Effect of solvents on the anionic polymerization of styrene by phenyllithium

Abstract

AbstractThe anionic polymerization of styrene by phenyllithium was studied in various solvents The order of the decreasing molecular weights obtained depending on the solvent, was: tetrahydrofuran » toluene > benzene > petroleum ether > diethyl ether. The molecular weight, due to absence of termination, is governed by the ratio of the rate propagation to the rate of initiation (Rp/Ri). Both these rates are influenced by the dielectric constant and the solvating power of the solvent for the phenyllithium and for the growing end, solvation of the growing end being greater due to the resonance of the propagating styryl anions. This leads to the fact that on passing from petroleum ether (low dielectric constant) to tetrahydrofuran (relatively high dielectric constant and high solvating power), Rp increases much more than Ri, leading to higher molecular weight. The polymerization of styrene in mixtures of petroleum ether and tetrahydrofuran was studied by the addition of increasing amounts of tetrahydrofuran while keeping the catalyst and monomer concentrations constant. Initially (up to 2 vol.‐% tetrahydrofuran) there was a sharp decrease in molecular weight due to a greater dissociation of the phenyllithium, with a corresponding increase in Ri. On further addition of tetrahydrofuran (up to about 25 vol.‐%), the molecular weight increased and approached the order of that obtained in petroleum ether. This is due to a greater increase in Rp over Ri due to greater solvation of the propagating end. At this relatively high tetrahydrofuran concentration we have maximum solvation both of the catalyst and of the growing end. With further addition of tetrahydrofuran there is a linear dependence of the molecular weight both on the concentration of the tetrahydrofuran and on the dielectric constant of the system. Ether gave the lowest molecular weights, in spite of its relatively high dielectric constant, due to greater dissociation of the catalyst associates (C6H5Li)x leading to an increased efficiency of the catalyst (greater Ri). Toluene and benzene led to higher molecular weights than petroleum ether, although the dielectric constants are of approximately the same order, due to the homogeneous nature of the polymerization in these solvents as compared with the heterogeneous polymerization in petroleum ether. The molecular weight increased with decrease in temperature in various solvents due to the increase in the dielectric constant and the lowering in Ri