Recent developments in high polymers

Abstract

Polymers range in properties from almost completely amorphous to highly crystalline solids. Crystallinity is of great importance because it determines the physical and technological properties of the substance. Physical properties such as melting point or softening point and the mechanical properties of the polymer depend on chain orientation and crystallinity. The ability of polymers to crystallize depends, among other things, upon the regularity of the structure and the chemical nature of the chains. In the crystalline state the chains are arranged in an orderly and regular arrangement and any factor which decreases the regularity reduces the ability to crystallize. The recent discovery of stereospecific polymerization of the olefin hydrocarbons and its extension to other types of non-hydrocarbon monomers has led to many new classes of regular, generally crystalline polymers, some of which are already of great practical interest. The first stereoregular polymers obtained since the discovery of stereoregular catalysts in 1956 were the vinyl polymers with a hydrocarbon main chain such as the ones prepared from the α-olefins, conjugated dolefins and vinyl ethers. The major consequence of the regularity of steric structures of polymers from monomers CH 2 ═CH X in which the X -groups all occupy the same spatial arrangement along the backbone chain is that the polymer molecules pack together in an ordered three-dimensional structure in which they display the properties of crystalline materials. The most striking property consequent upon crystallinity is the melting point—a property of fundamental interest, for instance, in the spinning of a fibre and in the manufacture of plastics of good high-temperature properties. Table 14 summarizes the melting points of a few of the isotactic polyolefins (Bawn & Ledwith 1962; Natta 1960) and these properties may be satisfactorily interpreted in terms of the geometrical shapes of polymeric molecules in the crystallites. While much of the earlier study on stereospecific polymers and that which clearly emphasized the relation between structure and physical properties was concerned with the α-olefins and other hydrocarbon polymers, more recent work has shown similar dependence among the more polar polymers (Bawn & Ledwith 1962), most of which in the crystalline state adopt the helical chain arrangement first observed for the poly-α-olefins. Table 15 summarizes some typical results.