Ring-opening polymerization of heterocycles bearing substituted and unsubstituted exo-methylene groups - a review

Abstract

Monocyclic monomers bearing exo-methylene groups are likely to undergo ring-opening polymerization under the conditions of radical or cationic initiation. Several structural features will determine the course of reaction after initiation by species that exclusively attack the exo-methylene functionality. Reviewing some of the most common classes of monocyclic heterocycles it becomes obvious that a monomer has to meet certain structural preconditions that makes them suitable for isomerizations. Among the considered heterocycles there are some types of monomers possessing an outstanding ring-opening behavior following polymerization mechanisms free from side-reactions. The combination of strain release due to ring-opening isomerization, an energy decrease in the transition state caused by the formation of carbonyl-functionalities and an appropriate stabilization of the growing chain end by steric and electronic effects may lead to clear ring-opening mechanisms. However, this property remains restricted to only some special monomers with simple aromatic substituents being attached to the cycle. Hence a variety of cyclic ketene acetals, enolethers, (meth)acrylates, carbonates, lactones and lactames have been chosen in order to demonstrate both the possibilities and limits of ring-opening.