The University of New South Wales Faculty of Engineering School of Chemical Sciences and Engineering Centre for Advanced Macromolecular Design (CAMD)
An investigation into the Synthesis of Poly(co-maleic anhydride/iso-butyl vinyl ether) with RAFT polymerisation
A Thesis in Industrial Chemistry by S. C. Lea
Submitted for the degree of Masters of Science in Industrial Chemistry October 2006 Table of contents
List of symbols...... i Chapter 1. Introduction ...... 1 References...... 3
Chapter 2. Chemistry and mechanism of radical polymerisation...... 4 2.1 Free radical polymerisation ………………………………………...4 2.1.1 Initiation ...... 4 2.1.2 Propagation...... 6 2.1.3 Termination ...... 7 2.1.4 Chain transfer...... 8 2.1.4.1 Transfer agents...... 9 2.1.4.2 The transfer constant Ctr and the re-initiation constant Cri... 9 2.1.5 Number and weight average molecular weight ...... 10 2.1.6 Kinetics of FRP ...... 11 2.1.7 Kinetic chain length and degree of polymerisation ...... 14 2.1.8 Effect of the transfer agent on the degree of polymerisation.. 15 2.1.9 Molecular weight distribution...... 16 2.2 Living radical polymerisation (LRP)……………………………...18 2.3 Features of living radical polymerisation(LRP)…………………..18 2.3.1 LRP processes: Nitrogen Mediated Polymerisation (NMP)... 20 2.3.2 LRP processes: Atom Transfer Radical Polymerisation (ATRP) ………………………………………………………………………..21 2.3.3 LRP processes: Reversible Addition-Fragmentation chain Transfer (RAFT) ...... 21 2.3.3.1 Mechanism of the RAFT polymerisation...... 22 2.3.3.2 Choice of the RAFT agent...... 25 2.3.3.3 Side reactions in RAFT polymerisation ...... 27 2.3.3.4 Copolymerisation using the RAFT system ...... 28 2.3.3.4.1 Random copolymers...... 28 2.3.3.4.2Diblock and triblock copolymers...... 29 References...... 30 Chapter 3. Copolymerisation systems in radical polymerisation...... 32 3.1 Definitions………………………………………………………...32 3.2 Copolymer composition…………………………………………..33 3.2.1 The Terminal model ...... 34 3.2.2 Explicit and Implicit Penultimate Models...... 37 3.2.3 The Q-e approach to the theory of reactivity...... 39 3.3 Influence of the reaction medium…………………………………41 3.4 Alternating copolymers…………………………………………...42 3.4.1 Copolymers of vinyl ethers and maleic anhydride...... 43 3.4.1.1 Maleic anhydride (MAn)...... 44 3.4.1.2 Vinyl ethers...... 45 3.4.1.3 Co (maleic anhydride-alt-methyl vinyl ether)...... 46 3.4.1.4 The synthesis of co (maleic anhydride-alt-iso-butyl vinyl ether)……………………………………………………………….47 References……………………………………………………………...48
Chapter 4. The free radical polymerisation of iso-butyl vinyl ether and maleic anhydride...... 50 4.1 Experimental procedure...... 50 4.1.1 Purification of reagents and solvents...... 50 4.2 Polymerisation procedures ...... 51 4.2.1 Polymerisation in Schlenck tubes...... 51 4.2.2 Polymerisation in the presence of DDM as a chain transfer agent…………………………………………………………………………..51 4.2.3 In situ NIR-FTIR spectroscopy polymerisation...... 52 4.3 Polymerisation kinetics ...... 52 4.3.1 In situ FT- NIR spectroscopy ...... 52 4.3.2 1H NMR spectroscopy ...... 54 4.3.3 Gravimetric analysis ...... 55 4.4 Copolymer characterisation...... 56 4.4.1 13C NMR spectroscopy ...... 56 4.4.2 Size Exclusion Chromatography ...... 56 4.5 Results and discussion...... 58 4.5.1 FRP in dioxane as a solvent...... 58 4.5.2 FRP in MEK and Acetone as solvents...... 61 4.6 Reliability of FT-NIR spectroscopy for the evaluation of the conversion...... 66 4.7 Reliability of NMR spectroscopy for the determination of the conversion...... 70 4.8 Experiments with dodecylmercaptan (DDM) as a chain transfer agents ...... 72 4.9 Conclusions ...... 75 References...... 75
Chapter 5. Use of RAFT agents to control the radical copolymerisation of MAn and IBVE...... 76 5.1 Experimental procedure...... 76 5.1.1. Purification of reagents and solvents...... 76 5.2 RAFT agents synthesis...... 76 5.2.1. Synthesis of benzyl dithiobenzoate (BDTB) ...... 76 5.2.2. Synthesis of 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) ...... 76 5.2.3. Synthesis of dibenzyl trithiocarbonate (DBTTC) ...... 77 5.3 Polymerisation procedures ...... 77 5.3.1. Polymerisation in Schlenck tubes...... 77 5.3.2. In situ NIR-FTIR spectroscopy polymerisation...... 78 5.4 Polymerisation kinetics ...... 78 5.4.1. In situ FT-NIR spectroscopy ...... 78 5.4.2. 1H NMR spectroscopy ...... 78 5.4.3. Gravimetric analysis ...... 78 5.5 Copolymer characterisation...... 79 5.5.1. 13C NMR spectroscopy ...... 79 5.5.2. Size Exclusion Chromatography…………………………………79 5.6 Use of RAFT agents for the control of the molecular weight...... 79 5.6.1. 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) and dibenzyl-trithiocarbonate (DBTTC) as RAFT agents...... 83 5.7 BDTB as a RAFT agent ...... 89 5.7.1. RAFT polymerisation with BDTB in polar solvents...... 89 5.7.1.1 Acetone...... 89 5.7.1.2 Ethyl acetate and N, N, dimethyl acetamide...... 97 5.7.1.3 Characterisation of the two phases...... 98 5.7.2. RAFT polymerisation with BDTB in 1,4 dioxane as a solvent ………………………………………………………………………105 5.7.3. Tetrahydrofuran as a solvent ...... 117 5.8 Investigation into the colour alteration of a polymerisation mixture containing BDTB as a RAFT agent...... 125 5.9 Conclusions ...... 127 References...... 129 List of symbols and abbreviations
Ac = acetone AM = acrylamide
CM = chain transfer constant for chain transfer to the monomer
CS = chain transfer constant for chain transfer to the solvent or chain transfer agent DMAc = N, N dimethyl acetamide DEE – diethyl ether
DPn = number average degree of polymerisation
DPw = weight average degree of polymerisation d = number of chains generated by radical-radical termination Eth Ac = ethyl acetate f =initiator efficiency I = generic initiator molecule I = generic primary radical
I0 = integral of the monomer peak at initial time in NMR spectroscopy
It = integral of the monomer peak at time t in NMR spectroscopy IBVE= iso-butyl vinyl ether k = rate constant = fraction of chains termination by disproportionation mM = molecular weight of the monomer mRAFT = molecular weight of the RAFT agent M = generic monomer molecule [M] = concentration of a generic monomer molecule M = generic monomer radical
M0= molecular weight of the monomer
Mi = molecular weight of a chain of length i
M n = number average molecular weight
M w = weight average molecular weight MA = Methacrylates MAH= Maleic acid. MAn =Maleic anhydride MMA = Methyl Methacrylate P = polymer chain
i Pn = generic growing n-meric chain PDI = polydispersity index
Rp = rate of polymerisation S = chain transfer agent Sty = styrene v = reaction rate. wi = weight of a chain of length i wP = mass of the polymer wS = mass of the sample
M = weight fraction of monomer in the reaction mixture at time t=0 conversion
ii Cinzia Lea Introduction
Chapter 1. Introduction
Membranes are selective separation barriers, they occur naturally or they can be synthesised. Synthetic membranes have been produced for applications ranging from desalination, dialysis, filtration, to gas separation. Depending on the specific application, different membrane morphologies and driving forces are required for the separation process to take place.
Microfiltration (MF) and ultrafiltration (UF) membrane technology has been used extensively in drinking water treatment and also for the recycling of municipal, industrial and commercial wastewater, due to the increasing shortage of readily available fresh water. These separation processes are based on sieving; under the action of pressure the membrane pores retain the solid foreign matter contaminating the water.
While the membranes pore sizes have a large influence on the membrane retention characteristics, other parameters play an important role too, among them the fouling rate of the membrane. This parameter can be defined as the volume of product filtered up to reversible or irreversible blocking of the membrane. The membrane material, the pores structure and the surface properties all influence both the physical and chemical adsorption properties of the membrane, which in turn, influence its fouling rate. While membranes made of hydrophobic materials offer better chemical, thermal and biological stability, it has been demonstrated that hydrophilic polymers are generally less susceptible to adsorptive fouling1.
By introducing hydrophilic groups onto the surface of a hydrophobic polymeric membrane it is possible to combine the advantages offered by both hydrophobic and hydrophilic materials. Surface modification can be achieved in many ways; the coating of the material with hydrophilic surfactants2,3,4 and the graft-polymerisation of hydrophilic monomers5,6,7 are among the most popular procedures. Another method consists in blending small amounts of an additive bearing desirable functional groups with a host polymer melt or solution8,9,10,11. During polymer processing, physical phenomena like diffusion, spontaneous phase segregation and shear are likely to induce the migration of the additive to the host surface. This procedure leaves the properties of the membrane core substantially unchanged and since it can be integrated into a melt extrusion process it has the advantage of being cost effective.
1 Cinzia Lea Introduction
A previous study conducted at the University of New South Wales12 explored the possibility of introducing surface modification to a commercial membrane made out of a hydrophobic polymer, polyvinylidene fluoride (PVdF). This method consisted of adding small amounts of Gantrez® (poly (co methyl vinyl ether-alt-maleic anhydride )) to the original membrane formulation. Gantrez® contains carbonyl groups that in virtue of their polarity confer hydrophilicity to this compound. Preliminary results showed that this method had the potential to impart hydrophilic character to the host membrane.
The additive used in the present work is poly (co iso-butyl vinyl ether-alt-maleic anhydride). The difference between this copolymer and Gantrez® consists in the fact that the comonomer of maleic anhydride is in this case iso-butyl vinyl ether rather than methyl ether. Iso-butyl vinyl ether is a liquid and therefore it is easier to handle respect to its lower homologous which is a gas. We prepared poly (co iso-butyl vinyl ether-alt- maleic anhydride ) through the radical process; by using a RAFT agent we obtained a polymer with differing molecular weights and low polydispersity. The next stage consisted in blending the copolymer/additive through a PVdF polymer matrix and then study the relation between the additive molecular weight and its ability to migrate to the host surface and impart to it hydrophilic properties.
Chapter 2 commences by reviewing mechanisms, basic concepts and definitions of the free radical polymerisation, which was employed to synthesise the copolymer. It then goes on to describe the main features of living radical polymerisation and in particular of the Reversible Addition-Fragmentation chain Transfer (RAFT) radical polymerisation process. In this chapter we also discuss the ways this technique has revolutionised the free radical polymerisation process by extending its possibilities and applications.
Chapter 3 deals with the principles of the radicalic copolymerisation and reviews the various models developed with the aim of predicting the product composition. It then focuses on the properties of maleic anhydride and iso-butyl vinyl ether, their homopolymers and on some of the copolymers, which have these monomers as one of the components.
Chapter 4 reports on the procedures adopted and the experiments carried out to synthesise poly(co MAn-alt-IBVE) through the free radical process (FRP). It looks at the kinetics of this reaction and at the characteristics of the polymer produced. At the end of the chapter a set of experiments were carried out with a mercaptan as a chain
2 Cinzia Lea Introduction
transfer agent and its effect on the copolymer molecular weight and PDI has been analysed.
Chapter 5 finally explores ways of preparing the same copolymer with various molecular weights and low polydispersity (< 1.5) through the Reversible Addition- Fragmentation chain Transfer (RAFT) radical polymerisation process. We investigated the efficacy of various RAFT agents: benzyl dithiobenzoate, 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) and dibenzyl trithiocarbonate in providing living conditions to the alternating copolymerisation of MAn and IBVE.
This project succeeds in identifying conditions that enable to control the radical copolymerisation of the two comonomers by using RAFT agents, however, side reactions have been found to take place alongside the formation of the alternating copolymer. This affects the repeatability of the experimental results and the yield of the reaction. In the course of this study, experiments were carried out with the aim to identify sources of interference and when possible remove them.
Such emphasis on the synthesis of the copolymer/additive, allowed little time to be allocated for the development of the second part of the project, relative to the incorporation of the copolymer product into the hydrophobic matrix. Although some techniques for the surface analysis of the modified polymer were set up and some polymer blends were tested, the results obtained were not considered conclusive and therefore they have not been included in this dissertation.
References
[1] J. M. Laine, J. P. Hagstrom, M. M. Clark, J. Mallevialle; J. AWWA 80, 61 1989. [2] A. G. Fane, C. J. D. Fell; Desalination 62, 117, 1987. [3] K. J. Kim, A. G. Fane, C. J. D. Fell; Desalination 70, 229, 1988. [4] A. Maartens, P. Swart, E. P. Jacobs; J. Colloid Interface sci. 221, 137, 2000. [5] M. Ulbricht, G. Belfort; J. Membr. Sci.111,193 1996. [6] M. Yamagishi, J. Crivello, G. Belfort; J. Membr. Sci.105, 249 1995. [7] J. Pieracci, J. Crivello, G. Belfort; J. Membr. Sci.156,223 1999. [8] F. Garbassi, M. Morra, E. Occhiello, Polymer surfaces, physics to technology. New York,: Wiley 1994. [9] D. M. Brewis, D. J Briggs; Polymers, 22, 7 1981. [10] C. M. Chan; Polymer surface modification and characterisation. New York: Hanser, 1994. [11] H. Lee, L. A. Archer; Polymer, 43, 2721, 2002. [12] E. Floyd; Honors thesis, UNSW, 2001.
3 Cinzia Lea Chemistry and mechanism of radical polymerisation
Chapter 2. Chemistry and mechanism of radical polymerisation
2.1 Free radical polymerisation
Radical polymerisation is one of the most commonly used processes for the production of commercial high molecular weight polymers. This reaction proceeds through the chain reaction of radical species. A radical is a molecule fragment bearing an unpaired electron, it is generated by homolitic scission of a covalent bond; the presence of the unpaired electron confers to it a high energy state which makes it extremely reactive. A radical polymerisation starts with the Initiation reaction, during which radicals form, and ends up with their extinction in the Termination reaction. In between these two events a multitude of Propagation reactions takes place, which is responsible for the growth of the polymeric chains1.
2.1.1 Initiation
Initiation takes place when a primary radical turns a monomer molecule into a radical able to propagate and form a macromolecule. Primary radicals can be produced in many ways:
5 Through the self-initiating ability of monomers like butadiene and styrene 2;
5 Through thermal decomposition of an unstable molecule appositely added to the system, called initiator and represented in Scheme 2. by I. Commonly used initiating agents are organic or inorganic peroxides, azo and diazocompounds mono and disulphides. The molecule of 2,2’ azobis(2-isobutyronitrile) dissociates releasing
nitrogen with dissociation rate coefficient kd and rate of dissociation vI•, as shown in scheme 2.1;
5 Through photoinitiators, compounds that decompose upon irradiation with electromagnetic radiation (UV and visible light). Peroxides and azo-compounds are examples of photoinitiators that can also undergo thermal decomposition;
5 Through redox initiators in the aqueous medium.
4 Cinzia Lea Chemistry and mechanism of radical polymerisation
I 2 I
CH 3 CH3 kd CH2 H CN 3 N CH3 2 CH 3 + N2 CN CN CN
d[ I ] -2 d[ I ] vI = ==2 f kd [ I ] dt dt
Scheme 2.1. Formation of primary radicals for a generic initiator and for 2,2’ azobis(2-isobutyronitrile) with corresponding kinetic equation.
The factor f in scheme 2.1 is the initiator efficiency and it represents the fraction of primary radicals that effectively initiate macromolecular growths. The primary radicals so formed promptly react with the monomer M with rate of initiation vi and related constant ki:
ki I + M P1 CH3
CH3 CH 3 + R CH3 R CN CN
vi = ki [ M] [ I ]
Scheme 2.2. Reaction of initiation through primary radical deriving from the generic initiator, from 2,2’ azobis(2-isobutyronitrile) with corresponding kinetic equation.
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2.1.2 Propagation
Once the monomer has been activated through the initiation reaction, more rapid additions of monomer units will follow, accompanied by the simultaneous progression of the active centre onto the latest unit of the polymer chain. It is assumed that the reactivity of each chain towards the monomer is independent from the length of the n- • meric growing chain Pn ; therefore a unique value of the kinetic constant kp is assigned to the propagation events which take place with rate of propagation vp.
kp Pn + M P n+1
CH 3 CH 3
CH 3 R + R CH R CN 3 CN R
CH 3 CH 3 n-1
CH 3 R + n R CH CN 3 R CN R R v = k [ P ][ M ] p p 1
Scheme 2.3. The propagation reaction with its corresponding kinetic equation.
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2.1.3 Termination
Active growing chains can be stopped in many ways: by irreversible chain transfer reactions, by reaction with a primary radical or with other radicals present in the system as impurities. However, the most important termination mechanism is the reaction between two radical chains; this encounter can lead either to termination by
Combination or by Disproportionation, described respectively by the rate constants kt,c and kt,d. In the former case the product is an inactive chain with a number of monomer units given by the sum of the units present on each active chain. In the latter case an inactive macro-monomer and an inactive chain are formed, as a result of the removal of a hydrogen atom situated in respect to the carbon bearing the radical. Although most monomers undergo both termination processes, in some cases the nature of the monomer dictates which process is the most favoured one. In the case of Methyl Methacrylate, for instance, the steric impedance caused by the presence of the methyl group in the position causes the disproportionation reaction to prevail1.
Termination by Combination:
kt,c Pm + Pn Pm+n CH CH3 CH3 3 H3C
+
O OCH3 O O OCH3 O OCH 3 OCH3 Termination by Disproportionation:
kt,d Pm + Pn Pm + Pn
CH 3 CH 2 H 3C H 3C
+ +
O OCH3 O OCH O OCH 3 3 O OCH3
Scheme 2.4. The termination reaction for MMA as the monomer; the combination and the disproportionation mechanism.
The overall kinetic equation is the following :
2 vt = 2 kt [ P ] and the overall rate constant of termination kt , is equal to the sum of kt,c and kt,d.
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2.1.4 Chain transfer
As previously mentioned, polymeric radicals can also terminate by reacting with molecules called Transfer Agents. Species present in the system such as impurities, the solvent, the monomer, or a substance added in small amounts in order to regulate the molecular weight can behave like transfer agents. Scheme 2. illustrates the reaction of the polymeric radicals with the transfer agent AB; in the AB molecule A may represent an atom of hydrogen, an atom of halogen or a group of atoms. This reaction is characterised by two steps: Step 1 is the transfer reaction, distinguished by rate constant ktr and Step 2 is the re-initiation reaction with rate constant kri. During Step 1 the polymeric radicals transfer their active centre to the transfer agent, generating an inactive chain and a new radical. Depending then upon its stability, the new radical can react with monomer molecules and therefore initiate a new chain, as illustrated in Step 2. As a result, the transfer reaction shortens the average life of the active polymeric chains, while keeping the kinetic chain uninterrupted. The polymer generated has a significantly lower molecular weight, as it will be discussed in the kinetics section.
Step 1. Transfer:
ktr Pn + A BAPn + B
vtr = ktr [ P ] [ AB ] Step 2. Re-initiation:
kri B + M P1
vri = kri [ M ] [B ]
Scheme 2.5. The mechanism of chain transfer.
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2.1.4.1 Transfer agents
During the transfer reaction, the extraction of the A-part of the chain transfer agent takes place in conjunction with the production of a B radical. Thermodynamic considerations suggest that elements contributing towards the stability of the B radicals increase the likelihood of a substance to act as a transfer agent. As a consequence, the ease of removal of the A group will increase, for instance, at increasing number of substituents on the aliphatic carbon atom it is bonded to; this is in virtue of the formation of radicals characterised by increased stability due to resonance. However, the higher the stability of the radical B formed, the less likely it is to react with monomer molecules and therefore re-initiate the polymerisation reaction, this can lead to Retardation or to Degenerative Transfer; as discussed below. An increase of the operational temperature usually favours the transfer reaction over the propagation reaction, since the difference between the activation energy of the transfer process and the one of the propagation process is positive (Etr-Ep>0).
2.1.4.2 The transfer constant Ctr and the re-initiation constant Cri
The transfer and the re-initiation step compete for the consumption of monomer with the propagation reaction, the overall effect of the transfer agent on the polymerisation can be expressed through Ctr = ktr/kp and Cri= kri/kp, respectively the transfer and the re- initiation constants. The transfer constant is a measure of the effectiveness of the transfer agent: the higher the value of Ctr the lower the amount of transfer agent required for obtaining a targeted molecular weight3,4. The re-initiation constant is a measure of the reactivity of the radical B with the monomer; if Cri is low, the radical is very stable and the re-initiation reaction may not take place leading to the extinction of the polymerisation reaction. In this case the transfer agent behaves like an Inhibitor. When the re-initiation reaction is very slow with respect to the transfer reaction the substance
AB is defined as a Retardant. As a result, the concentration of B will increase and the termination reactions of these radicals with themselves or with other radicals present in the system will become predominant. This phenomenon is denominated Degenerative Transfer; it is characterised by a decrease in the amount of polymeric radicals in the system, which ultimately leads to a decrease in the polymerisation rate. In the ideal chain transfer process the radical B has a reactivity which is comparable to that of the
9 Cinzia Lea Chemistry and mechanism of radical polymerisation
polymeric radical; in such conditions their concentration will not be affected significantly and therefore the rate of polymerisation will not vary appreciably1.
2.1.5 Number and weight average molecular weight
So far we have seen how, in radical polymerisation, the occurrence of certain reactions is influenced by the operating conditions adopted. Growing radicals, furthermore, behave individually and undertake a sequence of events, which can differ sensibly from one another. In this scenario it is predictable that each chain is characterised by a distribution of differing number of structural units and therefore differing chain length, degree of polymerisation and molecular weight. For this reason average values of such parameters are adopted to characterise the product. The number average molecular weight M n is the total weight of the sample divided by the number of molecules in the sample. The weight average molecular weight M w is defined as the sum of the weights of the chains with a certain molecular weight multiplied by their molecular weight and divided by the total weight of the sample1:
8 8 niMi Wi i=1 i=1 M = = (2.1) n 8 8 ni ni i=1 i=1 8 8 2 wiMi niMi i=1 i=1 M = = (2.2) w 8 8 wi niMi i=1 i=1
ni is the number of chains of length i, wi is the weight of chains of length i, Mi is the molecular weight of chains of length i.
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2.1.6 Kinetics of FRP
The development of a global kinetic scheme is essential for understanding the association between process parameters (temperature, components concentration, time) and polymer characteristics (molecular weight, molecular weight distribution, polydispersity, conversion). This enables to set up experimental conditions that favour the synthesis of a polymer with the desired properties5. Ideally a comprehensive scheme should take into consideration the contributions given by each reaction to the overall process, however, depending on the experimental conditions adopted, some events are less likely to impact on the final properties of the polymer and they can be overlooked. This approach leads to the development of a simplified comprehensive kinetic model called the Classical Scheme which is built on the assumptions shown below and on the kinetics associated to each reaction step (Scheme 2.)1,5:
Assumptions: (a) Reaction steps compose the overall polymerisation process; (a) Each step represents an irreversible reaction; (b) Reactions involving the transfer and the termination between growing macroradicals and primary radicals are negligible; (c) Intramolecular transfer and depropagation are negligible; (d) The disappearance of monomer caused by the initiation reaction is negligible
respect to the amount consumed in the propagation reaction (vp » vi); (e) All macroradicals display equal reactivity, regardless of their length; (f) At low conversion values, when diffusion phenomena can be considered negligible, the propagation and the termination kinetic constants of the growing
radicals kp and kt are independent from their chain length and from the value of the conversion; (g) The rate of disappearance of radicals is equal to their rate of formation (Steady
state assumption) and ki » kd.
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Initiation:
kd I 2 I d[ I ] -2 d[ I ] v I = ==2 kd [ I ] dt dt
ki I + M P 1
vi = ki [ M] [ I ] Propagation:
kp Pn + M Pn+1 v = k [ P ][ M ] p p 1 Termination:
kt,d Pm + Pn Pm + Pn
Pm+n kt,c 2 vt = 2 kt [ P ] Transfer:
ktr Pn + A BPn A + B
vtr = ktr [ R ] [ AB ]
kri B + M R1
vri = kri [ M ] [B ]
Scheme 2.6. A summary of the reactions characterising FRP.
The rate of polymerisation, vp, corresponds to the rate of consumption of the monomer. This is given by the sum between the rate of initiation and the rate of propagation, however, based on assumption (e), vp » vi and therefore:
dM Rp= - = vp= kp[ P ][ M ] (2.3) dt
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From the radicals mass balance equation and based on the steady state assumption: d[ P ] = v - v = k [ I ][ M] - 2 k [ P ]2 = 0 (2.4) dt i t i t
d[ I ] = vd - vi = vd - ki[ I ][M] = 0 (2.5) dt
When we combine the two equations and substitute the kinetic equation of the thermal initiation process, vd=2fkd[I], we obtain:
dM kp 1/2 1/2 Rp= - = 1/2 (2fkd) [ I2 ] [ M ] (2.6) dt kt
When we substitute for the fractional conversion to polymer, :
[M]0-[M] = [M]0 and considering that the volume contraction is negligible, equation (2.6) becomes: k d p 1/2 1/2 Rp= = 1/2 (2fkd) [ I2 ] [ M ]0(1- ) (2.7) dt kt This equation is able to describe effectively the rate of polymerisation for high efficiency initiators and at low conversions. It predicts that the rate of polymerisation is directly proportional to the concentration of the monomer and to the square root of the concentration of the initiator. It also shows that the rate of polymerisation is inversely proportional to the value of the conversion.
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2.1.7 Kinetic chain length and degree of polymerisation
The simple kinetic scheme obtained in the previous paragraph allows to deduce the kinetic chain length , defined as the average number of monomer molecules which has added at each initiating event. At low values of the conversion, when the concentration of the monomer and the initiator can be considered to be constant and in the absence of transfer events:
k [ M ] p vp/vi = 1/2 (2.8) kt 1/2 (2fkd[ I2 ])
This equation predicts that the chain length is directly proportional to the concentration of monomer and inversely proportional to the square root of the concentration of the initiator.
The degree of polymerisation D p is linked to the chain length; it expresses the average number of molecules contained in each polymer chain. At low values of conversion and it is given by:
1/2 k k t [ M ] D = v /v = p (2.9) p p t (k +(1/2k ) (2fk [ I ])1/2 t,d t,a d 2
It can be noted that the two parameters coincide when the termination events are due to disproportionation. In this case, in fact, the rate at which the macroradicals disappear equals the rate at which they form. When termination is caused by combination, the formation of a dead polymeric chain happens at the expense of two macroradicals, thus the value of xn corresponds to half the value of .
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2.1.8 Effect of the transfer agent on the degree of polymerisation
As discussed in 2.1.4, the presence of a transfer agents in the polymerisation medium does not cause a change in the radical concentration so long as kri»ktr,. It will impact, however, on the value of the molecular weight, since transfer reactions shorten the life of the chains, as illustrated in the following scheme:
ktr Pn + A BPn A + B
vtr = ktr [ P ] [ AB ]
kri B + M P1
vri = kri [ M ] [B ]
The Mayo equation (2.10) provides a means for the evaluation of the molecular weights in the presence of the transfer agent. The equation expresses the degree of
polymerisation DPn as a function of the rates of propagation and termination:
1 (1+ kt [ P ] [ S ] = + CM + CS (2.10) k [ M ] [M] DPn p
represents the fraction of termination by disproportionation, CM the chain transfer constant for chain transfer to monomer ( ktr,M/kp), CS the chain transfer constant for chain transfer to the solvent or to a chain transfer agent ( ktr,S/kp), and [S] the concentration of chain transfer agent. This equation is often used for the measurement of CS for a chain transfer agent; for high values of CS, it is possible to overlook the other terms and calculate the degree of polymerisation as a linear function of [S]/[M].
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2.1.9 Molecular weight distribution
As already discussed in 2.1.5 the polymer generated with the FRP process is characterised by a distribution of molecular weights. In order to calculate this parameter we first define the r-moment of the distribution of the degree of polymerisation Qr: 8 Q = ir n (2.11) r i=1 i
ni is the fraction of chains containing n monomeric units. The number and weight
DP DP average degree of polymerisation n and w are given by: 8 i ni i=1 Q1 ( 2.12 ) DPn = 8 = Q0 ni i=1
8 2 i ni i=1 Q2 ( 2.13 ) DPw = 8 = Q1 i ni i=1 where Q0=1. We now introduce a probability factor representing the probability of a radical to propagate rather than to terminate:
vp = ( 2.14 ) vp + vt + vtr
Here vt and vtr represent respectively the sums of all the termination and of all the transfer events. is independent of the chain length and is a constant for the steady state assumption and at low conversions ( this latter condition assures that the values of the concentrations keep constant). If we assume that all the termination events can be ascribed to disproportionation reactions and that transfer reactions are due to one agent only, then (1-) represents the probability of a chain to terminate. The numerical fraction of chains with a number i of monomer units is represented by the numerical distribution function ni:
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(i ) ni = ( ) ( 2.15 ) While the weight distribution function is given by:
(i ) 2 wi = ( ) ( 2.16 )
By replacement of equation (2.15) in equations (2.12) and (2.13) we obtain:
1 DP = ( 2.17 ) n 1
1+ DP = ( 2.18 ) w 1
DPw PDI = = 1+ ( 2.19 ) DPn Equation 2.15 shows how the probability of formation of chains with an i-number of monomer units decreases for increasing values of i, leading to the numerical predominance of the short chains. From equation 1.16 it can be seen, instead, that the contribution of the short chains to the weight distribution function is not relevant. As a consequence, for long chains (vp»vt, vtr) equation 2.14 shows that tends to 1; the value of the polydispersity index PDI, therefore tends to 2. PDI provides an indication of how homogeneous the length of the chains is in the system.
By applying the same procedure to the case of a chain with a chain length P formed exclusively by combination of two chains of length m and n and introducing the condition that m+n=P, the following equation for PDI is obtained:
DPw PDI = = 1 + ( 2.20 ) 2 DPn
This equation shows that in the case of termination generated solely by combination, for long chains, the polydispersity index tends to 1.5, thus narrower distributions are achieved respect to the previous case in which termination was caused solely by disproportionation. This is a consequence of the statistical coupling of 2 chains with different lengths. In the more common case of both combination and disproportionation
17 Cinzia Lea Chemistry and mechanism of radical polymerisation
taking place it can be demonstrated that the value of the PDI for long chains varies between 1.5 and 2. It can also be demonstrated that, when a chain transfer agent is added to the system, the polydispersity index does not affect the distribution in the case of termination by disproportionation, however this parameter increases in the case of combination.
2.2 Living radical polymerisation (LRP)
Introduction
Living polymerisation has attracted remarkable interest in polymer science and engineering in recent years. Szwarc6 first introduced the term living to describe the behaviour of the anionic polymerisation of Styrene and Butadiene. He noticed that the mentioned systems displayed the ability to resume the polymerisation with the same rate when a new batch of monomer was added, furthermore the number average molecular weight M n increased linearly with conversion. The first phenomenon indicates that the number of active chains keeps constant, the second one is symptomatic of the absence of chain transfer reactions. He concluded that, in such processes, reactions leading to chain- breakage are absent, eg no termination and transfer events take place. It was demonstrated that in such systems free ions coexist with inactive aggregated ions and the two species undertake rapid interchange and the theoretically predicted MW and MWD are achieved.
2.3 Features of living radical polymerisation(LRP)
Radical polymerisation is characterised by the fact that active chains are continuously generated, they propagate for 5-10 s and then undergo termination by radical-radical reaction. In ideal living processes all chains form at the same time at the beginning of the polymerisation, they propagate at the same rate and do not undergo termination. This is obtained by introducing into the system specific reagents, which are able to temporarily suppress the reactivity of the growing radicals in the early stages of the polymerisation and therefore counteract all processes leading to irreversible chain termination7. Such reagents, also called mediators, reversibly deactivate the growing chains through a reversible deactivation (termination) in Nitrogen Mediated Polymerisation process (NMP) and Atom Transfer Radical Polymerisation (ATRP) and through reversible (degenerative) chain transfer reaction in Reversible Addition-
18 Cinzia Lea Chemistry and mechanism of radical polymerisation
Fragmentation Transfer (RAFT) 8. The deactivated (dormant) and the active centres rapidly interchange; ensuring that active centres which formed at the same time grow in turn, therefore forming chains of similar length. At any given time in LRP the concentration of the dormant species is conveniently dominant over the concentration of radicals, this condition ensures that termination reactions are suppressed.
kda Pn +X (Y) P n X + (Y) ka +M
k p
Scheme 2.7. Activation and deactivation process in LRP.
X-(Y) deactivates the active growing species R with a rate constant of deactivation kda.
The process is reversible and R X can regain its status of active centre by reacting with Y or by decomposing under the action of radiation. It will then be able to propagate with a constant of propagation kp. In line with what has been discussed so far living radical polymerisation denotes a process in which:
Active chain ends are retained;
Chain growth starts at the beginning of the polymerisation and continues until all the monomer is consumed;
The polymer produced has a molecular weight which increases linearly with conversion. Ideally, the number average molecular weight is given by the ratio between the concentration of converted monomer over the initial concentration of mediator I: [M]0 t M n = mM (2.21) [I]0 Negative deviations from this theoretical value may indicate that transfer reactions take place, positive deviations may be characteristic of inefficient initiation or chain coupling.
Poisson distribution closely describes the polydispersity, provided that the initiation process is instantaneous9 and that the exchange between dormant and active species is slow:
19 Cinzia Lea Chemistry and mechanism of radical polymerisation
DPw/DPn 1+1/DPn (2.22)
According to this equation PDI tends to 1 with increasing conversion. An opposite trend is observed when chain transfer reactions become dominant.
The plot of ln[M]0/[M] vs time can be used to check on the reliability of the process. For a process which is first order with respect to the monomer concentration the plot is expected to be linear. Positive deviations may indicate slow initiation, negative deviations may indicate termination or deactivation of the catalyst.
Nitroxide Mediated Polymerisation (NMP), Atom Transfer Radical Polymerisation (ATRP) and Reversible Addition-Fragmentation chain Transfer (RAFT) are considered to be the most effective living radical polymerisation techniques available to date.
2.3.1 LRP processes: Nitrogen Mediated Polymerisation (NMP)
In the Nitrogen Mediated Polymerisation process (NMP) the species X-Y in Scheme 2 is an alkoxyamine derivative (an alkylated nytroxide) which decomposes on heating to generate an initiating radical and a capping nitroxide radical. The nitroxide radical mediates the living radical polymerisation. By nature, mediating radicals do not react with one another, are unable to initiate the polymerisation and do not undergo termination by disproportionation with growing radicals; they are also called persistent radicals. The best examples of these stable nitroxide mediators are: TEMPO (2,2,6,6 tetramethyl piperidinyl-1-oxy) derivatives (1), the family of arenes (2) and phosphonate derivatives (3). TEMPO and its derivatives were the first compounds developed as mediators for radical polymerisation; today they are mainly used for the living polymerisation of styrene derivatives10. The last two classes of alkylamines have been demonstrated to be very versatile, being able to control the polymerisation of acrylates, acrylamides, 1-3 dienes and acrylonitrile derived monomers11. The process is compatible with waterborne systems, however it has the disadvantage of requiring high operating temperature, making use of relatively expensive mediating agents and being sensitive to the presence of oxygen.
20 Cinzia Lea Chemistry and mechanism of radical polymerisation
O N O N
O N P OEt O OEt
(1) (2) (3)
2.3.2 LRP processes: Atom Transfer Radical Polymerisation (ATRP)
The name atom transfer radical polymerisation refers to the step that is critical for the establishment of living conditions in this technique. The deactivating species/persistent n+1 radical is represented by a transition metal complex with a suitable ligand Mt –Y / ligand. This catalyst reacts with a growing radical, undergoes a one electron reduction and generates a dormant species by transfer of a pseudo halogen. The presence of an initiator with a transferable pseudo halogen is also required. The success of this technique can be ascribed to the constant research into catalyst formulations. Different transition metals (predominantly from group VI and VIII-XI) have been combined with various kinds of ligands (mono or multi-dentate) in order to suit the monomer of interest. A definite advantage associated with this technique is represented by the fact that it is compatible with waterborne systems, it is functional over a wide temperature range and it is tolerant to some extent to the presence of oxygen. Its downsides are represented by the necessity to remove the catalyst from the final product in order to avoid interference with its final use and to prevent the polymer degradation. Care is also required in order to avoid interactions between the catalyst and the monomer.
2.3.3 LRP processes: Reversible Addition-Fragmentation chain Transfer (RAFT)
This process is one of the latest additions to the field of living radical polymerisation. The mediating agent in this case reacts by addition fragmentation chain transfer. In the mid 1980s the first reports appeared on the use of macromonomers12, allyl sulphides13, allyl bromides14, allyl peroxides15 vinyl ethers16 and thionoesters17 as addition- fragmentation chain transfer agents for the control of free radical polymerisation. Later, it was found that simple compounds containing the thiocarbonylthio moiety were much
21 Cinzia Lea Chemistry and mechanism of radical polymerisation
18 more effective at imparting a living behaviour to these systems than the previous ones . The RAFT process has demonstrated to be quite a versatile technique; it has been successfully carried out to control the FRP of a variety of vinyl monomers, in particular, it is very effective in controlling the polymerisation of fast propagating monomers. Operating conditions include low temperatures19, while initiation is provided by thermal initiators, UV20 or -radiation21. Reactions can be conducted in bulk and in a range of reaction media, including supercritical carbon dioxide22 and protic solvents like water23,24 and alcohol25. RAFT has proved to be suitable in dispersed media, in emulsion and microemulsion polymerisation26,27,28 where ATRP and NMP have failed. The process has also been shown to be compatible with the syntheses of monomers bearing
-OH, -SO3 and -CO2H functionality.
2.3.3.1 Mechanism of the RAFT polymerisation
The key step in this technique is represented by a sequence of addition fragmentation equilibria as shown in Scheme 2.829. After the initiation step, at the early stage of the polymerisation, the RAFT agent, a thiocarbonylthio compound in this instance, reacts with the active polymeric chain forming the adduct (A). This in turn undergoes fragmentation, with formation of a macro RAFT agent and a new initiating radical. For living conditions to be established it is believed that the rapid exchange between the active propagating radicals Pn and Pm and the dormant thiocarbonylthio compound plays a fundamental role by providing equal probability to all chains to grow in turn. For this system the transfer coefficient is given by Ctr = ktr/kp, as for transfer agents in free radical polymerisation, however, the rate constant for chain transfer (ktr) takes into account the fragmentation step and the reversibility of the process. Assuming steady state conditions and no side reaction of the intermediate radical7:
k kf ktr = add k-add + kf For RAFT systems the Mayo method does not provide an accurate estimation of this constant, due to the high value of Ctr.
22 Cinzia Lea Chemistry and mechanism of radical polymerisation
Initiation:
ki I + M Pn
Reversible chain transfer:
k kf S S R add Pn S S R Pn S S Pn + + R M k k-f -add Z kp Z Z
RAFT agent (A) (M)
Re-initiation:
kri R + M R M
Chain equilibration:
k P S S P k S S R - m n Pm S S Pm + + Pn k M k Z - kp Z Z
RAFT agent (A') (M)
Termination:
kt P + P m n dead polymer
Scheme 2.8. Mechanism of the RAFT polymerisation.
23 Cinzia Lea Chemistry and mechanism of radical polymerisation
In general, for a RAFT agent to be effective the following conditions must be realised:
5 The C=S double bond must be highly reactive (high kadd);
5 The radical adduct (A) must promptly undergo fragmentation kf»k-add;
5 The radical R formed as a result of the fragmentation must be able to efficiently re-
initiate the polymerisation (high kri).
The interaction of the substituent Z with the C=S double bond can either activate or deactivate the RAFT agent toward free radical addition. In general, substituents that stabilise the intermediate radical (A) increase the growing radical addition rate. However, the choice of a Z group has to take into consideration the fact that a highly stabilising substituent would slow down the rate of fragmentation of the adduct radical and therefore cause unwanted retardation. Depending on the nature of Z we can distinguish four classes of thiocarbonylthio compounds:
1. Dithioesters; 2. Trithiocarbonates; 3. Dithiocarbonates (xanthates); 4. Dithiocarbamates,
They originate when Z is respectively: an aryl/alkyl group (1), substituted sulphur (2), substituted oxygen (3) or substituted nitrogen (4). In the case of Styrene, which is considered to be a radical with moderate reactivity, the chain transfer constant decreases when going from dithioesters to dithiocarbamates; this is in line with what discussed above. The low reactivity of dithiocarbonates and dithiocarbamates can be ascribed to the fact that the delocalisation of the oxygen and nitrogen lone pairs with the C=S double bond increases the compound stability. These substituents are highly effective for highly reactive monomer radicals, such as vinyl esters. The dithiocarbonates and dithiocarbamates efficiency as RAFT agents is enhanced when they become part of, or are adjacent to, an electron-withdrawing compound. In such circumstances, in fact, the ability of the lone pair to interact with the C=S30 group is diminished.
24 Cinzia Lea Chemistry and mechanism of radical polymerisation
The intermediate radical rate of fragmentation k is favoured by factors which render the radical R a good homolitic leaving group with respect to the attacking growing
P radical n . RAFT agents bearing a benzyl (CH2 Ph) group as a leaving radical R , for instance, have been found to be effective in controlling the polymerisation of styryl and acrylyl propagating radicals, but almost inert when applied to the polymerisation of methyl methacrylate; the explanation for this phenomenon lay in the fact that the benzyl radical is more stable than the styryl and acrylyl radicals, but a rather poor leaving group with respect to the methacrylyl growing radical31 and, as a consequence, in this latter case, the rate of fragmentation of the intermediate radical A is affected. It is also important to note that the stability of the radical R formed should not compromise the ability of the new radical to re-initiate polymerisation without delay, as already seen for the role of transfer agents in free radical polymerisation.
2.3.3.2 Choice of the RAFT agent
The selection of the RAFT agent decides the success or failure of the living process. In order to provide guidelines on how to select the right agent for a certain monomer, we can divide monomers in three groups. The first one includes methylmethacrylates (MMA), the second one includes styrene (S), methylacrylate (MA), acrylamide (AM) and acrylonitrile (AN) and finally the third one including vinyl monomers like vinyl acetate. Scheme 2.97 can provide an indication for choosing a suitable agent for a particular monomer type. On the top line the addition rate decreases from left to right when the activating group Z varies, on the bottom line the fragmentation rate decreases from left to right when the leaving group R changes. The dashed line indicates poor control, i.e., a linear increase of the molecular weight with conversion, but high polydispersity or, for vinyl acetate, retardation. Z groups which highly activate the C=S double bond towards addition and good leaving groups are needed when the growing radical is MMA, characterised by high steric hindrance and moderate reactivity32. So, 32 good control is obtained for R= C(alkyl)2Ph and C(alkyl )2CN and Z=Ph . The second group offers more flexibility in the choice of the RAFT agent, in particular, in regard to the leaving agent R. This can be explained by the lower stability of the monomer radical with respect to the methylmethacrylate radical. At last, vinyl acetate is a very reactive radical; only Z substituents which confer low stability to the intermediate radical (A’) are able to enhance its rate of fragmentation, therefore xanthates and
25 Cinzia Lea Chemistry and mechanism of radical polymerisation
dithiocarbamates32,33 make effective RAFT agents for the polymerisation of this class of monomers. O
Z: Ph >> SCH3 ~ CH3 N >> N > OPh > OEt ~ N(Ph)(CH3) > N(Et)2
VAc MMA S, MA, AM, AN
CH3 CH CH CH 3 H 3 CH3 3 CH3 H CH3 H R: CN ~ Ph > COOEt>> Ph > CH2 CH 3 ~ CN ~ Ph > CH 3 ~ Ph CH 3 CH CH3 CH CH3 H CH 3 CO2H 3 CH3 3 H
MMA S, MA, AM, AN
VAc
Scheme 2.9. Variation of addition and fragmentation rates for different monomers when the composition of R and Z varies.
Once a suitable RAFT agent has been selected, the concentration of the components and their ratios need to be considered. The experimental conditions must favour the generation of the largest number of possible living chains and discourage the occurrence of termination events. In order to achieve this it is appropriate to adopt a high ratio of RAFT agent to initiator concentration and to reduce the concentration of radical species in the system by conducting the reaction at low radical flux. Of all the polymeric chains in the RAFT process, some are RAFT agent derived, whereas others are initiator derived. The former ones carry the thiocarbonylthio end group, while the second ones did not undergo addition reaction with the RAFT agent and therefore they did not acquire a living character. The number of living chains, characterised by the thiocarbonylthio end group, is equal to the number of radicals R generated during the fragmentation step. If a significant fraction of initiated chains does not undergo addition, their contribution must be taken into account when calculating the average number molecular weight M n :
26 Cinzia Lea Chemistry and mechanism of radical polymerisation
[M] t Mn = mM + mRAFT [RAFT]0 + df ([I]0-[I]t)
where [M]0–[M]t and [I]0–[I]t are respectively the amount of monomer and the amount of initiator consumed, mM and mRAFT are respectively the molecular weight of the monomer and of the RAFT agent, d is the number of chains generated from radical- radical termination and f is the initiator efficiency. In the ideal case that each chain has living properties, the molecular weight is given by the ratio between the converted monomer and the initial RAFT agent concentration:
[M] t M n = mM + mRAFT [RAFT]0
2.3.3.3 Side reactions in RAFT polymerisation
The occurrence of side reactions during the polymerisation reaction can lead to retardation, formation of by-products and anomalies in the molecular weight distributions. Significant retardation (decrease in the polymerisation rate with respect to the FRP process) has been noticed in the RAFT polymerisation of acrylate esters in the presence of dithiobenzoate esters. An initial inhibition period (no polymerisation activity) and retardation has been reported for the polymerisation of MA mediated by the following dithiobenzoates: 1-phenylethyl (1-PEDB) and 2-(2cyanopropyl) (CPDB), benzyl (BDTB), , and also with cumyl dithiophenylacetate 34,35,36. Less retardation for the mediated polymerisation of the acrylate esters was instead experienced when the RAFT agents were aliphatic dithioesters and trithiocarbonates34,35,37.. Also in the case of the polymerisation of styrene and methacrylates retardation effects have been reported when RAFT mediating agents are cumyl dithiobenzoates7,38,39, 40. This effect becomes more pronounced the higher the concentration of the RAFT agent. While it has been suggested that the inhibition period may correspond to the time taken to convert the RAFT agent into the macroRAFT agent, the causes of retardation are still a controversial subject. Furthermore inconsistencies in the reported rates of polymerisation suggests that RAFT polymerisation is sensitive to the presence of oxygen and impurities in the system. It has been shown that the polymerisation of MMA with S-dodecyl S-(2-cyano- 4-carboxy)but-2-yltrithiocarbonate in the presence of oxygen provides narrow molecular
27 Cinzia Lea Chemistry and mechanism of radical polymerisation
weight distribution and some molecular weight control, however retardation is present and the product is characterised by lower than predicted molecular weights7. It is possible that slow fragmentation increases the chances of the intermediate radicals undergoing side reactions among themselves or with other radical species like initiator derived or propagating radicals.
2.3.3.4 Copolymerisation using the RAFT system
2.3.3.4.1 Random copolymers
When the RAFT process is used for the synthesis of copolymers, the choice of the agent must take into account its compatibility with both monomers. The copolymerisation of styrene and MMA has been successfully conducted in the presence of benzyldithiobenzoate, this agent, however does not make a good mediator for the homopolymerisation of MMA7. Therefore it is not required that the agent be suitable for the controlled homopolymerisation of each one of the monomers involved in the copolymerisation. 1H-NMR spectroscopy has demonstrated that the copolymers sequence distribution in a random copolymerisation is not modified by the introduction of a RAFT agent41,42. Nevertheless, at very low conversions, a drift from the copolymer composition obtained in the conventional process has been noticed. This effect could result from the preference of the initiating species R towards one of the components43,44. The advantage associated with the RAFT copolymerisation is that all chains have similar composition; they are called tapered or gradient copolymers, whereas the product obtained in FRP is composed of chains with different molecular composition. The presence of the RAFT agent also does not alter the composition of alternating polymers, as seen for the synthesis of poly(styrene-alt-maleic anhydride)45 and poly(styrene-alt- methylmethacrylate)46.
28 Cinzia Lea Chemistry and mechanism of radical polymerisation
2.3.3.4.2 Diblock and triblock copolymers
The presence of the thiocarbonylthio group in the living chains makes it also possible to synthesise di-block copolymers of composition AB47,48,49. This is obtained by adding a different monomer to a pre-formed living homopolymer chain. Consideration must be paid to the sequence according to which the monomers are fed to the mixture, it has been demonstrated50 that the monomer with better leaving ability should constitute the first building block in order to obtain low polydispersity. In a similar way, by adding in sequence further monomer(s) we can chain extend an AB block and generate ABA or ABC blocks. A more efficient way to generate ABA blocks is to take advantage of a RAFT agent with double functionality (bis-RAFT agent) and synthesise in one step a di- block copolymer with 2 arms of equal composition and length. Two types of bis-RAFT agent: type A and type B have been used to synthesise tri-blocks. As shown in Scheme 2.2, in bis-RAFT A the thiocarbonylthio functionality is situated at the extremities of the molecule, whereas in type B it is situated in the centre. As a consequence the polymer produced with type A bis-RAFT bears the RAFT functionality at the chain ends, while in the second case it is located in the centre. Type A bis-dithioesters51, bis- trithiocarbonates52, bis-dithiocarbonates53 and bis-dithiocarbamates54 (in which both Z and R substituents are respectively alkyl or aryl groups, substituted sulphur groups or substituted oxygen groups) and type B bis-trithiocarbonates55,56 (in which both R and Z substituents are substituted sulphur groups) have been reported to afford control over the formation of tri-blocks.
S S nM S S Z=R: Alkyl Z S R S bis-dithioesters Z Z S M S n Z Aryl
Bis-RAFT A S nY S Z=R: Substituted sulphur bis-trithiocarbonates Z S M Y S n n Z Substituted oxygen bis-dithiocarbonates S S nM Z M M Z Substituted nitrogen bis-dithiocarbamates
Z Z n/2 n/2
Bis-RAFT B S nY Z=R: Substituted sulphur bis-trithiocarbonates Z Y M M Y Z n/2 n/2 n/2 n/2
Scheme 2.2. Mechanism of formation of a tri-block copolymer with bis-RAFT agents.
29 Cinzia Lea Chemistry and mechanism of radical polymerisation
References
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31 Cinzia Lea Copolymerisation systems in radical polymerisation
Chapter 3. Copolymerisation systems in radical polymerisation
3.1 Definitions A macromolecule having two or more disparate monomers incorporated in its structure is defined as a copolymer. Copolymers can differ either by their microstructure, which is the order by which the comonomers are aligned along the chain, or by their composition, which is the relative amount of each monomer in the copolymer chain. In the simplest case that two monomers only participate to the synthesis, copolymers can be classified on the basis of their microstructures in:
Random copolymers. This is the most common microstructure and it is characterised by a casual sequence of monomer units. The ratio between the monomers in the reaction mixture and their relative reactivity determines the copolymer composition.
Alternating copolymers featuring regularly alternating monomer units along the chain;
Block copolymers characterised by long sequences of a monomer joining long homopolymer chains of the other monomer.
Grafted copolymers in which homopolymer side chains of a monomer are attached to a backbone made of repeating units of the other monomer.
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3.2 Copolymer composition
Copolymerisation provides a versatile way to prepare polymeric materials with predetermined chemical and physical properties. These properties are closely related to the copolymer components chemical nature, their distribution in the copolymer (copolymer microstructure) and relative amount (copolymer composition). Staudinger in 1930 discovered that the monomers percentage in the copolymer is very often different from their percentage in the initial mixture. This is due to the fact that each monomer has different reactivity, intended as its tendency to enter the polymeric chain; which determines a variation in the feed composition as a function of conversion during the course of the reaction. Kinetic models have been created with the aim to predict the instantaneous copolymer composition and sequence distribution. Although copolymerisation proceeds through the initiation, the propagation and the termination steps, as seen for homopolymerisation, kinetic models take into consideration the propagation reaction only, since this is the step which controls the monomers rate of admission into the growing chain. A kinetic model able to describe such a complex system must rely on some simplifying assumptions:
1. Propagating radicals with a chain length greater than 3 units are considered kinetically equal. These chains are long enough for short radical addition to be negligible and for the monomers consumption to be ascribed only to propagation reactions1. For the same reason termination and chain transfer reactions can be overlooked for active propagating radicals with chain length greater than 10 unit;
2. The radical concentration is constant (quasi-steady state assumption)2;
3. Only side reactions specific to the conditions employed are taken into account.
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3.2.1 The Terminal model
In the Terminal model the following further simplifying assumption are made: Only the latest monomeric unit influences the reactivity of the growing chain, regardless of its composition and length; The propagation reaction is the result of irreversible addition events. In the presence of two monomers M1 and M2, and their respective growing species M1 and M2 , the following addition reactions are possible:
k11 + M 1 M1M1
M1 k12 + M 2 M1M2
k21 + M 1 M2M1
M2 k22 M M + M2 2 2
Scheme 3.1. Possible propagation reactions according to the Terminal model.
The first and the second index associated to the rate constants k indicate respectively the active centre and the monomer involved in the reaction. So, k1,1 and k2,2. are the self- propagation rate constants and k1,2 and k2,1 are the corresponding cross-propagation rate constants. The rate of consumption of the monomer is expressed by:
d[M1] - = k11 M1 M1 + k21 M2 M1 dt (3.1)
d[M2] - = k12 M1 M2 + k22 M2 M2 dt
The ratio between the two equations is:
M M M M d[M ] k11 1 1 + k21 2 1 1 = (3.2) k M M + k M M d[M2] 12 1 2 22 2 2
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In accordance with the steady state assumption, for the concentration of M1 and M2 to be constant it is necessary that their rate of cross-propagation is equal:
k21 [M2 ][M1] = k12 [M1 ][M2] (3.3)
By substituting in equation 3.2 the value of [M2 ] obtained from equation (3.3) and introducing the relative reactivity ratios r1 and r2, which express the propensity of each monomer to react with the same monomer (homopropagate) rather than with the other monomer (cross-propagate):
r1= k11/ k12
r2=k22/ k21
3,4,5 The following equation is derived :
d[M ] f1 r f + f 1 = 1 1 2 (3.4) + r f d[M2] f2 f2 2 2
Equation (3.4) expresses the dependence of the instantaneous copolymer composition on the monomer feed fractions f1 and f2 and on the relative reactivity ratios of the monomers r1 and r2. This equation applies also to the anionic and cationic copolymerisation processes (anions and cations being the relevant active species) as long as the simplifying assumptions still suit the systems. Depending on the value of the product between the relative reactivity ratios r1 r2, three different systems can be identified: the random, the alternating and the block copolymer. As a broad approximation, a random copolymer is formed when r1 r2 approximates unity. This condition is fulfilled either when r1r21 or when r11 and r21 or viceversa. When r1r21 the monomers manifest equal reactivity towards each other and the units distribution along the chain tends to be purely random, whereas for r11 and r21 or viceversa, the copolymer composition is more incline to become randomly enriched with the most reactive monomer. Monomers manifest a preference to react with the other monomer when r1 r2 1; in general the closer the value of this product to zero, the stronger the alternating character of the monomers. For perfectly alternating systems the value of both self-propagation rate constants r1 and r2 has to be close to zero, in which case the copolymer composition is independent from the feed composition At last, the
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case of r1 r2 1 is the most common and favours the formation of a block copolymer. Equation (3.4) was later applied also to the copolymerisation of three or more monomers. Based on a previous kinetic analysis of copolymerisation systems through the terminal model carried out by Melville et al.6 and Walling7, Fukuda derived the value of the rate constant of propagation:
2 2 r1f1 +2f1f2+r2f2 (3.5) kp= r1f1/k11+ r2f2/k22
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3.2.2 Explicit and Implicit Penultimate Models
For those instances when the terminal model fails to predict the copolymer composition, a more complex model has to be adopted. The Explicit Penultimate model, developed by Merz et al. 8, assumes that not only the ultimate unit, but the combined effect of the penultimate and the ultimate unit determines the reactivity of the growing chain. In this case eight different reactions must be taken into account during the propagation step:
k111 + M 1 M1M1M1
M1M1
k112 + M 2 M1M1M2
k121 + M1 M1M2M1
M1M2
k122 + M 2 M1M2M2
k211 + M 1 M2M1M1
M2M1
k212 + M 2 M2M1M2
k221 + M 1 M2M2M1
M2M2
k222 + M2 M2M2M2
Scheme 3.2. Possible propagation reactions according to the Explicit Penultimate model.
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For these reactions we can define four reactivity ratios associated with the monomer:
k111/ k112 = 1
k222/ k221 = 2
k211/ k212 = ’1
k122/ k121 = ’2
And two reactivity ratios associated with the growing radical:
k211/ k111 = s1
k122/ k222 = s2
By introducing the adjusted parameters: r1 , r2 , k11 and k22
f +f r = ' 1 1 2 1 1 1'f1+f2 f +f r = ' 2 2 1 2 2 2'f2+f1
f +f k = k 1 1 2 11 111 1f1+f2/s1 f +f k = k 2 2 1 22 222 2f2+f1/s2
And placing them instead of r1, r2, k11 and k22 in equations 3.4 and 3.5, we can derive the composition and the
38 Cinzia Lea Copolymerisation systems in radical polymerisation
model by imposing to the Explicit Penultimate model the condition that the reactivity ratios of the monomer are equivalent to their Terminal model forms:
1 (= k111/ k112)= ’1 (= k211/ k212)=r1
2 (= k222/ k221)= ’2 (= k211/ k212)=r2
In the Implicit Terminal model, like in the Terminal model, the tendency of the growing radical to favour the addition of one monomer over the other is not influenced by the penultimate unit. However, because of the presence in the
3.2.3 The Q-e approach to the theory of reactivity
The Q-e scheme was developed by Alfrey and Price in 1947, in an attempt to rationalise the copolymers microstructure. It is based on semi-empirical assumptions and it is regarded as a useful approximation rather than as a proper model. According to this scheme the equations for the cross-propagation rate constant and the reactivity ratios are given by:
k1,2 = P1Q2 exp (-e1e2) (3.7)
r1 = (Q1/Q2)exp[-e1(e1-e2)] (3.8)
r2 = (Q2/Q1)exp[-e2(e2-e1)] (3.9)
2 r1 r2 = exp[(-e1-e2) ] (3.10)
P and Q express the reactivity due to resonance respectively of the radical M and of the monomer M. The polarity is equal for both the radicals and the monomer and is described by the parameter e. By assigning arbitrary values of Q and e to styrene: Q =1.0 e = -0.8 the relative values of Q and e for all the other monomers has been calculated. The limitations of this scheme lay in the fact that:
39 Cinzia Lea Copolymerisation systems in radical polymerisation
a) It assumes that the propensity of the monomers to alternate depends only on the mutual ability of the radicals and of the monomers to polarise their permanent charge; b) It overlooks steric factors; c) It does not differentiate between the polarity of the monomers and the polarity of the radicals.
Nevertheless this scheme has provided appropriate values of r1 and r2 for monosubstituted ethylenic monomers.
3.3 Influence of the reaction medium.
Ito and Otsu10 first analysed the effect of benzene, benzyl alcohol and phenol on the free-radical copolymerisation parameters of styrene and methylmethacrylate. Solvents effects were also reported by Bontà et al.11 by Madruga et al.12,13and Harwood14. Not only the reactivity ratios of ionogenic monomers like acrylic and methacrylic acid are sensitive to pH condition, but also the sequence distribution of non ionazable monomers (Sty, MMA, AM, AN etc) appears to be affected. In general, the reaction medium alters the monomers reactivity ratios, however the extent of this phenomenon has been found to depend on the nature of the monomers and solvents involved in the copolymerisation systems. Deviations from the base models experienced by systems in which a polar solvent is present can be interpreted in the following way: it is assumed that, during the propagation step, the reacting species form polarised structures. Being such intermediates stabilised by the interaction with a polar solvent15, the activation energy of the reaction will lower and the chain growth will be advantaged. The solvent influences also selectivity; since polar interactions arise between monomers of different polarity, cross-propagation reactions and therefore alternation will be inevitably favoured over homopropagation. For instance, in the free-radical copolymerisation of styrene and methylmethacrylate an increase of solvent dielectric constant corresponds to a decrease of the reactivity ratios of both monomers which leads to an increased propensity of the two monomers towards alternation, as outlined in Table 3.111. In this instance, styrene bears a partial negative charge due to the electron-donor properties of the aromatic ring, while methylmethacrylate appears to have a partial positive charge due to the electron withdrawing properties of the ester group:
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CH3
COOCH3
Sty MMA
Table 3.1. Reactivity ratios for the copolymerisation of styrene and methylmethacrylate at 50º C in various solvents11.
Solvent 25°C rSty rMMA rSty4rMMA Dioxane 2.21 0.56 0.53 0.29 Acetone 20.70 0.49 0.49 0.24 Dimethylformamide 36.71 0.38 0.45 0.17 Bulk _ 0.47 0.45 0.21
These monomers can form radical-solvent or monomer-solvent complexes, which in general present a different rate of propagation from their not complexed counterparts, since this interaction causes changes in their bulkiness and in their stability. Kamachi16 developed a terminal model to predict the copolymer composition in the presence of radical-solvent complexes, while for a monomer-solvent complex a slight modification of the so-called monomer-monomer complex dissociation applies17 (see below). In the case of the formation of a monomer-monomer complex, the complex presents the characteristics of a donor-acceptor complex. During the propagation step the following situations can take place: a) The monomer-monomer complex acts as a monomer and competes with the propagation of the free monomer. The terminal model based on this assumption is denominated the monomer –monomer complex participation model (MCP)18; b) The monomer-monomer complex acts as a monomer, but dissociates upon reaction with the growing chain adding only one monomer unit each time. The terminal model based on this assumption is denominated the monomer –monomer complex dissociation model (MCD);
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c) The formation of the complex causes inhibition. In the extreme case that the growing species are prevented from propagating by their interaction with the solvent, solvent partitioning will take place and the actual values of the reactivity ratios will deviate from the predicted ones. This phenomenon goes under the name of bootstrap effect14,19; it arises when circumstances cause the local free monomer, radical concentration or both, to vary from the average initial feed composition. Preferential absorption of one of the monomers on the growing chain end or poor solubility of the copolymer formed in the reaction medium can all lead to the bootstrap effect. Experimental evidence 20,21,22,23,24 has shown the versatility of the bootstrap model for simulating general copolymerisation systems.
3.4 Alternating copolymers
Alternating copolymers have been studied extensively because of the orderly alternation of the monomer units in their structure. Two theories have originated, as illustrated in Scheme 3.3; one theory suggests that, under the influence of polarity effects, monomers position themselves into an alternating pattern or matrix in the polymerisation mixture and this pattern is then transferred onto the polymer structure. Another popular theory is that the alternating structure is a consequence of the formation of a charge transfer complex (CT) between the comonomers. Indeed, upon mixing electron-rich (donor) monomers and electron depleted (acceptor) monomers, a coloured solution often results, which is indicative of the formation of a charge transfer complex25, in addition, the complex equilibrium constants have been measured by UV and NMR spectroscopy. It has also been proposed that the complex may act as a monomer itself and may participate directly to the propagation reaction; the fact that the colour of the solution fades as the polymerisation reaction proceeds seems to support this argument. However, this may be a result of side reactions and /or of the consumption of one of the components, furthermore, in some cases leading to the formation of an alternating copolymer no absorption in UV-VIS spectrum is noticed when the two monomers are mixed26, like in the copolymerisation of norbornene and maleic anhydride27 and in the copolymerisation between 2,3 dimethylbutadiene with acrylonitrile. This suggests that the formation of a complex is not a stringent requirement for the generation of an alternating structure. Several studies have been conducted to demonstrate the influence of polar effects in free radical chemistry. Pioneering work from Giese et al28,29 has showed that the electron-rich cyclohexyl free radical manifests a marked preference to react with an electron-depleted olefin like maleic anhydride rather than with 1-hexene.
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Jones and Tirrell30,31, on the basis of these findings, transposed this concept to the reaction of an electron-rich free radical with a mixture of an electron-rich monomer and an electron poor monomer. They also noticed that the product was almost exclusively a result of the reaction of the electron-rich free radical with the electron poor monomer and that no copolymer had been produced. These and similar results32, 27 have led many researchers to conclude that, at this time, there is no evidence that the chain transfer complex plays a role in the alternating propagation of donor and acceptor monomers 32,27,33. It is instead believed that polar effects are likely to be responsible for the alternating structure since they have been shown to determine the order of monomer addition to a growing radical33. 1 Theory of the formation of a CTC adding as a single unit to the growing radical.
D + A D DAD A + D A ADA
2. Theory of the formation of a matrix.
D A D A D A D-A -D-A -D-A
D = donor monomer A= acceptor monomer
33 Scheme 3.3. Two theories for interpreting alternating copolymerisation .
3.4.1 Copolymers of vinyl ethers and maleic anhydride
Both alkyl and aryl vinyl ethers and some unsaturated cyclic ethers form an alternating copolymer when reacted with MAn under mild conditions. Spontaneous polymerisation can take place with some vinyl ethers: 1,2 dimethoxyethylene34, p-dioxane35, conjugated dihydroanisole36. The presence of amide-MAn mixtures has also been shown to initiate copolymerisation of MAn with this class of monomers37. For this particular study, co- (maleic anhydride-alt-methyl vinylether) and co-(maleic anhydride-alt-iso-butyl vinylether) are of interest and they’ll be discussed after reviewing the comonomers properties.
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3.4.1.1 Maleic anhydride (MAn)
MAn is a dicarboxylic cyclic anydride also called 2,5 furandione or cis-butene-dioic anhydride with the formula:
O O O The anhydride functionality takes part to reactions of nucleophilic addition to the carbon of the carboxylic group: hydrolysis, formation of peracids, esterification, reaction with amines and ammonia. The unsaturated bond not only participates to reactions characteristic of an olefin, but also to addition reactions with nucleophiles, Diels-Alder reaction and photo-reactions. The electrophilic character of the double bond derives from the electron-withdrawing action of the two carbonyl groups in its vicinity. Due to the different electronic properties of its reactive groups MAn can therefore behave as a donor or as an acceptor; the two carboxylic oxygen atoms, the anhydride oxygen and the double bond function as donor groups, while the -electron system acts as an acceptor38. The formation of an electron donor-acceptor (EDA) complex is invoked in many reactions where MAn is one of the reagents: Diels-Alder, photochemical, radical and polymerisation reactions. In general, the formation of a complex manifests through the appearance of a broad intense band in the ultraviolet or visible absorption spectrum at wavelengths longer than those assigned to the single components. The determination of the complex constant is executed by varying the ratio between the acceptor and the donor; an absorption maximum indicates the composition corresponding to the formation of the complex67. In some instances, like for the acetone- MAn complex, however the formation of an EDA complex may not be accompanied by a distinct UV or visible absorption. In the acetone and the benzene-MAn systems Silber et al39 noticed temperature and concentration-dependent NMR chemical shifts as well as concentration dependent heat of solution and dilution and they used these parameters to calculate the equilibrium constants for the complexes. Until the early 1960s it was established knowledge that MAn was not able to homopolymerise40,41,42,43 and this was cited as an example of the ability of steric hindrance and polar effects to deter the homopolymerisation of 1,2 substituted olefins. This all changed when in 1961 it was reported that MAn had been homopolymerised44 and indeed further research revealed that UV and radiation, free radical initiators45, pyridine based compounds, shock waves and electrochemical processes were all able to polymerise MAn. Indeed the free radical polymerisation of MAn is challenging under conditions that are, instead,
44 Cinzia Lea Copolymerisation systems in radical polymerisation
common for vinyl monomers and this has been attributed to the electrostatic repulsion between the monomer and the monomer radical in the propagation step. The bulk homopolymerisation of MAn requires the monomer to be in the molten state (T 53 C ) or in solution, a large amounts of initiator (5-10 wt % ) must also be present. In order to reach high conversions, incremental additions of the initiator are required at temperatures at which the initiator half-life is very short (< 60 min)46. It had already been noticed that the reaction of MAn with conjugated olefins, which would normally lead to the formation of Diels-Alder products, produced polymers instead, upon addition of peroxides or UV light in the presence of photosensitisers47,48. On the basis of these considerations, Gaylord and co-workers49 advanced the hypothesis that the presence of excited species in the reaction medium is necessary in order for MAn to homopolymerise. When peroxides and azo compounds decompose rapidly, energy is released50 that is transferred to the monomers to generate excited monomers or dimers (excimers) responsible for the propagation of the reaction. A series of experiments carried out in the presence of peroxides and photosensitisers proved the necessity of having a sufficient concentration of excimers for MAn to homopolymerise51. Maleic anhydride readily copolymerises with many 1,2 substituted monomers, which do not easily undergo free-radical polymerisation. The presence of MAn in the copolymer backbone has been found to modify the physico-chemical properties of polymers and improve their adhesion, hydrophilicity, heat distortion and dyeability, as well as provide functional groups for chemical modifications.
3.4.1.2 Vinyl ethers
Vinyl ethers respond to the generic formula: R O These compounds were among the first to be found to exhibit living behaviour in carbocationic polymerisation systems. In 197652 Johnson and Young reported that n- butyl vinyl ether at –60 C in CH2Cl2 in the presence of iodine (I2) produced poly(n-butyl vinyl ether) with living characteristics; data showed a linear increase of the molecular weight with monomer conversion and relatively narrow molecular weight distribution. It was later discovered that binary initiating systems consisting of a protic acid, HB and a weak Lewis acid, MXn, were suitable for the preparation of living polyvinyl ethers.
Examples of MXn/HB initiating systems for vinyl ethers are: HI/I2 which was used for 53 54 the living cationic polymerisation of iso-butyl vinyl ether (IBVE) , HI/ZnX2 (X=I, Br,
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- - - - 55 Cl), HI/R4NX (X=ClO4 , NO3 BF4 or Cl ), RCO2H/ZnCl2 . Living carbocationic polymerisation was then extended to the homopolymerisation of other resonance stabilised monomers like styrene and para tert-butylstyrene and also the to homopolymerisation of iso-butylene, which is not considered to be a resonance stabilised monomer. Many other initiating systems have been extensively studied for the cationic homopolymerisation of vinyl ethers, despite of the fact that these compounds primary use is the production of additives and that they accounts only for 1% of the cationically produced polymers56. In analogy to other living systems, it is generally accepted that the key factor in living cationic polymerisation is the existence of a dynamic equilibrium between the dormant covalent adduct (1) and the active ionic species (2) according to the following scheme56:
k k 1 + - 2 + - P //MXm+1 P + MX Pm X + MXn m m m+1 k-1 k-2 1 2
Scheme 3.4. Dynamic equilibrium in living polymerisation
It has also been reported that vinyl ether can be cationically homopolymerised in the presence of poly(maleic anhydride) and monomeric MAn. This has been attributed to the fact that poly(maleic anhydride) prepared through radical initiators like AIBN and BPO contains trapped radicals which can initiate the cationic homopolymerisation of vinyl ethers, styrene and copolymerise MMA-styrene mixtures. The initiation mechanism is illustrated in the following scheme:
- + (P IBVE) P + IBVE P + IBVE complex
Scheme 3.5. Mechanism of initiation of IBVE and Styrene homopolymerisation with MAn as an initiator
3.4.1.3 Co (maleic anhydride-alt-methyl vinyl ether)
The alternating copolymers of methyl vinyl ether and MAn and its acid and ester forms have commercial relevance and are known under the trade name of Gantrez ®. Due to their low toxicity they are extensively employed as bio-adhesives, drugs carriers, additives for all purpose cleaners and industrial coatings and as coatings for digital printing.57. The low molecular weight equimolar methyl vinyl ether–MAn copolymer
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(Gantrez AN) is prepared by slow addition of the vinyl ether to a refluxing benzene solution of the anhydride and BPO at a temperature ranging between 50 and 80º C. A high molecular weight product is obtained when operating with a large excess of the vinyl ether, with methylene chloride as a solvent and lauryl peroxide as an initiator.
3.4.1.4 The synthesis of co (maleic anhydride-alt-iso-butyl vinyl ether)
When Maleic anhydride and iso-butyl vinyl ether are dissolved in a common solvent and mixed together in the presence of a radicalic initiator, an alternating copolymer originates.MAn has been assigned an e value equal to +3.69; this value expresses the electron deficiency of its double bond, induced by the electron-withdrawing effect of the 2 carbonyl groups in its vicinity. IBVE e value, instead, is equal to –1.2758, it derives from the presence of an electron donating alkoxyl substituent adjacent to the double bond, which renders this compound an electron donor able to promptly react with electron deficient systems. On the basis of what already discussed in the previous paragraphs, alternation may originate from: a) The alternating addition of the two free monomers attracted to each other by their polarity difference 59,60; b) The addition of the free vinyl monomers; c) The competing addition of the CTC complex61,62, or d) The homopolymerisation of the donor-acceptor complex acting itself as a monomer63.
In 1965, Baldwin64 carried out kinetic studies on the copolymerisation of IBVE with MAn, alongside with other alternating systems; he employed dilatometry to calculate the value of the conversions. At constant monomer concentration, in ethyl acetate and at 60°C, he found that the maximum rate of reaction of the IBVE/MAn copolymerisation occurs at a 1:4 IBVE:MAn mole ratio and very low rates were obtained in the presence of a large excess of MAn. The composition of the copolymer was found to be equimolar. Later on, Ratzsch 65 studied the reaction of several alkenes with MAn and the influence of the solvent used on the copolymerisation rate. Results showed that the use of solvents with increasing donor number, being this parameter the qualitative measure of their
47 Cinzia Lea Copolymerisation systems in radical polymerisation
Lewis basicity66, leads to a decrease of the rate of reaction and also causes the maximum copolymerisation rate to shift towards higher initial concentrations of IBVE. He reached the conclusion that solvents with high donor number are able to form complexes with the MAn chain ends, therefore hindering the addition of IBVE. Fujimori et al67 have given evidence of the formation of a donor-acceptor complex, in the copolymerisation of IBVE with MAn. They determined the complex constant respectively in chloroform (dielectric constant = 4.81, donor number DN = 4 Kcal mol- 1) and in methyl ethyl ketone (MEK) (dielectric constant = 18.51, donor number DN = 17.4 Kcal mol-1)68 by application of UV and NMR spectroscopy. They found that the complex equilibrium constant has the lowest value when MEK is the solvent in use. The fact that this parameter is influenced by the nature of the solvent, however, does not seem to affect the microstructure of the copolymer, as found by 13C NMR spectroscopy69. Alternation in fact, is retained for values of conversion up to 10%, regardless of the monomer composition and regardless of the solvent used for the copolymerisation. The same research group also interpreted the large presence of cis- linkage configuration at the cyclic MAn units in the copolymer, as evidence of the participation of the complex in the copolymerisation. More recently Zhu et al70 have applied the RAFT process to the copolymerisation of IBVE and MAn using BDTB as a RAFT agent and 1,4 dioxane as a solvent. They claim to have achieved a living behaviour and low polydispersities of the polymer product.
References
[1] M . Deady, A.W. H. Mau, G. Moad, T. H. Spurling; Makromol. Chem.1691, 194 1993. [2] M Farina Makromol. Chem.191,2795, 1990. [3] E. Jenkel; Z Phys. Chem Abstr A 190,24,1942. [4]F. R. Mayo and F. M. Lewis; J. Am. Chem. Soc., 66, 1594, 1944. [5] T. Alfrey and G. Goldfinger; J. Chem Phys., 12, 205 1944. [6] H. W. Melville, B. Noble, W. F. Watson; J. Polym. Sci. 2, 229, 1947. [7] C. Walling; J. Am. Chem. Soc. 71, 1930 1949. [8] E. Merz, T Alfrey Jr, G. Goldfinger;J. Polym. Sci. 1,75, 1946 [9] T. Fukuda, Y. D. Ma, H. Inagaki; Macromolecules, 18,17, 1985. [10] Ito T, and Otsu T.; J. Macromol. Sci. Chem., A3, 197, 1969. [11] Bontà G., Gallo B.M., Russo S.; Polymer16, 429, 1975. [12] E. L. Madruga, J. San Roman, M. A. Del Puerto; J. Macromol. Sci. Chem. A13, 1105, 1979. [13] J. San Román, E. L. Madruga, M. A. Del Puerto; Angew Makromol. Chem. 86, 1, 1980. [14] H. J. Harwood; Makromol. Chem. Macromol Symp.10/11, 17 1987. [15] A. Pross; Theoretical and Physical principles of Organic Reactivity, Wiley, New York, 1995. [16] M. Kamachi; Adv. Polym. Sci.38, 56, 1981. [17] P. Karad C. Schneider; J. Polym. Sci.; Part A: Polym. Chem.16, 1295, 1983. [18] P. D. Bartlett, K. Nozaki; J. Am. Chem. Soc., 68, 1495 1946. [19] Y. D. Semchikov; Macromol. Symp., 111, 317, 1996. [20] T. P. Davis; Polym. Commun. 31, 442, 1990. [21] B. Klumpermann, K. F. O’Driscoll; Polymer, 34, 1032,1993.
48 Cinzia Lea Copolymerisation systems in radical polymerisation
[22] M. L. Coote, T. Davis, B. Klumpermann, M. Monteiro; J. Macromol. Sci. Rev. Macromol. Chem. Phys. C38, 567, 1998. [23] B. Klumpermann, I. R. Kraeger; Macromolecules, 27, 1529, 1994. [24] E. L. Madruga; Makromol. Chem. Rapid Commun. , 14, 581, 1993. [25] P. Pfeifer, T. Bottler, Ber. 51, 1819, 1918. [26] Y. Li , A. B. Padias, H. K. Hall Jr; J. Org. Chem. 58(25), 7049-7058, 1993. [27] H. Ito, D. Miller, N. Sveum, M. J. Sherwood; J. Polym. Sci. Part A: Polym. Chem, 38(19), 3521-3542, 2000. [28] B. Giese, J. Meixner; Angew. Chem. 91(2), 167-168, 1979. [29] B. Giese; Angew. Chem. 95(10), 771-782, 1983. [30] S. A. Jones , D. A. Tirrell, J. Polym. Sci. Part A:Polym. Chem. 25(11), 3177-3180, 1987. [31] S. A. Jones , D. A. Tirrell, Macromolecules, 19(7) 2080-2082 1986. [32] R. Breslow; Acc. Chem. Res. 24(6), 159-164, 1991. [33] H. K Hall, A. B. Padias; J. Polym. Sci. Part A: Polym. Chem, 39, 2069-2077, 2001. [34] T. Kokubo, S. Iwatsuki, Y. Yamashita; Makromol. Chem., 123, 256, 1969. [35] S. Iwatsuki, S. Iguchi, Y. Yamashita; Kogyo Kagazu Zasshi, 69(1), 145 1966.; Chem. Abstr. 65, 10673g 1966. [36] T. Yamaguchi, T. Ono, S. Swagishi; Kobunshi Kagaku 26(286), 187, 1969; Chem. Abstr.71, 3741u, 1969. [37] H. Tamura, M. Tanaka, N. Murata; Kogyo Kagazu Zasshi, 72(11), 2506, 1969. [38] B. C. Trivedi, B. M. Culbertson; Maleic Anhydride, Plenum Press, New York and London 1982. [39] E. Silber, S. Park, W. C. Herndon; J. C. S. Chem. Comm., 933, 1978. [40] T. Alfrey, E. Lavin; J. Am. Chem. Soc. 67, 2044, 1945. [41] F. M. Lewis, F. R. Mayo; J. Am. Chem. Soc. 70, 1530, 1948. [42] C. H. Bamford, M. G. Barb, A. D. Jenkins, P. F. Onyon; The Kinetics of Vinyl Polymerisation by Radical Mechanisms, p. 183, Academic Press, Butterworths, N. Y., London 1958. [43] P. J. Flory; Principles of Polymer Chemistry, p.55, Cornell University Press, Ithaca, N. Y. 1957. [44] J. L. Lang, W. A. Pavelich, H. C. Clarey; J. Polym. Sci. 55, 531, 1961. [45] P.D. Bartlett, K. Nozaki; J. Am.Chem. Soc. 68, 1497, 1946. [46] N. G. Gaylord, S. Maiti; J. Poym. Sci. Polym Lett. Ed., 11 , 253, 1973. [47] N. G. Gaylord, M. Stolka, A. Takahashi, S. Maiti; J. Macromol. Sci. Chem. A 5, 867, 1971. [48] N. G. Gaylord, S. Maiti; J. Polym. Sci. B 9, 359, 1971. [49] N. G. Gaylord, J.Y. Koo; J. Polym. Sci. Polym. Lett. Ed., 19,107, 1981. [50] Y.Nakayama, K.Kondo, K.Takakura, K. Hayashi, S.Okamura; J. Appl.Polym. Sci. 18, 3661, 1974. [51] N. G. Gaylord, S. Maiti; J. Polym. Sci. Polym. Lett. Ed., 11, 253, 1973. [52] A. F. Johnson, R. N. Young; J. Polym. Sci. polym. Symp., 56, 211, 1976. [53] M. Miyamoto, M. Sawamoto, T. Higashimura; Macromolecules, 17, 265, 1984. [54] M. Sawamoto, C. Okamoto, T. Higashimura; Macromolecules, 20, 2693, 1987. [55] M Kamigaito, M. Sawamoto, T. Higashimura; Macromolecules, 24, 3988, 1991. [56] J. E. Puskas , G. Kaszas; Prog. Polym. Sci. 25, 403, 2000. [57] ISP (Internation specialty products) Products technical information [58] R. Z. Greenley J. Macromol. Sci., Chem , A14, 427 1980. [59] K. Dodgson and J.R. Ebdon, Eur. Pol J.,13, 791 1977. [60] R. A Sanayei, K. F. O’ Driscoll, B. Klumpermann, c 27, 5577, 1994. [61] K. G. Olson, G. B. Butler, Macromolecules, 16, 707, (1983) [62] G. B. Butler, C. H. Do, Zerner M.C., J. Macromol. Sci., Chem. A26, 1115, 1994. [63] P.D. Bartlet, K. Nozaki J. Am.Chem. Soc. 68, 1495 1946. [64] M.G. Baldwin; J. Polym.Sci. Part A, 3, 703-710, 1965. [65] M Ratzsch, Progr. Pol. Sci., 13(4), 277 (1988). [66] V. Gutmann, Coord. Chem. Rev.19, 225, 1976. [67] K. Fujimori, P.P. Organ, M.J. Costigan , I.E. Craven, J. Macromol. Sci., Chem., A23, 64, 1986. [68] G. Wipich Knovel solvents- A properties database, Chemtech publishing 2000. [69] X. Hao, K. Fujimori, D.J. Tucker, P.C. Henry, Eur. Pol. J., 36, 1145, 2000. [70] M. Zhu, L. Wei, P. Zhu, F. Du, Z. Li. F. Li, Gaofenzi Xuebao, 3, 418, 2001.
49 Cinzia Lea The free radical polymerisation of IBVE and MAn
Chapter 4. The free radical polymerisation of iso-butyl vinyl ether and maleic anhydride
4.1 Experimental procedure
4.1.1 Purification of reagents and solvents All reagents were reagent grade and used with no further purification, unless otherwise stated:
- Maleic Anhydride (MAn) (Fluka) was recrystallised from toluene and dried under vacuum.
- Iso-butyl vinyl ether (IBVE) (Fluka) was filtered through a column of activated basic aluminium oxide to remove eventual peroxides. Before addition of IBVE to the reaction mixture, the presence of these compounds was investigated by using Quantofix® Peroxide 25, a reagent which consists of test sticks able to provide a semi-quantitative determination of the peroxides present in a solvent/solution. The concentration of peroxides measured in IBVE according to this method was always determined to be lower than 5 ppm.
- 2,2’Azobisisobutyronitrile (AIBN ) was recrystallised from ethanol and dried under vacuum.
- Dioxane was purified by distillation in the presence of LiAlH4, stored under nitrogen atmosphere at a temperature of –4 º C and checked, prior to use, for the presence of peroxides with Quantofix® Peroxide 25, which consists of test sticks able to provide a semi-quantitative determination of the peroxides present in a solvent/solution. In no circumstances peroxides were ever detected.
- Methyl ethyl ketone (MEK) was distilled and stored over 4 Å activated molecular sieve.
- Acetone was stored over 4 Å activated molecular sieve.
- THF was filtered over alumina and stored over 4 Å activated molecular sieve.
50 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.2 Polymerisation procedures
4.2.1 Polymerisation in Schlenck tubes
Copolymerisation of IBVE and MAn in solution was carried out at 60 C using AIBN as a radical initiator and the selected solvent.
AIBN was first weighted, followed by the appropriate amount of MAn, IBVE and solvent. The polymerisation mixture was poured into a Schlenk tube and sealed with rubber septa, then 4 cycles of freeze-evacuate-thaw were executed. Part of the polymerising solution was transferred, under nitrogen atmosphere, into an IR cuvette previously freed from oxygen by nitrogen spurging, the rest of the solution was immersed into a thermostatic water bath and kept at a temperature of 60 C. Aliquots of the polymerisation mixture were sampled at intervals, according to the following procedure: an airtight syringe was flushed with nitrogen several times, part of the inert gas was then injected into the reaction vial and a small aliquot of the reaction mixture was withdrawn. The sample was immediately cooled in an ice bath and 1H NMR spectroscopy and gravimetric analysis were conducted to determine conversion values. While 1H NMR spectroscopy was carried out directly on the sampled solution, gravimetric analysis involved precipitation of the sample in diethyl ether, centrifugation and removal of the residual moisture in a reduced pressure oven.
4.2.2 Polymerisation in the presence of DDM as a chain transfer agent
A solution containing MAn and IBVE in a concentration equal to 2.8 mol L-1 each and 1 10-2 mol L-1 of AIBN in dioxane was prepared and used to fill 6 vials, each one up to the volume of about 3 ml. The appropriate amount of dodecyl mercaptane was then added to each of them to achieve a concentration of chain transfer agent ranging from 0.059 to -1 0.169 mol L . Each solution was then sealed with rubber septa, spurged with nitrogen for about 30 minutes and then immersed in a oil bath at 60 °C. Sampling occurred after about 7 minutes. The copolymer solutions were precipitated in diethyl ether, centrifuged and dried at reduced pressure for the evaluation of the conversion. For each vial it was found that the value of the monomer conversion was lower than 10%. SEC analysis was
carried out on the dry polymer and its number average molecular weight M n and weight
average molecular weight M w was obtained.
51 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.2.3 In situ NIR-FTIR spectroscopy polymerisation The FT-NIR instrument consisted of a Bruker IFS66\S Fourier transform spectrometer equipped with a tungsten halogen lamp, a CaF2 beam splitter and liquid nitrogen cooled InSb detector. After been degassed with 4 cycles of freeze–evacuate–thaw, part of the polymerisation solution was transferred under nitrogen atmosphere into an IR cuvette previously purged with nitrogen and placed in a FT-NIR spectrometer. Here the mixture polymerised at a temperature of 60°C and spectra were recorded at regular intervals.
4.3 Polymerisation kinetics
4.3.1 In situ FT- NIR spectroscopy
Monomer conversions were derived from FT-NIR spectra by monitoring the decrease of intensity of the overtone bands associated with the stretches of the C-H bonds of the vinyl groups present in the monomers. The bands were located at = 6169 cm-1 and = 6131 cm-1 respectively for IBVE and MAn. Figure 4.1a provides an example of a full spectrum characteristic of the system investigated while figure 4.1b is an enlargement of the monomers peaks region. A certain degree of overlapping between the two bands can be noticed and this can lead to errors in the quantification of the individual components1, however no other non overlapping overtones could be identified. In paragraph 4.6 the extent of the interaction between the polymerisable vinyl bonds of two monomers is going to be analysed. A linear baseline was selected for the IBVE peak between 6252 and 6175 cm-1 and for the MAn peak between 6175 and 6084 cm-1. The area included within these intervals was integrated and its variation was then used for the calculation of the monomers conversion according to the following equation:
(%) = (1- At/A0) 100
Where represents the conversion of either monomer into polymer, A0 the initial value of the absorbance associated to the monomer and At the value of the absorbance at the time t.
52 Cinzia Lea The free radical polymerisation of IBVE and MAn
5 a 4
3
2 Absorbance
1
0
6500 6000 5000 4750 4500 Wavenumber / cm-1
Figure 4.1a. Evolution with time of a typical NIR-FTIR full spectrum for the in-situ copolymerisation reaction between MAn and IBVE. The arrow points towards the decreasing absorbance with reaction time.
1.5 b
1.0
Absorbance 0.5
0.0
6240 6200 6150 6100 Wavenumber / cm-1
Figure 4.1b. Enlargement of the region between 6240-6100 cm-1 of the NIR-FTIR spectrum related to the in-situ copolymerisation between IBVE and MAn. The arrow points towards the decreasing absorbance with reaction time.
53 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.3.2 1H NMR spectroscopy
1 H NMR spectra were recorded in acetone-d6 using a Bruker ACF300 (300 MHz) spectrometer. Samples of approximately 0.6 ml in volume were taken by use of an airtight syringe previously purged with nitrogen, they were cooled in an ice bath and placed in NMR tubes. The monomer to polymer conversions was determined via 1H NMR spectroscopy immediately after the withdrawal of the samples. The decrease in 1 time of the peak integrals of the MAn (peak A, figure 4.2, H NMR acetone-d6 [ppm] 1 7.048 (s, CH)) and IBVE (peak B, figure 4.2, H NMR acetone-d6 [ppm] 6.47 (m, CH)) vinylic protons respect to the peak integrals of the methyl protons of IBVE (peak G, 1 figure 4.2, H NMR acetone-d6 [ppm] 0.95 (m, CH3)) were used for the calculation of the conversion according to the following formula:
(%) = (1- It/I0) 100.
I0 and It represent the values of the integrated areas of the peaks associated to either monomers at the initial time and at a time t respectively.
The MAn protons integrated area showed to increase at increasing recycle delay time and become constant at 40 s recycle delay time. This time interval between measurements was, therefore, adopted to acquire the 1H spectra of the copolymer and allow for the protons to fully relax.
54 Cinzia Lea The free radical polymerisation of IBVE and MAn
H G
H H G G 10 H 23 E H H HA HA H H O H F H C 78 1 4 O OH H E O O 5 6 H H OH O G 9 G O H D H B
H G
A B H C D E F G
876543210 ppm
1 Figure 4.2. H NMR spectrum recorded in acetone-d6.of the polymerisation mixture used to synthesise poly( MAn-co-IBVE)
4.3.3 Gravimetric analysis The gravimetric determination of the conversions was conducted in the following way: the sample was precipitated in petroleum spirit or diethyl ether, centrifuged, dissolved in acetone, re-precipitated in the same solvents, centrifuged and dried under vacuum. The recovered polymer was then weighted for the determination of the conversion and sampled for GPC analysis. The value of the conversion was calculated from the equation:
(%) = (WP/MWS) 100.
WP = mass of the polymer
M = weight fraction of monomer in the reaction mixture at time t =0
WS = mass of the sample
55 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.4 Copolymer characterisation
4.4.1 13C NMR spectroscopy
13 C NMR spectra were recorded in acetone-d6 using a Bruker ACF300 (300 MHz) spectrometer. Polymer solutions had a concentration of 0.2 g of copolymer/ 1 g acetone- d6. This technique was utilised for the determination of the triad sequence distribution of the copolymer. Sub-spectra of the methine, methylene and methyl carbons of the Man/i- BVE copolymer were acquired by using the Distortionless Enhancement Polarisation Transfer pulse sequence (DEPT). This method provides distortionless enhancement of 1 13 spectra through polarisation transfer from H to C nuclei. Experiments 1, 2, and 3 were run (number of scans = 2000), with final 1H pulses respectively of /4 (DEPT45) /2 (DEPT90) and 3 /4 (DEPT135). The application of the following equations provided respectively the methine, methylene and methyl carbons sub-spectra:
CH = 2 - c( 1+ a 3)
CH2 = 1-a 3
CH3 = 1 + a 3 - b 2
Being the theoretical value of a equal to 1, the value of b equal to 0.707, the value of c equal to 0.
4.4.2 Size Exclusion Chromatography
Molecular weights and molecular weight distributions were measured by size exclusion chromatography (SEC) with either N-N- dimethyl acetamide (DMAc) or tetrahydrofuran (THF) as eluents. In the former case, analyses were carried out in a Shimadzu modular LC system consisting of a DGU-12A solvent degassing device, a LC-10AT pump, a SIL-10AD autoinjector, a CTO-10A column oven and a RID-10A refractive index detector. The separation system comprised four 300 x 7.8 mm linear columns (Phenomenex 500, 103, 104,105 Å pore size; 5 nm particle size) their temperature was kept at 40°C. The eluent used was (HPLC, 0.03% w/v, LiBr, 0.05% BHT), at a flow rate of 1mL/min. The polymer solutions injected had a concentration of 5mg/mL. When THF was used as the eluent, the analytical set-up consisted of a Shimadzu modular system comprising an auto-injector, a Polymer Laboratories 5 m bead-size guard column followed by 3 linear columns (Phenomenex 500, 103, 104,105 Å pore size; 5 nm particle
56 Cinzia Lea The free radical polymerisation of IBVE and MAn
size) and a differential refractive index detector. The eluent had a flow rate of 1 ml min-1 at 25°C. Both systems were calibrated with narrow polydispersity polystyrene standards (Polymer laboratories) in the range 0.5-1000 kDa and SEC traces were analysed with Cirrus 2.0 software (PL). The evaluation of the copolymer molecular weight and molecular weight distribution was conducted, in some instances, directly on the polymerisation mixture previous dilution with the appropriate solvent. Alternatively the analyses were carried out on the polymer obtained by precipitation of the polymerisation mixture in either diethyl ether (DEE) or in petroleum spirit.
Due to the absence in literature of the Mark-Houwink-Sakurada (MHS) parameters relative to the MAn/IBVE copolymer, the values of the number average molecular weight M n of the MAn/IBVE copolymer reported in figures and tables correspond to styrene equivalents; this method can only provide a semi-quantitative determination2 of the M n and this uncertainty must be taken into account when analysing results.
57 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.5 Results and discussion
Prior to analysing the RAFT polymerisation of MAn and IBVE (chapter 5), the free radical polymerisation between these two monomers was carried out; we analysed its kinetics in different solvents and characterised the final product. This study aimed at providing a benchmark, from which the effects of the addition of a RAFT agent on the kinetics of this copolymerisation and on the characteristics of the polymer itself could be better appreciated. It has already been discussed in paragraph 3.4.1.4 that the product of this synthesis is an alternating copolymer, as represented in scheme 4.1:
I + 2 O O O O
O O O
OOOOOO O O O
Scheme 4 1. The free radical polymerisation between MAn and IBVE.
Three independent techniques were employed to monitor the kinetics of the copolymerisation of IBVE and MAn: in-situ FT-NIR, off-line 1H NMR spectroscopy and gravimetric analysis. While the use of the spectroscopic techniques enabled to observe the rate at which each monomer reacted, the gravimetric analysis monitored the cumulative copolymer conversion. In this study we observed the copolymerisation rate of IBVE and MAn in MEK, acetone-d6 and dioxane.
4.5.1 FRP in dioxane as a solvent
The polymerising solutions were prepared according to the procedure described in paragraphs 4.2.1 and the polymerisation process took place both in a water thermostatic bath and in a FT-NIR spectrophotometer. The initial solutions appeared to be clear in colour, overtime, however, they would assume a yellow tinge and hand sampling would become impossible at high conversions due to the increasing viscosity. Figures 4.3 and 4.4 show the kinetics for three different concentrations of AIBN in dioxane as a solvent; the experimental conditions are summarised in table 1. From the analysis of the values
58 Cinzia Lea The free radical polymerisation of IBVE and MAn
of the conversion, as determined by FT-NIR, it can be noticed in figure 4.5 that an induction period seems to be present for both runs a) and b), then auto-acceleration can be noticed for run c). Furthermore, it is evident that the two monomers react at different rates, with IBVE reacting at a faster rate respect to MAn in both run a) and b). In run c), instead, both monomers appear to react at the same rate for most of the reaction, however, at very high values of the conversion, the rate of reaction of MAn appears to be higher than IBVE. This result is in contradiction with the formation of an alternating copolymer. According to gravimetric analysis, however the values of the conversion overlap with the values of the conversion of MAn as derived by FT-NIR. In table 1 the copolymerisation rate Rp, is present, which corresponds with the rate of formation of polymer:
Rp= d(polymer)/dt= -d[MT]/dt (4.1)
[MT] represents the total monomer concentration at the time t, given by [MAn]+[IBVE].
Considered that the conversion is defined as:
= 1-[MT]/ [MT]0 (4.2) then its derivative d dt is given by:
d/dt = -(d[MT]/dt) [MT]0 (4.3)
Being [MT]0 the initial concentration of total monomer. Based on equation (4.3), therefore, Rp can be written as:
Rp = [MT]0 d/dt (4.4)
By substituting the known initial concentration of monomer [MT]0 and, by approximating to a line the initial portion of the conversion-time curve relative to MAn, d/dt can be calculated from the slope of the line:
d/dt = (y2-y1)/(x2-x1) (4.5)
For a polymerisation that proceeds according to the radical mechanism and for values of conversion <10%, direct proportionality between the value of the initial rate of polymerisation Rp and the square root of the concentration of initiator is expected
(2.1.6). The values of Rp in table 1, however, show that Rp is proportional to an higher
59 Cinzia Lea The free radical polymerisation of IBVE and MAn
power of [AIBN]0 It is likely that, in the course of the polymerisation process, perfect isothermal conditions were not maintained; causing the rate of reaction to be higher than expected.
Table 4.1. Free radical copolymerisation of MAn and IBVE in 1,4 dioxane as solvents at 60 ºC, [IBVE]:[MAn]=1:1, initiator AIBN.
Experiment [MT]0 [AIBN]0 Solvent RP Mn (mol L-1) (mol L-1) (mol L-1s-1) (g mol-1) X 102 FRP(Diox-1) 5.6 1.4 10-3 Dioxane 1.9 _ FRP(Diox-2) 5.6 1.4 10-2 Dioxane 11 378000 =14% FRP(Diox-3) 5.6 1 10-1 Dioxane 43 32563 =26% *FRP(Diox-2c) 2 1.4 10-2 Dioxane 4 NA
*The value of Rp was calculated from FRP(Diox-2).
100
80
c) b) a) 60
40
[AIBN] = 1.4 10-3 -2 20 [AIBN] = 1.4 10 [AIBN] = 1 10-1
0 0 100 200 300 400 500 600 Time / min
Figure 4.4. Free radical copolymerisation of MAn and IBVE at 60ºC, [IBVE]:[MAn]=1:1, initiator AIBN, 1,4 dioxane as a solvent. The solid line and the empty symbol represent respectively the conversion of MAn and of IBVE as calculated by NIR-FTIR, the bicolour symbol represents the conversion as calculated by gravimetric analysis.
60 Cinzia Lea The free radical polymerisation of IBVE and MAn
40
c) b) a)
20
[AIBN] = 1.4 10-3 [AIBN] = 1.4 10-2 [AIBN] = 1 10-1 0 04080120 Time / min
Figure 4.5. Enlargement of figure 4.4 showing the sections of the curves used for the calculation of the slope d/dt.
4.5.2 FRP in MEK and Acetone as solvents
The same copolymerisation was then run in the presence of MEK and Acetone. This was done to test the effect that solvents with different parameters can have on the kinetics. It has been reported in literature that the value of the initial rate of polymerisation, Rp, for the reaction between IBVE and MAn depends on the nature of the solvent; it increases for decreasing polarity and donor number of the solvent3,4. Table 2 illustrates the conditions adopted for the experiments, the conversion–time curves are illustrated in figures 4.6 and 4.8. It can be noticed that for these experiments also, IBVE seems to react faster than MAn according to data obtained from NIR-FTIR. Figure 4.7 suggests that the difference in the rate of reaction of the two comonomers is particularly pronounced in the initial stage of the reaction. Values of the conversions were also 1 calculated by H NMR spectroscopy for FRPAc-d6c (figure 4.8), according to the procedure described in 4.3.2. They suggest that both monomers react at the same rate. It can also be brought to attention that the sets of values obtained with the two techniques differ noticeably. Also, by comparing the values of Rp relative to runs FRPDiox-2c, -1 -1 FRPMEK and FRPAc-d6c ([MT]0=2 mol L , [AIBN]0 =1.5 mol L ), it can be seen that
Rp increases in the order MEK>Ac>Diox. This is in discordance with data in literature,
61 Cinzia Lea The free radical polymerisation of IBVE and MAn
which report that the fastest reaction should take place in dioxane (dielectric constant =2.209, donor number DN = 14.8 kcal mol-1) rather than in MEK ( = 18.51, DN = 17.4 kcal mol-1) due to the lowest polarity associated to this solvent5,6.
Table 4.2: Free radical copolymerisation of MAn and IBVE in MEK and acetone- d6 as solvents at 60 ºC, [IBVE]:[Man]=1:1, initiator AIBN.
Experiment [MT]0 [AIBN]0 Solvent RP (mol L-1) (mol L-1) (mol L-1s-1) X 102 FRP(MEK) 2 2·10-2 MEK 9.9 -2 *FRP(Ac- 2 2·10 Ac-d6 2.8 d6c1) -2 FRP(Ac-d6) 6 1.5·10 Ac-d6 6.3 -3 *FRP(Ac- 6 2·10 Ac-d6 2.3 d6c2)
* The value of Rp was calculated from FRPAc-d6.
100
NIR-FTIR Xi-BVE NIR-FTIR X MAn 80
60 / % / % 40
20
0 0 102030405060 Time / min
Figure 4.6. Experiment FRP(MEK): monomers conversion versus time plots as calculated by in-situ NIR-FTIR spectroscopy for the FR copolymerisation of -1 -2 -1 IBVE and MAn. T=60ºC, [MT]0=2 mol L , [AIBN]0=2·10 mol L .
62 Cinzia Lea The free radical polymerisation of IBVE and MAn
0.16 IBVE 0.14 MAn
0.12
0.10 -1 s -1 0.08
mol l
/ 0.06 p R 0.04
0.02
0.00 0 102030405060 Time / min
Figure 4.7. Experiment FRP(MEK): rate of FR copolymerisation of IBVE and MAn versus time. Conversion values obtained by in-situ NIR-FTIR -1 -2 -1 spectroscopy. T=60ºC,[M]0=2 mol L ,[AIBN]0=2·10 mol L .
NIR-FTIR 100 i-BVE NIR-FTIR MAn 1 H MAn 80 1H MAn
60 / % 40
20
0 0 20406080100120 Time / min
Figure 4.8. Experiment FRP(Ac-d6): monomers conversion versus time plots as calculated by in-situ NIR-FTIR spectroscopy for the FR copolymerisation of IBVE -1 -2 -1 and MAn. T=60ºC, [MT]0=6 mol L , [AIBN]0=1.5·10 mol L .
63 Cinzia Lea The free radical polymerisation of IBVE and MAn
In regard to the SEC analysis of the polymer prepared by FRP, figures 4.9 and 4.10 show that the molecular weight varies slightly with increasing values of the conversion. The value of the polydispersity, represented in figures 4.11 and 4.12 is comprised between 1.45 and 2.3, suggesting that both termination mechanisms (combination and disproportionation) might take place.
X = 13 % poly(co-iBVE/MAn) X = 27 % 1.5 poly(co-iBVE/MAn) X = 44 % poly(co-iBVE/MAn) X = 52 % poly(co-iBVE/MAn) Conversion 1.0 dW/d(Log M) dW/d(Log 0.5
0.0
4.0 4.5 5.0 5.5 6.0 6.5 Log(M) / g mol-1
Figure 4.9. Experiment FRP(MEK): evolution of the SEC chromatograms for the -1 solution FR copolymerisation of IBVE and MAn. T=60ºC, [M]0=2 mol L , -3 -1 [AIBN]0=1·10 mol L . The values of the conversions were calculated by in-situ NIR-FTIR spectroscopy.
64 Cinzia Lea The free radical polymerisation of IBVE and MAn
1.6 X = 4.5 % poly(co-iBVE/MAn) X = 19 % poly(co-iBVE/MAn) X = 40 % poly(co-iBVE/MAn) X = 57 % 1.2 poly(co-iBVE/MAn) Conversion
0.8
dW/d(LogM) 0.4
0.0
4.5 5.0 5.5 6.0 6.5 Log M / g mol-1
Figure 4.10. Experiment FRP(MEK): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot for the solution FR copolymerisation -1 -3 -1 of IBVE and MAn. T=60ºC, [M] 0 = 2 mol l , [AIBN] 0 = 1·10 mol l .
2.0 PDI
1.5 150000
125000
100000 -1 75000
g mol / 50000 Mn
25000
0 0 102030405060 / %
Figure 4.11. Experiment FRP(Ac-d6): evolution of the SEC chromatograms for the -1 solution FR copolymerisation of IBVE and MAn. T=60ºC, [M]0=6 mol L , -2 -1 [AIBN]0=1.5·10 mol L . The values of the conversions were calculated by in-situ NIR-FTIR spectroscopy.
65 Cinzia Lea The free radical polymerisation of IBVE and MAn
3.0 PDI 1.5
160000 -1 120000 / gr mol / gr n M 80000
40000
SEC M n 0 0 102030405060 poly(co-i-BVE/MAn)
Figure 4.12. Experiment FRP(Ac-d6): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot for the solution FR copolymerisation -1 -2 -1 of IBVE and MAn. T=60ºC, [M]0=6 mol L , [AIBN]0=1.5·10 mol L .
4.6 Reliability of FT-NIR spectroscopy for the evaluation of the conversion.
Due to the peculiarity of the results obtained so far, which suggest that the rate of polymerisation of IBVE (RIBVE) is faster than the rate of polymerisation of MAn (RMAn) we decided to test the assumption that the FT-NIR spectrum obtained from the reacting solution at any instant is a linear superposition of spectral contributions from the two comonomers (Beer’s law). Let A() be the absorbance (or emission) signal at wavelength given by:
A( = ( c b
where b is the sample cell path length in cm, and, for experimental convenience, c is the solute concentration in g cm-3, so that ( is the mass concentration extinction
2 -1 coefficient (cm g ). Let IBVE( andMAn( andcIBVE and cMAn be the extinction coefficient and mass concentration of IBVE and MAn respectively at wavelength . If the Beer’s law holds for these measurements, then the absorbance A( of the mixture of the two comonomers is given by the sum of the component absorbances:
66 Cinzia Lea The free radical polymerisation of IBVE and MAn
A( = b [IBVE( cIBVE +MAn( cMan]. (4.1)
Individual solutions of known concentration were prepared for each monomer in both acetone and in dioxane and analysed at the FT-NIR spectrophotometer. Then, solutions composed by a mixture of both monomers in a concentration identical to the individual ones were analysed. An additional curve was also calculated, based on equation 4.1 and denominated simulated overall curve in figures 4.13-4.16.
Results show that the peaks of the two monomers display some extent of overlapping, as already discussed in paragraph 4.3.1, but most importantly, it appears that the Beer’s law does not hold for these measurements. Figures 4.13-4.16 demonstrate that the spectrum related to the mixture of the two comonomers in solution is not, at every wavelength, equivalent to the sum of the absorbances of the spectra of the constituents (simulated overall spectrum). In particular the spectrum associated with a ratio of the mixture MAn:IBVE 1 appears to underestimate both comonomers vinyl bonds contributions. This is still true for solutions in dioxane composed of a mixture of the comonomers in a ratio MAn:IBVE = 1.5:1, while the same mixture in acetone appears to underestimate IBVE vinyl bonds contributions and overestimate the absorbance due to the MAn vinyl bonds. These results reveal that the comonomers polymerisable vinyl bonds interact with one another and that the extent of this interaction depends on the solvent used, being more pronounced in the case of solutions prepared in acetone as a solvent, and on the ratio between the comonomers, increasing at increasing molar ratio between MAn and IBVE. It also seems to affect to a greater extent the wavelength of absorbance of the IBVE monomer, while it is less relevant for the MAn monomer. As a result, the measurement of both monomers concentrations in acetone leads inevitably to error, when these are present in equal molar amount, however it provides an acceptable estimation of the amount of MAn in the other cases and in particular for dioxane solutions. Figure 4.4 shows the good agreement between the conversion of the copolymer as calculated by gravimetric analysis and the value of the conversion of MAn as calculated by FT-NIR spectroscopy.
67 Cinzia Lea The free radical polymerisation of IBVE and MAn
0.5 MAn IBVE 0.4 Simulated overall MAn + IBVE
0.3
0.2 Absorbance
0.1
0.0 6250 6200 6150 6100 6050 Wavenumber / cm-1
Figure 4.13. FT-NIR spectra at T=60ºC in acetone of a 1 M solution of MAn, of a 0.9 M solution of IBVE, of a solution containing both monomers in a concentration 1 M respect to MAn and 0.9 M respect to IBVE and spectrum calculated with the assumption that the Beer’s law applies (simulated overall).
0.6 MAn IBVE 0.5 Simulated overall MAn + IBVE 0.4
0.3
Absorbance 0.2
0.1
0.0
6250 6200 6150 6100 6050 -1 Wavenumber / cm
Figure 4.14. FT-NIR spectra at T=60ºC in dioxane of a 0.9 M solution of MAn, of a 1 M solution of IBVE, of a solution containing both monomers in a concentration 0.9 M respect to MAn and 1 M respect to IBVE and spectrum calculated with the assumption that the Beer’s law applies (simulated overall).
68 Cinzia Lea The free radical polymerisation of IBVE and MAn
0.28 MAn IBVE 0.24 Simulated overall MAn + IBVE 0.20
0.16
0.12
Absorbance 0.08
0.04
0.00
6250 6200 6150 6100 6050 Wavenumber / cm-1
Figure 4.15. FT-NIR spectra at T=60ºC in acetone of a 3 M solution of MAn, of a 2 M solution of IBVE, of a solution containing both monomers in a concentration 3 M respect to MAn and 2 M respect to IBVE and spectrum calculated with the assumption that the Beer’s law applies (simulated overall).
0.8 MAn 0.7 IBVE Simulated overall 0.6 MAn + IBVE
0.5
0.4
0.3 Absorbance
0.2
0.1
0.0
6250 6200 6150 6100 6050 Wavenumber / cm-1
Figure 4.16. FT-NIR spectra at T=60ºC in dioxane of a 3 M solution of MAn, of a 2 M solution of IBVE, of a solution containing both monomers in a concentration 3 M respect to MAn and 2 M respect to IBVE and spectrum calculated with the assumption that the Beer’s law applies (simulated overall).
69 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.7 Reliability of NMR spectroscopy for the determination of the conversion
As already done for FT-NIR spectroscopy, the reliability of 1H NMR spectroscopy for calculating the values of the conversion for this specific copolymerisation system was tested. Solutions of known concentration of the two monomers were mixed in different ratios and analysed in acetone-d6, according to the procedure described in paragraph 4.3.2. The ratios between the peak integrals of the two monomers vinylic protons ( for 1 MAn peak A, figure 4.2, H NMR acetone-d6 [ppm] 7.048 (s, CH), for IBVE peak B, 1 figure 5.10, H NMR acetone-d6 [ppm] 6.47 (m, CH))) respect to the peak integrals of 1 the methyl protons of IBVE (peak G, figure 5.10, H NMR acetone-d6 [ppm] 0.95 (m,
CH3)) were then compared with the known values and the error related to each measurement was calculated. When the calculation of the conversion is conducted with 1H NMR spectroscopy, results in table 5.3 demonstrate that the technique overestimates the values of the conversions of both monomers and in particular the one related to IBVE. The minimum error is associated with a solution characterised by a ratio between the concentration of MAn and IBVE ([MAn]/[IBVE]) equal to 0.5, while the highest uncertainty is associated with the solution containing the comonomers in a ratio [MAn]/[IBVE] equal to 1.25. Figure 4.17 shows the spectrum of a solution containing a ratio [MAn]/ [IBVE] equal to 1.25. Based on this ratio between the two comonomes, the ratio between the peak integral of the vinyl protons of MAn and the methyl protons of IBVE should be equal to 0.4125; however the value calculated from the spectrum is equivalent to 0.376 which corresponds to a value 8.8% lower than the nominal value. When the same procedure was repeated for the peak integral of the vinyl protons of IBVE, which is expected to be equal to 1/6 of the peak integral of the methyl protons of IBVE, the value obtained is 20.69% lower than the nominal value. It was concluded that the measurement of the conversion through 1H NMR is accompanied by large error. Only in the case that the peak integral of MAn is used to carry out the calculations, an approximate estimate of the conversion of the copolymer can be obtained.
Table 4.3. Error associated with the determination of the conversion by 1H NMR spectroscopy for different ratios ([MAn]/ [IBVE]).
([MAn]/[IBVE]) Error associated with MAn Error associated with IBVE (%) (%) 0.5 5.3 7.38 0.726 6.8 9.5 1.25 8.8 20.69 5 2.7 9.4
70 Cinzia Lea The free radical polymerisation of IBVE and MAn
HG
H H G 10 G H 23 E H H HA HA H H O HF HC 78 1 4 O OH HE O O 5 6 H H OH O G 9 G O HD HB
HG
2.26 0.79 6.00 BA H C D E F G 876543210 / ppm
Figure 4.17. NMR spectrum of a solution in acetone-d6 containing a ratio [MAn]/[IBVE] equal to 1.25 and peak integrals of MAn (2.26) and IBVE (0.79) vinyl protons and of IBVE methyl protons (6).
71 Cinzia Lea The free radical polymerisation of IBVE and MAn
4.8 Experiments with dodecylmercaptan (DDM) as a chain transfer agents
The IBVE-MAn copolymer afforded so far by FRP has been characterised by an average
-1 number molecular weight M n of about 150000 g mol and polydispersity index PDI comprised between 1.45 and 2.3. As a means to obtain polymers with differing M n and lower value of polydispersity index a chain transfer agent was employed: dodecyl mercaptan (DDM) (see paragraph 2.1.4 for a discussion on chain transfer agents). Mercaptans have been successfully applied to regulate the chain length of copolymers of MAn with methacrylates and acrylates7. In this paragraph we describe how DDM can also be used to regulate the molecular weight of the copolymer of MAn with IBVE. Six polymer solutions were prepared according to the procedure described in 4.2.2, and their molecular weight distributions were obtained from the SEC analysis of the samples (figure 4.17). The Mayo equation (4.6) was then used to calculate the value of the chain transfer constant CS: 1 (1+ kt [ P ] [ S ] = + CM + CS (4.6) kp [ M ] [M] DPn
In the Mayo’s equation DPn represents the degree of polymerisation, which can be calculated either from the Mn or from the weight average molecular weight M w .
expresses the fraction of termination by disproportionation, CM the chain transfer constant for chain transfer to monomer ( ktr,M/kp), CS the chain transfer constant for chain transfer to a chain transfer agent ( ktr,S/kp), and [S] the concentration of the chain transfer agent. In transfer dominated systems, in which polymers do not present very low
M M molecular weights, the value of DPn can be calculated as n/M0 or as w /2M0, therefore, provided that the value of CS is high enough to allow overlooking the other terms, this constant is given by the slope of the plot of 1/ DPn versus [DDM]/[MT] (figure 4.18).
72 Cinzia Lea The free radical polymerisation of IBVE and MAn
Table 4.4: Data relative to the calculation of the chain transfer constant,Cs,T=60 °C, dioxane as a solvent, dodecyl mercaptan as a chain transfer agent. -1 [DDM]/ mol l [DDM]/[MT] M -1 -1 PDI n /g mol M w /g mol 0 0 209000 518000 2.48 0.0169 3 10-3 65000 167000 2.57 0.03 5.36 10-3 44000 104000 2.35 0.062 1.1 10-2 50000 81000 1.63 0.098 1.75 10-2 29000 53000 1.87 0.159 2.8 10-2 21000 37000 1.77
-4 -4 CS 0.14±4.3 10 0.17±2.2 10
0.0169 M 1.0 0.03 M 0.062 M 0.8 0.098 M 0.159 M 0 M 0.6
0.4 dW/d(Log M) dW/d(Log
0.2
0.0
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Log M / g mol-1
Figure 4.17: Progression of the MWD for increasing concentration of DDM, [MT]0 =5.6 mol L-1 [AIBN]=1 10-2 mol L-1 , dioxane as a solvent, conversion <10 %.
73 Cinzia Lea The free radical polymerisation of IBVE and MAn
6.0x10-3 1/DP n 5.0x10-3
4.0x10-3
3.0x10-3
2.0x10-3
-3 1.0x10 M n M w 0.0 0.0 1.0x10-2 2.0x10-2 3.0x10-2 4.0x10-2 [DDM]/[M ] T
Figure 4.18: Mayo plots for the determination of the chain transfer constant Cs, T=60 °C, dioxane as a solvent, dodecyl mercaptan as a chain transfer agent.
As already mentioned in paragraph 4.4.2, relative to the description of the method utilised to carry out the SEC analysis, the determination of both the value of the number
M average molecular weight n and the value of the weight average molecular weight
M w is semi-quantitative, due to the fact that both values are reported in polystyrene equivalents. As a consequence, the value of CS is also inevitably affected by error.
M The value of Cs obtained from calculations DPn = n /M0 is equal to 0.14 with a -4 standard deviation of ±4.3 10 , while Cs equals 0.17 with a standard deviation of ±2.2
-4 M 10 when derived from DPn = w /2M0. The better reproducibility of the data obtained from M w relates to the fact that baseline error in GPC analysis affects to a greater
8, 9 extent the value of M n . It can be seen in figure 4.17 that the molecular weight distribution curves manifest bimodality, however, by increasing the concentration of DDM the value of the molecular weight decreases and so does the value of the polydispersity index (table 4.4). This leads to the disappearance of the shoulder for a concentration of DDM equal to 0.159 M. This phenomenon can be explained in the following way: in the absence of a
74 Cinzia Lea The free radical polymerisation of IBVE and MAn
chain transfer agent the rate of termination decreases, leading to the formation of polymer chains with high molecular weight. The introduction of increasing concentrations of a chain transfer agent favours a decrease of the molecular weight of the polymer formed, which in turn decreases the viscosity of the system and minimises the phenomenon of auto-acceleration.
Despite the uncertainty inherent to the method adopted to carry out the SEC analysis, we can conclude that dodecyl mercaptan is an efficient chain transfer agent for this system.
The value of CS obtained by using the procedure described is of the same order of magnitude of the one obtained for methyl methacrylate in bulk and at 60 ºC, equal to 0.6310.
4.9 Conclusions
The free radical copolymerisation of maleic anhydride and iso-butyl vinyl ether was carried out in dioxane, methyl ethyl ketone and acetone. FT-NIR analysis was found not to be able to provide reliable values of the comonomers conversion when acetone was used as a solvent. The determination of the conversion of the comonomers is also uncertain when 1H NMR is used. Gravimetric analysis was found to be the most suitable technique to record the conversion versus time curves of the experiments carried out. The polymer produced has
-1 an average number molecular weight M n of about 150000 g mol and polydispersity index PDI comprised between 1.45 and 2.3. Polymers with lower M n and lower value of the PDI can be synthesised by adding increasing amounts of dodecyl mercaptan to the polymerisation mixture.
References
[1] T. E.Long, J. H.Y.Liu, B. A. Schell, D. M. Teegarden, D. S. Uerz; Macromolecules, 26, 6237, 2003. [2] M. D. Zammit, T. P. Davis; Polymer 38, 17, 4455-4468 1997. [3]X. Hao, Ph.D. Thesis, University of New England, Armidale – Australia September 2000.. [4] M Ratzsch; Progr. Pol. Sci., 13(4), 277, 1988. [5] X. Hao, K. Fujimori, D.J. Tucker, P.C. Henry, Eur. Pol. J., 36, 1145, 2000. [6] K. Fujimori, P.P. Organ, M.J. Costigan , I.E. Craven, J. Macromol. Sci., Chem., A23, 647 1986. [7] B. C. Trivedi, B. M. Culbertson; Maleic Anhydride, p. 279. Plenum Press, New York and London 1982. [8] J. P. A. Heuts, T. P. Davis, G. T. Russell; Macromolecules, 32, 6019, 1999. [9] M. Stickler, G. Meyerhoff; Macromol. Chem. Phys., 199, 2403 1998. [10] R. A. Hutchinson, D. A. Paquet, J. H. McMinn; Macromolecules 28, 5655 1995.
75 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
Chapter 5. Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.1 Experimental procedure
5.1.1. Purification of reagents and solvents
- THF was filtered over alumina and stored over 4 Å activated molecular sieve. For all other reagents and solvents purification procedures please refer to paragraph 4.1.1.
5.2 RAFT agents synthesis
5.2.1. Synthesis of benzyl dithiobenzoate (BDTB)
This reagent was synthesised according to the method described in Chernikova et al1: 5.3 g of S-(Thiobenzoyl)thioglycolic acid were dissolved in 200 ml of a solution of NaOH 0.25M. The solution was stirred at room temperature and 3.4 g of benzylmercaptan were added to it. BDTB started precipitating as dark red heavy oil from a pink solution. After 15 hours the product was extracted with ether and washed with a solution 0.1 M of sodium hydroxide followed by water. The organic phase was then dried over MgSO4, filtered on paper and evaporated on Rotavapor. Thin layer chromatography (TLC) (Rf=0.37) c-hexane/ ethyl acetate (499/1) was carried out and revealed the presence of negligible amounts of contaminants. NMR analysis confirmed this result and the yield of the synthesis was estimated to be approximately 95%:
1 H-NMR (CDCl3) : 4.6 (s, 2H, CH2-Ph), 7.3–7.6 (m, 8H, Ar–H), 8.02 (d, 2H Ar–H).
5.2.2. Synthesis of 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid)
This RAFT agent was prepared according to the procedure described by Stenzel and Davis 2: 3-mercapto propionic acid (10.35 g, 0.115 mol) was added to a solution of potassium hydroxide (13 g, 0.23 mol) in water (125 ml). Carbon disulfide (19 g) was added dropwise and the resulting orange solution was stirred for 5 hours. Benzyl bromide (19.6 g, 0.115 mol) was then added and allowed to react for 12 hours at 80ºC. The reaction
76 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
mixture was then allowed to cool down and diluted with chloroform (150 ml), it was subsequently acidified with hydrochloric acid until the organic layer turned into a yellow colour. The water layer was then extracted with chloroform (2x 50ml). The organic layers were combined and washed with a 10% solution of sodium carbonate in water (2x 50ml). After being dried over magnesium sulfate, the solvent was evaporated and the obtained product purified by gel column chromatography with a 3:1 hexane/ethyl acetate mixture as an eluent. The resulting yellow powder (90% yield) was characterised by NMR spectroscopy:
1 H-NMR (CDCl3) : 2.84 (t, 2H, CH2–C=O), 3.62 (t, 2 H CH2–S), 4.61 (s, 2H CH2–Ph), 7.27 (m, 5 H, Ar–H) 10.1(b, 1H, OH).
5.2.3. Synthesis of dibenzyl trithiocarbonate (DBTTC)
DBTTC was synthesised according to the method described by R. K. Bai 3 with a yield of approximately 80%. Dry anion exchange resin was added to carbon disulfide (100 mL) and stirred at room temperature for about 5 min. The colour of the resin changed from light yellow to deep red. Then benzyl bromide (0.020 mol) was added, and the reaction mixture was stirred under reflux for 1.5 h. After this time, the mixture was filtered and then washed with carbon disulfide. The filtrate was dried over anhydrous sodium sulphate overnight, and the solvent was removed under reduced pressure to afford 2.32 g of the pure product (yield: 80%).
1 H–NMR (CDCl3) !ppm : 4.67(s, 4H, 2 CH2–), 7.15–7.25 (m, 10 H, 2 Ar–H).
5.3 Polymerisation procedures
5.3.1. Polymerisation in Schlenck tubes
Copolymerisation of IBVE and MAn in solution was carried out at 60 C using AIBN as a radical initiator, the RAFT agent of choice and the selected solvent.
The RAFT agent was first weighed, followed by the appropriate amount of AIBN, MAn, IBVE and solvent. The polymerisation mixture was poured into a Schlenk tube and sealed with rubber septa, then 4 cycles of freeze-evacuate-thaw were executed. Part of the polymerising solution was transferred, under nitrogen atmosphere, into an IR cuvette previously freed from oxygen by nitrogen spurging. The rest of the solution was
77 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
immersed into a thermostatic water bath and kept at a temperature of 60 C. Aliquots of the polymerisation mixture were sampled at intervals according to the following procedure: an airtight syringe was flushed with nitrogen several times, part of the inert gas was then injected into the reaction vial and a small aliquot of the reaction mixture was withdrawn. The sample was immediately cooled in an ice bath and 1H NMR spectroscopy and gravimetric analysis were conducted to determine conversion values. While 1H NMR spectroscopy was carried out directly on the sampled solution, in the case of gravimetric analysis the sample was precipitated in diethyl ether, centrifuged and the residual moisture removed in a reduced pressure oven.
5.3.2. In situ NIR-FTIR spectroscopy polymerisation
The FT-NIR instrument and the procedure utilised in this chapter are identical to the ones described in paragraph 4.4.2.
5.4 Polymerisation kinetics
5.4.1. In situ FT-NIR spectroscopy
Monomer conversions were derived from FT-NIR spectra according to the procedure reported in paragraph 4.2.3.
5.4.2. 1H NMR spectroscopy
1 H NMR spectra were recorded in acetone-d6 using a Bruker ACF300 (300 MHz) spectrometer according to the procedure detailed in paragraph 4.3.2.
5.4.3. Gravimetric analysis
The gravimetric determination of the conversions was conducted in the same way described in paragraph 4.3.3.
78 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.5 Copolymer characterisation
5.5.1. 13C NMR spectroscopy The same procedure followed in paragraph 4.4.1 was also applied to characterise the polymer synthesised through controlled radical polymerisation.
5.5.2. Size Exclusion Chromatography
Molecular weights and molecular weight distributions were measured by size exclusion chromatography (SEC) with either N-N- dimethyl acetamide (DMAc) or tetrahydrofuran (THF) as eluents. A detailed description of the machinery and the procedure utilised has already been provided in paragraph 4.4.2.
5.6 Use of RAFT agents for the control of the molecular weight
Introduction
As already discussed in paragraph 2.3.3, the RAFT process provides a versatile and efficient way to synthesise a polymer with living characteristics. This can be achieved by adding an appropriate thiocarbonylthio compound called RAFT agent to the formulation of a classical free radical polymerisation (FRP) reaction. Scheme 5.1 illustrates the reversible addition-fragmentation mechanism peculiar of the RAFT process for the copolymerisation of MAn and IBVE. A growing chain adds to the thiocarbonylthio compound, forming an intermediate radical (1). This radical then fragments into a macro RAFT agent and a new radical R•, which re-initiates the polymerisation. The step which is believed to be responsible for imparting living characteristics to the process4 is the chain equilibration step; in this passage the growing chains add to the macro-RAFT agent forming an intermediate adduct which fragments releasing a new macro-RAFT compound and a growing chain. As already discussed, this equilibrium allows for the chains to grow in turn and with equal probability.
The monomer radicals are in this case IBVE and MAn (figure 5.1). An analysis of the MAn radical suggests that it is likely to present moderate reactivity towards the addition to the C=S double bond due to the electron-withdrawing feature of the carbonyl groups,
79 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
able to delocalise the negative charge and partially stabilise it, it also appears to be bulky. If we suppose that this radical behaves like styrene and its group, as discussed in paragraph 2.3.3.2, then it will require Z groups which are able to increase the rate of addition to the C=S double bond of the RAFT agent. Alternatively it might be similar to methyl methacrylates because of its bulkiness and therefore require highly activating Z groups in order to react with the RAFT agent. IBVE radical on the other hand is highly unstable and therefore reactive, due to the electron donor properties of the buthoxy substituent. The intermediate (1’) formed is therefore little prone to fragmentation when the leaving chains Pn or Pm are led by an IBVE radical. This radical is likely to resemble the characteristics and preferences of vinyl acetate monomers. A consultation of Scheme 2.9 indicates that there is no Z group able to provide good control over the copolymerisation of MAn and IBVE. IBVE, in fact requires the Z group to be either a xanthate or a carbamate, in order to facilitate the fragmentation of the intermediate (1’), however, based on scheme 2.9, such compounds do not seem to suit MAn. Three RAFT agents differing for their Z group were employed in an attempt to provide living conditions or molecular weight control to the copolymerisation of IBVE and MAn: dibenzyl-trithiocarbonate (DBTTC), 3-benzyl sulfanyl thiocarbonyl sulfanyl- propionic acid (RAFT acid) and benzyl dithiobenzoate (BDTB). Their activating groups (Z groups), as noted in scheme 5.1 are respectively Ph, Alkyl and Alkylthio for BDTB, RAFT-acid and DBTTC. As mentioned in 2.3.3.1, the ability of the Z group to stabilise the intermediate (1’) and (1’’) decreases in the order: Ph»Alkyl~Alkylthio 5 (SCH2Ph/SCH3); due to the increasing electron donor character of these substituents. This characteristic could be favourable for the IBVE radical, but detrimental for the MAn radical. Therefore, by using the mentioned RAFT agents, the influence of the Z group on the ability of the agent to control the copolymerisation of IBVE and MAn can be tested. It was speculated that a xanthate could be employed on this system, indeed it might have been useful for the control of the polymerisation of the IBVE radical, being the Z group in this compound substituted oxygen. However, on the basis of scheme 2.9 this family of control agents was not considered to be suitable to control the polymerisation of a radical with intermediate reactivity like the MAn radical.
80 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
O O O O
A B
Figure 5.1. The maleic anhydride (A) and the iso-butyl vinyl ether (B) radicals.
Last, it is worth mentioning that the introduction of a RAFT agent into a free radical copolymerisation system is not expected to cause any appreciable modification to the copolymer microstructure. In the alternating copolymer obtained by the free radical copolymerisation of MAn and styrene, for instance, the addition of the RAFT agent BDTB is reported not to alter the microstructure of the copolymers formed at low conversion and alternation could be equally observed by 13C NMR spectroscopy6. The same behaviour has also been reported when the RAFT agent BDTB mediates the free radical polymerisation of IBVE and MAn in dioxane7. However, in statistical copolymerisation, a cumyl dithiobenzoate (CBD) RAFT agent mediated copolymerisation has been reported to cause a slight modification of the copolymer microstructure in respect to the product obtained with conventional free radical polymerisation. A preference for the monomer with larger reactivity ratio has been observed for the composition of three statistical copolymer systems8.
81 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
Initiation and propagation I (Initiator) + Monomer Pn
Addition to RAFT agent S P S S S S R Pn S R n + R Pn + Z Z Z RAFT agent (1) Reinitiation
R + M Pm Chain equilibration
S S S P Pm S S Pn Pm S n + Pn Pm + Z Z Z
RAFT agent (1)
Z = Ph BDTB: R = CH2Ph
O O Z = CH CH COOH RAFT- acid: 2 2 R = CH2Ph Pm : or and RAFT agent Z = SCH Ph OO O O DBTTC: 2 O O R = CH2Ph
(2) (2')
O O O S S Pn P + S C S Pn S C S + n O O O O OOO O O
(1')
O O S S Pn O + S C S P n S C S + P O O n O O O O O O O
(1'')
Scheme 5.1. Mechanism of the generic reversible addition-fragmentation process3 and of the copolymerisation of IBVE and MAn.
82 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.6.1. 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) and dibenzyl-trithiocarbonate (DBTTC) as RAFT agents.
The table below provides the conditions employed for the experiments in which RAFT- acid and DBTTC were used as RAFT agents.
Table 5.1. Experimental conditions for the RAFT solution copolymerisation of IBVE and MAn conducted with benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) and benzyl-trithiobenzoate as RAFT agents.
Experiment [MT]0 [AIBN]0 [RAFT]/[AIBN] [MT]0/[RAFT] RP (mol l-1) (mol l-1) (mol L-1s-1) X 102 RAFT-acid(Diox-1) 6 1.5·10-2 2 200 9.5 RAFT-acid(Diox-2) 6 2·10-3 10 280 6.12 DBTTC(Ac-1) 6 2 10-3 10 280 0.58
RAFT-acid has already been successfully used for the RAFT mediated polymerisation of butyl acrylate9, so this RAFT agent was chosen in virtue of the similarity between the structure of the MAn and the acrylyl radicals. In regard to DBTTC, it has been reported to be successful in providing living conditions to the synthesis of poly(MAn-co-Sty)10. According to what was discussed in the introduction, however, the behaviour of the IBVE radical towards this RAFT agent might differ substantially from the styrene radical.
The addition of both RAFT agents imparted a yellow colour to the polymerisation mixture.FT-NIR conversion versus time plot for experiment RAFT-acid(Diox-1) can be viewed in figure 5.2. As experienced for the kinetics of the free radical polymerisation, in this case also the rate of reaction of IBVE appears to be slightly higher than the one associated to MAn. This phenomenon has already been analysed in paragraph 4.7.1 and as a consequence, the conversion of the copolymerisation reaction has been calculated from the curve that represents the conversion of MAn. The SEC chromatograms relative
to the same experiment (figure 5.3) demonstrate that the value of M n has decreased considerably: from about 378000 g mol-1 in FRPDiox-2 (table 4.1) to about 20,000 g mol-1 in the present conditions (see figure 5.4). In the same figure, it can be noticed that in the course of the copolymerisation with RAFT-acid no increase of the molecular
weight takes place. Furthermore, a comparison between the Rp of these two experiments suggests that, subject to errors in the determination of the conversion, only a slight -2 -1 -1 retardation is present in this RAFT process (Rp=9.5 10 mol L s for RAFT-acid(Diox-
83 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
-2 -1 -1 1) and Rp=11 10 mol L s for RAFT-acid(Diox-1)). These observations indicate that the RAFT agent acts as a conventional chain transfer agent and that the termination process still dominates this system, causing the formation of dead polymer. In order to reduce the amount of termination events, experiment RAFT-acid(Diox-2) was carried out at a higher [RAFT]/[AIBN] ratio. Figure 5.5 shows the conversion-time curve in which both comonomers appear to react at the same ratio, however figures 5.6 and 5.7 show that living conditions have not been achieved, since the value of the M n does not increase in the course of the reaction. Analogous considerations to the ones made for the previous experiments can also be made in regard to reaction DBTTC(Ac-1), for which dibenzyl trithiocarbonate was used. Figure 5.8 represents the conversion vs time curve; it can be noticed that this reaction reached a value of conversion equal to about 45% after about 500 minutes and no further increase was registered after 1300 minutes. This result shows that this reaction is remarkably retarded respect to the free radical process * -2 -1 -1 FRP(Ac-d6c2) and also respect to RAFT-acid(Diox-1) (Rp=2.3 10 mol L s for * -2 -1 -1 FRP(Ac-d6c2) and Rp=0.58 10 mol L s for DBTTC(Ac-1)). Figure 5.9 represents the evolution of the M n over time, again this parameter stays constant with increasing conversion. The fact that PDI increases demonstrates that the termination mechanism dominates the process.
From the above observations we can deduce that both RAFT compounds act as conventional chain transfer agents for the system under study; this is demonstrated by the lower molecular weight of the copolymer obtained in both experiments with respect to the FRP process. The retardation noticed to a minor extent in the case of RAFT-acid and to a greater extent in the case of DBTTC may suggest that the Z constituents (an alkyl thioester for RAFT-acid and a thiocarbonate for DBTTC ) are not able to adequately activate the C=S double bond of the RAFT agent towards addition by the MAn radical. On the other hand IBVE radical may easily add to the RAFT agent, however the adduct generated by this reaction is not likely to fragment. All these factors contribute to inhibit the chain equilibration step from taking place resulting in these
RAFT agents not be able to control the value of the M n and therefore develop systems with living properties.
84 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
100 NIR-FTIR X i-BVE
NIR-FTIR XMAn 80
60
40 Conversion %
20
0 0 20406080100120140 time / min
Figure 5.2. Experiment RAFT-acid(Diox-1):monomers conversion versus time plots as calculated by in-situ NIR-FTIR spectroscopy for the RAFT copolymerisation of -2 -1 IBVE and MAn. T=60˚C, [MT]0/[RAFT-acid]=200, [AIBN]0=1.5·10 mol L , [RAFT-acid]=3·10-2 mol L-1.
2.0 Conversion 14% 56% 74% 1.5 78%
1.0
dW/d(LogM) 0.5
0.0
3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 Log M / gr mol-1
Figure 5.3: Experiment RAFT-acid(Diox-1): evolution of the SEC chromatograms for the solution RAFT copolymerisation of IBVE and MAn in dioxane as a solvent. -2 -1 -2 T=60˚C, [MT]0/[RAFT-acid]=200, [AIBN] 0=1.5·10 mol L , [RAFT-acid]= 3·10 mol L-1.
85 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
"$
"# PDI
20000
15000 -1
10000
Mn / g mol 5000
0 0 20406080100 / %
Figure 5.4. Experiment RAFT-acid(Diox-1): variation of the molecular weight with conversion for the solution RAFT copolymerisation of IBVE and MAn in dioxane -2 -1 as a solvent. T=60˚C, [MT]0/[RAFT-acid]=200, [AIBN] 0=1.5·10 mol L , [RAFT- acid]=3·10-2 mol L-1.
75
60
45
30 Conversion / %
X 15 i-BVE X MAn
0 0 50 100 150 200 time / min
Figure 5.5. Experiment RAFT-acid(Diox-2): monomers conversion versus time plots as calculated by in-situ NIR-FTIR spectroscopy for the RAFT copolymerisation of IBVE and Man in dioxane as a solvent. T=60˚C, [MT]0/[RAFT- -1 -3 -1 2 -1 acid]=280, [M]0=6 mol L , [AIBN] 0=2·10 mol L , [RAFT-acid]0=2·10- mol L .
86 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.6 = 26 % co(poly-i-BVE/MAn) 1.4 co(poly-i-BVE/MAn)= 57 %
1.2
1.0
0.8
0.6 dW/d(LogM)
0.4
0.2
0.0
-0.2 3.5 4.0 4.5 5.0 5.5 Log M / gr mol-1
Figure 5.6. Experiment RAFT-acid(Diox-2):evolution of the SEC chromatograms for the solution RAFT copolymerisation of IBVE and MAn in dioxane as a solvent. -1 -3 -1 T=60˚C, [MT]0/[RAFT-acid]=280, [M]0=6 mol L , [AIBN] 0=2·10 mol L , [RAFT- 2 -1 acid]0=2·10- mol L .
"%
"$
PDI "# 45000
30000 -1
15000 Mn / g mol
0 0 20 40 60 80 100 / %
Figure 5.7. Experiment RAFT-acid(Diox-2): variation of the molecular weight with conversion for the solution RAFT copolymerisation of IBVE and MAn in dioxane -1 -3 as a solvent. T=60˚C, [MT]0/[RAFT-acid]=280, [M]0=6 mol L , [AIBN]0=2·10 mol -1 2 -1 L , [RAFT-acid]0=2·10- mol L .
87 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
50
40
30
/ % 20
10
0 0 200 400 1250 1300 1350 Time / min
Figure 5.8. Experiment DBTTC(Ac-1): conversion versus time plot for the solution RAFT copolymerisation of IBVE and MAn in acetone as a solvent. T = 60ºC, -3 -1 -2 -1 [AIBN]=2 10 mol L [DBTTC]=2 10 mol L , [MT]/ [BDTB]=280, [BTTB]/[AIBN]=10.
1.50
1.35
PDI 1.20
24000
-1 16000
Mn / g mol 8000
0 0 1020304050 / %
Figure 5.9. Experiment DBTTC(Ac-1): variation of the molecular weight with conversion for the solution RAFT copolymerisation of IBVE and MAn in acetone -3 -1 -2 -1 as a solvent. T=60ºC, [AIBN]=2 10 mol L , [BTTB]=2 10 mol L , [MT]/ [BDTB]=280, [BTTB]/[AIBN]=10.
88 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.7 BDTB as a RAFT agent
On the basis of the conclusions reached in the previous paragraph it was decided to adopt a RAFT agent bearing a Z group able to strongly activate the RAFT agent towards the addition of growing radicals. Since a phenyl substituent is able to highly stabilise the intermediate species generated by the addition of a growing radical to a RAFT agent, some experiments with benzyl dithiobenzoate (BDTB) as a controlling agent were carried out.
5.7.1. RAFT polymerisation with BDTB in polar solvents
Table 5.2 summarises the experimental conditions of the experiments carried out in this section. Copolymerisation experiments were carried out in polar solvents: acetone-d6 ( = 20.7, DN = 17 kcal mol-1) ethyl acetate (EtAc) ( = 6.02, DN = 17.1 kcal mol-1) and N, N dimethylacetamide (DMAc) ( = 37.78, DN = 27.8 kcal mol-1).
Table 5.2. Experimental conditions for the copolymerisation of IBVE and MAn at -1 60 °C with BDTB as a RAFT agent, [IBVE]/[MAn] =1, [M] 0 = 6 mol L .
Experiment [AIBN] 0 [RAFT] 0 [RAFT]/[AIBN] Macrophase (mol L-1) (mol L-1) separation BDTB(Ac-200) 1.5·10-2 3·10-2 2 Yes BDTB(Ac-400) 1.5·10-2 1.5·10-2 1 No BDTB(Ac-800) 1.5·10-2 7.5·10-3 0.5 No BDTB(EtAc-1) 2·10-3 2·10-2 10 Yes BDTB(DMAc-1) 2·10-3 2·10-2 10 Yes
5.7.1.1 Acetone
During the course of polymerisation BDTB(Ac-200) visual macrophase separation occurred. It manifested with the appearance of a thin clear layer on top of the red- coloured polymerisation mixture. E. Chernikova et al1 also report the occurrence of the same phenomenon during their RAFT polymerisation of Styrene and MAn in dioxane with BDTB.
89 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
The polymerisation mixture also experienced a change in colour: from the characteristic red colour imparted by BDTB to an orange or even brown colour for high ratios [M]/ [BDTB]. During experiment BDTB(Ac-200) for instance, the solution turned to a brown colour and subsequently back to orange. When the same experiment was repeated, the polymerisation solution turned to a dark brown and no polymer formed. Figure 5.10 is the conversion versus time plot of BDTB(Ac-1); it shows clearly that IBVE is reacting much faster than MAn is. The values of conversion were calculated with NIR-FTIR and also with 1H NMR spectroscopy, according to the method described respectively in paragraphs 4.2.3 and 4.3.2. The marked difference between the values of conversion of the two comonomers is likely to be due to the fact that phase separation alters the ratio between the components. We also know, from previous experiments, that the presence of acetone in the reaction mixture is responsible for inaccuracy in the NIR-FTIR measurement, and that error increases at increasing molar ratio between MAn and IBVE. Values of the conversion calculated through 1H NMR spectroscopy also suggest that the rate of disappearance of IBVE comonomer from the lower phase is much higher than the rate of disappearance of MAn comonomer. It was concluded that perhaps along with the formation of the alternating copolymer of MAn and IBVE, IBVE homopolymer could be forming.
In regard to the SEC analysis of the copolymers, samples were drawn at intervals and analysed by SEC previous dilution with DMAc. The chromatograms obtained are monomodal (figure 5.11) and an increment in molecular weight can be noticed for increasing values of the conversions in figure 5.12. The values of conversion for this plot have been calculated by NIR-FTIR and are related to MAn; they should therefore be considered an approximation. The value of the polydispersity index, (PDI) of the samples is around 1.3. When this data is compared with the FRP process FRP(Ac-d6)
(table 4.2) we observe that in BDTB(Ac-1) M n increases with conversion and its value -1 has decreased from about 125000 g mol to about 12000, while PDI has decreased from about 1.6 to 1.3. A comparison with the previous RAFT experiments, run with RAFT- acid and with DBTTC also indicates that control over the M n has been achieved with the use of BDTB. Although the uncertainty associated with the value of the conversions limits the extent of the discussion, it can be commented that the value of the M n is higher than the theoretical one (equation 2.21). This could be due to the fact that the determination of the M n relies on calibration with polystyrene standards and it could therefore be inaccurate, or it might hint to the possibility that BDTB is not all consumed
90 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
and therefore that this is not the ideal agent for the system in consideration. In any case, the value of the M n achieved through the use of BDTB is the closest to the theoretical value, suggesting that this is the most effective agent among the ones used so far. The value of PDI has also decreased respect to the previous agents, proving that the occurrence of termination events has been reduced.
NIR-FTIR i-BVE 100 NIR-FTIR MAn 1 H MAn 1 H i-BVE 80
60
40
20
0 0 100 200 300 400 Time / min
Figure 5.10. Experiment BDTB(Ac-200): monomers conversion versus time plots as calculated by in-situ NIR-FTIR spectroscopy for the RAFT copolymerisation of -1 -2 -1 IBVE and MAn in acetone as a solvent. [M]0 = 6 mol L , [AIBN]0=1.5·10 mol L , -2 -1 [BDTB]0=3·10 mol L .
91 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
Conversion 1.0
0.5 dW/d(LogM)
0.0
3.5 4.0 4.5 5.0 Log M / gr mol-1
Figure 5.11. Experiment BDTB(Ac-200): evolution of the SEC chromatograms for the solution RAFT copolymerisation of IBVE and MAn in acetone as a solvent. -1 -2 -1 -2 -1 [M]0 = 6 mol L , [AIBN]0=1.5·10 mol L , [BDTB]0=3·10 mol L . Conversions are 28%, 41% and 42% respectively.
1.5
PDI 1.0
16000
12000 -1
8000 gr mol / n M 4000
Theoretical Mn Mn 0 0 10203040 X / % poly(co-iBVE/MAn)
Figure 5.12. Experiment BDTB(Ac-200): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot for the solution RAFT -1 copolymerisation of IBVE and MAn in acetone as a solvent. [M]0 = 6 mol L , -2 -1 -2 -1 [AIBN]0=1.5·10 mol L , [BDTB]0=3·10 mol L . Conversions are 28%, 41% and 42% respectively.
92 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
Both phenomena: the discolouration of the polymerisation mixture and the formation of two phases seemed to suggest that the presence of BDTB could be interfering with the copolymerisation system and perhaps with the formation of the complex that is believed to be crucial for the copolymerisation to take place. In order to prove this theory, the ratio between the monomer and the RAFT agent concentration ([M]/[RAFT]) was varied from 200 to 800 by decreasing the concentration of RAFT agent. Each experiment with ratio between the monomer and the RAFT agent concentration [M]/[RAFT] equal to 200, 400 and 800 was at least repeated twice. For each experiment, in some instances the onset of visual macrophase separation did not manifest, which resulted in the difference between the values of conversions of the two comonomers to become smaller. However, in some others, discoloration and phase separation clearly appeared. It had to be concluded that the occurrence of phase separation, for the considered range of the ratio monomer:catalyst was purely incidental and did not depend on this parameter. Figure 5.13 is the plot in semi-logarithmic coordinates of runs BDTB(Ac-400) and BDTB(Ac- 800); the fact that these plots are linear indicates that a constant concentration of active sites is maintained in the course of the reactions. All reactions appear to be accelerated in respect to a linear plot, indicating that perhaps slow initiation characterises these systems. In regard to the evolution of the molecular weight with conversion, from the
M n versus conversion plot depicted in figure 5.14, we can see how a decrease in the concentration of BDTB causes the M n of the formed copolymer to increase. However, even the lowest ratio shows the ability to keep some control over the reaction, as demonstrated by the closeness of the values of the M n to the theoretical ones and by the low values of the PDI.
93 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
[M]/[BDTB]=400 NMR 2.4 IBVE [M]/[BDTB]=400 NMR MAn [M]/[BDTB]=800 NMR IBVE 2.0 [M]/[BDTB]=800 NMR MAn [M]/[BDTB]=280 NIR-FTIR MAn 1.6 ) 1.2 ln(1/1- 0.8
0.4
0.0 0 50 100 150 200 250 300 350 400 time /min
Figure 5.13. Experiments BDTB(Ac-400) and BDTB(Ac-800): monomers conversion versus time plots as calculated by 1H NMR for the RAFT -1 copolymerisation of IBVE and MAn in acetone as a solvent. [M]0 = 6 mol L , -2 -1 -2 -1 [AIBN]0=1.5·10 mol L . For BDTB(Ac-400) [BDTB]0=1.5 10 mol L , for BDTB -3 -1 (Ac-800) [BDTB]0=7.5 10 mol L .
94 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.50
1.25 PDI
75000 [M] / [BDTB]=800 [M] / [BDTB]=400 [M] / [BDTB]=200 60000
-1 45000 mol / g n 30000 M
15000
0 0 20406080 / %
Figure 5.14. Experiments BDTB(Ac-200), BDTB(Ac-400) and BDTB(Ac-800): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot as calculated by 1H NMR for the RAFT copolymerisation of IBVE -1 -2 -1 and MAn in acetone as a solvent. [M]0 = 6 mol L , [AIBN]0=1.5·10 mol L . For -2 -1 -2 - BDTB(Ac-200) [BDTB]0=3·10 mol L , for BDTB(Ac-400) [BDTB]0=1.5 10 mol L 1 --3 -1 , for BDTB(Ac-800) [BDTB]0=7.5 10 mol L .
From this set of experiments it can be concluded that:
• The polymerisation in acetone is characterised by phase separation of the polymerising mixture. This phenomenon is accompanied by faster rate of reaction of IBVE respect to MAn and it therefore prevents the copolymer from reaching high values of conversion;
• The fact that visual phase separation was not observed in FRP experiments inevitably links the insurgence of this phenomenon to the addition of the RAFT agent to the reaction mixture. Later on in this chapter, in paragraph 5.7.1.3, both phases are going to be analysed in an attempt to comprehend the reasons for this behaviour;
• For values of the ratio between the monomer and the RAFT agent concentration [M]/[RAFT] comprised between 200 and 800, the onset of phase separation does not depend on this parameter;
95 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
• When side-effects are not present good control is maintained even at high ratios [M]/[BDTB].
96 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.7.1.2 Ethyl acetate and N, N, dimethyl acetamide
It is known from literature that the equilibrium constant of the complex formed between MAn and IBVE varies according to the solvents in use; it has been found to increase the less polar the solvent11,12. The value of the constant, in turn, influences the rate of reaction and faster copolymerisation reactions have been seen to take place in the least polar solvent. In order to investigate the factors influencing the phase separation of the reaction medium, the reaction was run in EtAc ( = 6.02, DN = 17.1 Kcal mol-1) and in DMAc ( = 37.78, DN = 27.8 Kcal mol-1) as solvents. Figures 5.15 and 5.16 provide the conversion versus time curve for BDTBEtAc-1 and for BDTBDMAc-1, visual macrophase separation was again experienced in both experiments and in the course of the experiment in DMAc the polymerising solution turned to a dark brown colour. In regard to the value of the rate of copolymerisation, data suggest that the rate of copolymerisation in ethyl acetate is comparatively the fastest one (once the difference in initiator concentration has been taken into account) and the reaction in N,N dimethylacetamide appears to be the slowest one. This result is in agreement with the theory discussed above, however it does not rule out that the presence of the RAFT agent may affect the equilibrium constant of complex formation between MAn and IBVE.
70
60
50
40
30
Conversion / % Conversion 20
NIR-FTIR 10 i-BVE NIR-FTIR MAn 0 0 200 400 600 800 1000 time / min
Figure 5.15. Experiment BDTBEAc-1: monomers conversion versus time plot as calculated by in-situ NIR-FTIR spectroscopy for the RAFT copolymerisation of -1 -3 -1 -2 IBVE and MAn: T=60 ˚, [M]0=6 mol L , [AIBN]0=2·10 mol L , [BDTB]=2·10 mol L-1.
97 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
80 NIR-FTIR i-BVE NIR-FTIR MAn
60
40 Conversion / % 20
0 0 50 100 150 200 time/min
Figure 5.16. Experiment BDTBDMAc-1: monomers conversion versus time plot as calculated by in-situ NIR-FTIR spectroscopy for the RAFT copolymerisation of -1 -3 -1 -2 -1 IBVE and MAn. [M]0=6 mol L , [AIBN]0=2·10 mol L , [BDTB]=2·10 mol L .
5.7.1.3 Characterisation of the two phases
At this point the experiment which had been carried out in ethyl acetate was utilised to investigate the nature of the two phases. This was done by analysing the microstructure of the polymers formed by 13C NMR spectroscopy. The procedure followed is described below and it is also represented in scheme 5.2:
a. A reaction vial displaying phase separation (1) was vigorously agitated to blend the two phases together,
b. A sample was quickly withdrawn from vial 2 and poured in anhydrous diethyl ether;
Both MAn and IBVE monomer and their respective homopolymers are soluble in diethyl ether, while Poly(co-maleic anhydride/iso-butyl vinyl ether) is insoluble in this solvent. 13
98 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
c. The precipitate formed (3’) was analysed by NMR,
The distortionless method described in the experimental section 4.4.1 was applied to the precipitate formed (3’) and the spectra obtained were compared with previous assignments for this copolymer 14. The full 13C NMR spectrum and the methylene sub- spectrum are displayed respectively in figures 5.17 and 5.18. In the methylene spectrum, figure 5.18, the C5 resonance at 30.9-36.5 ppm was assigned to the alternating MAn/IBVE/MAn (010) triad. The sequence at 38.8-42 ppm, indicative of the presence of the (110) triad and the sequences at 36.5-38.8 ppm of C5, characteristic of the presence of the semi-alternating triads (110) and (011) are absent and so is the triad (111). This proved that the reaction had produced a copolymer with alternating characteristics between the monomeric units.
d. The surnatant solution (3) obtained from the precipitation of the copolymer was added with methanol, in order to check for the presence of eventual other compounds.
In this case also, we can appeal to the difference in solubility of poly (MAn) and poly- (IBVE) to separate the two compounds; between the two compounds only poly-(IBVE) is in fact insoluble in methanol. Upon addition of this solvent, the formation of a white gelatinous precipitate (4’) was noticed.
e. The precipitate formed (4’) was dried in a vacuum oven and then solubilised in d-chloroform;
The resulting solution was analysed by 13C NMR spectroscopy, the DEPT 13C sub- spectrum of the methylene carbons can be viewed in figure 5.19.
f. The remaining surnatant solution (4) was first concentrated then chloroform added, in order to precipitate any eventual poly(MAn) present.
No insoluble compounds formed in this case. This is not surprising considering that the homopolymerisation of MAn is not likely to take place under the low initiator concentration conditions we are operating at (Copolymerisation systems 3.4.1.1). A comparison between the spectrum of the unknown precipitate (4’) and the spectrum of poly(IBVE), cationically synthesised, provides the evidence that the shift of the C5 peak at 38.8-42 ppm is the same one of the C5 peak of poly-IBVE. The triad (111) at 38.8-42 ppm dominates this spectrum and no other features can be noticed to indicate the
99 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
presence of further microstructures. Figure 5.20 compares the DEPT sub-spectra shifts of the methylene carbons C5 and C7 in the different compounds.
a b c
1 2 3 4
3’ 4’
Scheme 5.2. Procedure followed for the identification of the products of the -1 copolymerisation between MAn and IBVE. T=60˚C, [M]0 = 6 mol L , [AIBN] 0 = 2·10-3 mol L-1, [BDTB]= 2·10-2 mol L-1.
100 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
O OR O O C C 5 CH CH 2 6 HC CH CH HA HA 2 CH 2 C C 3
1 4 C C OR O O O
H H
7 89, 10 R = C C ( C H 2) H H H C8, acetone d acetone d 6 6
C9,10
C1,4 C6 C3C5 C7 C2
200 150 100 50 0 ppm
13 Figure 5.17. Experiment BDTB(EtAc-1): full C NMR spectrum in acetone-d6 of the copolymer obtained by precipitating the reation mixture in diethyl ether. -1 -3 -1 -2 -1 T=60˚C, [M]0 = 6 mol L , [AIBN] 0 = 2·10 mol L , [BDTB]= 2·10 mol L solvent: ethyl acetate.
C 5 (010) 36.5-30.9 ppm C 7 Residue of C , C 9 10
C 5
(011)+(110) (010) 38.8-36.5 ppm
50 45 40 35 30 25
250 200 150 100 50 0 ppm
Figure 5.18. Experiment BDTB(EtAc-1): DEPT 13C NMR sub-spectrum in acetone- d6 of the methylene carbon of MAn (0)/IBVE(1) copolymer obtained by -1 precipitating the reation mixture in diethyl ether. T=60˚C, [M]0 = 6 mol L , [AIBN] -3 -1 -2 -1 0 = 2·10 mol L , [BDTB]= 2·10 mol L .
101 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
C 7 C (111) 5 42.0-38.8 ppm
C 5
55 50 45 40 35 30
250 200 150 100 50 0 / ppm
Figure 5.19. Experiment BDTB(EtAc-1): DEPT 13C NMR sub-spectrum in d- chloroform of the methylene carbon of the substance obtained by precipitating the -1 -3 -1 surnatant solution in methanol. T=60˚C, [M]0=6 mol L , [AIBN]0=2·10 mol L , [BDTB]=2·10-2 mol L-1.
102102 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
C 7
C 5
MAn/IBVE copolymer
80 60 40 20 ppm
C 7
C 5
unknown precipitate
80 60 40 20 / ppm
O OR O O C C C 7 5 CH CH 2 6 HC CH CH HA HA 2 CH 2 C C 3
1 4 C C OR O O O
H H
7 89, 10 R = C C ( C H 2) C 5 H H H
IBVE homopolymer
80 60 40 20 / ppm
Figure 5.20. Experiment BDTB(EtAc-1): comparison of the DEPT CH2 sub-spectra of the different compounds obtained when phase separation occurs. T=60˚C, [M]0 = -1 -3 -1 -2 -1 6 mol L , [AIBN] 0 = 2·10 mol L , [BDTB]= 2·10 mol L .
103 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
The same procedure was repeated on both phases separately and demonstrated that MAn /IBVE copolymer forms in the red coloured lower phase while poly-IBVE is present in the upper, clear layer. Based on these results, the conclusion that can be drawn up to this point is that two processes take place in parallel when phase separation occurs:
1) The alternating copolymerisation between IBVE and MAn.
This reaction takes place in the lower, red-coloured layer. It produces poly(IBVE-co-
MAn) with an M n that increases with conversion and PDI lower than 1.5. The reagents conversion can be determined gravimetrically by estimating the amount of polymer produced upon precipitation of the reaction mixture in DEE.
2) The homopolymerisation of IBVE.
This reaction produces poly-IBVE which forms a separate top layer, when it is insoluble in the solvent employed. The amount of IBVE involved in the homopolymerisation has been determined gravimetrically on many occasions, according to the procedure described in paragraph 5.7.1.3. For severely inhibited copolymerisations, which were characterised by copolymer conversion around 10 % after about 20 hours, it was observed that the ratio between the two polymers was close to 1. Furthermore, the homopolymer seemed to degrade over time, as demonstrated by the fact that the highest value of copolymer conversion would be recorded after about 3 hours and it would then be lower on the final sampling, which would occur after about 20 hours from the start of the reaction.
It is uncertain what causes IBVE to homopolymerise. The presence of BDTB appears to be necessary for such reaction to take place, since this behaviour was not noticed in the course of FRP reactions. It is known from literature that IBVE homopolymerises through cationic polymerisation mechanism and therefore a cationic initiator must originate in the reaction mixture, due to the presence of the RAFT agent. It is possible that impurities in the system initiate this reaction; for instance Maleic acid (MAH) (3% by 1H NMR spectroscopy), may protonate the RAFT agent and form a stable cationic compound able to initiate the cationic polymerisation of IBVE.
Since solvents with increasing polarity have been found to increase the cationic propagation rate (kp) by separating the ions (propagating and counterion) and therefore
104 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
lowering the steric restrictions to the incoming monomer2. As a consequence we assumed that the employment of a solvent characterised by lower polarity and lower DN value, like dioxane ( =2.209, DN =14.8 kcal·mol-1) would repress the cationic homopolymerisation of IBVE and eliminate the phase separation.
5.7.2. RAFT polymerisation with BDTB in 1,4 dioxane as a solvent
Some experiments were conducted with dioxane in virtue of its low polarity ( =2.209, DN =14.8 kcal·mol-1). By doing so, as discussed at the end of paragraph 5.7.1.3 it was intended to deter the formation of IBVE homopolymer. 1,4 dioxane was carefully purified and stored, according to the modality described in paragraph 4.1.1. This was done to prevent the peroxides present in 1,4 dioxane from interfering with the RAFT agent15.
No phase separation was noticed when dioxane was used as a solvent, although the presence of poly-IBVE was detected in some circumstances, according to the method described in the previous paragraph. Rather than a contradiction this is a consequence of the fact that poly(IBVE) has an increased solubility in this solvent. Experiments
characterised by different [MT]/[BDTB] ratio and initiator concentrations were run and compared, their conditions are described in table 5.3:
Table 5.3: Experimental conditions and Rp for the solution RAFT copolymerisation of IBVE and MAn conducted in 1,4 dioxane as a solvent.
Experiment [MT]0 [AIBN]0 [BDTB]/[AIBN] [MT]/[BDTB] RP (mol L-1) (mol L-1) (mol L-1s-1) BDTB(Diox-1) 5.6 5·10-3 4 400 0.078 BDTB(Diox-2) 5.6 1·10-2 2 280 0.04 BDTB(Diox-3) 5.6 1·10-2 2 400 0.126 BDTB(Diox-4) 5.6 5·10-3 4 280 0.001 BDTB(Diox-5) 6 2·10-3 10 300 0.011 *FRP(Diox-1c) 5.6 5 10-3 NA NA 0.035 *FRP(Diox-2c) 5.6 1 10-2 NA NA 0.034
*Rp values were theoretically derived from FRPDiox-1 and FRPDiox-2 (table 4.1).
Reaction mixtures characterised by low [MT]/[BDTB] ratio ( 280) in conjunction with low initiator concentration ( 5 10-3) experience a rather erratic outcome. In some
105 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
instances, as in the case of experiment BDTBDiox-1 (figure 5.21) results repeatability is a critical issue and in the two distinct occasions this reaction was run it proceeded at quite differing rates. Furthermore, even when the reaction occurred at the fastest rate, IBVE homopolymer was present as a side-product, therefore high conversions could not be achieved even after a long period of time. The polymerisation mixture also experiences a change in colour: from the characteristic red colour imparted by BDTB, to an orange colour. At lower ratios ( 187) not even a high initiator concentration is able to initiate the polymerisation and the reaction mixture may turn to a dark brown colour. These phenomena seem to indicate that perhaps, in such conditions, a reaction involving the RAFT agent takes place.
At a [MT]/[BDTB] ratio equal to 400 as in experiment BDTB(Diox-1) (figure 5.21) the rate of polymerisation is not affected by retardation respect to the free radical reaction, *FRP(Diox-1c) (see table 5.3). The kinetic plot in semi-logarithmic coordinates (figure 5.21) exhibits an acceleration with respect to the linear trend, which might derive from a slow initiation mechanism. The values of the M n (figure 5.22) appear to increase with the value of conversion, however it is higher than the calculated one. The discordance between the two values could either derive from lack of accuracy in the evaluation of
M n , due to the fact that the calibration curve was obtained from polystyrene standards, or it could confirm that slow initiation occurs. This hypothesis would also provide an explanation to the fact that the initial value of the polydispersity index is relatively high (PDI = 1.3 at 16% conversion). A slow initiation mechanism causes the polymeric chains to form at different times; as a consequence of the delay, chains differ in length, generating a high initial polydispersity index. PDI is then expected to decrease with increasing conversion. In this present case, however the value of the PDI increases to 1.4 at~60% conversion indicating that some termination may take place (figure 5.22). Termination derives from the fact that some active chains are present in the system. This suggests that the chain deactivation step, either the addition step or the chain equilibration step (refer to scheme5.1) may not be optimal.
Figure 5.25 compares experiment BDTB(Diox2) and BDTB(Diox3), run with
[MT]/[BDTB]=280 and 400 respectively and with initial concentration of AIBN equal to -2 -1 1·10 mol L . The trend for [MT]/[BDTB]=400 seems to indicate that deceleration occurs, it is therefore likely that the amount of BDTB adopted is not sufficient to control the reaction. The M n of the polymer obtained increases with conversion (figure 5.26),
106 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
however the value of PDI is high, ranging from 1.39 to 1.49. The pseudo first-order rate plot for [MT]/[BDTB]=280 (figure 5.25) on the other hand increases linearly and so does Mn versus (figure 5.26)"&he value of PDI (figure 5.26) slightly increases, but it keeps lower than in BDTB(Diox-4) and below the value of 1.5. We can conclude that in these conditions good control has been achieved.
Figure 5.29 compares the conversion versus time plots of experiments BDTB(Diox-1) and BDTB(Diox-3), characterised by [MT]/[BDTB]=400 and initial concentration of AIBN respectively equal to 5 10-3 and 1 10-2 mol L-1. As reported in table 5.3, the initial rate of reaction, Rp, of BDTB(Diox-1) appears to be faster than the correspondent calculated value of the free radical polymerisation *FRP(Diox-1), while experiment
BDTB(Diox-3) appears to be retarded with respect to *FRP(Diox-2c). The value of Rp of BDTB(Diox-1) and BDTB(Diox-3), with a little approximation, increases linearly with the square root of the initiator concentration. For both runs the values of the M n (figure 5.30) are higher than the calculated ones and very similar to each other. This property is distinctive of living systems.
Figure 5.31 is the conversion versus time plot of reactions BDTB(Diox-2) and
BDTB(Diox-4) with a [MT]/[BDTB]=280 and initial AIBN concentration equal respectively to 5 10-3 mol L-1 and 1 10-2 mol L-1. This plot shows that no appreciable increase in conversion has happened after 4 hours in the reaction with the least amount of initiator (BDTB(Diox-2), this reaction is therefore retarded in respect to the FRP run (table 5.3) and also in respect to BDTB(Diox-4) once the difference in initiator concentration has been taken into account.
107 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.0 [M ]/[BDTB]=400 T [M ]/[BDTB]=280 T 0.8 ) 0.6
ln(1/1- 0.4
0.2
0.0 0 50 100 150 200 250 Time / min
Figure 5.21. Experiments BDTB(Diox-1) and BDTB(Diox-4): pseudo first-order rate -1 -3 -1 plot. T=60º C, [MT]= 5.6 mol L , [AIBN]=5·10 mol L . For BDTB(Diox-1), [BDTB]/[MT]=400, for BDTB(Diox-4)[BDTB]/[MT] =280.
1.6
PDI 1.2 60000
45000 -1
30000 g mol / n M 15000 [M] / [BDTB]=280 T [M] / [BDTB]=400 T 0 0 20406080 / % Figure 5.22. Experiments BDTB(Diox-1) and BDTB(Diox-4): number average molecular -1 weight Mn and polydispersity index (PDI) vs. conversion plot. T=60ºC, [MT]=5.6 mol L , -3 -1 [AIBN]=5·10 mol L . For BDTB(Diox-1) [BDTB]/[MT]=400, for BDTB(Diox-4) [MT]/[BDTB ]=280
108 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.4 = 16.57 % 1.2 = 33.25 % Conversion = 52.47 % = 60.92 % 1.0 = 58.35 % 0.8
0.6 dW/d(LogM) 0.4
0.2
0.0
3.9 4.2 4.5 4.8 5.1 5.4 5.7 Log M
Figure 5.23. Experiment BDTB(Diox-1): evolution of the SEC chromatograms; -3 -1 T=60º C, [MT]= 5.6 M, [AIBN]=5·10 mol L , [MT]/[ BDTB ]=400.
1.4
1.2 = 6% Conversion = 7% = 6% 1.0 = 6% = 8% 0.8
0.6 dW/d(LogM) 0.4
0.2
0.0
3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 Log M / g mol-1
Figure 5.24. Experiment BDTB(Diox-4): evolution of the SEC chromatograms; -3 -1 T=60º C, [MT]= 5.6 M, [AIBN]=5·10 .mol L , [[MT]/[ BDTB ]=280.
109 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
0.8
0.7
0.6
0.5
0.4
ln1/1- 0.3
0.2 [M]/[BDTB]=400 0.1 [M]/[BDTB]=280 0.0 0 30 60 90 120 150 180 210 Time / min
Figure 5.25. Experiments BDTB(Diox-2) and BDTB(Diox-3): pseudo first-order rate. -2 -1 T=60ºC, [MT]=5.6 M, [AIBN]=1·10 mol L . For BDTB(Diox-2), [BDTB]/[MT]=280, for BDTB(Diox-3) [MT]/[BDTB ]=400.
1.6 1.4 PDI 1.2 1.0 75000
60000
45000 n
M 30000
15000 [BDTB]/[M ]=280 T [BDTB]/[M ]=400 T 0 0 20406080100 Time / min
Figure 5.26. Experiments BDTB(Diox-2) and BDTB(Diox-3): number average molecular -1 weight Mn and polydispersity index (PDI) vs. conversion plot. T=60ºC, [MT]= 5.6 mol L , -2 -1 [AIBN]=1·10 mol L . For BDTB(Diox-2) [BDTB]/[MT]=280, for BDTB(Diox-3) [MT]/[ BDTB ]=400.
110 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.2 = 27% Conversion = 44.4% = 59% 1.0 = 77.78% = 94.64% 0.8
0.6
dW/d(LogM) 0.4
0.2
0.0
3.5 4.0 4.5 5.0 5.5 6.0 Log M /g mol-1
Figure 5.27. Experiments BDTB(Diox-3): evolution of the SEC chromatograms. -1 -2 -1 T=60ºC, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[ BDTB ]=400.
1.4 = 7 % 1.2 Conversion = 29 % = 37 % = 1.0 42 % = 78 % = 95 % 0.8
0.6
dW/d(LogM) 0.4
0.2
0.0
3.9 4.2 4.5 4.8 5.1 5.4 5.7 Log M /g mol-1
Figure 5.28. Experiment BDTB(Diox-2) Evolution of the SEC chromatograms; T=60º -1 -2 -1 C, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[ BDTB ]=280. 111 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
100
80
60
40
20 [I]=5 10-3 [I]=1 10-2 0 0 20406080100 Time / min
Figure 5.29. Experiments BDTB(Diox-1) and BDTB(Diox-3): conversion versus -1 time plot. T=60ºC, [MT]=5.6 mol L , [BDTB]/[MT]=400. For BDTB(Diox-1) [AIBN]=5·10-3 mol L-1, for BDTB(Diox-3) [AIBN]=1·10-2 mol L-1.
60000
50000
40000 -1
30000 / g mol n
M 20000
[ I ] = 5 10-3 10000 -2 [ I ] = 1 10 Theoretical MW 0 0 20406080100 Time / min
Figure 5.30. Experiments BDTB(Diox-1) and BDTB(Diox-3): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot. T=60ºC, -1 -3 -1 [MT]=5.6 mol L , [BDTB]/[MT]=400. For BDTB(Diox-1) [AIBN]=5·10 mol L , for BDTB(Diox-3) [AIBN]=1·10-2 mol L-1.
112 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
60
[I]=1 10-2 50 [I]=5 10-3
40
30 %
20
10
0 0 50 100 150 200 250 time / min
Figure 5.31. Experiments BDTB(Diox-2) and BDTB(Diox-4): conversion -1 -1 versus time plot. T=60ºC, [MT]=5.6 mol L , mol L , [BDTB]/[MT]=280. For BDTB(Diox-2) [AIBN]=1·10-2 mol L-1, for BDTB(Diox-4) [AIBN]=5·10-3 mol L- 1 , [MT]/[BDTB]=280.
1.6
PDI 1.2
45000 -1
30000 / g mol n M
[I]=1 10-2 [I]=5 10-3 15000 0 102030405060 / %
Figure 5.32. Experiments BDTB(Diox-2) and BDTB(Diox-4): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot. T=60ºC, -1 -2 -1 [MT]=5.6 mol L , [BDTB]/[MT]=280. For BDTB(Diox-2) [AIBN]=1·10 mol L , for experiment BDTB(Diox-4) [AIBN]=5·10-3 mol L-1.
113 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
50
X i-BVE X 40 MAn
30
20 Conversion / % Conversion
10
0 0 50 100 150 200 250 300 350 400 time / min
Figure 5.33. Experiment BDTB(Diox-5): monomers conversion versus time plots as calculated by in-situ NIR-FTIR spectroscopy for the RAFT copolymerisation of -1 -3 IBVE and MAn in dioxane as a solvent. T=60˚C, [M]0=6 mol L , [AIBN]0=2·10 mol L-1, [BDTB]=2·10-2 mol L-1.
In order to improve chain deactivation and limit termination in the copolymerisation, the initiator concentration was lowered and in so doing the ratio between the concentration of RAFT agent and the initiator concentration ([BDTB]/[AIBN]) was increased from 4 to 10 in experiment BDTD(Diox-5). An analysis of the graphs related to this run (figures 5.33-5.36) suggests that retardation occurs at the beginning of the reaction (figure 5.33), this is followed by acceleration (figure 5.33 and 5.34). The value of M n increases with conversion (figure 5.37), however it appears that species with low molecular weight form (figure 5.36), causing the PDI to increase. These elements suggest that no improvement has been achieved with respect to the previous runs and that some termination still affects the reaction. Due to inhibition problems the concentration of AIBN could not be further lowered.
114 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
0.40
0.35
0.30
0.25
0.20
ln 1/1- 0.15
0.10
0.05
0.00 0 50 100 150 200 250 300 350 Time / min
Figure 5.34. Experiment BDTB(Diox-5): pseudo first-order rate. T=60˚C, [M]0=6 -1 -3 -1 -2 -1 mol L , [AIBN]0=2·10 mol L , [BDTB]=2·10 mol L .
1.4 = 14% 1.2 = 21% = 30% Conversion 1.0
0.8
0.6 dW/d(LogM)
0.4
0.2
0.0 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 Log M / g mol-1
Figure 5.35. Experiment BDTB(Diox-5): evolution of the SEC chromatograms for the solution RAFT copolymerisation of IBVE and MAn in dioxane as a solvent. -1 -3 -1 -2 -1 T=60˚C, [M]0=6 mol L , [AIBN]0=2·10 mol L , [BDTB]0=2·10 mol L .
115 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.40 1.35
PDI 1.30 16000
12000 -1 g mol
/ 8000 Mn
4000
GPC M n Theoretical M n 0 010203040
Figure 5.36. Experiment BDTB(Diox-5): evolution of the SEC chromatograms for the solution RAFT copolymerisation of IBVE and MAn in dioxane as a solvent. -1 -3 -1 -2 -1 T=60˚C, [M]0=6 mol L , [AIBN]0=2·10 mol L , [BDTB]0=2·10 mol L .
116 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.7.3. Tetrahydrofuran as a solvent
Some experiments were also conducted in parallel using THF and dioxane as solvents; the conditions adopted are summarised in table 5.4 and the results obtained are reported in the graphs that follow. In general, the results obtained in the two solvents are very similar; in this case also, as experienced when dioxane was the solvent, reactions with
low [MT]/[BDTD] ratio ( 280) often undergo inhibition or retardation for no apparent reason. In figure 5.37 an example of this behaviour is provided; the two reactions, although run in identical conditions, are characterised by different reaction rates and IBVE homopolymer formed in the slowest reaction. We can see in figure 5.38 that the
polymer sampled presents M n close to the theoretical values; however PDI increases over time, indicating that termination events take place. Figure 5.39 shows that the conversion versus time plot are very similar for BDTB(Diox-6) and BDTB(THF-3), which have ratio [M]/{BDTB] equal to 400 and which differ only for the solvent used.
M Furthermore both reactions have values of n similar to the theoretical ones (figure 40) and ln(1/1-) versus time is for both reactions a fairly linear plot (figures 5.47 and 5.49). In regard to reactions with ratio [M]/{BDTB] equal to 800, the experimental values of
M n have become closer to the theoretical value, (figure 5.44) suggesting that in such conditions the RAFT agent gets entirely used. However, the fact that the value of PDI is higher (but still below 1.5) in respect to the experiments run with [M]/{BDTB] equal to 400 provides an indication that these systems are not well controlled, (figure 5.44). This is also confirmed by the fact that the ln(1/1-) versus time plots are not linear (figures 5.49 and 5.51).
Table 5.4: Experimental conditions and Rp for the solution RAFT copolymerisation of IBVE and MAn conducted in tetrahydrofuran as a solvent.
Experiment [MT]0 [AIBN]0 [BDTB]/[AIBN] [MT]/[ BDTB] (mol l-1) (mol l-1) BDTB(THF-1) 5.6 1·10-2 2 280 BDTB(THF-2) 5.6 1·10-2 2 280 BDTB(THF-3) 5.6 5 10-3 1.4 400 BDTB(THF-4) 5.6 1·10-2 .7 800 BDTBDiox-6 5.6 1·10-2 1.4 400 BDTBDiox-7 5.6 1 10-2 .7 800
117 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
100 BDTBTHF-1 BDTBTHF-2 80
60
/ % 40
20
0 0 100 200 1200 1300 1400 Time / min
Figure 5.37. Experiments BDTB(THF-1) and BDTB(THF-2): conversion versus -1 -2 -1 time plot. T=60ºC, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[ BDTB ]=280 [BDTB]/[AIBN]=2.
1.3
PDI 1.2 30000
20000 -1
10000 Mn / g mol
0 0 1020304050 / %
Figure 5.38. Experiments BDTB(THF-1): number average molecular weight Mn and -1 polydispersity index (PDI) vs. conversion plot. T=60ºC, [MT]= 5.6 mol L , -2 -1 [AIBN]=1·10 mol L , [MT]/[ BDTB ]=280 [BDTB]/[AIBN]=2. 118 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
100 90 80 70 60 50
/ % 40 30 20 10 Dioxane THF 0 0 50 100 6000 7000 8000 time / min
Figure 5.39. Experiments BDTB(Diox-6) and BDTB(THF-3): conversion versus -1 -2 -1 time plot. T=60ºC, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB]= 400 [BDTB]/[AIBN]=1.4.
1.40
1.35
PD 1.30
30000
25000
-1 20000
15000 Dioxane Mn / g mol 10000 THF 5000 Th Mn
0 0 20 40 60 80 100
Figure 5.40. Experiments BDTB(Diox-6) and BDTB(THF-3): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot. T=60º C, -1 -2 -1 [MT]=5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB]= 400 [BDTB]/[AIBN]=1.4, solvents dioxane and tetrahydrofuran.
119 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.2 Conversion = 6 % 1.0 = 17 % = 23 % 0.8 = 32 % = 77 %
0.6
dW /d(LogM) dW 0.4
0.2
0.0
3.5 4.0 4.5 5.0 5.5 Log M /g mol-1
Figure 5.41. Experiment BDTB(Diox-6): evolution of the SEC chromatograms for -1 -2 -1 T=60ºC, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB]= 400 [BDTB]/[AIBN]=1.4.
1.2 Conversion 1.0 = 21 % = 28 % = 33 % 0.8 = 41% = 99%
0.6
dW / d(LogdW M) 0.4
0.2
0.0
3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 Log M /g mol-1
Figure 5.42. Experiment BDTB(THF-2): evolution of the SEC chromatograms. -1 -2 -1 T=60º C, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB]=400 [BDTB]/[AIBN]=1.4. solvent: THF.
120 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
70
60
50
40
30
20
10 THF Dioxane 0 0 30 60 90 1200 1350 1500 Time / min
Figure 5.43. Experiments BDTB(Diox-7) and BDTB(THF-4): conversion versus -1 -2 -1 time plots T=60ºC, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB] =800 [BDTB]/[AIBN]=0.7.
1.5
1.4
PD 1.3 55000
44000
-1 33000
22000 Mn / g mol
11000 THF Dioxane Th Mn 0 0 10203040506070 / %
Figure 5.44. Experiments BDTB(Diox-7) and BDTB(THF-4): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot for T=60º C, -1 -2 -1 [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB] =800, [BDTB]/[AIBN]=0.7.
121 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.2 Conversion = 12 % 1.0 = 19 % = 32 % = 56 % 0.8 = 69 %
0.6
dW / dLog(M) dW 0.4
0.2
0.0
3.2 3.6 4.0 4.4 4.8 5.2 5.6 Log (M)
Figure 5.45. Experiment BDTB(THF-4): evolution of the SEC chromatograms. -1 -2 -1 Solvent THF, T=60ºC, [MT]=5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB]=800 [BDTB]/[AIBN]=0.7.
1.0 = 7 % = 12 % = 18 % 0.8 = 22 % = 26 % 0.6
dW / d(logM) dW 0.4
0.2
0.0
3.33.63.94.24.54.85.15.4 Log ( M)
Figure 5.46. Experiment BDTB(Diox-7): evolution of the SEC chromatograms. -1 -2 -1 Solvent Dioxane, T=60ºC, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB] =800 [BDTB]/[AIBN]=0.7 [M]T/[RAFT]=800.
122 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5 [M] / [BDTB]=400 [M] / [BDTB]=800
4
ln 1 / 1- 0.5
0.0 0 50 100 150 2000 4000 6000 8000 Time / min
Figure 5.47. Experiments BDTB(THF-3) and BDTB(THF-4): pseudo first order -1 -2 -1 plot. T=60º C, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [MT]/[BDTB]=400 solvent THF. For BDTB(THF-3) [MT]/[BDTB] =400, for BDTB(THF-4) [MT]/[BDTB] =800.
1.50 1.35
PDI 1.20
30000
25000
20000 -1
15000
Mn /g mol 10000
[M ]/[BDTB]=400 5000 T [M ]/[BDTB]=800 T 0 020406080100 / %
Figure 5.48. Experiments BDTB(THF-3) and BDTB(THF-4): number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot. T=60º C, -1 -2 -1 [MT]= 5.6 mol L , [AIBN]=1·10 mol L . For BDTB(THF-3) [MT]/[BDTB] =400, for BDTB(THF-4) [MT]/[BDTB] =800.
123 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
1.6 [M ]/[BDTB]=400 T 1.4 [M ]/[BDTB]=800 T 1.2
1.0
0.8
ln1/1- 0.6
0.4
0.2
0.0 0 50 100 1200 1300 1400 1500 Time / min
Figure 5.49 Experiment BDTB(Diox-6) and BDTB(Diox-7): pseudo first order plot. -1 -2 -1 T=60º C, [MT]= 5.6 mol L , [AIBN]=1·10 mol L , For BDTB(Diox-6) [BDTB]/[MT] =400, for BDTB(Diox-7) [BDTB]/[MT =800.
1.50 PDI 1.35
35000
28000
-1 21000
/ g mol / g 14000 n M [M ]/[BDTB]=400 T 7000 [M ]/[BDTB]=800 T
0 010203070 75 80 / %
Figure 5.50. Experiments BDTB(Diox-6) and BDTB(Diox-7) Number average molecular weight Mn and polydispersity index (PDI) vs. conversion plot. T=60º C, -1 -2 -1 [MT]= 5.6 mol L , [AIBN]=1·10 mol L , [BDTB]/[AIBN]=0.7. [MT]/[BDTB] =400 [MT]/[BDTB] =800
124 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.8 Investigation into the colour alteration of a polymerisation mixture containing BDTB as a RAFT agent
It has been mentioned that a slight discolouration of the reaction medium was noticeable in FRP reactions; this could be considered as an indication of the fact that a charge transfer complex formed between the two comonomers, as discussed in paragraph 3.4. In the RAFT polymerisation, however, for elevated concentrations of BDTB, the solution would turn into a dark brown colour and the reaction would undergo inhibition. An experiment aimed at identifying the factors responsible for this phenomenon was carried out.
Separate solutions were prepared and placed in a thermostatic bath at 60º C, each one contained BDTB, AIBN, acetone-d6 and one of the monomers in a ratio [M]/[BDTB] equal to about 56. After about 40 minutes, each solution was checked for discoloration. It was found that only the solution containing IBVE had changed its colour to brown. 1H NMR analysis was conducted on this sample; it revealed that iso-butyl hemiacetal had formed in the polymerisation mixture. The presence of this compound is likely to be an indication of the fact that auto-oxidation of IBVE has taken place. Ethers are extremely susceptible to auto-oxidation; this reaction leads to a complex mixture of compounds, among them peroxides, which have been reported to react with the C=S double bond of the dithioethers and cause the oxidation of the sulphur to sulphines. These compounds in turn decompose and generate thioesters and elemental sulphur16,17,18. The reaction described would explain the dramatic loss of colour experienced by the reaction mixture, since it disrupts the C=S double bond and the electronic transition ' '* that affords colourful solutions. In order to confirm this theory, the NMR spectrum of the reaction mixture should have been checked for the presence of thiobenzoates, as described by Vana et al.19
125 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
HF
H F H F OH H A H D
H A O HE
H A H B HC HF HF
H F
B DC E A F 876543210 / ppm
Figure 5.51. 1H NMR spectrum of a solution displaying brown discoloration, showing the peaks associated with iso-butyl hemiacetal.
126 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
5.9 Conclusions
The free radical copolymerisation, FRP, of maleic anhydride and iso-butyl vinyl ether was carried out in dioxane, methyl ethyl ketone and acetone. The following features were noted: 1. Auto-acceleration could be seen for high initiator concentrations (1 10-1);
2. FT_NIR analysis was found not to provide reliable values of the comonomers conversion when acetone was used as a solvent. 1H NMR was found to overestimate, in most cases, the values of the conversions of both monomers and in particular IBVE. Gravimetric analysis was found to be the most suitable technique to record the conversion versus time curves of the experiments carried out. The copolymer produced was found to be alternating and characterised by
-1 an average number molecular weight M n of about 150000 g mol and polydispersity index PDI comprised between 1.45 and 2.3;
3. Dodecyl mercaptan is a suitable chain transfer agent for the copolymer of MAn
and IBVE; it affords polymers with lower M n with respect to the FRP process and values of PDI as low as 1.6 for high concentration of this chain transfer agent.
When RAFT agents were added to the system, to impart living characteristics to the copolymer of MAn and IBVE, the following observations were made:
1 Both 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) and benzyl trithiobenzoate RAFT compounds act as conventional chain transfer agents for the system under study; this is demonstrated by the lower molecular weight of the copolymer obtained in both experiments respect to the FRP process. The retardation noticed to a minor extent in the case of RAFT-acid and to a greater extent in the case of DBTTC may suggest that the Z constituents (an alkyl thioester for RAFT-acid and a thiocarbonate for DBTTC ) are not able to adequately activate the C=S double bond of the RAFT agent towards addition by the MAn radical. On the other hand, while IBVE radical may easily add to the RAFT agent, however, the adduct generated by this reaction is not likely to fragment. All these factors contribute to inhibit the chain equilibration step from taking place, resulting in
127 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
these RAFT agents not being able to control the value of the M n and therefore develop systems with living properties;
2 The use of BDTB as a RAFT agent is able to provide controlled radical polymerisation conditions to the copolymerisation of MAn and IBVE. Many solvents can be used, however in the case of polar mediums, (MEK, acetone, ethyl acetate, DMAc) macrophase separation occurs, due to the formation of IBVE homopolymer in the upper phase and of copolymer in the lower phase. This phenomenon strongly affects the accuracy of spectroscopic methods for the determination of conversion values. This has been experienced with both 1H NMR and NIR-FTIR spectroscopy;
3 Poly(IBVE) also forms when the RAFT polymerisation is conducted with BDTB as a RAFT agent in less polar solvents such as tetrahydrofuran and dioxane, however due to the increased solubility of this polymer in these solvents, this side reaction does not manifest with phase separation. Interaction between the RAFT agent and impurities present in the reaction mixture are believed to be responsible for such behaviour.. It could also be possible that the system IBVE/MAn behaves similarly to the Sty/MAn copolymer and that care should be taken to carry out the reaction in conditions that do not alter the value of the equilibrium constant of the complex formed between MAn an IBVE in a way that could compromise the formation of an alternating copolymer. In the case of Sty/MAn copolymer, a work by Kokubo et al.20 reports that an alternating copolymer between these two comonomers can only be formed for a certain range of values of the constant (Kc > 0:1 L/mol), while for 1 < Kc < 5 L/mol, spontaneous ionic polymerization occurs;
4 The occurrence of inhibition has been attributed to the reaction between BDTB and the peroxides that form as a result of the auto-oxidation of IBVE, according to the mechanism described in paragraph 5.8.
5 The number average molecular weight M n of the copolymer increases with increasing conversion, however it is higher than the calculated value. This factor could be related to the fact that styrene standards were used for the calibration of the SEC instrument, or it might suggest that termination is still present to some extent. This latest hypothesis finds confirmation in the fact that the value of the PDI increases with conversion, although it remains always lower than 1.5. These
128 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
observations lead to the conclusion that BDTB is able to control the copolymerisation of MAn and IBVE, but it might not be able to provide living conditions to this system.
Based on the findings described in this dissertation the following recommendations can be made for future research to be carried out on this subject:
1 The inhibition of the polymerisation mixture could be further investigated by determining and quantifying the side products through GC-MS or HPLC-MS;
2 The degradation of the RAFT agent could be proved by searching the NMR spectrum for the presence of sulphines, which are the products of oxidation of dithioethers;
3 IBVE monomer should be thoroughly purified in order to minimise the introduction of peroxides into the polymerisation system;
4 It would be advantageous to be able to determine the absolute molecular weight of the polymer synthesised. This can be done by employing in the course of the SEC analysis a light scattering detector.
5 TGA should be carried out in order to ascertain the stability of the IBVE/MAn additive at high temperatures;
6 Copolymers synthesised and characterised by differing molecular weights could be blended into the PVDF matrix through a procedure which resembles as much as possible the membrane making process;
7 The MAn groups could be hydrolised and the hydrophilicity of membrane surface could be analysed through contact angle measurements; preliminary data showed encouraging results.
References
1 [ ] E Chernikova, P. Terpugova, C. Bui, B Charleux; Polymer, 44, 4101, 2003. [2] M.H Stenzel, T.P Davis; J. of Polym. Sci. Part A: Polymer Chemistry, Vol.40, 4498, 2002. [3] R.K. Bai, Y.Z. You, C.Y. Pan; Macromol Chem Phys Rapid Commun., 22, 315, 2001. [4] E. Rizzardo, J. Chiefari, R. Mayadunne, G. Moad , S. H. Thang, ACS 2000. [5] J. Chiefari, R. T. A. Mayadunne, C. L. Moad, G. Moad, E. Rizzardo, A. Postma, M. A. Skidmore, S. H. Thang; Macromolecules, 36, 2273, 2003. [6] E Chernikova, P. Terpugova, C. Bui, B. Charleux; Polymer, 44, 410, 1993. [7] M. Zhu, L. Wei, P. Zhu, F. Du, Z. Li. F. Li, Gaofenzi Xuebao, 3, 418, 2001. [8] A. Feldermann, A. Ah Toy, H. Phan, M. Stenzel, T. P. Davis, C. Barner-Kowollik, Polymer, 45, 3997, 2004.
[9] M. Jesberger, L. Barner, M. H. Stenzel, E. Malmström, T.P. Davis, C. Barner-Kowollik, J. of Polym. Sci. Part A: Polymer Chemistry, 41, 3847 2003. [10] Y. Z. You, C. H. Hong, C. Y. Pan, Eur. Polym. J., 38, 1289, 2002. [11] X. Hao, K. Fujimori, D.J. Tucker, P.C. Henry, Eur. Pol. J., 36, 1145, 2000.
129 Cinzia Lea Use of RAFT agents to control the radical copolymerisation of MAn and IBVE
[12] K. Fujimori, P.P. Organ, M.J. Costigan , I.E. Craven, J. Macromol. Sci., Chem., A23, 647 1986. [13]J. Brandrup, E. H. Immergut, E. A. Grulke, A. Abe, D. Bloch; Polymer Handbook-J. Wiley and sons 2005. [14] X. Hao, Ph.D. Thesis University of New England, Armidale – Australia September 2000.
[15] P. Vana, L. Albertin, L. Barner, T. P. Davis, C. Barner-Kowollik; J. Polym. Sci.Polym. Chem. 40, 4032-4037, 2002.
[16]H.Alper, C.Kwiatkowska, J. F. Petrignani, F. Sibtain; Tetrahedron Lett. 27, 5449, 1986. [17]K. Buggle, B. Fallon; Monatsh Chem 118, 1197, 1987. [18]F. Carreta, A.-M. Le Nocher, C. Leriverend, P. Metzner, T. N. Pham; Bull. Soc. Chim Fr. 132, 67, 1995. [19] P. Vana, L. Albertin, L. Barner, T. P. Davis, C. Barner-Kowollik; J. of Polymer Sci. Part A: Polymer Chemistry 40, 4032, 2002. 20 [ ] Kokubo T, Iwatsuki SH, Yamashita Y. Macromol. 1, 482, 1968.
130 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES
Thesis/Dissertation Sheet Surname or Family name: Lea First name: Santa Other name/s: Cinzia Abbreviation for degree as given in the University calendar: MSc School: Chemical Sciences and Engineering Faculty: Engineering
Title: An investigation into the Synthesis of Poly(co-maleic anhydride/iso-butyl vinyl ether)with RAFT polymerization.
Abstract 350 words maximum: (PLEASE TYPE)
Poly (co iso-butyl vinyl ether-alt-maleic anhydride), an alternating copolymer, was synthesised. For this class of copolymers the formation of an electron-donor complex is invoked to explain their microstructure in which the two comonomers strictly alternate. Due to its polarity, this copolymer constitutes a potential additive for imparting hydrophilic properties to a hydrophobic matrix. In order to obtain narrow molecular weight polymers and study the relation between the molecular weight of this additive and its ability to migrate to the host polymer surface, chain transfer agents were introduced in the system and also the Reversible Addition-Fragmentation chain Transfer (RAFT) process was employed. Free radical polymerisation was first carried out to allow for a comparison with the RAFT process and kinetics of copolymerisation was studied by NIR-FTIR and 1H NMR spectroscopy in order to analyse the rate of reaction of each comonomer. Dibenzyl trithiobenzoate, 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid and dibenzyl trithiobenzoate were used as RAFT agents. Results demonstrate that only benzyl dithiobenzoate is able to control the molecular weight of this copolymer and decrease its polydispersity index; possible reasons laying behind this result are discussed. It was also found that, in particular in the presence of benzyl dithiobenzoate, poly(iso-butyl vinyl ether) forms. This is an unusual phenomenon considering that the free radical polymerisation affords alternating copolymers and that iso-butyl vinyl ether is a monomer that polymerises through the cationic process. Experiments were carried out in various solvents in an attempt to counteract this side reaction, but no appreciable correlation between the properties of the solvents and the formation of homopolymer were found. Various hypothesis are considered, however it is likely that, in the conditions adopted, the presence of the RAFT agents alters the equilibrium constant of complex formation favouring the synthesis of the homopolymer. In addition to this side–reaction also inhibition of the copolymerisation reaction was at times encountered and an investigation into this phenomenon was also conducted.
Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only).
…………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS Thesis corrections-Examiner 1
(D)E1: The equation at the bottom of page 26 has been changed from:
[M] t M n = mM + mRAFT [RAFT]0 + df ([I]0-[I]t) to:
[M] t Mn = mM + mRAFT [RAFT]0 + df ([I]0-[I]t)
(E) E1: page 24. The examiner claims that the influence of the substituent R is discussed, however there is little mention of the effect of substituent Z. Since on page 24 the effect of substituent Z is treated, I assume that the examiner, in reality, intended to suggest that the effect of substituent R needs to be added to the discussion. Therefore, the following paragraph has been included:
The intermediate radical rate of fragmentation k is favoured by factors which render the radical R a good homolitic leaving group with respect to the attacking growing radical Pn .
RAFT agents bearing a benzyl (CH2 Ph) group as a leaving radical R , for instance, have been found to be effective in controlling the polymerisation of styryl and acrylyl propagating radicals, but almost inert when applied to the polymerisation of methyl methacrylate; the explanation for this phenomenon lay in the fact that the benzyl radical is more stable than the styryl and acrylyl radicals, but a rather poor leaving group with respect to the methacrylyl growing radicali and, as a consequence, in this latter case, the rate of fragmentation of the intermediate radical A is affected. It is also important to note that the stability of the radical
R formed should not compromise the ability of the new radical to re-initiate polymerisation without delay, as already seen for the role of transfer agents in free radical polymerisation.
(F) Page 30. The section on star polymers has been erased from the text. (G) Page 33. The word disparate has been added to the sentence. (I) Page 35. According to S. Russo in chapter II of Macromolecole: scienza e tecnologia AIM 1983, kinetic models for the prediction of the instantaneous copolymer composition and sequence distribution assume that no depropagation reactions take place. This assumption allows to exclude all thermodynamic equations that regulate the concentrations at the equilibrium of the reagents and also those that relate the rate of depropagation to the reaction temperature. (J) Page 37. The term ultimate has been replaced by the term terminal.
(K) The term radicalic has been replaced by the term radical.
(L) The expression growing radical polarised effect has been deleted.
(M) The first sentence in paragraph 3.4 has been replaced by:
Alternating copolymers have been studied extensively because of the orderly alternation of the monomer units in their structure.
(N) The following part was added at the end of paragraph 4.4.2 on SEC analysis:
Due to the absence in literature of the Mark-Houwink-Sakurada (MHS) parameter relative to the MAn/IBVE copolymer, the values of the number average molecular weight M n of the MAn/IBVE copolymer reported in figures and tables correspond to styrene equivalents. This
ii M method can only provide a semi-quantitative determination of the n and this uncertainty must be taken into account when analysing results.
(O) Paragraph 4.6 on the Reliability of FT-NIR spectroscopy was amended in the following way:
Due to the peculiarity of the results obtained so far, which suggest that the rate of polymerisation of IBVE (RIBVE) is faster than the rate of polymerisation of MAn (RMAn) we decided to test the assumption that the FT-NIR spectrum, obtained from the reacting solution at any instant, is a linear superposition of spectral contributions from the two comonomers (Beer’s law). Let A() be the absorbance (or emission) signal at wavelength given by:
A( = ( c b
where b is the sample cell path length in cm, and, for experimental convenience, c is the solute concentration in g cm-3, so that ( is the mass concentration extinction coefficient
2 -1 (cm g ). Let IBVE( andMAn( andci-BVE and cMAn be the extinction coefficient and mass concentration of IBVE and MAn respectively at wavelength . If the Beer’s law holds for these measurements, then the absorbance A( of the mixture of the two comonomers is given by the sum of the component absorbances:
A( = b [IBVE( cIBVE +MAn( cMAn]. (4.1)
Individual solutions of known concentration were prepared for each monomer in both acetone and in dioxane and analysed at the FT-NIR spectrophotometer. Then, solutions composed by a mixture of both monomers in a concentration identical to the individual ones were analysed. An additional curve was also calculated, based on equation 4.1 and denominated simulated overall curve in figures 4.13-4.16.
Results show that the peaks of the two monomers display some extent of overlapping, as already discussed in paragraph 4.3.1, but most importantly, it appears that the Beer’s law does not hold for these measurements. Figures 4.13-4.16 demonstrate that the spectrum related to the mixture of the two comonomers in solution is not, at every wavelength, equivalent to the sum of the absorbances of the spectra of the constituents (simulated overall spectum). In particular the spectrum associated with a ratio of the mixture MAn:IBVE 1 appears to underestimate both comonomers vinyl bonds contributions. This is still true for solutions in dioxane composed of a mixture of the comonomers in a ratio MAn:IBVE = 1.5:1, while the same mixture in acetone appears to underestimate IBVE vinyl bonds contributions and overestimate the absorbance due to the MAn vinyl bonds. These results reveal that the comonomers polymerisable vinyl bonds interact with one another and that the extent of this interaction depends on the solvent used, being more pronounced in the case of solutions prepared in acetone as a solvent, and on the ratio between the comonomers, increasing at increasing molar ratio between MAn and IBVE. It also seems to affect to a greater extent the wavelength of absorbance of the IBVE monomer, while it is less relevant for the MAn monomer. As a result, the measurement of both monomers concentrations in acetone leads inevitably to error, when these are present in equal molar amount, however it provides an acceptable estimation of the amount of MAn in the other cases and in particular for dioxane solutions. Figure 4.4 shows the good agreement between the conversion of the copolymer as calculated by gravimetric analysis and the value of the conversion of MAn as calculated by FT-NIR spectroscopy.
(P) Page 73, discussion of CS values. The last sentence in paragraph 4.8 has been erased and amended to: Despite the uncertainty inherent to the method adopted to carry out the SEC analysis, we can conclude that dodecyl mercaptan is an efficient chain transfer agent for this system.
The value of CS obtained by using the procedure described is of the same order of magnitude of the one obtained for methyl methacrylate in bulk and at 60 ºC, equal to 0.63iii. As stated in the experimental procedure, paragraph 4.2.2, each comonomer is present in equal amount in the reaction mixture. A concentration ratio equal to 1:1 between the two comonomers in this experiment is analogous to the experimental conditions adopted to carry out the conventional free radical copolymerisation. The aim of this experiment has been further clarified in the first part of the paragraph:
The IBVE-MAn copolymer afforded so far by FRP has been characterised by an average
-1 number molecular weight M n of about 150000 g mol and polydispersity index PDI comprised between 1.45 and 2.3. As a means to obtain polymers with differing M n and lower value of polydispersity index a chain transfer agent was employed: dodecyl mercaptan (DDM) (see paragraph 2.1.4 for a discussion on chain transfer agents). Mercaptans have been successfully applied to regulate the chain length of copolymers of MAn with methacrylates and acrylatesiv. In this paragraph we describe how DDM can also be used to regulate the molecular weight of the copolymer of MAn with IBVE.
Results have been commented further:
As already mentioned in paragraph 4.4.2, relative to the description of the method utilised to carry out the SEC analysis, the determination of both the value of the number average
M M molecular weight n and the value of the weight average molecular weight w is semi- quantitative, due to the fact that both values are reported in polystyrene equivalents. As a consequence, the value of CS is also inevitably affected by error.
M The value of Cs obtained from calculations DPn = n /M0 is equal to 0.14 with a standard -4 -4 deviation of ±4.3 10 , while Cs equals 0.17 with a standard deviation of ±2.2 10 when
M M derived from DPn = w /2M0. The better reproducibility of the data obtained from w relates to the fact that baseline error in GPC analysis affects to a greater extent the value of
v, vi M n . It can be seen in figure 4.17 that the molecular weight distribution curves manifest bimodality, however, by increasing the concentration of DDM the value of the molecular weight decreases and so does the value of the polydispersity index (table 4.4). This leads to the disappearance of the shoulder for a concentration of DDM equal to 0.159 M. This phenomenon can be explained in the following way: in the absence of a chain transfer agent the rate of termination decreases, leading to the formation of polymer chains with high molecular weight. The introduction of increasing concentrations of a chain transfer agent favours a decrease of the molecular weight of the polymer formed, which in turn decreases the viscosity of the system and minimises the phenomenon of auto-acceleration.
(Q) I was made aware of the possible detrimental effect that the use of a solvent like dioxane could have had on the RAFT system; this is the reason why, on paragraph 4.1.1 Purification of reagents and solvents, I mention that:
Dioxane was purified by distillation in the presence of LiAlH4, stored under nitrogen atmosphere at a temperature of –4 º C and checked, prior to use, for the presence of peroxides, with Quantofix® Peroxide 2, a reagent which consists of test sticks able to provide a semi-quantitative determination of the peroxides present in a solvent/solution. In no circumstances peroxides were ever detected.
Also at the beginning of paragraph 5.7.2 RAFT polymerisation with BDTB and 1,4 dioxane as a solvent it is stated that:
Some experiments were conducted with dioxane in virtue of its low polarity ( =2.209, DN =14.8 kcal·mol-1). By doing so, as discussed at the end of paragraph 5.7.1.3 it was intended to deter the formation of IBVE homopolymer. 1,4 dioxane was carefully purified and stored, according to the modality described in paragraph 4.1.1. This was done to prevent the peroxides present in 1,4 dioxane from reacting with the RAFT agentvii and interfering with the process.
I have now added the citation.
Although I cannot rule out that oxidation of RAFT end group might have taken place, however the employment of this solvent does not appear to compromise the efficacy of the RAFT agent and a comparison with the results obtained when alternative solvents were used does not suggest that 1,4 dioxane is directly responsible for any of the side effects observed in the course of the reactions.
(R) Figure 5.3. The truncation of the GPC trace is purely incidental. A past attempt to rectify this distribution revealed that the original trace was no longer available. A hard copy of the trace has been attached for reference.
(S) The sentence on page 91 has been amended to:
The fact that these plots are linear indicates that a constant concentration of active sites is maintained in the course of the reactions. The conclusions have been modified as it follows:
4.1 Conclusions
The free radical copolymerisation, FRP, of maleic anhydride and iso-butyl vinyl ether was carried out in dioxane, methyl ethyl ketone and acetone. The following features were noted: 1. Auto-acceleration could be seen for high initiator concentrations (1 10-1);
2. FT_NIR analysis was found not to provide reliable values of the comonomers conversion when acetone was used as a solvent. 1H NMR was found to overestimate, in most cases, the values of the conversions of both monomers and in particular IBVE. Gravimetric analysis was found to be the most suitable technique to record the conversion versus time curves of the experiments carried out. The copolymer produced was found to be alternating and characterised by an average number
-1 molecular weight M n of about 150000 g mol and polydispersity index PDI comprised between 1.45 and 2.3;
3. Dodecyl mercaptan is a suitable chain transfer agent for the copolymer of MAn and
IBVE; it affords polymers with lower M n with respect to the FRP process and values of PDI as low as 1.6 for high concentration of this chain transfer agent.
When RAFT agents were added to the system, to impart living characteristics to the copolymer of MAn and IBVE, the following observations were made:
1 Both 3-benzyl sulfanyl thiocarbonyl sulfanyl-propionic acid (RAFT acid) and benzyl trithiobenzoate RAFT compounds act as conventional chain transfer agents for the system under study; this is demonstrated by the lower molecular weight of the copolymer obtained in both experiments respect to the FRP process. The retardation noticed to a minor extent in the case of RAFT-acid and to a greater extent in the case of DBTTC may suggest that the Z constituents (an alkyl thioester for RAFT-acid and a thiocarbonate for DBTTC ) are not able to adequately activate the C=S double bond of the RAFT agent towards addition by the MAn radical. On the other hand, while IBVE radical may easily add to the RAFT agent, however, the adduct generated by this reaction is not likely to fragment. All these factors contribute to inhibit the chain equilibration step from taking place, resulting in these RAFT agents not being able to
control the value of the M n and therefore develop systems with living properties;
2 The use of BDTB as a RAFT agent is able to provide controlled radical polymerisation conditions to the copolymerisation of MAn and IBVE. Many solvents can be used, however in the case of polar mediums, (MEK, acetone, ethyl acetate, DMAc) macrophase separation occurs, due to the formation of IBVE homopolymer in the upper phase and of copolymer in the lower phase. This phenomenon strongly affects the accuracy of spectroscopic methods for the determination of conversion values. This has been experienced with both 1H NMR and NIR-FTIR spectroscopy;
3 Poly(IBVE) also forms when the RAFT polymerisation is conducted with BDTB as a RAFT agent in less polar solvents such as tetrahydrofuran and dioxane, however due to the increased solubility of this polymer in these solvents, this side reaction does not manifest with phase separation. Interaction between the RAFT agent and impurities present in the reaction mixture are believed to be responsible for such behaviour.. It could also be possible that the system IBVE/MAn behaves similarly to the Sty/MAn copolymer and that care should be taken to carry out the reaction in conditions that do not alter the value of the equilibrium constant of the complex formed between MAn an IBVE in a way that could compromise the formation of an alternating copolymer. In the case of Sty/MAn copolymer, a work by Kokubo et al.viii reports that an alternating copolymer between these two comonomers can only be formed for a certain range of values of the constant (Kc > 0:1 L/mol), while for 1 < Kc < 5 L/mol, spontaneous ionic polymerization occurs;
4 The occurrence of inhibition has been attributed to the reaction between BDTB and the peroxides that form as a result of the auto-oxidation of IBVE, according to the mechanism described in paragraph 5.8.
5 The number average molecular weight M n of the copolymer increases with increasing conversion, however it is higher than the calculated value. This factor could be related to the fact that styrene standards were used for the calibration of the SEC instrument, or it might suggest that termination is still present to some extent. This latest hypothesis finds confirmation in the fact that the value of the PDI increases with conversion, although it remains always lower than 1.5. These observations lead to the conclusion that BDTB is able to control the copolymerisation of MAn and IBVE, but it might not be able to provide living conditions to this system. Based on the findings described in this dissertation the following recommendations can be made for future research to be carried out on this subject:
1 The inhibition of the polymerisation mixture could be further investigated by determining and quantifying the side products through GC-MS or HPLC-MS;
2 The degradation of the RAFT agent could be proved by searching the NMR spectrum for the presence of sulphines, which are the products of oxidation of dithioethers;
3 IBVE monomer should be thoroughly purified in order to minimise the introduction of peroxides into the polymerisation system;
4 It would be advantageous to be able to determine the absolute molecular weight of the polymer synthesised. This can be done by employing in the course of the SEC analysis a light scattering detector.
5 TGA should be carried out in order to ascertain the stability of the IBVE/MAn additive at high temperatures;
6 Copolymers synthesised and characterised by differing molecular weights could be blended into the PVDF matrix through a procedure which resembles as much as possible the membrane making process;
7 The MAn groups could be hydrolised and the hydrophilicity of membrane surface could be analysed through contact angle measurements; preliminary data showed encouraging results.
[i]B. Y. K. Chong, J. Krstina, T. P. T. Le, G. Moad, A. Postma, E. Rizzardo, S. H. Thang; Macromolecules 36, 2256, 2003. [ii] M. D. Zammit, T. P. Davis; Polymer 38,17, 4455-4468 1997. [iii] R. A. Hutchinson, D. A. Paquet, J. H. McMinn; Macromolecules 28, 5655 1995. [iv] B. C. Trivedi, B. M. Culbertson; Maleic Anhydride, p. 279.Plenum Press, New York and London 1982. [v] J. P. A. Heuts, T. P. Davis, G. T. Russell; Macromolecules, 32, 6019, 1999. [vi]M. Stickler, G. Meyerhoff; Macromol. Chem. Phys., 199, 2403 1998. [vii] P. Vana, L. Albertin, L. Barner, T. P. Davis, C. Barner-Kowollik; J. Polym. Sci.Polym. Chem. 40, 4032-4037, 2002. [viii] Kokubo T, Iwatsuki SH, Yamashita Y. Macromol. 1, 482, 1968. Thesis corrections- Examiner 2
With regard to the comment on compounds characterization, on page 75 the synthesis of a few RAFT agents is described: BDTB, RAFT- acid, BDTTC. Such compounds have already been synthesised and referenced have been cited for each one of them.
Page 91. The sentence was amended to :
In order to prove this theory, the ratio between the monomer and the RAFT agent concentration ([M]/[RAFT]) was varied from 200 to 800 by decreasing the concentration of RAFT agent. Each experiment with ratio between the monomer and the RAFT agent concentration [M]/[RAFT] equal to 200, 400 and 800 was at least repeated twice. For each experiment, in some instances no onset of visual macrophase separation manifested, while, in some others, discoloration and phase separation clearly appeared. It had to be concluded that the occurrence of phase separation, for the considered range of the ratio monomer: catalyst was purely incidental.
Page 93. The conclusions have been amended to :
• The polymerisation in acetone is characterised by phase separation of the polymerising mixture. This phenomenon is accompanied by faster rate of reaction of IBVE respect to MAn and it therefore prevents the copolymer from reaching high values of conversion;
• The fact that visual phase separation was not observed in FRP experiments inevitably links the insurgence of this phenomenon to the addition of the RAFT agent to the reaction mixture. Later on in this chapter, in paragraph 5.7.1.3, both phases are going to be analysed in an attempt to comprehend the reasons for this behaviour;
• For values of the ratio between the monomer and the RAFT agent concentration [M]/[RAFT] comprised between 200 and 800, the onset of phase separation does not depend on this parameter; • When side-effects are not present good control is maintained even at high ratios [M]/[BDTB].
Page 96. The layout of paragraph 5.7.1.3 Characterisation of the two phases, has been modified for the purpose of clarity and a graphic scheme has been added:
“At this point, the experiment which had been carried out in ethyl acetate was utilised to investigate the nature of the two phases. This was done by analysing the microstructure of the polymers formed, with 13C NMR spectroscopy. The procedure followed is described below and it is also represented in scheme 5.2:
a. A reaction vial displaying phase separation (1) was vigorously agitated to blend the two phases together,
b. A sample was quickly withdrawn from vial 2 and poured in anhydrous diethyl ether;
Both MAn and IBVE monomer and their respective homopolymers are soluble in diethyl ether, while poly(co-maleic anhydride/iso-butyl vinyl ether) is insoluble in this solvent. i
c. The precipitate formed (3’) was analysed by NMR,
The distortionless method described in the experimental section 4.4.1 was applied to the precipitate formed (3’) and the spectra obtained were compared with previous assignments for this copolymer ii. The full 13C NMR spectrum and the methylene sub-spectrum are displayed respectively in figures 5.17 and 5.18. In the methylene spectrum, figure 5.18, the C5 resonance at 30.9-36.5 ppm was assigned to the alternating MAn/IBVE/MAn (010) triad. The sequence at 38.8-42 ppm, indicative of the presence of the (110) triad and the sequences at
36.5-38.8 ppm of C5, characteristic of the presence of the semi-alternating triads (110) and (011) are absent and so is the triad (111). This proved that the reaction had produced a copolymer with alternating characteristics between the monomeric units.
d. The surnatant solution (3) obtained from the precipitation of the copolymer was added with methanol, in order to check for the presence of eventual other compounds. In this case also we can appeal to the difference in solubility of poly (MAn) and poly-(IBVE) to separate the two compounds; between the two compounds only poly-(IBVE) is in fact insoluble in methanol. Upon addition of this solvent, the formation of a white gelatinous precipitate (4’) was noticed.
e. The precipitate formed (4’) was dried in a vacuum oven and then solubilised in d- chloroform;
The resulting solution was analysed by 13C NMR spectroscopy, the DEPT 13C sub-spectrum of the methylene carbons can be viewed in figure 5.19.
f. The remaining surnatant solution (4) was first concentrated then chloroform added, in order to precipitate any eventual poly(MAn) present.
No insoluble compounds formed in this case. This is not surprising considering that the homopolymerisation of MAn is not likely to take place under the low initiator concentration conditions we are operating at (Copolymerisation systems 3.4.1.1). A comparison between the spectrum of the unknown precipitate (4’) and the spectrum of poly(IBVE), cationically synthesised, provides the evidence that the shift of the C5 peak at 38.8-42 ppm is the same one of the C5 peak of poly-IBVE. The triad (111) at 38.8-42 ppm dominates this spectrum and no other features can be noticed to indicate the presence of further microstructures. Figure 5.20 compares the DEPT sub-spectra shifts of the methylene carbons C5 and C7 in the different compounds.
a b c
1 2 3 4
3’ 4’
Scheme 5.2. Procedure followed for the identification of the products of the -1 -3 copolymerisation between MAn and IBVE. T=60˚C, [M]0 = 6 mol L , [AIBN] 0 = 2·10 mol L-1, [BDTB]= 2·10-2 mol L-1.
Page 99. Indeed the ratio between poly-IBVE and poly(co-maleic anhydride/iso-butyl vinyl ether) has been measured on several occasions, however not on a regular basis. This has been added to the text:
This reaction produces poly-IBVE which forms a separate top layer, when it is insoluble in the solvent employed. The amount of IBVE involved in the homopolymerisation has been determined gravimetrically on many occasions, according to the procedure described in paragraph 5.7.1.3. For severely inhibited copolymerisations, characterised by copolymer conversion around 10 % after about 20 hours, it was observed that the ratio between the two polymers was close to 1. Furthermore, the IBVE homopolymer seemed to degrade over time, as demonstrated by the fact that the highest value of copolymer conversion would be recorded after about 3 hours and it would then be lower when the final sample was taken.
Page 99. The explanation to the presence of IBVE homopolymer has been modified as follows:
It is uncertain what causes IBVE to homopolymerise. The presence of BDTB appears to be necessary for such reaction to take place, since it was not noticed in the course of FRP reactions. It is known from literature that IBVE homopolymerises through cationic polymerisation mechanism and therefore a cationic initiator must originate in the reaction mixture, due to the presence of the RAFT agent. It is possible that impurities in the system initiate this reaction; for instance Maleic acid (MAH) (3% by 1H NMR spectroscopy), may protonate the RAFT agent and form a stable cationic compound able to initiates the cationic polymerisation of IBVE.
Paragraph 5.8- Investigation into the colour alteration of a polymerisation mixture containing BDTB as a RAFT agent-has been modified in the following way:
It has been mentioned that a slight discolouration of the reaction medium was noticeable in FRP reactions; this could be considered as an indication of the fact that a charge transfer complex formed between the two comonomers, as discussed in paragraph 3.4. In the RAFT polymerisation, however, for elevated concentrations of BDTB, the solution would turn into a dark brown colour and the reaction would undergo inhibition. An experiment aimed at identifying the factors responsible for this phenomenon was carried out.
Separate solutions were prepared and placed in a thermostatic bath at 60º C, each one contained BDTB, AIBN, acetone-d6 and one of the monomers in a ratio [M]/[BDTB] equal to about 56. After about 40 minutes, each solution was checked for discoloration. It was found that only the solution containing IBVE had changed its colour to brown. 1H NMR analysis was conducted on this sample; it revealed that iso-butyl hemiacetal had formed in the polymerisation mixture. The presence of this compound is likely to be an indication of the fact that auto-oxidation of IBVE has taken place. Ethers are extremely susceptible to auto- oxidation; this reaction leads to a complex mixture of compounds, among them peroxides, which have been reported to react with the C=S double bond of the dithioethers and cause the oxidation of the sulphur to sulphines. These compounds in turn decompose and generate thioesters and elemental sulphuriii,iv,v. The reaction described would explain the dramatic loss of colour experienced by the reaction mixture, since it disrupts the C=S double bond and the electronic transition ' '* that affords colourful solutions. In order to confirm this theory, the NMR spectrum of the reaction mixture should have been checked for the presence of thiobenzoates, as described by Vana et al.vi
[i] J. Brandrup, E. H. Immergut, E. A. Grulke, A. Abe, D. Bloch; Polymer Handbook-J. Wiley and sons 2005. [ii] X. Hao, Ph.D. Thesis University of New England, Armidale – Australia September 2000. [iii]H.Alper, C.Kwiatkowska, J. F. Petrignani, F. Sibtain; Tetrahedron Lett. 27, 5449, 1986. [iv]K. Buggle, B. Fallon; Monatsh Chem 118, 1197, 1987. [v]F. Carreta, A.-M. Le Nocher, C. Leriverend, P. Metzner, T. N. Pham; Bull. Soc. Chim Fr. 132, 67, 1995. [vi] P. Vana, L. Albertin, L. Barner, T. P. Davis, C. Barner-Kowollik; J. of Polymer Sci. Part A: Polymer Chemistry 40, 4032, 2002.