VALENCE ELECTRONIC-STRUCTURE OF SELECTED POLYORGANOSILOXANES - X-RAY PHOTOELECTRON-SPECTROSCOPY AND QUANTUM-CHEMICAL STUDIES

The valence electronic structures of poly(dimethylsiloxane) (PDMeSO), poly(methyl-silsesquioxane) (PMeSSO), and poly(phenylsilsesquioxane) ( PPhSSO) were calculated by the ab initio Hartree-Fock LCAO crystal orb ital method and were studied experimentally by x-ray photoelectron spe ctroscopy. The basic features of the experimental spectra could be rep roduced by the calculations, and thus the peaks could be assigned to s tructural elements. The electrons of the methyl side-groups of these p olymers are basically localized. The aromatic electrons of the phenyl side-groups interact with the sigma-bonds of the silsesquioxane skelet on. This skeleton-side-group interaction is similar to what has been r eported for polyorganosilanes, though in PPhSSO these states are deloc alized to a lesser extent as their smaller k-dispersion shows. The met hyl side-groups of PDMeSO and PMeSSO contribute to the spectra only in the inner valence regions (between 16 eV and 28 eV), while the contri bution of the phenyl groups in PPhSSO is important also in the outer v alence region. These observations may facilitate the interpretation of chemical changes occurring during the transformation of these polymer s into ceramic-like materials.