Anionic polymerization mechanism of acrylonitrile trimer anions: key branching point between cyclization and chain propagation.

A cluster anion of vinyl compounds in the gaseous phase has served as one of the simplest microscopic models of the initial stages of anionic polymerization. Herein, we describe our investigations into the initial stage mechanisms of anionic polymerization of acrylonitrile (AN; CH2═CHCN) trimer anions. While the cyclic oligomer is found in mass and photoelectron spectroscopic studies of (AN)3(-), only the chain oligomer is found in the infrared photodissociation (IRPD) spectrum of Ar-tagged (AN)3(-). On the basis of the calculated polymerization pathway of (AN)3(-), we consider that the chain oligomers are the reaction intermediates in the cyclization of (AN)3(-). The rotational isomerization of the (AN)3(-) chain oligomer is found to be the bottleneck in the cyclization of (AN)3(-). To form the (AN)4(-) chain oligomer by chain propagation, the addition of an AN molecule to (AN)3(-) should occur prior to the rotational isomerization. We conclude that the rotational isomerization in the (AN)3(-) chain oligomer is the key branching point between cyclization (termination) or chain propagation in the anionic polymerization.