Macromolecular polymorphism and stereoregular synthetic polymers

Abstract

Definitions and general concepts on polymorphism are briefly reviewed and specific aspects of the polymorphism of macromolecular substances are emphasized. A list of synthetic crystallizable polymers for which polymorphism phenomena have been observed to date is presented. For each polymer are briefly reported or discussed: the number of polymorphic forms discovered, the types of unit cell found in the crystallographic research and brief information about thermodynamic stability, kinetic transformations, physical properties and conditions of preparation of single crystalline modifications. Either chain conformation or mode of packing of chains can be responsible for structural differences among crystalline modifications. More detailed information is then given and a number of experimental results is presented relating to typical cases of enantiotropy and of monotropy found among stereoregular polymers. An extended analysis is made of the enantiotropy of transtactic polybutadiene, which can appear in two crystalline modifications, and shows a reversible solid-solid transition from one form to the other. The results of a detailed experimental investigation are also reported on the monotropy of isotactic polybutene-1, for which three polymorphic modifications were recognized and thermodynamic and kinetic transformations were examined in different experimental conditions. A discussion finally deals with the theoretical problem of the prediction of the existence of different polymorphic modifications of given macromolecular chain structures. Satisfactory results have so far been obtained in the chain conformational polymorphism of isotactic and syndiotactic polypropylene by means of a recent analysis of the energy minima of an isolated chain. This kind of approach appears to be of general application and to be the most convenient for the analysis of even more complicated chains.