A vibrational spectral study of the structure of poly(dimethyl)- and poly(diethyl)silanes

Abstract

The structures of linear high-molecular-weight polysilanes holds fundamental interest relative to their unusual properties [I]. Poly(dimethyl)silane (PDMS), synthesized by Miller et al. [2], and poly(diethyl)silane (PDES), synthesized in our laboratory, are solid, insoluble, rather stable polymers, whose structure has not been extensively studied. We have obtained the IR and Raman spectra of these crystalline polymers, which show that their homoatomic silicon chain is a planar, zigzag trans chain analogous to the carbon chain of crystalline polyethylene. This conclusion was drawn on the basis of the selection rules operative in the spectra. Indeed, in accord with Dzh symmetry for the monomer unit of the planar, zigzag polysilane trans chain, the vSi-Si stretching vibrations of the A1g and Bzg types appear only in the Raman spectra (373 and 480 cm -I for PDMS and 395 and 471 cm -I for PDES) and do not have analogs in the IR spectrum, while the alternative prohibition is observed for the four vSi-C stretching vibrations. The vSi-Si and vSi-C stretching bands in the spectra of PDMS and PDES are anomalously narrow, which indicates a high degree of orderedness of the macromolecules in these polymers. Heating to 200°C does not lead to fundamental changes in the spectrum of PDMS, while significant changes are observed in the spectrum of PDES, which indicate "thawing" of the rotational isomerism related to rotation of the side ethyl substituents about the Si-C bonds with retention of the planar trans conformation of the chain. The electronic absorption spectra of solid PDMS and PDES have bands at Am~ 341 and 352 nm, respectively. Heating of these polymers does not give a thermochromic effect, which was observed in the UV spectra of poly(di-n-butyl)-, poly(di-n-pentyl)-, and poly(di-n-hexyl)silanes [3]. This discrepancy is attributed to the ordered planar trans structure of PDMS and PDES in the crystal. These results are not in accord with the viewpoint presented by Burkhard, Wesson, and Williams [i] that the planar trans configuration of the polysilane chain arises only when the length of the side hydrocarbon chain n > 5.