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Investigation of the Pyrolytic Conversion of Poly(silylenemethylene) to Silicon Carbide

1999; American Chemical Society; Volume: 11; Issue: 8 Linguagem: Inglês

10.1021/cm981067c

1520-5002

Quan Liu, H.-J. Wu, Russell H. Lewis, Gary E. Maciel, Leonard V. Interrante,

Fiber-reinforced polymer composites

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 D2 from the deuterio-derivative of PSE, [SiD2CH2]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 H2 from the Si is a key step in the cross-linking process. A reaction pathway is postulated for the cross-linking and pyrolysis of PSE in which both 1,1-H2 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%).