Structural characterization of poly(diethylsiloxane) in the crystalline, liquid crystalline and isotropic phases by solid-state 17O NMR spectroscopy and ab initio MO calculations

The structure of poly(diethylsiloxane) (PDES) has been characterized using solid‐state NMR of 17O. The sample studied had a weight‐average molecular weight of 2.45 × 105. The sample was prepared by utilizing the cationic ring‐opening polymerization of 17O‐enriched hexacyclotrisiloxane. Solid‐state NMR of 17O‐enriched PDES was measured on the low‐temperature β1 phase, the high‐temperature β2 phase, the two‐phase system consisting of the liquid crystal and isotropic liquid phase and the isotropic phase. From these data, the molecular structure and dynamics of PDES in the various phases were characterized via the chemical shifts of 17O, and electric field gradient parameters were determined from NMR and ab initio molecular orbital (MO) calculations. In addition to the solid‐state NMR of 1H, 13C and 29Si previously reported on these samples, knowledge of the dynamic behavior of PDES as inferred from the NMR of 17O in the present study was enhanced significantly. Further, the potential of combining the experimental NMR of 17O with ab initio MO calculations to characterize the dynamics of polymers containing oxygen is demonstrated. Copyright © 2004 John Wiley & Sons, Ltd.