The pyrolysis and photolysis of some oligosilanes.

The thermolysis of octamethyltrisilane yields trimethy1silane as the major product. Other prominent products are cyclic carbosilanes; evidence is presented that one of these is formed in a novel cyclisation/elimination reaction. The thermolysis of n-decamethyltetrasilane and i-decamethyltetrasilane also yield trimethylsilane as the major product. Other major products include cyclic carbosilanes and high molecular weight multiple ring disilacyclobutane compounds. Kinetic and trapping experiments show that all these pyrolysis mechanisms are predominantly radical in nature; there is also a notable contribution from silylenes, silenes and disilenes. The difference in product composition between the high and low sample pressure reactions is rationalised in terms of the relative importance of uni- and bimolecular reactions in the pyrolysis mechanisms. The main silicon containing product in the pyrolysis of 2,2-diethyl-hexamethyltrisilane apart from trimethylsilane was vinyltrimethylsilane. Unlike the pyrolyses of the permethylated oligosilanes cyclic carbosilanes were found to be minor products. Mechanistic suggestions, of relevance to polysilane laser photoablation, explain this difference. Computer modelling by numerical integration, using the KINAL and ACUCHEM packages, was used to validate the pyrolysis mechanisms of octamethyltrisilane and 2,2-diethy1-hexamethyl- trisilane. Three primary photochemical pathways have been identified in the irradiation of three oligosilanes at 254 and 228 nm. These pathways include a reductive elimination not previously observed in oligosilanes but analogous to a chain scission process that occurs in polysilanes.