Effect of ethyl aluminium sesquichloride on the relative reactivity of methyl methacrylate and buta-1,3-dienes towards initiator radicals

The end-group method of Bevington et al. was applied to the investigation of methyl methacrylate (MMA) copolymerisation with a series of buta-1,3-dienes (butadiene, 2,3-dimethylbutadiene, isoprene, and 2,3-diphenylbutadiene) in the presence of relatively low concentrations of the Lewis acid ethylaluminium sesquichloride (EASC). The reactivity of the radical species produced by decomposition of azo-bis-methylisobutyrate methyl-13C (AIBMe), azo-bis-isobutyronitrile methyl-13C (AIBN) and azo-bis-1-phenylethane methyl-13C (APE) were investigated. The relative reactivity of the radical derived from AIBMe was consistent with the reactivity of the poly(MMA) radical in these systems, and showed a consistent behaviour in addition to all donor monomers that could be fit adequately in terms of the Q–e scheme using Q=24–28 and e=2.1. Little change was seen in the reactivity of the radical derived from AIBN, in contrast to the copolymerisation of MMA and styrene (S) in the presence of EASC, where the selectivity of this radical is comparable to that derived from AIBMe. This could not be interpreted in terms of the Q–e scheme and indicates an interaction with styrene giving anomalously high selectivity, which will be the topic of future investigation. The radical derived from APE showed an extremely high selectivity towards MMA, as expected, and the magnitude of 1-phenylethyl radical addition to styrene was consistent with previous work on the MMA–S–EASC system.