Study on the mechanism of anionic polymerization with mixed RLi-R'OK initiators, 1 Polymerization of butadiene

Butadiene polymerization in toluene with n-BuLi/t-AmOK mixed initiator at K /Li molar ratio, x, from 0 to 6 has been studied. The results observed subs tantially depend on the composition of the initiator. The usual scheme assu ming the accumulation of highly reactive organopotassium species due to a m etal-metal exchange between the initiator components is qualitatively appli cable only at 0 < x < 1, although its quantitative agreement with the exper iment still remains disputable. In the overstoichiometric region (x > 1), t he general pattern is completely inconsistent with this scheme. At x = 2, t he relative constant of chain trans fer to toluene is 30 times higher than that with potassium alkyls (RK), and only low-molecular-weight polymer (<(M )over bar(n)> approximate to 500) is formed, the polymerization rate being 10 times lower than the rate observed at x = 1. At even higher excess of t- AmOK, x greater than or equal to 5, <(M)over bar(n)> remains very low but < (M)over bar(w)> greatly increases. As a result, the polydispersity of the p olymer is unusually high, <(M)over bar(w)>/<(M)over bar(n)> greater than or equal to 50, indicating the participation of at least two new types of gro wing species with a fairly slow exchange between them. The study of several model mixed initiators suggests that these new species are complex aggrega tes containing simultaneously K, Li, and OR moieties rather than single-met al species. Surprisingly, it was found that neither free RK nor its complex es with excess t-AmOK play any important role in the overstoichiometric reg ion.