The Catalytic Synthesis of Inorganic Polymers for High Temperature Applications and as Ceramic Precursors
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THE CATALYTIC SYNTHESIS OF INORGANIC POLYMERS FOR HIGH TEMPERATURE APPLICATIONS AND AS CERAMIC PRECURSORS Jeffrey A. Rahn, Richard M. Laine* and Zhi-Fan Zhang Contribution from the Department of Materials Science and Engineering and the Polymeric Materials Program of the Washington Technology Center, University of Washington, Seattle, WA 98195 ABSRACT: Polysilsesquioxanes,-[RSi(O) 1 .5 ]x-, exhibit many properties that are potentially quite useful for industrial applications. These properties include high temperature stability (- 6000 C in 02); good adhesion and, liquid crystal-like behavior for some derivatives. Moreover, [MeSi(O)l. 5 ]x, polymethylsilsesquioxane has been used successfully as a precursor for the fabrication of carbon fiber/"black glass" (SiO 2/SiC/C) composites and "black glass" fibers. Current methods of preparation depend on hydrolysis of RSiCI3 or RSi(OR) 3. Unfortunately, this approach leads to several products that are difficult to purify because polysilsesquioxanes exhibit a great propensity for forming gels. We describe here a simple catalytic approach to the synthesis of polymethylsilsesquioxane copolymers of the type -[MeRSiO].3[MeSi(O). 5].7- where R - H, OMe, OEt, OnPr and OnBu. The R - H copolymer is produced by catalytic redistribution of -[MeHSiO]xoligomers using dimethyltitanocene, Cp2"iMe 2 as the catalyst precursor. Following catalytic redistribution, the resulting copolymer, -[MeHSiO]. 3[MeSi(O) 1.5].7-, is reacted in situ with alcohols to produce -[Me(R'O)SiO].3[MeSi(0)i.5].7- (where R' - Me, Et, nPr and nBu) which serve as masked forms of the polymethylsilsesquioxane. These new copolymers have been characterized by 1H, 13 C and 29Si NMR TGA and DTA. The NMR studies allow us to assign structures for the copolymer. These new copolymers exhibit improved tractability. Their high temperature properties are all quite similar; although, the MeO-, EtO- and especially the nPrO- derivatives give much higher ceramic yields than expected.
Mat. Res. Soc. Symp. Proc. Vol. 171. '11990 Materials Research Society
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INTRODUCTION Polysilsesquioxane polymers,1 -[RSi(O) 1 .5]x-, represent a very poorly exploited area of polysiloxane chemistry despite the fact that they exhibit a variety of potentially useful properties including: high temperature stability in air; 2 good adhesion to a wide variety of substrates 3 and, in some instances, liquid crystal-like behavior.1 Moreover, -[MeSi(O)1.5]x-, polymethylsilsesquioxane has been used as a preceramic polymer for fabrication of silicon carbide powders, 4 "black glass" (70% SiO 2/20% SiC/1 0% C) composite matrices for carbon fibers 5 and for the fabrication 6 of black glass fibers. The primary problems associated with using polysilsesquioxanes for engineering applications are: (1) the lack of good, high yield synthetic routes and, (2) the highly crosslinked nature of the polymers which limits their tractability and ease of purification. Literature syntheses generally rely on the hydrolysis of RSiC13 or RSi(OR') 3:1 RSiC13 [RSi(OR') 3] + H20
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