A structural study of substituted polysilanes in the solid state by 29Si NMR

Si-29 spin-lock cross-polarization NMR measurements have been performed to characterize the solid structure of different polysilanes in the solid state. The polysilane samples are (1) symmetrically substituted polysilane derivatives, dimethyl (PDMS) and diethyl (PDES) derivatives, and (2) the copolymer formed from two symmetrically substituted monomers, PM-co-ES with dimethyl and diethyl silane units, and (3) the unsymmetric polysilane, poly(ethylmethylsilane) (PEMS). Finite perturbation theory calculations of the Si-29 shielding constants within the CNDO/2 MO framework were carried out in order to understand and discuss the observed Si-29 NMR chemical shifts. As a result, the conformation of PDMS takes small, statistical deviations from exact trans planarity as an increase of temperature. PDES takes one ordered phase at low temperature, with an increase of temperature takes disordered and ordered phases and then the temperature more increased, a transformation to more disordered (gauche rich conformation) phase occur slowly. For PM-co-ES, the Si-29 chemical shift of the PDMS moiety is almost independent of temperature, while that of the PDES moiety gradually shifts downfield. The conformation of Si backbone for PEMS does not change almost during -70 to 120 degrees C. (C) 1998 Elsevier Science B.V.