Polymerization of methylmethacrylate by organometallic compounds—vii. The n-butyl magnesium bromide-di- n-butyl magnesium system in THF + toluene solution

Abstract

The molar-mass distributions of the products at 223°K and below were multimodal having three distinct and separate components, viz. volatile side-products, methanol-soluble polymer ( DP n ≈ 50 ) and methanol-insoluble polymer ( DP n = 200–600 ). At low THF concentrations, with n-BuMgBr, small amounts of a third distinct and independent polymeric product appeared at higher molar mass. The distributions of configurational triads of the chains of the methanol-soluble and insoluble polymers were non-Bernoullian but different. It is proposed that the two components are formed at two distinct and independent active centres. This eneidic mechanism explains the very high values of DP w / DP n reported in previous studies of Grignard initiation of MMA. The configurational triad distribution of the methanol-soluble and insoluble components remained unchanged both when n-Bu 2Mg replaced n-BuMgBr as initiator and when excess MgBr 2 was present. It is proposed that the same active centres operated throughout. In contrast to t-BuMg systems, there is no evidence that MgBr 2 or BrMg- groups play any other role in the polymerization than to reduce the initiator efficiency. The effects of temperature and the addition of strongly solvating solvents were tested. Tetraethylene glycol dimethyl ether caused the precipitation of MgBr 2 from the initiator solution but did not otherwise affect the mechanism. However when hexamethylphosphoroamide was added at 223°K the polymerization took a different mechanism. Sixteen per cent of n-BuMg- groups remain unreacted during the polymerization. It is proposed that the presence of these groups at the active centre accounts for the influence of the Bu group on the stereospecificity of the polymerization.