STRUCTURAL REGULARITY AND CRYSTALLINITY IN MACROMOLECULES

Structural regularity is a primary requisite for the crystallization of a polymer. Fulfillment of this requirement is dictated by the long-range order and symmetry that is characteristic of the crystalline state. It follows that, in order to crystallize, a polymer chain must consist of identical units regularly repeated throughout its length or over long sequences thereof. Stereochemical regularity must be maintained as well if a high degree of crystallization is to be realized. This latter assertion is vividly demonstrated in the epochal investigations of stereoregular vinyl polymers by Professor Giulio Natta, to whose memory this International Symposium is so fittingly dedicated, and by his collaborators and followers throughout the world. It finds illustration also in comparisons of polymers of biological origin with their synthetic analogs; the former generally are highly stereoregular, the latter ordinarily are not, vinyl polymers obtained by stereospecific polymerization excepted. Presence of structural or stereochemical irregularities in macromolecules lowers the melting point and decreases the degree of crystallinity. In the case of block copolymers the crystallizable blocks may be selectively crystallized; the melting point is little depressed if the blocks are long. These aspects find abundant illustration also in the polymers extensively investigated by Professor Natta and his colleagues. In order to trace the intricate connections between structural regularity and physical properties, it is necessary to comprehend the conformational preferences among the many conformations accessible to a polymer chain. Methods are well established for this purpose. Their application to vinyl polymer chains emphasizes the role of stereochemical structure in affecting the spatial form of the macromolecule and, hence, the properties of the polymer.