Role of polymer–catalyst bond in coordinated anionic polymerization of diolefins

Abstract

The variations in the microstructure of polydiolefins made with alkali metal catalysts can be adequately explained in terms of the nature of the bond between the metallic catalyst and the growing polymer chain. This bond, in alkali metal polymerization, is not a resonance hybrid of ionic and covalent states, but is a tautomeric equilibrium of covalent bonds and ion pairs. The size of the equilibrium constant is one of the factors controlling the microstructure of the final polymer. An additional factor is the degree of p character of the covalent bond. Variations in microstructure between polyisoprene and polybutadiene can be attributed to differing proportions of ion pairs. Similarities between Ziegler‐Natta and alkali metal catalysis are shown. Some predictions of relative catalytic activity of alkali metals were shown to correspond to observed reactions.

Lithium Catalysis in Industrial Polymerization

“…R. .1. Orr (85) has indicated, in a study of the abundance of ion pairs at the growing end of polymer chains in alkali metal-catalyzed polymerizations and in solvents of low dielectric constant, that the carbon-lithium bond exists as a tautomeric equilibrium between ion pairs and covalent bonds, and not as a resonance hybrid of these two types. An extremely small number of ion pairs is actually present at any one time.…”