Anionic homogeneous and heterogeneous polymerizations of acrylonitrile by alkali metal alkoxides

The homogeneous (in dimethylformamide) and heterogeneous (in petroleum ether) polymerizations of acrylonitrile by methanolic solutions of alkali metal methoxides were compared. In the homogeneous system termination of polymerization was by chain transfer to monomer and degree of polymerization (DP) was constant, while in the heterogeneous system termination was by the acidic hydrogen of the alcohol and DP increased with monomer concentration according to the equation DP = K_p[M]/K_t[ROH]. Decrease in temperature led to increase in molecular weight in the heterogeneous polymerization but not in the homogeneous. The positive counterion influenced the molecular weight only under the heterogeneous conditions; the more electropositive potassium gave smaller molecular weights than the sodium catalyst. Owing to the high dielectric constant of dimethylformamide and its high solvating power, the growing ion pairs   C⁻Li⁺,   C⁻Na⁺, and   C⁻K⁺ in this solvent were fully dissociated and showed their “limiting anionic behavior,” leading to the molecular weights' being constant irrespective of polymerization temperature and alkali metal counterions. Because of the high ionic character of the  C⁻K⁺ bond, the potassium catalyst shows its “limiting anionic behavior” even under heterogeneous conditions, leading to molecular weights of the same order in both the homogeneous and the heterogeneous systems.