Trans content in atactic polystyrene estimated by double-quantum solid-state NMR.
The conformational statistics of atactic polystyrene have been investigated by solid-state NMR, on a sample labeled with 25% 13CH2 groups. Double-quantum pulse sequences are used to select statistically formed 13C-13C spin pairs. An experiment correlating the 13C-13C internuclear vector with the 13C chemical-shift anisotropy determines the conformational statistics of a single backbone torsion angle. Double-quantum experiments correlating the 13CH2 chemical-shift anisotropies of two adjacent segments provide information on two consecutive torsion angles. The analysis of the spectra yields a trans content of 68% (+/-10%). This represents the first direct experimental estimate of the trans/gauche ratio in atactic polystyrene. The trans content found here agrees well with rotational-isomeric-state models but is inconsistent with results from more elaborate atomistic models that take packing effects into account.
Dunbar
M G
MG
Department of Polymer Science and Engineering, University of Massachusetts, Amherst 01003, USA.
Novak
B M
BM
Schmidt-Rohr
K
K
D016880:Anisotropy; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008968:Molecular Conformation; D011137:Polystyrenes; D011336:Probability; D011789:Quantum Theory; D012399:Rotation; D013237:Stereoisomerism; D014102:Torsion Abnormality
D011137:Polystyrenes