Characterization of Polystyrene Conformation by Solid-State NMR Correlation of C−2H and 13C−13C Bond Orientations

Abstract

The backbone conformation of polystyrene (PS) is investigated by two-dimensional solid-state NMR. The experiment correlates well-defined nuclear spin interactions, the 13C−13C dipolar coupling along a backbone bond, and the 2H quadrupolar coupling along the C−2H 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, 13C−13C dipolar decoupling is applied during excitation and reconversion of the heteronuclear coherences, and the 13C−13C dipolar coupling is detected. The two-dimensional spectral patterns obtained for gauche-bar (ḡ) conformations are clearly distinct from the gauche (g) and trans (t) patterns. The fraction of the ḡ 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°, within ±10°. This contradicts ...