (for PHEMA samples), and a Shimadzu RID-1 OA differential refractive index detector. Tetrahydrofuran (BDH, HPLC grade) and dimethyl acetamide (Aldrich, HPLC grade) containing 0.05% LiBr were used as eluents at 1.0 and 0.7 mL/min, respectively. Calibration of the SEC equipment was performed
4-7 with narrow poly(methyl methacrylate) or poly(styrene) standards (Polymer
Laboratories).
4.2.7 Molecular Weight Analysis of PHEMA Samples
In the absence of PHEMA standards and adequate Mark-Houwink constants for our available SEC system, a calibration curve was determined via an indirect manner.
Pulsed laser polymerization[l 1] is an established experimental technique in which propagation rate coefficients are determined from a molecular weight distribution by relating the propagation rate coefficients to a characteristic chain length:
Minf=kp'M-td-mo (4-1
where Mjnf is the low molecular weight inflection point on the w(log M) distribution, kp the propagation rate coefficient, M the monomer concentration, td the time between two laser pulses and mo is the monomer mass. Since kp has been accurately measured recently by Buback and co-workers[l Ic], pulsed laser polymerization under controlled conditions results in a predetermined value of Minf.
Hence by relating the M^f obtained against a styrene calibration curve to the predetermined real values an adequate calibration curve is obtained.
4-8 r-4
0.2 -
-3
0.0- -2
First -0.2 - - 1 Derivative C7) O
-0.4 - -0
--1 -0.6-
—I \ ' 1 " 1 > 1 1 \ i 1 1— -2 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
log(M)
Figure 4-1 Typical molecular weight distribution and its first derivative of PHEMA produced by a pulsed laser polymerization at 20°C. [AIBN] = 1x10'^ mol pulsing frequency = 20 Hz.
Pulsed laser polymerization was carried out using an experimental set-up described extensively in previous publications[12]. Reaction cells (10mm diameter
60mm height) were charged with the HEMA/ethanol solution and photo initiator (AIBN
MO"^mol-L'^), sealed with rubber septa and then deoxygenated using a flow of nitrogen for 5 min. The samples were allowed to equilibrate at the experimental temperature
(20°C) prior to laser exposure. The polymer was precipitated into a THF/«-hexane (1:1) mixture and dried under vacuum at room temperature until the attainment of constant weight. Monomer conversion was always below 1%. The resulting polymers were subsequently analyzed using SEC against a styrene calibration as described above and a
4-9 typical molecular weight distribution and its first derivative are shown in Fig. 4-1. The resulting values for Minf were plotted against those obtained using Equation 4-1 and the results are shown in Fig. 2; the corresponding calibration curve is given by Equation 4-2.
log(MpHema) = 1.1061og(Mpstyrene) " 0.04614 (4-2
'Og (Mpstyrene)
Figure 4-2: Theoretical molecular weight (from PLP experiment) versus those obtained against styrene standards.
4-10 4.3 Results and Discussion
4.3.1 Chain transfer constants
The chain transfer constants of COBF in the bulk free-radical polymerization of
HEMA were determined at 40 and 60°C using both the Mayo[13] (Eq. (3)) and CLD[15]
(Eq. (4)) procedures.
(4-3
-^peak (4-4
In these expressions, DPn is the number average degree of polymerization (here estimated both as Mn/mo and Mw/(2-mo), where Mn and Mw are the number and weight average molecular weights, respectively), X is the fraction of termination by disproportionation, (kt> is the average termination rate coefficient, [R*] the overall radical concentration, Cm the chain transfer constant to monomer, Cs the chain transfer constant to COBF and Apeak the slope of a log number distribution (i.e., ln(P(M)) vs M) taken in the region of the peak molecular weight. These procedures and their respective advantages and disadvantages have been discussed in detail previously[15].
4-11 5.0x10-
4.5x10'-
4.0x10"'-J
3.5x10"'-
3.0x10'-
^ 2.5x10"'-
2.0x10"'-
1.5x10"'-
1.0x10"'-
5.0x10"^-
0.0 1x10"' 2x10"' 3x10^ 4x10' 5x10"* 6x10"'
fCOBFI [HEMA]
Figure 4-3 ; Mayo and CLD plots for the COBF-mediated free-radical polymerization of HEMA at 60°C. (A) ^ :-Apeak-mo; (o) ^ = DPn'^ = 2mo/Mw;
(•) : DP„-^ = mo/M„.
The molecular weight results are summarized in Table 4-3 and the Mayo and CLD plots for the 60°C data are shown in Figure 4-3. Resulting chain transfer constants for HEMA and those for MMA, obtained under similar conditions, are listed in Table 2. Comparison of the MMA values with those of HEMA shows that the latter are about 50 times smaller. This lower value is partly due to the larger propagation rate coefficient (a factor of about 4-5), but also to a lower chain transfer rate coefficient, ktr
(a factor of about 10).
4-12 Table 4-3: Molecular weight characteristics of polymers produced for the chain transfer constant determination of COBF in the bulk free-radical polymerization ofHEMA at 40 and 60°C.'
40X 60°C
[Co]/[M] Mn Mw "•^peak [Co]/[M] Mn "•^peak /lO"^ /lO^ /lO^ /lO-^ /lo-^ /lO^ /lO^ /lO'^
1.18 608 1168 2.37 1.20 150 259 9.77
2.30 139 233 14.5 2.34 91 145 17.2
3.37 75 103 19.5 3.43 63 96 24.4
4.39 63 67 18.8 4.47 51 77 27.2
5.36 77 106 23.2 5.46 42 65 31.9
1.18 172 316 7.4 1.20 88 137 15.4
2.30 141 260 8.2 2.34 74 112 19.2
3.37 91 127 15.8 3.43 61 92 21.2
4.39 91 130 17.9 4.47 44 67 28.8
5.36 58 64 34.4 5.46 39 60 34.4
2.49 132 221 11.2 1.03 93 127 12.0
4.86 70 114 20.2 2.01 78 107 21.8
7.12 47 77 33.8 2.94 65 85 27.4
9.27 58 49 49.9 3.83 64 79 29.6
11.31 32 41 61.2 4.68 49 60 36.5
2.49 106 225 11.9 1.03 125 185 14.8
4.86 78 97 24.7 2.01 88 117 20.8
7.12 58 65 37.6 2.94 64 84 28.0
9.27 30 45 46.8 3.83 59 74 30.9
11.31 27 34 59.0 4.68 58 71 31.5
Listed data are values after correction of the MWD using Eq. 4-2.
4-13 Table 4-4: Summary of chain transfer constants of HEMA and MMA^ at 40 and
60°C measured via the Mayo and CLD methods
Cs
Monomer Temp /°C Mn^ Mw' A^pea, k^
HEMA 60 (4.2±0.8)-10^ (6.1 ± (6.5±L2>10^
40 (3.6±0.7)-10^ (6.2±0.9)-10^ (7.1 ± 0.6>10^
MMA 60 40-10^ 34-10^
40 40-10^ 33-10^
^ MMA data taken from reference [17b]; ^ Chain transfer constant determined by the
Mayo method (Eq. (3)) using DPn = Mn/mo; ^ Chain transfer constant determined by the
Mayo method (Eq. (3)) using DPn = Mw/(2-mo); ^ Chain transfer constant determined by
the CLD method (Eq. (4)).
It has previously been speculated that the rate determining step in the catalytic
chain transfer polymerization of methacrylates could be diffusion-controlled[16] and we
observed the following relationship between monomer viscosity (rjmonomer) and chain transfer rate coefficient (ktr = Csxkp) for a range of methacrylates[17]:
Ca^kp^ rjmonomer - constant (4-5
This relationship has a physical basis in the Smoluchowski expression for diffusion-controlled rate coefficients and the Stokes-Einstein relationship between diffusion coefficients and viscosity[18]. Although this relationship appears to be applicable to most methacrylates studied to date, no direct proof exists that the rate determining step is indeed diffusion-controlled.
4-14 Table 4-5: Summary of kinetic and viscosity data of the COBF-mediated free-
radical polymerization of HEMA and MMA at 40 and 60°C
Monomer Temp/°C c/ kp/(L-mor'-s^) Tlmonomer/cP C S ^ kp X T) rnonomer
HEMA 60 6-10^ 3375*^ 2.10 4.3-10^
HEMA 40 6-10^ 1980^ 3.44 4.1-10^
MMA' 60 34-10^ 833^^ 0.37 10-10^
MMA' 40 33-10^ 497^ 0.45 7.4-10^
Chain transfer constant determined by the Mayo method (Eq. (3)) using DPn =
Mw/(2-mo); ^ Value of propagation rate coefficient of HEMA taken from reference
[11c]; Chain transfer constants and viscosities for MMA taken from reference [17b];
^ Value of propagation rate coefficient of MMA taken from reference [lid]
On the basis of the measured monomer viscosities (i.e., 3.44 cP and 2.10 cP for
HEMA, and 0.45 cP and 0.37 cP for MMA at 40 and 60°C, respectively), and assuming that there is indeed a causal relationship between ktr and monomer viscosity, the lower values for ktr in HEMA may not be surprising. However, as can be seen from the data in Table 3, the values for ktr in HEMA are about 2 times smaller than predicted by Eq.
(4-5). From this result we can conceivably conclude that viscosity effects alone cannot explain the experimental observations and that other effects must play a role. Clearly this argument is only meaningful if the relationship of Eq. (4-5) is correct.
The most important difference between HEMA and MMA (and the series of methacrylates "obeying" Eq. (4-5)) is the presence of the OH group, which is known to form complexes with the Co(II) centre - the initial catalyst is prepared as the bis(methanol) complex [12, 13]. Furthermore, Haddleton and co-workers previously found a significant reduction in the chain transfer constant of MMA when methanol was
4-15 present[7]. In order to explore the role of the hydroxyl group, we further investigated
the catalytic chain transfer polymerization of MMA in the presence of ethanol and
studied possible ligation effects by UV/Vis spectroscopy.
4.3.2 Effect of the hydroxyl functionality
The chain transfer constant of COBF in MMA/ethanol mixtures was measured
over a range of monomer concentrations and the results are summarized in Table 4-6. It
is immediately clear that the chain transfer constant shows a significant decrease as the
concentration of ethanol is increased (considering the purity of the used ethanol, it is
unlikely that this decrease is caused by catalyst poisoning). Since PLP experiments
suggest that kp is insensitive to the presence of ethanol[19], the observed decrease in Cs must be due to a reduction in ktr. It should also be noted that the reduction in ktr caused by the ethanol is a factor of ~2, which is similar to the factor that was unaccounted for in HEMA after taking into consideration the differences in propagation rate coefficient and monomer viscosity.
Table 4-6 : Chain transfer constants of COBF in the free-radical polymerization of
MMA at 60°C with varying ethanol concentrations
[MMA] /(mol-L"0 Mayo Method' CLD Method'
939^ 34-10^
5.36 26-10^ 25-10^
4.02 17-10^ 17-10^
^ Chain transfer constant determined by the Mayo method (Eq. (3)) using DPn =
Mw/(2-mo); ^ Chain transfer constant determined via the CLD method (Eq. (4)); Bulk polymerization of MMA - chain transfer constant taken from reference [17b].
4-16 1.0-
Q) o c: 0.8^ XCOI wo JD < 0.6- •O NQ ) I 0.4-
0.2-
0.0 —I ' 1— —I— —I 350 400 450 500 550 600 Wavelength (nm)
Figure 4-4 : UVA^is absorbance spectra of COBF in various solutions at 60°C;
( ) MMA, ( ) MMA:ethanol 3:1 (v/v), (—) ethanol, ( ) HEMA.
Table 4-7 :Summary of absorbance maxima of COBF in various solutions
Absorbance Maximum Solution (nm)
HEMA 457.5
MMA 444.5
MMA:Ethanol 3:1 (v/v) 457.0
MMAiEthanol 1:1 (v/v) 458.5
MMA:Ethanol 1:7 (v/v) 460.0
Ethanol 460.5
4-17 Since the presence of different axial ligands will change the electronic environment around the cobalt centre, possible OH complexation should be observable in the UV/Vis spectrum of the catalyst. Fig. 4-4 and the data in Table 4-7 clearly show a distinct shift in the absorbance spectrum of COBF in methyl methacrylate when ethanol is added. Furthermore, it can be seen that the absorbance spectrum of COBF in
HEMA is very similar to that in MMA/ethanol, suggesting a very similar electronic environment of the catalyst. Considering this result it is conceivable that the hydroxyl group in HEMA causes a similar reduction in ktr in HEMA polymerization as the ethanol does in MMA polymerization. This would in turn account for the "missing factor 2" in the HEMA data in Table 3.
4,4 Conclusions
The bulk free-radical polymerization of 2-hydroxyethyl methacrylate in the presence of COBF was studied at 40°C and 60°C, and the chain transfer rate coefficients were found to be much smaller than those seen in methyl methacrylate. Although no definite proof exists for the following statement, the observed difference between the
HEMA and MMA data appears to correspond to a six to eightfold reduction because of the higher viscosity of HEMA and a further twofold reduction because of the presence of hydroxyl groups. The former factor is predicted on the basis of Eq. (4-5), which we found to be applicable for a range of methacrylate monomers, and the second factor corresponds to the reduction in ktr of MMA in the presence of ethanol. It should be stressed here that we cannot be certain of this explanation, but clearly the results are consistent with it. Finally, the apparent presence of electronic substituent effects suggests that the rate determining step is not fully controlled by a transport step.
4-18 4,5 References
[1] Adamsons, K,, et al., Oligomers in the evolution of automotive clearcoats:
mechanical performance testing as a function of exposure. Prog. Org. Coat., 1998.
34(1-4): p. 64-74.
[2] Huybrechts, J., et al, Surfactant-free emulsions for waterborne, two-component polyurethane coatings. Prog. Org. Coat., 2000. 38(2): p. 67-77.
[3] Muratore, L.M. and T.P. Davis, Self-reinforcing hydrogels comprised of
hydrophobic methyl methacrylate macromers copolymerized with N,N-
dimethylacrylamide. Journal of Polymer Science, Part A: Polymer Chemistry, 2000.
38(5): p. 810-817.
[4] Yamashita Y: Chemistry and Industry of Macromonomers. Basel: Hiithig &
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[5] For general reviews on catalytic chain transfer polymerization, see
(a) Karmilova, L.V., et al., Metalloporphyrins as chain transfer catalysts in radical polymerization and stereoselective oxidation. Uspekhi Khimii, 1984. 53(2): p.
223-35.;
(b) Davis, T.P., D.M. Haddleton, and S.N. Richards, Controlled polymerization of acrylates and methacrylates. J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1994.
€34(2): p. 243-324.
(c) Davis, T.P., et al., Cobalt-mediated free-radical polymerization of acrylic monomers. Trends Polym. Sci. (Cambridge, U. K.), 1995. 3(11): p. 365-73.
(d) Gridnev, A., The 25th anniversary of catalytic chain transfer. J. Polym. Sci.,
Part A: Polym. Chem., 2000. 38(10): p. 1753-1766.
4-19 (e) Gridnev, A. and S.D. Ittel, Catalytic Chain Transfer In Free Radical
Polymerization. Chemical Reviews, 2001.;
(f) Heuts, J.P.A., G.E. Roberts, and J.D. Biasutti, Catalytic chain transfer polymerization: an overview. Australian Journal of Chemistry, 2002. 55(6 & 7): p. 381-
398..
[6] Cacioli, P., et al., Copolymerization of w-unsaturated oligo(MMA). J. Macromol.
Sci.-Chem., 1986. A23: p. 839.
[7] Haddleton, D.M., et al., .alpha.,.omega.-dihydroxy telechelicpoly(methyl methacrylate) via .beta.-scission (radical addition-fragmentation) chain transfer polymerization by macromonomer chain transfer agents, as prepared by catalytic chain transfer polymerization. Macromol. Chem. Phys., 1996. 197(9): p. 3027-3042.
[8] Heuts, J.P.A., L.M. Muratore, and T.P. Davis, Preparation and characterization of oligomeric terpolymers of styrene, methyl methacrylate and 2-hydroxyethyl methacrylate: a comparison of conventional and catalytic chain transfer. Macromol.
Chem. Phys., 2000. 201(18): p. 2780-2788.
[9] (a)Haddleton, D.M., et al.. Aqueous solution cobalt mediated catalytic chain transfer polymerization. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 1999.
40(1): p. 381-382.
(b)Bon, S.A.F., et al.. Advances in catalytic chain transfer polymerization mediated by cobaloximes. Macromolecular Symposia, 2001. 165(Developments in
Polymer Synthesis and Characterization): p. 29-42.
(c)Haddleton, D.M., et al.. Cobalt-mediated catalytic chain-transfer polymerization in water and water/alcohol solution. Journal of Polymer Science, Part
A: Polymer Chemistry, 2001. 39(14): p. 2378-2384.
4-20 [10] (a)Bakac, A., M.E. Brynildson, and J.H. Espenson, Characterization of the
structure, properties, and reactivity of a cobalt(II) macrocyclic complex. Inorg. Chem.,
1986. 25(23): p. 4108-14.
(b)Suddaby, K.G., D.R. Maloney, and D.M. Haddleton, Homopolymerizations of
methyl methacrylate and styrene: chain transfer constants from the Mayo equation and
number distributions for catalytic chain transfer, and the chain length dependence of
the average termination rate coefficient. Macromolecules, 1997. 30(4): p. 702-713.
[11] (a)Olaj, O.F., I. Bitai, and F. Hinkelmann, The laser-flash-initiated polymerization as a tool of evaluating (individual) kinetic constants offree-radical polymerization. 2. The direct determination of the rate constant of chain propagation.
Makromol. Chem., 1987.188(7): p. 1689-702.
(b)Van Herk, A.M., Pulsed Initiation polymerization as a means of obtaining propagation rate coeficients in free radical polymerizations. Macromol. Theroy. SImul,
2000. 9: p. 433-441.
(c)Buback, M. and C.H. Kurz, Free-radical propagation rate coefficients for cyclohexyl methacrylate, glycidyl methacrylate, and 2-hydroxyethyl methacrylate homopolymerizations. Macromol. Chem. Phys., 1998. 199(10): p. 2301-2310.
(d)Beuermann, S., et al., Critically evaluated rate coefficients for free-radical polymerization. Part 2. Propagation rate coefficients for methyl methacrylate.
Macromol. Chem. Phys., 1997. 198(5): p. 1545-1560.
[12] (a)Coote, M.L., et al., Copolymerization propagation kinetics of styrene and methyl methacrylate - revisited -1 - pulsed laser polymerization study.
Macromolecules, 1997. 30(26): p. 8182-8190.
4-21 (b)Zammit, M.D., et al., Effect of the ester side-chain on the propagation kinetics of alky I methacrylates - an entropic or enthalpic effect? Macromolecules, 1998.
31(4): p. 955-963.
(c)Yee, L.H., J.P.A. Heuts, and T.P. Davis, Copolymerization propagation kinetics of dimethyl itaconate and styrene: Strong entropic contributions to the penultimate unit effect. Macromolecules, 2001. 34(11): p. 3581-3586.
[13] Mayo, F.R., Chain Transfer in the polymerisation of styrene: The reation of solvents with free radicals. Journal of the American Chemical Society, 1943. 65: p.
2324.
[14] (a)Whang, B.C.Y., et al.. Molecular weight distributions in emulsion polymerizations: evidence for coagulative nucleation. Australian Journal of Chemistry,
1991.44(8): p. 1133-7.
(b)Clay, P. A. and R.G. Gilbert, Molecular Weight Distributions in Free-Radical
Polymerizations. I. Model Development and Implications for Data Interpretation.
Macromolecules, 1995. 28(2): p. 552-69.
(c)Christie, D.I. and R.G. Gilbert, Transfer constants from complete molecular weight distributions. Macromol. Chem. Phys., 1996.197(1): p. 403-12.
[15] (a)Moad, G. and C.L. Moad, Use of chain length distributions in determining chain transfer constants and termination mechanisms. Macromolecules, 1996. 29(24): p. 7727-7733.
(b)Heuts, J.P.A., et al., Copolymerization of styrene and methyl methacrylate in the presence of a catalytic chain transfer agent. Macromolecules, 1998. 31(9): p. 2894-
2905.
4-22 (c)Heuts, J.P.A., T.P. Davis, and G.T. Russell, Comparison of the Mayo and
chain length distribution procedures for the measurement of chain transfer constants.
Macromolecules, 1999. 32(19): p. 6019-6030.
[16] Haddleton, D.M., et al., Catalytic chain transfer polymerization (CCTP) of methyl methacrylate. Effects of catalyst structure and reaction conditions on chain transfer coefficient. Macromol. Symp., 1996. Ill: p. 37-46.
[17] (a)Forster, D.J., et al.. Catalytic Chain Transfer Polymerization of Methyl
Methacrylate in Supercritical Carbon Dioxide: Evidence for a Diffusion-Controlled
Transfer Process. Macromolecules, 1999. 32(17): p. 5514-5518.
(b)Forster, D.J., J.P.A, Heuts, and T.P. Davis, Conventional and catalytic chain transfer in the free-radical polymerization of 2-phenoxyethyl methacrylate. Polymer,
1999. 41(4): p. 1385-1390.
(c)Heuts, J.P.A., D.J. Forster, and T.P. Davis, The effects of ester chain length and temperature on the catalytic chain transfer polymerization of methacrylates.
Macromolecules, 1999. 32(12): p. 3907-3912.
(d)Roberts, G.E., et al., Viscosity effects in cobaloxime-mediated catalytic chain- transfer polymerization of methacrylates. Journal of Polymer Science, Part A: Polymer
Chemistry, 2002. 40(6): p. 782-792.
[18] Pilling MJ, Seakins PW. Reaction Kinetics. Oxford: Oxford University Press;
1996.
[19] Morrison BR, Piton MC, Winnik MA, Gilbert RG, Napper DH. Macromolecules
1993;26:4368.
4-23 5 Reversible Addition-Fragmentation Chain Transfer Polymerization of Methyl Methacrylate in Suspension
5.7 Introduction
The advent of nitroxide-mediated (NMP), atom transfer (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization over the past decade has revolutionized polymer synthesis, making increasingly complex structures with controlled architectures readily accessible[l]. The use of these polymerization techniques in water-borne systems potentially provides environmental benefits and a better process control. For these reasons, research into water-borne living radical polymerization techniques has intensified in recent years[2]. In general, it has been found that the application of living radical polymerization techniques to water-borne
(disperse) systems is not straightforward and that some control over the molecular weight distribution is lost. This is mainly caused by problems intrinsic to the colloidal nature of the systems and which include the aqueous phase partitioning of the "control agent", transport of the control agent from monomer droplets to latex particles in the case of emulsion polymerization and droplet/particle stability. Progress has been made
5-1 in solving some of these problems, but still many issues remain to be solved[2,3]. A more detailed account of the research into these areas is presented in chapter 3.
In this chapter the suspension polymerization of methyl methacrylate (MMA) at
70°C mediated by the RAFT agent 2-cyanoprop-2-yl dithiobenzoate (CPDB) is investigated and compared with the bulk polymerization of MMA at 60°C mediated by the same RAFT agent. ,
5.2 Experimental Section
5.2.1 Materials
Methyl methacrylate (Aldrich, 99%) was passed through a column of activated basic alumina (ACROS, 50-200 jum) in order to remove the MEHQ inhibitor.
Hydroquinone (Aldrich, 99%), 1% aqueous solution of poly(sodium methacrylate) (ICI), sodium dihydrogen orthophosphate (Ajax, 99+%), tetrahydrofuran (THF; BDH, HPLC grade) and dimethyl acetamide (DMAc; Aldrich, HPLC grade) were used as received.
2,2'-Azobis(isobutyronitrile) (AIBN; DuPont) was recrystallized twice from methanol and used as initiator. The RAFT agent 2-cyanoprop-2-yl dithiobenzoate was synthesized from bis(thiobenzoyl) disulfide using a procedure described in Figure 5-1 [4] and its purity checked using NMR.
5.2.2 Bulk Polymerization
Firstly, stock solutions containing MMA, CPDB and AIBN were prepared with
[CPDB]/[AIBN] = 5, and were subsequently diluted with MMA to obtain several solutions with different [MMA]/[CPDB] ratios. Each of these solutions was distributed over 5 vials (2 mL) after which they were deoxygenated by purging with high purity nitrogen, sealed and placed in a thermostatted water bath at 60°C. Vials were removed 5-2 after 1, 2, 4, 6 and 8 hours and polymerization was stopped by pouring the reaction mixture into pre-weighed pans containing a few milligrams of hydroquinone. After evaporation under ambient conditions for twelve hours in a fume cupboard the samples were dried under vacuum at 40°C until constant weight was obtained. Conversions were determined by gravimetry.
12 % Sodium Methoxide in methanol s s I I + NaCI 80°C 24 hours s-s salt removed by filtration methanol removed under vaccuum
diethyl ether over aqueous soltion \ '= s NaCI \ HCI HS Na'^'
diethyl ether over aqueous soltion S, ^ HoO NaOH Na'^ HS
K3Fe(CN)6 S. \\ ^ Ma's ambient temperature, dimer recovered by filtration
1.5x excess AIBN ethyl acetate
reflux, 18 hours ethyl acetate revmoved under vaccum, chromatography over silica 2:3 ethyl acetate:hexane
Figure 5-1: Synthesis of RAFT agent 2-cyanoprop-2-yl dithiobenzoate.
5-3 5.2.3 Suspension Polymerization
All suspension reactions were carried out in a 500 mL wide neck flask modified
to include four 20 mm radial baffles equipped with a four bladed turbine (40 mm x 10
mm) driven by a 200-2000 rpm Heidolph overhead stirrer, heating mantle with
feedback control, nitrogen inlet and a condenser as shown in Figure 5-2. Two stock
solutions were made up consisting of: (I) 190 g of distilled deionised water, 10 g of 1%
poly(sodium methacrylate) solution and 2 g of sodium dihydrogen orthophosphate; (II)
50 g of methyl methacrylate containing 5x10"^ M AIBN and CPDB in the range of 0-55
xlO'^ M. The aqueous phase (solution I) was added to the reactor and purged with nitrogen for 1 hour at ambient temperature. The organic phase (solution II) was purged with nitrogen for 1 hour in a separate vessel before being added to the reactor. The reaction was considered to have started (time = 0 s) once the reactor reached the operating temperature of 70°C. Stirring speeds of 800 rpm were used in all polymerizations. Samples were taken at regular intervals using standard syringe techniques to monitor monomer conversion by gravimetry and molecular weight evolution by size-exclusion chromatography.
5.2.4 Block Copolymer Synthesis
Solutions of the polymer prepared by suspension polymerization, the block- forming second monomer and AIBN were prepared in toluene or N,N- dimethylacetamide, depending on the nature of the second monomer. These solutions were then divided over five reaction vials, deoxygenated by at least three freeze pump thaw cycles and placed in a water bath at 60°C. Vials were removed at 2, 4, 8, 12 and
24 hours and polymerization was stopped by pouring the reaction mixture into pre- weighed pans containing a few milligrams of hydroquinone. After evaporation under
5-4 ambient conditions for 24 hours in a fume cupboard, the samples were dried under vacuum at room temperature until a constant weight was obtained. Conversions were determined by gravimetry.
Figure 5-2:Reactor design and cross section of reactor
5-5 5.2.5 Molecular Weight Analysis
Molecular weight distributions were determined by size exclusion chromatography with a Shimadzu LC-10 AT VP pump, a Shimadzu SIL-IOAD VP
Autoinjector, a Shimadzu RID-1 OA differential refractive index detector and a column set consisting of a Polymer Laboratories 5.0 jam bead-size guard column (50 x 7.5 mm) followed by three linear PL columns (10^ ,10"^ and 10^ A) in a column oven at 40°C with
THF eluent or at 60°C with DMAc as the eluent. In all cases the elution rate was 1.0 mL/min. Calibration of the SEC equipment was performed with narrow poly(methyl methacrylate) or poly(styrene) standards (Polymer Laboratories, molecular weight range:
200- 1.6x lO^gmor^).
5.2.6 Particle Size Distributions
Particle size distributions were determined via optical miscroscopy. Samples were prepared by first washing the particles in water for 12 hours to remove any of the remaining surfactant after which the particles were filtered and dried. The particles were then placed on a microscope slide and photographed using a Leica DM LB Optical
Microscope in reflection mode. Particle size distributions were determined using the
Scion Image™ image processing software. 550-1150 particles were counted for each determination of the particle size distribution.
5-6 5,3 Results and Discussion
5.3.1 Bulk RAFT Polymerizations
The bulk polymerization of MM A mediated by CPDB at 60°C was investigated before carrying out any suspension polymerization. A range of initiator concentrations was used and a [CPDB]/[AIBN] ratio of 5:1 was maintained. The results are summarized in Table 5-1 and the dependence of the number-average molecular weight
(Mn) on conversion (X) is shown in Figure 5-3. In all cases, living behaviour is observed, and except for series V (i.e., the lowest [CPDB] concentration), the slopes of the lines are about equal to [MMA]o/[CPDB]o, as expected for a living radical polymerization. Furthermore, it should be noted in Figure 5-3 shows that none of the series appears to originate in Mn « 0 for x = 0, which suggests that in the early stages
CPDB acts as a "conventional" chain transfer agent in MMA polymerization and that
RAFT behaviour is only observed when the CPDB has been converted into a polymeric
RAFT agent.[5]
There are apparently only three other studies in which a methacrylate was polymerized in the presence of CPDB, i.e., the solution polymerization of MMA in benzene [6], the bulk polymerization of 3-[tris(trimethylsilyloxy)silyl)]propyl methacrylate[7] and the solution polymerization of 6-[4-(4'-methoxyphenyl)- phenoxyjhexyl methacrylate in toluene [8], Only for the latter polymerization sufficient data are available to draw conclusions about the low-conversion molecular weight behaviour and this appears to be similar to what is observed in this study.
5-7 Table 5-1: Experimental Results of AIBN-Initiated Bulk Methyl Methacrylate Polymerizations at 60°C mediated by CPDB.
# Time/h X/% Mn / g-mol PDI Derived Results^ la 1 9.2 7.1x10^ 1.40 lb 2 23.8 12.6x10^ 1.22 kp[R']= 1.6x10'^ s"' Ic 4 75.4 23.3x10^ 1.18 Mn,x-.o = 5.5x 10^ g-mor' Id 6 90.0 30.6x10^ 1.20 (based on a-e) le 8 97.9 29.7x10^ 1.26 Ila 1 8.3 8.5x10^ 1.47 lib 2 17.3 16.0x10^ 1.25 kp[R'] = l.lxlO-^s-^ lie 4 63.7 34.5x10^ 1.17 Mn.x^o = 5.4x 10^ g-mor^ lid 6 63.4 33.5x10^ 1.12 (d omitted from analysis) He 8 92.8 52.7x10^ 1.18 Ilia 1 3.8 10.5x10^ 1.47 lllb 2 35.3 20.2x10^ 1.28 kp[R']-4.8xlO'^s'^ IIIc 4 37.6 40.0x10^ 1.17 M„,x^O = 8.2X 10^ g-mor^ Illd 6 51.1 43.0x10^ 1.15 (b omitted from analysis) Ille 8 72.8 64.2x10^ 1.17 IVa 1 3.5 18.7x10^ 1.45 IVb 2 8.1 26.0x10^ 1.40 kp[R'] = 1.9x10-^ s-^ IVc 4 23.7 55.0x10^ 1.17 Mn,x-^o=12.7x 10^ g-mol"^ IVd 6 30.3 63.8x10^ 1.17 (e omitted from analysis) IVe 8 57.9 99.4x10^ 1.13 Va 1 1.6 69.3x10^ 1.52 Vb 2 4.1 88.1x10^ 1.54 kp[R'] = 9.1 xlO-^-^ Vc 4 8.5 120.0x10^ 1.48 Mn,x->o = 66.1 x 10^ g-mol"^ Vd 6 8.3 106.4x10^ 1.44 (d omitted from analysis) Ve 8 21.7 179.1x10^ 1.36 ^ kp[R*] is the slope of a first order kinetic plot, Mn,x->0 the intercept of the Mn vs X plot. In these analyses the obvious outliers were not considered as indicated. For all Polymerizations: [CPDB]/[AIBN] = 5. [CPDB] : Series I: 21.2 mM; Series II: 15.9 mM, Series III: 10.6 mM, Series IV: 5.3 mM; Series V: 0.5 mM 5-8 The observed trend of a decrease in PDI from an initial relatively high value is
consistent with the argument that the system first behaves like a conventional chain
transfer system, and afterwards the RAFT process assumes control, with a resultant
decrease in PDI. To obtain a rough estimate of the chain transfer constant of the initial
RAFT agent a Mayo plot was constructed with the values of Mn,x^o(i.e. the intercept of
an Mn v's conversion plot; it should be a reasonable estimate of the chain transfer
derived Mn value at 0% conversion). An apparent chain transfer constant to CPDB of
about 8 is obtained. This is in reasonable agreement with a value of 13 reported by
Moad et al[9] for the same system.
The observed increase in PDI at high conversions can be attributed to an
increase in bimolecular termination reactions. At higher conversions a peak appears in
the molecular weight distribution at twice the chain length of the main peak. (See Figure
5-4). This is consistent with the combination or disproportionation product of two propagating radicals or the disproportionation product of a propagating radical with the
intermediate RAFT radical. Similar reports have been reported for other systems, such
as styrene (STY; also predicted from modeling)[ 10], dimethylacrylamide and 3-
[tris(trimethylsilyloxy)silyl]] propyl methacrylate. The results for these last two
examples suggest that the formation of the higher molecular weight polymer depends on both the type and concentration of RAFT agent; the higher molecular weight peak is more pronounced for lower concentrations of RAFT agent, and this is also seen in this
study.
5-9 200000 n
150000-
o 100000-
50000 -
—r- 20 40 60 80 100 X%
Figure 5-3 iMolecular weight evolutions of AIBN-initiated bulk polymerizations
of methyl methacrylate mediated by CPDB at 60°C. (•) I, (O) II, (A) III, (A) IV,
.(•) V. Conditions as listed in Table 5-1. Solid lines are the theoretical
predictions based on [MMA]o/[CPDB]o.
2.5 n
2.0- Time
1.5-
1.0-
O 0.5- /
0.0-
-0.5 —r 1 3.0 3.5 4.0 4.5 5.0 5.5
log(Mg mor')
Figure 5-4: Typical evolution of the molecular weight distributions in the bulk
RAFT polymerization of MMA mediated by CPDB. The data correspond to
entries la - le in Table 5-1. Molecular weight distributions are scaled with
conversion.
5-10 5.3.2 RAFT Polymerization in Suspension
To determine if the presence of the aqueous phase affects either the rate of polymerization or the molecular weight distribution in a suspension polymerization, an experiment was first conducted in which the same solution of monomer, CPDB and initiator was concurrently polymerized in bulk and in suspension at 70°C. At the same time intervals both polymerizations were sampled and the rate and molecular weight results are compared in Figures 5-5 and 5-6.
These data show that the RAFT behaviour in bulk is very similar to that in suspension, with possible slight discrepancies being caused by the fact that the bulk polymerization solutions reached the polymerization temperature more quickly than the suspension polymerization. The evolutions of the molecular weight distributions in general follow the theoretical predictions fairly well, but a sudden increase in the molecular weight and a subsequent leveling off can be seen around and above 40% conversion. Because similar behaviour has been observed for the subsequent suspension polymerizations at similar initial CPDB concentrations (the [CPDB]o = 19 mM data are shown later in figure 5-9), this behaviour is probably characteristic of lower RAFT agent concentrations. However, at this point, there are no obvious reasons to ascribe this observation to any heterogeneous phase effects, and it is possible that this effect is caused by changing reaction dynamics of the RAFT process at higher conversions.
The effect of changing [CPDB] on the RAFT polymerization in suspension at
70°C was then investigated and the results are shown in Figures 5-7 to 5-9. Figure 5-7 shows that, at least for low and medium conversions, the [CPDB] does not affect the rate of polymerization, with all RAFT experiments having a rate similar to that of the conventional free-radical polymerization (i.e., [CPDB] = 0 mM). At higher conversions,
5-11 the rates of polymerization in systems with lower amounts of RAFT agent (i.e., those systems with polymers of relatively high molecular weight and corresponding higher viscosities) seem to increase, which may indicate the presence of a gel effect. The development of the molecular weight distributions, for which an increase in the molecular weight is observed, followed by a broadening (see figure 5-8 and 5-9), is consistent with this argument. This observation only seems significant at low initial
CPDB concentrations.
1.0-
0.8-
0.6-
0.4 H o
0.2-J O •
0.0 100 200 300 400 500 Time (minutes)
Figure 5-5:First-order kinetic plots of the AIBN-initiated radical polymerization
of MMA at 70°C mediated by CPDB in bulk (•) and in suspension (o). General
conditions: [AIBN] = 4.4 mM and [CPDB] = 15 mM (based on bulk MMA).
5-12 D,000 H
60,000 o o
40,000 o 20,000
1.4 o o 1.2 o c*
1.0. ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' • I 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Figure 5-6 : Molecular weight and polydispersity index evolutions of the AIBN- initiated radical polymerization of MMA at 70°C mediated by CPDB in bulk (•) and in suspension (o). General conditions: [AIBN] = 4.4 mM and [CPDB] = 15 mM (based on bulk MMA). Solid line is the theoretical prediction based on
[MMA]o/[CPDB]o.
2.3-
2.0-
1.5- A V• o o
0.5-
0.0 —T —T —I— —I— —I— —1 0 50 100 150 200 250 300 350 Time (Minutes)
Figure 5-7: First-order kinetic plots of the AIBN initiated suspension polymerization of MMA at TOT mediated by CPDB. In all cases [AIBN] = 7 mM. [CPDB] :(T)0 mM, (A)and (V)19 mM, (•) and (0)37 mM, (A)57 mM.
5-13 3.5-
3.0-
2.5-
2.0-
1.5-
1.0-
0.5-
0.0-
—1—1—I—1—1—1—I—1—I—1—1—I—I—I—I—1—I—1—r 2 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8
log(A//g mol')
Figure 5-8:Typical molecular weight distributions for a CPDB-mediated suspension polymerization at 70 °C. In the shown example [CPDB] = 37 mM, the first sample was taken after 0.5 hours, with every subsequent sample following at an hour's difference.
50000
40000
30000
20000
10000-
1.4- o V
—I— 20 40 60 80 100 A'%
Figure 5-9 : Molecular weight and polydispersity index results for the CPDB- mediated suspension polymerization of MMA at 70°C. Conditions as in Figure
5, solid lines are the theoretical predictions based on [MMA]o/[CPDB]o.
[CPDB] :(T)0 mM, (A)and (V)19 mM, (•) and (0)37 mM, (A)57 mM.
5-14 It is somewhat unclear as to the exact nature of what is happening at high
conversion. Chains are less likely to terminate, but if they do, they will probably do this with small, newly formed radicals rather than with re-activated dormant chains.
Furthermore it could be envisaged that the RAFT process itself may be less effective due to the fact that it involves two long chain reactants which both suffer restrictions in their motions. High conversion electron spin resonance studies on bulk systems may shed some further light into this problem. Finally, it is interesting to note that in these suspension polymerizations all the Mn-X curves appear to originate in Mn ~ 0 for x = 0, which is conceivably caused by the fact that in general [CPDB]/[MMA] ratios used in these systems are higher than those used in the bulk studies used listed in table 5-1.
5.3.3 Particle Size Distributions
All RAFT suspension polymerizations were well-behaved in the sense that in all cases less than 5% coagulum was formed. The particle size distributions of the final products were determined by optical microscopy and image analysis. As can be seen from Table 5-2, it was found that the initial amount of RAFT agent has a very large effect on the particle size distribution. In general it can be concluded that the higher the initial [CPDB], the smaller the final particle size. This result can conceivably be explained by the fact that due to the lower viscosity of the particles/droplets in the case of higher [CPDB] the droplets are more easily broken up by the high shear stresses.
Furthermore, the dormant chains with a RAFT endgroup may have surface-active properties or undergo specific interactions with the stabilizing agents, as was suggested by Monteiro and co-workers in RAFT miniemulsion polymerization systems[ll].
Representative microscope images of the particles are shown in Figure 5-10.
5-15 Table 5-2 : Particle size results for the AIBN-initiated suspension polymerization of methyl methacrylate at TOT mediated by CPDB. [RAFT] / mM Xfinal / % Mn / g-mol"' PDI Dn / |im Dw/D„ 19 98 45.9x10^ 1.17 550 1.34 19 92 42.3x10^ 1.19 510 1.39 37 93 21.6x10^ 1.09 210 1.71 37 94 25.1x10^ 1.12 225 1.33 53 83 13.5x10^ 1.08 160 1.46
Figure 5-10: Microscope images of particles produced via RAFT suspension polymerization of MMA at TOT using [CPDB] of 53 mM. The scale bar indicates a length of 1000 [im.
5-16 Figure 5-11. Microscope images of particles produced via RAFT suspension
polymerization of MMA at TOT using [CPDB] of 37 mM. The scale bar indicates
a length of 1000 |im.
Figure 5-12 : Microscope images of particles produced via RAFT suspension polymerization of MMA at TOT using [CPDB] of 19 mM. The scale bar in all cases indicates a length of 1000 jim.
5-17 5.3.4 Chain Extension Experiments
To probe the presence of RAFT end groups in the final product of the
suspension polymerization, these polymers were subjected to further polymerization
with methyl methacrylate, styrene (STY) and 2-hydroxyethyl methacrylate (HEMA),
respectively. The reaction conditions and results of these experiments are summarized
in Table 5-3.
Table 5-3 : Reaction conditions and summary of the results of the chain extension of poly(methyl methacrylate) by RAFT suspension polymerization (Mn = 20.9x10^ gmol'^)
Reaction Time / h X/% Mn / gmol'^ PDI Conditions 3.59gPMMA- 0 0.0 20.9x10^ 1.09 RAFT
4.50 g MMA 2 18.6 22.6x10^ 1.08
7.47 g Toluene 5 33.7 25.5x10^ 1.07
4 mg AIBN 9 61.2 33.1x10^ 1.06
T = 60 24 84.6 33.6x10^ 1.07
4.11 gPMMA- 0 0.0 20.9x10^ 1.09 RAFT
4.50 g STY 2 14.4 21.4x10^ 1.11
8.70 g Toluene 5 25.8 22.3x10^ 1.10
4 mg AIBN 9 34.7 24.5x10^ 1.08
T = 60 24 42.6 26.7x10^ 1.07
12.69 gPMMA- 0 0.0 20.9x10^ 1.09 RAFT
20.0 g HEMA 2 36.8 38.4x10^ 1.12
40.0 g DMAc 5 47.8 47.8x10^ 1.09
23 mg AIBN 9 57.3 59.3x10^ 1.08
T = 60 24 68.2 85.7x10^ 1.02
5-18 All the reactions resulted in a simultaneous linear increase in molecular weight
and a reduction in PDI, which is indicative of a living radical polymerization. An
examination of the molecular weight distributions, for example those for the chain
extension with HEMA (see figure 5-11) shows that only a very small fraction of dead polymer was formed in the initial suspension polymerization, i.e., the small hump on the
low-molecular weight side of the distribution at high conversion. The hump at the high molecular weight side of the distribution is indicative of some irreversible bimoleculecular termination by recombination of growing radicals, forming polymers with twice the molecular weight of the major peak
4-
£o (DD
oOX ) 2 -
0-
—I ' 1 ' 1 ^ T" —I—•—1—•—T" 3.6 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 log (A//g mol"')
Figure 5-13 : Molecular weight distributions for the chain extension of a PMMA suspension polymer (Mn = 20.9x103 g mol-1) with HEMA at different reaction times.
5-19 5.4 Conclusions
The RAFT polymerization of methyl methacrylate mediated by CPDB was studied in bulk and in suspension, and in general, up to relatively high conversions, a good control over number average molecular weight and polydispersity indices are obtained in agreement with theoretical predictions. For lower [CPDB] the initial molecular weights are higher than those predicted from theory, which is indicative of a relatively low chain transfer constant of the initial CPDB RAFT agent; a value of about
8 was estimated for the chain transfer constant of CPDB at 60°C. It was shown that the kinetic and molecular weight characteristics of suspension and bulk RAFT polymerizations are not significantly different and that over a relatively wide range of
[CPDB], stable suspension polymerizations could be performed with a relatively good molecular weight control and low PDI (< 1.25) at high conversion. A small gel effect was observed for lower [CPDB] which resulted in a slight broadening of the molecular weight distribution and a rate increase. The dormant character of the suspension polymers has been proved by successful chain extensions with methyl methacrylate, styrene and 2-hydroxyethyl methacrylate, respectively. Finally, it was found that the final particle size decreases with increasing concentration of the raft agent, an effect which is probably due to the easier break-up of droplets in the early stages of the polymerization, which is in turn is due to the lower molecular weight of the polymer and thus reduced viscosity in the monomer phase.
5-20 5.5 References
[1] Matyjaszewski, K.; Davis, T. P. Handbook of Radical Polymerization; Wiley-
Interscience: New York, 2002.
[2](a)Cunningham, M.F., Living/controlled radical polymerizations in dispersed phase
systems. Progress in Polymer Science, 2002. 27(6): p. 1039-1067.
(b) Qui, J., B. Charleux, and K. Matyjaszewski, Controlled/Living Polymerization in
aqueous media: homogeneous and heterogeneous systems. Prog. Polym. Sci., 2001. 26:
p. 2083-2134.
[3] Prescott, S.W., et al, RAFT in emulsion polymerization: what makes it different?
Australian Journal of Chemistry, 2002. 55(6 & 7): p. 415-424.
[4] Thang, S.H., et al., A novel synthesis of functional dithioesters, dithiocarbamates,
xanthates and trithiocarbonates. Tetrahedron Lett., 1999. 40(12): p. 2435-2438.
[5]Bamer-Kowollik, C., et al.. Kinetic investigations of reversible addition fragmentation chain transfer polymerizations: Cumyl phenyldithioacetate mediated
homopolymerizations of styrene and methyl methacrylate. Macromolecules, 2001.
34(22): p. 7849-7857.
[6]Rizzardo, E., S.H. Thang, and G. Moad, Synthesis of dithioester chain-transfer
agents and use of bis(thioacyl) disulfides or dithioesters as chain-transfer agents in
radical polymerization, in PCTInt. Appl. (Commonwealth E.I. Du Pont De Nemours
and Company). W098/10478 1998
[7] Saricilar, S., et al. Reversible addition fragmentation chain transfer polymerization
of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate. Polymer, 2003. 44(18): p. 5169-
5176.
5-21 [8] Hao, X., et al., Living free-radical polymerization (reversible addition- fragmentation chain transfer) of 5-[4-(4'-methoxyphenyl)phenoxy]hexyl methacrylate: A route to architectural control of side-chain liquid-crystalline polymers. Journal of
Polymer Science, Part A: Polymer Chemistry, 2003. 41(19): p. 2949-2963.
[9]Moad, G., et al.. Living free radical polymerization with reversible addition - fragmentation chain transfer (the life of RAFT). Polym. Int., 2000. 49(9): p. 993-1001.
[10] Bamer-Kowollik, C., et al.. Modeling the reversible addition-fragmentation chain transfer process in cumyl dithiobenzoate-mediated styrene homopolymerizations: assessing rate coefficients for the addition-fragmentation equilibrium. Journal of
Polymer Science, Part A: Polymer Chemistry, 2001. 39(9): p. 1353-1365.
[11] Monteiro, M.J., M. Hodgson, and H. De Brouwer, The influence of RAFT on the rates and molecular weight distributions of styrene in seeded emulsion polymerizations.
Journal of Polymer Science, Part A: Polymer Chemistry, 2000. 38(21): p. 3864-3874.
5-22 6 Conclusions
In the experimental section of this thesis two aspects of controlled radical polymerization were investigated. Both techniques showed that it was possible to create novel structures using controlled radical polymerization, in particular the use of catalytic chain transfer polymerization and reversible addition fragmentation chain transfer polymerization.
In chapter 4 the catalytic chain transfer constant of bis(difluoroboro dimethyl glyoximato)cobalt(II) was measured for 2-hydroxyethyl methacrylate at both 60°C and
40°C. It was found that for both systems the chain transfer constant was about 6x10^ and about a factor of 2 lower than might be expected from previous experience. The effect of the hydroxyl group was further examined by measuring the chain transfer constants of methyl methacrylate in the presence of ethanol. The chain transfer constants showed that the presence of ethanol resulted in a reduction of the chain transfer constant by about a factor of 2. Examination of the effect of ethanol on the
UV/Visible absorbtion spectra of COBF in MMA revealed that the absorbtion spectra shifted by significant amount with the introduction of ethanol. Comparison of these
6-1 spectra showed that the peak position of COBF in ethanol was very similar to that seen in 2-hydroxyethyl methacrylate, suggesting that the corrordination of the axial ligands on the cobalt chelate was via hydrogen on the hydroxyl group and that the prescence of the ligand changes the speed of catalytic chain transfer.
In chapter 5 the applicability of RAFT polymerization of MMA in aqueous suspension was examined. It was found that the rate of polymerization of MMA was very similar to that found in bulk and the MMA CPDTA system is not retarded. Particle morphology was found to be sensitive to the amount of RAFT agent present with particle sized reduced by almost an order of magnitude between 53 and 19mMolL"^ of CPDTA. It is postulated that the RAFT agent and the macro raft agents might be surface active and thus form an additional barrier to particle agglomeration. Additional effects involve the lowering of the viscosity of the reaction medium before the particle identity point, thus increasing the ease of droplet breakup and reducing the final particle size. It was found that there was very little dead polymer found in the final product (which was more than ninety percent conversion) and that the product would chain extend easily with methyl methacrylate, styrene and 2-hydroxyethyl methacrylate. Gel effects play a roll in the loss of molecular weight control at high conversion as it seems that molecular weight control may be lost at high conversion.
The synthesis of macromonomers and end functional species was possible in high conversions and reproducible molecular weight. These species show interesting behaviour and physical properties and further study will no doubt be useful in the development of new and novel technologies. Further work in this area will focus on applications of micro spheres synthesized by dispersed phase polymerization in the production of block copolymers, core shell structures for use in fields as diverse as high
6-2 impact polymers, surface active films and as well as medical uses as implants and drug delivery systems.
6-3
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