Stereospecificity and optical properties of polymers

Abstract

AbstractPractically the only physical method of determining stereoregularity is x‐ray analysis, permitting one to judge the presence of a crystalline structure in a given polymer. However, crystallinity is a phase property, only indirectly reflecting the molecular structure of a substance. It is well known that a stereospecific polymer may be both in the crystalline and in the amorphous states. It is therefore highly important to find a property of the polymer which would directly (and unilaterally) indicate stereoregularity in its structures. Attempts to use the geometrical properties of molecular chains for this purpose have up to the present been unsuccessful, the sizes of macromolecules in solution (as determined by viscometry or light scattering) being found to be independent of stereoregularity. In the present work another approach has been sought, the attempt being made to ascertain whether the optical anisotropy of a molecule would be a more sensitive index of the stereospecificity of the molecular chain. With this in mind the flow birefringence was investigated, of solutions of polystyrene (from Dzerzhinsk) and of polymethylmethacrylate (from the laboratory of A. A. Korotkov) with varying degrees of stereoregularity. Experimental investigation of both polymers showed a marked increase in the segmental anisotropy α1 – α2 of the macromolecules when passing from the atactic to the isotactic specimens. Thus for atactic polystyrene α1 – α2 = 140 × 1025 cm3. whereas for isotactic polymer, α1 – α2 = 200 × 10−25 cm3. In the case of polymethyl methacrylate the difference was still greater. The increase in α1 – α2 in the case of polystyrene may be explained by the increased hindrance to rotation of the phenyl side chains on transition from the atactic to the isotactic polymer. The interpretation in the case of polymethyl methacrylate is not so evident and requires further elucidation. At the same time the fact itself of the influence of the stereoregularity of a polymer chain on its optical properties lays open the possibility of utilizing the latter to characterize the former.