The synthesis and properties of two polycarbosilanes that have essentially a "SiH2CH2" composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of Cl3SiCH2Cl followed by LiAIH 4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC-and C-fiberreinforced composites. These polymers thermoset on heating at 200-400 oC (or at 100 °C with a catalyst) and give near stoichiometric SiC with low 0 content in ca. 80% yield on pyrolysis to 1000 oC.The second method involves ring-opening polymerization of 1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 oC, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec., solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-toceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described. BACKGROUND

The pyrolysis of poly(silylenemethylene) (PSE), [SiH 2 CH 2 ] n , a linear polycarbosilane with a regularly alternating Si-C backbone structure and a high-yield precursor to stoichiometric SiC, was investigated by using a combination of thermogravimetric analysis, evolved gas analysis, and solid-state NMR and IR spectroscopies. The observed evolution of D 2 from the deuterio-derivative of PSE, [SiD 2 CH 2 ] n , as the primary gaseous product in the range of ca. 250-400 °C, where cross-linking of the polymer occurs, suggests that loss of H 2 from the Si is a key step in the cross-linking process. A reaction pathway is postulated for the crosslinking and pyrolysis of PSE in which both 1,1-H 2 elimination and intramolecular H-transfer reactions lead to highly reactive silylene intermediates; these insert into Si-H bonds of neighboring polymer chains forming Si-Si bonds which rapidly rearrange to Si-C bonds at these temperatures to form Si-C interchain cross-links. The cross-links prevent extensive fragmentation of the polycarbosilane network as the temperature is increased further to the range (> ca. 420 °C) where homolytic bond cleavage occurs at an appreciable rate, leading to free radicals. These free radical processes are presumably the main mechanisms at higher temperatures (>475 °C) where extensive rearrangement of the Si/C network structure is evidenced by solid-state NMR spectroscopy. Further heating of the polymer to 1000 °C leads to the formation of silicon carbide (SiC) in high yield (ca. 85%).

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