Chem. Mater. 1990,2, 464-472
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Synthesis and High-Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligoand -polysiloxanes Richard M. Laine,**tJeffrey A. Rahn,t Kay A. Youngdahl,? Florence Babonneau,s Martin L. Hoppe,? Zhi-Fan Zhang,t and John F. Harrodt Department of Materials Science and Engineering and the Washington Technology Center, University of Washington, Seattle, Washington 98195; The Department of Chemistry, McGill University, Montreal, Canada; and Laboratoire de la Matisre Condensse UA CNRS 302, Uniuersits Pierre et Marie Curie, Paris, France Received November 27, 1989
Homogeneous,titanium-catalyzed redistribution of the cyclomers -[MeHSiO],- ( x = 4 or 5) or the linear oligomer -[MeHSiO],- (M,,= 2000 Da) generates MeSiHBand a copolymer of approximate composition -[MeHSiO]o.3[MeSi(0)l,5]o.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 Si02(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 = 0, C, H). The chemical evolution of the copolymer during heating is followed by MAS NMR, TGA, and DRIFT spectroscopiesand chemical analysis at selected temperatures up to 1000 "C. Heating to 400 "C results in a loss of 20 w t % of the starting copolymer. Chemical analysis and NMR indicate that this weight loss is associated with the disappearence of most of the -[MeHSiO],- portion of the copolymer and that the 400 "C material consists primarily of -[MeSi(0)l,5]x-. Sharp absorption bands in the FTIR and resonances in the various NMR spectra associated with recognizable structural features (e.g., Si-CHS) 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 CH3groups react with Si0 bonds to generate SiOH bonds, SiH bonds, and new Sic bonds. These reactions illustrate the basic chemistry involved in the carbothermal reduction of Si02and the polymer degradation pathways. The resulting glassy material consists of species containing four, three, or two Si0 bonds with the remaining bonds being either Sic OF SiH. Heating the copolymers in O2gives ceramic yields of >90% of SO2. On the basis of this result, silsesquioxanes offer an alternative to sol-gel processing of silica glasses.
Introduction Silsesquioxanes, [RSi(O),,,],, have recently been explored as models for silica surfaces' and particles,2 as precursors to nitrided glasses3 (in the presence of NH3) and silicon oxynitride (Si20N2),4as precursors to silicon carbide powders6and silicon carbide reinforced (black) glass composite matrices! as cladding materials for optical fiber^,^ and as photoresists.8 The typical synthesis involves the hydrolysis of the corresponding silyl chloride, RSiCl,, in the presence of a catalyst (acid or base) in an appropriate organic ~ o l v e n t : ~
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peratures with [MeSi(0)l.5]astable at =400 "C, [PhSi(0)1.5]8 stable to 500 "C, and poly(phenylsi1sesquioxane) stable to temperatures of 600 0C.9Thus, they represent a potential alternative to organic polymers for a wide variety of high-temperature applications, providing useful synthetic routes can be developed that permit the preparation of tractable polysilsesquioxanes. Second, the synthesis of poly(3-tolylsilsesquioxanes) results in the formation of mesomorphic lamellar microstructures, which indicates that silsesquioxanes may
(1) Feher, F. J.;Newman, D. A.; Walzer, J. F. J. Am. Chem. SOC.1989, 111, 1741. (2) Agashikar, P. A.; Day, V. W.; Klemperer, W. G. J.Am. Chem. SOC. RSiC13 2-3H20 3HC1 -[RSi(0)l,5]z(1) 1987,109,5545. (3) Kamiya, K.; Makoto, 0.;Yoko, T. J. Noncryst. Solids 1986,83,208. Although represented by -[RSi(0)l.5]x-, the products of (4) (a) Laine, R. M.; Blum, Y. D.; Hamlin, R. D.; Chow, A. Ultrareaction 1 are normally a mixture of polyhedral polysilstructure Processing of Advanced Ceramics; Mackenzie, D. J., Ulrich, D. R., Eds.; Wiley-Interscience: New York, 1988, p 761. (b) Yu, Y.-F.;Mah, sesquioxanes, polyhedral oligosilsesquioxanes, [RSi(0)l.5]x T.-I. Ultrastructure Processing of Advanced Ceramics; Mackenzie, D. J., (generally
