Effects of stereoregularity on molecular parameters of polyacrylonitrile polymerized by gamma-ray irradiation

Polyacrylonitrile (referred to as γ-PAN) was prepared by γ-ray irradiation on an acrylonitrile-urea canal complex at -78 deg C. The pentad tacticity was evaluated from 13C NMR spectra according to a method proposed by Kamide et al. The content of a mmmm sequence (m; meso) for γ-PAN was 0.37 ± 0.02 over a wide range of molecular weight, which was about 4.4 times larger than that for PAN, prepared by conventional redox polymerization (R-PAN). Light scattering and solution viscosity were measured in dimethyl sulfoxide (DMSO), 57 - 67 wt% aq nitric acid at 25 deg C. By extrapolation of the second virial coefficient A2 to zero 56.5 wt% aq nitric acid was expected to be a Flory theta solvent at 25 deg C. Mark-Houwink-Sakurada equations and empirical relations between the radius of gyration Z1/2 and M-barw were determined in various solvents. The unperturbed chain dimension A was determined by four methods. In DMSO γ-PAN has larger A than R-PAN. The conformation parameter σ was 2.6 - 2.8 for γ-PAN, which is some 35 % larger than the expected value when simple steric hindrance due to the side chain group is considered. Unexpectedly larger conformation parameters σ for γ- and R-PAN were interpreted by considering the rigidity of the chain due to intra-interactions between neighbouring CN groups in meso-sequence. (author)