Anionic heterogeneous polymerization of acrylonitrile by butyllithium. II. Effect of lewis bases on the molecular weight

The heterogeneous polymerization of acrylonitrile in petroleum ether by butyllithium in the presence of Lewis bases was investigated. Molecular weights increased sharply on addition of small amounts of Lewis bases. High, maximal molecular weights were obtained at optimal values of [Lewis base]/[BuLi] = 1–2. Beyond this optimum a sharp decrease of molecular weights was observed. The optimal [Lewis base]/[BuLi] ratio was dependent on the Lewis base used and was specifically related to the structure of the complex Li⁺ Lewis base. The effectiveness of the Lewis bases in obtaining high molecular weights was as follows: DMF ≫ THF ∼ dioxane > dimethylacetamide. A nonterminated “living” polymerization for which DP was given by DP = R_p/R_i was observed with low rates of monomer addition, whereas chain transfer to monomer occurred with high rates. This behavior was qualitatively the same as that found in the absence of any Lewis base. However, under comparable conditions, higher molecular weights were always obtained in the presence of a Lewis base, except for relatively high ratios of [Lewis base]/[BuLi]. It was suggested that at the optimal ratio of [Lewis base]/[BuLi] and beyond it, all the positive Li⁺ ions in the polymerization mixture were solvated by the Lewis base. This solvation increased the nucleophilic reactivity of the carbanions. On increasing the Lewis base concentrations up to the optimal value of [Lewis base]/[BuLi], R_p increased much more than R_i, which resulted in a sharp increase of molecular weights. Beyond the optimal value of [Lewis base]/[BuLi] where all the positive counterions of the propagating ends were completely solvated, added Lewisbase might cause further dissociation of the associated [BuLi]_n, leading to a larger increase of R_i as compared to R_p, and consequently to a decrease of DP.