Characterization of polystyrene conformation by solid-state NMR correlation of C-2H and 13C-13C bond orientations

The backbone conformation of polystyrene (PS) is investigated by two-dimensional solid-state NMR. The experiment correlates well-defined nuclear spin interactions, the C-13-C-13 dipolar coupling along a backbone bond, and the H-2 quadrupolar coupling along the C-H-2 bond in an adjacent repeat unit of a suitably isotopically labeled PS. The pulse sequence is based on a heteronuclear multiple-quantum coherence (HMQC) approach, in addition, C-13-C-13 dipolar decoupling is applied during excitation and reconversion of the heteronuclear coherences, and the C-13-C-13 dipolar coupling is detected. The two-dimensional spectral patterns obtained for gauche-bar ((g) over bar) conformations are dearly distinct from the gauche (g) and trans (t) patterns. The fraction of the (g) over bar conformation is found to be too low to be detected in the experimental spectrum. The patterns are highly sensitive to the torsion angle near trans. The maximum in the torsion angle distribution in PS is found to be centered at 180 degrees, within +/-10 degrees. This contradicts the predictions of an atomistic model of PS. The trans peak is relatively narrow (standard deviation < 30 degrees), indicating that the rotational-isomeric-state approximation is quite suitable.