DIENE POLYMERIZATION BY MIXED SODIUMALKYL AND LITHIUMALKYL INITIATORSIN HYDROCARBON MEDIUM

Polymerization of butadiene and isoprene initiated by the hydrocarbon- soluble mixture 2-ethylhexyllithium/2-ethylhexylsodium in toluene was studied. The components of the initiator when taken separately give ri se to polymers greatly different in both their molecular weight and mi crostructure: living polydienes with high 1,4-unit content are formed when alkyllithium is used while organosodium initiators produce polyme rs of high 1,2(3,4)-structure and of low molecular weight due to stron g chain transfer to toluene. Polymers obtained with mixed initiators w ere found to have the same structure as the ''sodium'' ones, while the ir molecular weights are either the same as those of the polymers obta ined with alkyllithium alone (in the case of butadiene) or decrease gr adually with increase of sodium content in the initiator (in the case of isoprene). The calculations performed show that the simple scheme a ssuming chain propagation through individual lithium and sodium active centres and fast exchange of counter-ions between them cannot explain the results obtained. It is suggested that the true active centres in mixed systems are bimetallic complexes containing both sodium and lit hium atoms simultaneously.