Observations on the effect of solvents on the anionic polymerization of methyl methacrylate by butyllithium

The anionic polymerization of methylmethacrylate by n-butyllithium in toluene was investigated in the presence of the following Lewis bases: diethylamine, pyridine, α-picoline, triethylamine, dioxane and tetrahydrofuran. Up to [Lewisbase]/[BuLi]{reversed tilde equals}2 the weight-average molecular weights, M_w, decreased to a minimum which did not change with further increase of Lewis base concentration. With diethylamine, pyridine and α-picoline a new N-Li catalyst was formed, which initiated polymerization. Accordingly the polymers contained amino end groups which were titrated by perchloric acid. Thus the M_n of the polymers were calculated and were found to increase with the [Lewis Base]/[Buli] ratio. The distribution of the molecular weights, as measured by M_w; M_n was high, as previously found with butyllithium, and this decreased with increase of the Lewis base concentration. The M_w and M_n values were correlated by the presence of low polymer fractions besides high ones, the proportion of the former being very high. The effect of increasing Lewis base concentration was to decrease the proportion of the low polymer fraction (with a corresponding increase in the high fraction) and the DP_n of the high polymer fraction, thus leading to the narrower molecular weight distribution observed. The fraction of the catalyst which was lost in side reaction also increased. These observations were correlated with solvation effects which decreased the fraction of low polymer formed due to factors such as "pseudotermination" of short chain polymers.