SYNTHESIS AND HIGH-TEMPERATURE CHEMISTRY OF METHYLSILSESQUIOXANE POLYMERS PRODUCED BY TITANIUM-CATALYZED REDISTRIBUTION OF METHYLHYDRIDOOLIGOSILOXANES AND METHYLPOLYSILOXANES

Homogeneous, titanium-catalyzed redistribution of the cyclomers-[MeHSiO]x– (x = 4 or 5) or the linear oligomer –[MeHSiO]x– (Mn 2000 Da) generates MeSiH3 and a copolymer of approximate composition –[MeHSiO]0.3[MeSi(O)1.5]0.7–. The high-temperature behavior of this copolymer follows closely that of similar polymers prepared by sol-gel processing. Heating to 900 °C at 5 °C/min in N2 gives a black glass (75% ceramic yield) with an apparent composition of SiO2 (70%), SiC (20%), and C (10%). This composition belies the true nature of this amorphous material, which is best described by the various Si-E bonding arrangements (E = O, C, H). The chemical evolution of the copolymer during heating is followed by MAS NMR, TGA, and DRIFT spectroscopies and chemical analysis at selected temperatures up to 1000 °C. Heating to 400 °C results in a loss of 20 wt % of the starting copolymer. Chemical analysis and NMR indicate that this weight loss is associated with the disappearence of most of the –[MeHSiO]x– portion of the copolymer and that the 400 °C material consists primarily of -[MeSi(O)15]x– Sharp absorption bands in the FTIR and resonances in the various NMR spectra associated with recognizable structural features (e.g., Si-CH3) in the low-temperature polymeric material give way to broad, poorly defined absorptions and resonances at temperatures above 600 °C. The changes are indicative of the transformation from a polymer to a glass. Above 600 °C, the silsesquioxane CH3 groups react with SiO bonds to generate SiOH bonds, SiH bonds, and new SiC bonds. These reactions illustrate the basic chemistry involved in the carbothermal reduction of SiO2 and the polymer degradation pathways. The resulting glassy material consists of species containing four, three, or two SiO bonds with the remaining bonds being either SiC or SiH. Heating the copolymers in O2 gives ceramic yields of >90% of SiO2. On the basis of this result, silsesquioxanes offer an alternative to sol-gel processing of silica glasses. © 1990, American Chemical Society. All rights reserved.