The role of silicon intermediates in the pyrolysis of oligosilanes and the dehalogenation of chlorofluorocarbons.

The pyrolysis mechanisms of the oligosilanes, octamethyltrisilane, n-decamethyltetrasilane, i-decamethyltetrasilane and 2,2-diethylhexamethyltrisilane were studied using the batch stirred flow technique. Product analysis by GC/MS-SFR and kinetic studies on the major products revealed that the thermolysis mechanisms were primarily radical in nature with product formation being dependent on the balance between bimolecular and unimolecular reactions. Detailed mechanisms are proposed for major product formation and their feasibilty is assessed by computer simulation by numerical integration. Numerical integration is also used to rationalise previous discrepancies between computer modelling and experiment. Recent developments in the thermochemistry of organosilicon compounds have been successfully applied to the isomerisation mechanisms of tetramethyldisilene and methyltrimethylsilylsilylene. The use of silicon intermediates to dehalogenate CFCs is studied in detail. Thermal sources of silylenes, silyl radicals and silenes are utilised to study their affect on Freon-11, Freon-12 and Freon-13. The Mercury photosensitisation of trimethylsilane is used to generate low temperature silyl radicals which react with the Freons in a radical chain reaction. A study into the possible applications of this chemistry to the Direct Synthesis, the industrial process for methylchlorosilane production, is discussed. The gas phase pyrolyses of the methylchlorosilanes Me2SiCl2 and MeSiCl3 are investigated. GC/MS product identification was supplemented by detailed kinetic experiments and simulation of the proposed thermolysis mechanisms by numerical integration. These suggested that the thermal decomposition was initiated by a radical mechanism though silylene insertions played a role in product composition. The kinetics for the major products formed in the pyrolysis of Tetraethoxysilane are studied. These are discussed along with previously proposed mechanisms for pyrolysis. The formation of the major product ethene is believed to be formed by a number of routes including molecular and b-elimination. The Arrhenius parameters for ethanol and acetaldehyde formation suggest a heterogeneous reaction mechanism at the lower end of the temperature range studied.