TRANS CONTENT IN ATACTIC POLYSTYRENE ESTIMATED BY DOUBLE-QUANTUM SOLID-STATE NMR

The conformational statistics of atactic polystyrene have been investi gated by solid-state NMR, on a sample labeled with 25% (CH2)-C-13 grou ps. Double-quantum pulse sequences are used to select statistically fo rmed C-13-C-13 spin pairs. An experiment correlating the C-13-C-13 inf emuelear vector with the C-13 chemical-shift anisotropy determines the conformational statistics of a single backbone torsion angle. Double- quantum experiments correlating the (CH2)-C-13 chemical-shift anisotro pies of two adjacent segments provide information on two consecutive t orsion angles. The analysis of the spectra yields a trans content of 6 8% (+/-10%). This represents the first direct experimental estimate of the trans/gauche ratio in atactic polystyrene. The trans content foun d here agrees well with rotational-isomeric-state models but is incons istent with results from more elaborate atomistic models that take pac king effects into account. (C) 1998 Elsevier Science B.V. All rights r eserved.