Aryl and cyclo-alkyl magnesium initiators of methyl methacrylate. NMR studies of the nature of the initiator solutions and its effect on stereospecificity and size distribution

Abstract

Stereoblock and isotactic-like poly(methyl methacrylates) have been prepared using phenyl-, mesitylenyl- or cyclohexylmagnesium bromide in toluene solutions containing various amounts of tetrahydrofuran (THF). The optimum mole fraction of THF for isotactic polymerization was 0.01–0.02 in the case of PhMgBr. It was shown that the decline in efficiency and isotacticity of more extensively de-etherated initiator is due to loss of soluble bromide. The effects of THF concentration and temperature on the mol. wt distributions and steric triad distributions have been investigated. As temperature is increased and THF content decreased, the polymers change from stereoblock to isotactic-like when the Grignard reagent is used and from Bernoullian-distribution syndiotactic-like to stereoblock with analogous diarylmagnesiums. Molecular weight distributions are polymodal and change in a complex manner. Mesitylenylmagnesium bromide was the most isotactic specific of the initiators but it was inferior to tert-butylmagnesium bromide. The isotactic-directing power of a particular system correlated inversely with lability of the Schlenk equilibrium as indicated by the coalescence temperature of the individual 13C-NMR peaks of the RMgBr and R 2Mg species.