Effect of solvents on the molecular weight of polyacrylonitrile obtained by anionic catalysis

The effect of solvents on the molecular weight of polyacrylonitrile obtained by anionic polymerization with butyllithium was investigated. The molecular weight was found to decrease with increase in the dielectric constant of the solvents or their solvating power in the following order: petroleum ether « dimethylaniline > diethyl ether > tetrahydrofuran > dimethylformamide. The termination step of the polymerization is by chain transfer to monomer. The rate of propagation increases with increase in the dielectric constant, and the decrease in molecular weight seems to be due to a greater increase in the rate of termination. This lowering in molecular weight in anionic polymerization with increase in the dielectric constant is contrary to what is generally known in cationic polymerization, where, due to termination by the gegenion, the molecular weight increases.