Mechanism of catalytic chain transfer in the free-radical polymerisation of methyl methacrylate and styrene

Abstract

The mechanism of catalytic chain transfer with bis(boron difluorodimethylglyoximate) cobaltate(II) (COBF) has been studied in the homopolymerisations of methyl methacrylate and styrene. The chain transfer constants were measured using both the Mayo and Chain Length Distribution (CLD) methods over a range of temperatures (40–70°C). The two methods generally agree within 10%. The high values of the chain transfer rate coefficients, ktr (∼107 for MMA), suggest the possibility that the reaction is approaching diffusion control. This is also supported by the high values obtained for the frequency factor (A ∼ 1010). The chain transfer rate coefficients for styrene are approximately two orders of magnitude lower than those obtained for MMA, which can be explained in terms of the formation of cobalt-carbon bonds and the accessibility of β-H sites for hydrogen abstraction from the two different radical chain ends in the case of styrene. High conversion, solution polymerisation experiments on methyl methacrylate in toluene reveal behaviour inconsistent with a simple catalytic mechanism and may suggest deactivation of the catalyst by solvent. On the assumption that the kinetics of catalytic chain transfer can be explained by a classical free-radical mechanism, it is possible to derive information on the chain length dependence of the average termination rate coefficient, 〈kt〉. Applying this approach to methyl methacrylate and styrene at different temperatures, we have found that the chain length effect on 〈kt〉 appears to be independent of both temperature and monomer type.