Moderated copolymerization. Part 4.—Penultimate unit effect in chain-transfer: system styrene/methyl methacrylate/1-butane thiol

Abstract

The technique of moderated copolymerization has been applied in determinations of the transfer constant of styryl radicals towards 1-butane thiol in the system styrene/methyl methacrylate/1-butane thiol at 60°C. It has been found that the transfer constant does not vary significantly (outside the limits of experimental error) with the ratio [styrene]/[methyl methacrylate] so that there is no evidence for the existence of a penultimate unit effect as found in the system styrene/methyl methacrylate/carbon tetrabromide. The mean value 22.4 lies in the range of literature values.

These results are consistent with the view that the penultimate unit effect in the system with CBr₄ arises from local steric interference between the α-methyl of the radical [graphic omitted] MMA—St· and the incoming bromine atom.