Kinetics and Mechanism of Living Cationic Polymerization of Olefins

Olefins such as isobutylene and styrene can be cationically polymerized in a controlled/living fashion under certain specific conditions. The best systems tend to employ either TiCl4 or BCl3 coinitiators. Lewis base additives are used to suppress both protic initiation and propagation by free ions. The growing chains are characterized by a reversible ionization equilibrium between a small concentration of active ion pairs and a much larger concentration of dormant chains. These systems deviate from ideal living behavior to a lesser or greater degree depending upon conditions. With select systems at low temperatures, the deviations are very small, and well defined polymers can be produced. Non-ideality results from unimolecular termination reactions, as opposed to bimolecular chain transfer to monomer. This makes clear the danger of excess aging of living chains, and it highlights the practical importance to synthetic procedures of a thorough understanding of the kinetics of propagation and chain breaking events.