Moderated copolymerization. Part 2.—Transfer constant of styryl radicals towards carbon tetrabromide: a penultimate unit effect in chain transfer

Abstract

We have studied further the chain-transfer reaction of styryl radicals with carbon tetrabromide using the moderated copolymerization technique. With methyl methacrylate as moderating monomer, the observed chain-transfer constant CB,obs increases markedly with the ratio [styrene]/[methyl methacrylate]. The results are in quantitative agreement with a relation based upon a penultimate-unit effect in the transfer reaction, the reaction rate being influenced by the nature of the penultimate unit, but not by the remaining units in the chain. When styrene is the penultimate unit the transfer constant has been found to have the value 368, in substantial agreement with the work of Thomson and Walters and Gleixner et al. Further support for the existence of the effect is adduced from experiments with methyl acrylate as moderating monomer, which gave a transfer constant of 169. Experiments in the presence of inactive diluents revealed only a small dependence on concentration, showing that complex formation between halide and styrene is not significant in this work.We consider possible origins of the penultimate unit effect, which is reported here for the first time in chain-transfer, and conclude that steric obstruction between the entering bromine atom and the α-methyl group of the penultimate methyl methacrylate unit is the major cause.