Unperturbed dimensions of atactic poly(phenylacetylene)

Dilute solution characteristics of atactic (ca. 30% trans) poly( phenylacetylene), PPA, were determined by using the SEC/MALS technique in order to obtain a deeper insight into discrepancy between presumed rigidity of conjugated polyvinylene molecules and their random-coil behavior in solutions resulting from earlier viscometry measurements. PPA was found to exhibit the molecular-weight dependence of the radius of gyration in tetrahydrofuran solution similar to that of atactic polystyrene ( saturated analogue of PPA); also unperturbed dimensions of these polymers are almost equal. This rather surprising observation is explained as a consequence of the increased pi - pi attraction between highly polarizable PPA segments with well-delocalized pi-electrons, which compensates the effect of stiffening of these segments due to conjugation. Possible influence of chain branching is also discussed.