Valence electronic structure of selected polyorganosiloxanes; x-ray photoelectron spectroscopy and quantum chemical studies

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Abstract
The valence electronic structures of poly(dimethylsiloxane) (PDMeSO), poly(methylsilsesquioxane) (PMeSSO), and poly(phenylsilsesquioxane) (PPhSSO) were calculated by the ab initio Hartree - Fock LCAO crystal orbital method and were studied experimentally by x-ray photoelectron spectroscopy. The basic features of the experimental spectra could be reproduced by the calculations, and thus the peaks could be assigned to structural elements. The electrons of the methyl side-groups of these polymers are basically localized. The aromatic electrons of the phenyl side-groups interact with the -bonds of the silsesquioxane skeleton. This skeleton - side-group interaction is similar to what has been reported for polyorganosilanes, though in PPhSSO these states are delocalized to a lesser extent as their smaller k-dispersion shows. The methyl side-groups of PDMeSO and PMeSSO contribute to the spectra only in the inner valence regions (between 16 eV and 28 eV), while the contribution of the phenyl groups in PPhSSO is important also in the outer valence region. These observations may facilitate the interpretation of chemical changes occurring during the transformation of these polymers into ceramic-like materials.

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