Investigation of the pyrolytic conversion of poly(silylenemethylene) to silicon carbide

The pyrolysis of poly(silylenemethylene) (PSE), [SiH2CH2](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, [SiD2CH2](n), as the primary gaseous product in the range of ca. 250-400 degrees C, where cross-linking of the polymer occurs, suggests that loss of Hz 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 I,l-Hz 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 degrees 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 degrees C) where extensive rearrangement of the Si/C network structure is evidenced by solid-state NMR spectroscopy. Further heating of the polymer to 1000 degrees C leads to the formation of silicon carbide (SiC) in high yield (ca. 85%).