Study Of Emulsion Polymerization With Chain Transfer Agents Based On Irreversible Addition-Fragment Chain Transfer Mechanism

Molecular weight regulators or chain transfer agents (CTAs) based onirreversible addition-fragmentation chain transfer (AFCT) mechanism werefirst reported in the late1980s. Among these CTAs mentioned in thereferences in the following research, some allyl CTAs can be used formolecular weight control. The chain transfer constant for some particularvinyl monomers can be easily changed by designing the structure of theCTAs; It were also found no retardation during polymerization when theseallyl compounds were used in bulk or solution system for molecular weightcontrol of polystyrene. Therefore, they were ideal CTAs in bulk andsolution polymerization. Compared with mercaptans, the advantages ofirreversible AFCT agents are obvious: they have no odors; the chaintransfer constant for particular vinyl monomers can be regulated easily bychanging the activity group. Thus, these CTAs have the prospect for application in the industrial areas. Although various irreversible AFCTagents have been evaluated in bulk and solution polymerization, to the bestof our knowledge, the study on the emulsion polymerization of styrenewith these CTAs has never been published. Therefore, the research on theintrinsic kinetics of emulsion polymerization with irreversible AFCT agentsis important for the application of these new CTAs. This work mainlyfocused on the study of the kinetics for emulsion polymerization withirreversible AFCT agents as following description.Firstly, five kinds of CTAs based on allyl structure were synthesizedaccording to the literature or purchased from the chemical company. Theyare butyl(2-phenylallyl)sulfane (BPAS), dodecyl(2-phenylallyl)sulfane(DPAS), ethyl α-benzenesulfonyl-methylacrylate (EBSA), ethylα-p-toluenesulfonyl-methacrylate (ETSA) and2,3-dichloro-1-propene(DCP). These CTAs can be easily synthesized with high yield and have noobjectionable odors compared with mercaptans. They were alsocharacterized by~1H NMR, and the spectrums of compounds shown that thestructures were correct.Secondly, the emulsion polymerization of styrene with above fivedifferent CTAs based on irreversible AFCT mechanism was carried outusing conventional surfactant (sodium dodecyl sulfate, SDS) and initiator(potassium peroxodisulfate, KPS). The influences of these irreversibleAFCT agents on the rate of polymerization, particle size and average-number molecular weight were investigated. It was found that theintrinsic activity, diffusion and desorption behaviors of the CTAsdetermined the efficiency of molecular weight control, rate ofpolymerization and particle size in the emulsion polymerization. It has beendemonstrated that the irreversible AFCT agents with high chain transferconstant (EBSA and ETSA) decreased the reaction rate and practice size,meanwhile, the molecular weight of the polymers could not be controlledwell, whereas the irreversible AFCT agents with low chain transferconstant (BPAS, DPAS and DCP) had a slight effect on the polymerizationrate and particle size. DPAS failed to control the molecular weight, whichwas attributed to the low rate of diffusion through monomer droplets toaqueous phase. The average number of radicals per particle and thenumber-average molecular weight were calculated by classical radicalemulsion polymerization theory, and the experimental results were in goodagreement with the results of model calculations, when the irreversibleAFCT agents were used as CTAs. The effect of CTAs on the kinetics andnucleation in the emulsion polymerization of styrene can be attributed todesorption of chain-transfered radicals from the polymer particles. Theresults of this work show that BPAS as CTA in emulsion polymerization ofstyrene provides the best balance between the rate of polymerization andthe efficiency for molecular weight control conflicting tendencies.Thirdly, the emulsion polymerizations of styrene in presence of BPAS were carried out using various amount of sodium dodecyl sulfate assurfactant and potassium peroxodisulfate as initiator. The effects ofsurfactant, initiator and BPAS on the course of the emulsion polymerization,and the number-average molecular weight were studied. The relationshipsbetween the rates of polymerization and the number of the particles pervolume with respect to the concentrations of KPS, SDS and BPAS werefound. The obtained relationships can be attributed to the exit of leavinggroup radicals on BPAS from the particle. The experimental values of theaverage number of radicals per particle strongly depended on theconcentration of BPAS and were in good agreement with the theoreticalvalues of model calculations. The number-average molecular weight can becontrolled by BPAS almost over the whole conversion range which is alsoin agreement with a mathematical model. Moreover, BPAS can also beused in the emulsion polymerization at lower temperature, and itsefficiency for molecular weight control is relatively independent of reactiontemperature.Finally, the emulsion polymerizations of styrene-butadiene rubber(E-SBR) were also carried out using three CTAs based on irreversibleAFCT mechanism. The polymerization kinetics is similar with theemulsion polymerization of styrene at high temperature. It was also foundthat the irreversible AFCT agents with high chain transfer constantdecreased the reaction rated of polymerization, due to desorption of chain-transfered radicals from the polymer particles. Meanwhile, CTAswith very low chain transfer constant would obtain a decreased efficiencyfor molecular weight control. Therefore, there is a conflicting tendencybetween the rate of polymerization and the efficiency for molecular weightcontrol in emulsion polymerization.