Stereoregular Polymers: Stereochemical Aspects and Synthetic Approaches

Abstract

Stereoregular polymers constitute a large class of macromolecular compounds that are endowed with many interesting and valuable properties. The natural ones are of vital importance. The properties that these polymers possess are determined in large part by their steric regularity. The essential role of stereochemistry in determining the physical and chemical properties of chemical compounds was envisaged by Wislicenus (1) more than one hundred years ago. That this applies to high as well as to low molecular weight compounds was recognized early during the development of macromolecular science. In the late twenties Meyer and Mark (2,3) interpreted the differences between natural rubber and gutta-percha by suggesting that the former represents the eis- and the latter the trans-form of 1,4-polyisoprene, and Staudinger proposed stereoirregularity as the cause of the non-crystallizability of polyindene (4,5), polystyrene (5) and polyvinylacetate (5). At that time researchers were gradually beginning to realize that all organic polymers found in nature and containing sites of stereoisomerism were stereoregular. The development in the understanding of the structure of various natural polymers has been described by Meyer (6). Thus it must have become obvious that stereoregularity is an essential condition in order for these natural polymers to carry out their specific biologic functions. However, fundamental studies on the influence of stereochemistry on the physical and chemical properties of polymers started only in the late forties, when the first stereoregular polymers were synthesized from monomers non containing stereoisomerism sites. Great impetus to developments in this area was given by the first synthesis of a highly stereoregular polypropylene by Natta and his group in 1955 (7). This originated a large interest in the synthesis and the study of other stereoregular polymers, thus forming the basis for the remarkable progresses achieved in this area in the last 20 years. From this development it clearly emerged that stereochemistry strongly influences the reactivity, the conformational equilibria and many physical properties of polymers in dilute solution and in the melt. For chemically regular polymers, stereochemistry determines the type of crystal structure and the degree of crystallinity, and thus determines the properties of these polymers in the solid state. Therefore, it is not surprising that, in some cases (e.g. polypropylene) stereochemistry may even determine the success of the industrial application of a polymeric product. The relevance of synthetic stereoregular polymers is by no means limited to the study and applications of the relationships between polymer stereochemistry and properties. Progress in many other areas of polymer science has been made possible or has been accelerated by studies on stereoregular synthetic polymers. In particular, significant advances in the understanding of the mechanism of various polymerization processes have been achieved through the efforts made to elucidate the factors that determine the formation of stereoregular polymers. Furthermore, some factors influencing the mode of crystallization of the polymer chains and the morphology of the polymers have been clarified by the investigation of the large number of stereoregular polymers which have been synthesized.