Determination Of The Chain Transfer Constant In The RAFT Copolymerization

Reversible addition-fragmentation chain transfer(RAFT)polymerization is one of three major controlled/living free radical polymerization techniques.In the past decade,RAFT homopolymerization has been extensively investigated. Copolymerization is of significant importance for both academic and industry. Copolymerization of two or more different types of monomers allows us to synthesize a large number of polymer grades with various chain properties simply by adjusting the eomonomer ratios.However,the fundamental investigations on RAFT copolymerization have been little reported.In this thesis,I tried to shed insights on the RAFT copolymerization by developing a chain transfer constant model of the RAFT copolymerization,which could provide a guidance to RAFT copolymerization applications.The theoretical chain transfer constants(C_tr)model was developed under terminal model(TM)and penultimate unit model(PUM)of radicals,which describes the changing rule of C_trwith monomer compositions.The model show that C_tris a complicated function of monomer composition,reactivity ratios,homo- and crosschain transfer constant(C_tri·iand C_tri·j(i≠j=1,2)).In order to testify the model, C_tri·i,C_tri·jand the chain transfer constants with various monomer compositions were experimentally determined for the copolymerization systems of MMA/BA mediated by RAFT with Z=-CH₂C₆H₅ and of St/BA mediated by RAFT with Z=-CH(CH₃)₂ at 60℃.The results showed that the predictions of the model with the assumption of the terminal model of radicals were in excellent agreement with the experimental data for both investigated systems.Further discussion revealed that the addition reaction of RAFT has the penultimate unit effects.However,the degree of the effect on the transfer reaction rate is very close to that of propagation rate.Thus,the terminal model is effective in modeling the chain transfer constant of the RAFT copolymerization.The results showed that the dependence of chain transfer constants on monomer compositions is nonlinear in both copolymerization of MMA and BA with Z=-CH₂C₆H₅ and copolymerization of St and BA with Z=-CH(CH₃)₂.However, two systems have different changing rules.In the copolymerization of MMA and BA, C_trdecreases monotonously with the increase of f_MMA,therefore we can prepare polymers with much narrower molecular weight distribution with the decrease of f_MMA. But in the copolymerization of BA and St,C_trdecreases firstly,and then reaches a minimum value at f_st≈0.25,which is smaller than both C_tr1·1and C_tr2·2,then, increases again with the increasing of f_St.It is suggested that for the RAFT copolymerization,the molecular weight distribution could be broader than their homo-polymerization counterparts within some specific composition ranges.The relative addition and fragmentation probability in the copolymerization of MMA and BA with Z=-CH₂C₆H₅ indicated that in the synthesis of the block copolymer of MMA and BA,the block of PMMA should be synthesized firstly in order to gain a fast block transition.However,in the case of St/BA,the adding sequence does not matter in the synthesis of the block copolymer.The effect of temperature on C_trwas also investigated.It was found that the C_trdecreased with the rise of temperature.However,the influence is less pronounced, indicating a small apparent activating energy.