Moderated copolymerization. Part 5.—Penultimate unit effect in chain-transfer. Systems styrene/methyl acrylate/carbon tetrabromide and styrene/methyl acrylate/1-butane thiol and some general considerations

Abstract

Investigation of the transfer constants of styryl radicals in copolymerizations of styrene (St) has been continued by the technique of moderated copolymerization with methyl acrylate (MA) as comonomer. Using a low value of [styrene]/[methyl acrylate](8.75 × 10–3) we find that the transfer constant towards carbon tetrabromide is 302 at 60°C; this value, which refers essentially to [graphic omitted] MA—St· radicals, approximates to that found previously for [graphic omitted]St—St· radicals (337). The results therefore support the view that the low transfer constant found for [graphic omitted] MMA—St· radicals (MMA = methyl methacrylate) is attributable to steric interference between the α-methyl of the penultimate MMA unit and the incoming bromine atom. With 1-butane thiol as transfer agent, the transfer constant found for [graphic omitted] MA—St·(20.0) is close to that of [graphic omitted]MMA—St·(22.4), a result which also strongly supports the hypothesis of local steric interference mentioned above.The general case of copolymerization of monomers A and B in which both [graphic omitted]A· and [graphic omitted]B· radicals are subject to penultimate unit effects is considered and general equations are developed.Finally, the possibility of penultimate unit effects in chain-transfer in homopolymerizations is mentioned.