Polymerization of butadiene and isoprene in hydrocarbon solvents initiated by unsolvated organosodium compounds was studied. It was found that the polymer molecular weight and the MWD are determined mainly by chain transfer to solvent and polymer, and no chain transfer to monomer was observed even in the case of isoprene. The overall polymerization rate is proportional to the concentrations of the monomer and the initiator. Apparent chain propagation rate constants were found to be 0.11 litre mo1-ls-l for butadiene and 0.065 litre mol-ls-' for isoprene polymerization in heptane at 30°C. It is suggested that associated (dimeric) forms of polydienylsodium active centres play an important role in chain propagation, being responsible for a stronger chain transfer and a greater l,Zbutadiene, or 3,4-isoprene, unit content than in polymerization with other alkali metals.

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 rise to polymers greatly different in both their molecular weight and microstructure: living polydienes with high 1 ,4-unit content are formed when alkyllithium is used while organosodium initiators produce polymers of high 1,2(3,4)-structure and of low molecular weight due to strong chain transfer to toluene. Polymers obtained with mixed initiators were found to have the same structure as the "sodium" ones, while their molecular weights are either the same as those of the polymers obtained with alkyllithium alone (in the case of butadiene) or decrease gradually with increase of sodium content in the initiator (in the case of isoprene). The calculations performed show that the simple scheme assuming 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 lithium atoms simultaneously.

It is frequently observed in Ziegler-Natta polymerization that the dependence of the overall polymerization rate on the catalyst/cocatalyst ratio has a more or less marked maximum (see, e.g., Porri and Giarusso 1 ). However, in lithium-initiated anionic polymerization, the addition of organometallic modifiers usually monotonically decreases the overall rate. As early as 1960, it was shown 2 that the addition of AlEt 3 or ZnEt 2 to n-BuLi gradually decreases the rate of styrene polymerization in benzene to full inactivity at Al/Li ≈ 1 and Zn/Li ≈ 10, respectively. This was easily explained by the formation of LiAlR 4 and LiZnR 3 "ate" complexes which are known to be inactive in polymerization of nonpolar monomers. 2,3 A similar monotonic dependence was observed later for butadiene polymerization in cyclohexane with the sec-BuLi-Bu 2 Mg initiator. 4 All of these findings did not encourage a search for new bimetallic anionic initiators for polymerization of nonpolar monomers.However, in the case of heavier alkali metals the situation seems to be different. True, a few earlier attempts to use a preformed stoichiometric NaMgR 3 complex in diene polymerization also were unsuccessful. From the data reported, 5 one can estimate that the firstorder rate constant [k′ ) -(1/M)(dM/dt)] for isoprene polymerization in cyclohexane at 20°C is on the order of 10 -6 s -1 , and at 50°C, it is on the order of 10 -5 s -1 in comparison to 3 × 10 -4 (30°C) for the uncomplexed organosodium initiator 6 (the initiator concentration was ca. 5 × 10 -3 M in all cases). According to a later publication, 7 only 16% yield of polybutadiene was achieved after "1 day or more" with the same initiator in hexane at 50°C.Much more interesting results were obtained with substoichiometric mixtures. It was observed recently that the addition of small amounts of various aluminum alkyls such as AlEt 3 , Al(i-Bu) 3 , HAl(i-Bu) 2 , and so on to the common disodium tetra-R-methylstyrene initiator greatly increased the rate of butadiene polymerization in hydrocarbon solvents up to a certain maximum value. 8 The Al/Na ratio at the maximum depended on the particular aluminum compound. The addition of an aluminum compound above the optimum ratio decreased the overall rate to the full cessation of the process.In all of these cases, small amounts of a polar solvent were introduced with the organosodium component; hence, the role of solvation effects and the solvation/ complexation competition could not be excluded. However, as was reported in the preliminary communication, 9 at least for one particular model system, namely, 1,1,3-triphenylpropylsodium-AlPh 3 , the dependence of the polymerization rate on the Al/Na ratio passed through a maximum even in a pure hydrocarbon medium (toluene) without any polar additives.Here, we reported the results of the investigation of butadiene polymerization with a RNa-R 2 Mg system in toluene. In this study the same R ) 2-ethylhexyl was used for both sodium and magnesium components. Because of the solubility of 2-ethylhexylsodium...

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SUMMARY: 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 substantially depend on the composition of the initiator. The usual scheme assuming the accumulation of highly reactive organopotassium species due to a metalmetal exchange between the initiator components is qualitatively applicable only at 0 a x a 1, although its quantitative agreement with the experiment still remains disputable. In the overstoichiometric region (x A 1), the general pattern is completely inconsistent with this scheme. At x = 2, the relative constant of chain transfer to toluene is 30 times higher than that with potassium alkyls (RK), and only low-molecular-weight polymer (M -n L 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 The study of several model mixed initiators suggests that these new species are complex aggregates containing simultaneously K, Li, and OR moieties rather than single-metal species. Surprisingly, it was found that neither free RK nor its complexes with excess t-AmOK play any important role in the overstoichiometric region.