Ni. Pakuro et al., DIENE POLYMERIZATION BY MIXED SODIUMALKYL AND LITHIUMALKYL INITIATORSIN HYDROCARBON MEDIUM, Macromolecular chemistry and physics, 196(1), 1995, pp. 375-383
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.