Conformation of Racemo and Meso Dyads in Glassy Polystyrenes from 13C Polarization-Transfer NMR

Abstract

NMR measurements of polarization-transfer between carbon-13 nuclei in specifically labeled amorphous atactic and isotactic polystyrenes allow the conformational characterization of meso and racemo dyads separately. For racemo dyads, the experimental data are well explained by considering only conformations near those two used in rotational-isomeric-state (RIS) models. At least half of the racemo dyads in atactic polystyrene are near the trans−trans conformation. Calculations based on different RIS models for meso dyads are compared with measurements in isotactic polystyrene. RIS models with exclusively trans−gauche and gauche−gauche states fail to account for the measurements. RIS models with a small (5−10%), but significant, amount of meso dyads near the trans−trans state agree better with the experimental results. A comparison of the experimental data with calculations from atomistic simulations of atactic polystyrene shows that in the simulations, conformations which do not correspond to those considered in RIS models are overpopulated for both types of dyads.