Optical anisotropies of para-halogenated polystyrenes and related molecules

Abstract

The optical anisotropies γ² and molar Kerr constants _mK of C₆H₅X, p-CH₃—C₆H₄—X and p-(CH₃)₂CH—C₆H₄—X where X = Cl or Br, and of atactic poly(p-chlorostyrene) have been determined from depolarized Rayleigh scattering and electric birefringence measurements in CCl₄ solutions at 25°C using λ= 633 nm radiation; γ² for poly(p-bromostyrene) was determined in a mixture of CCl₄ and CBr₄. The anisotropic part of the polarizability tensors for C₆H₅Cl and C₆H₅Br are well defined by γ² and the molar Cotton–Mouton constant _mC, values of which are available from the work of LeFèvre and co-workers for these two compounds. Owing to the orientation of the dipole moment along one of the principal axes of , this tensor cannot be evaluated unambiguously from _mK and γ² for the other compounds. The anisotropy attributable to the arylene group about the axis joining the substituents appears to be little affected by substitution, however. Asymmetry of the arylene group perpendicular thereto depends appreciably on substitution; bond and group polarizabilities are not additive. The anisotropies of the polymers are formulated as the sums of tensors for the corresponding p-halogenocumenes. Averages 〈γ²〉 and 〈_mK〉 over all conformations of the polymer chains are calculated as functions of the fraction w_m of meso dyads. Values of 〈γ²〉 decrease about three-fold from syndiotactic (w_m= 0) to isotactic (w_m= 1). 〈_mK〉 is extraordinarily sensitive to stereochemical composition; it decreases from a large positive value for w_m= 0 to a large negative value for w_m= 1. These results are interpreted in terms of structure and preferred conformations. Experimental measurements on atactic polymers support the calculations.