Polysilsesquioxanes Through Base-Catalyzed Redistribution of Oligohydridosiloxanes
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Polysilsesquioxanes Through Base-Catalyzed Redistribution of Oligohydridosiloxanes Kamyar Rahimian*1, Roger A. Assink1, David P. Lang1, Douglas A. Loy*2 1 Organic Materials Department, Sandia National Laboratories, Albuquerque, NM 87185-1407 2 Catalysis and Chemical Technologies Department, Sandia National Laboratories, Albuquerque, NM 87185-1407
INTRODUCTION Organopolysilsesquioxanes have recently gained much interest as materials for low-K dielectrics [1], ceramic precursors [2] and photoresists [3]. Typical sol-gel synthesis of polysilsesquioxanes involves the hydrolysis of organotricholorosilanes and/or organotrialkoxysilanes in the presence of acid or base catalysts and organic solvents. However, under sol-gel conditions most organotrialkoxysilanes do not afford silsesquioxane gels. This limits the range of organic functionalities that can be introduced into these hybrid organicinorganic materials. An alternative route to polysilsesquioxanes is through oligohydridosiloxanes. Catalytic disproportionation, by titanium complexes, of linear or cyclic oligomers of methylhydridosiloxanes can lead to polymethylsilsesquioxanes [4]. We have shown that disproportionation of oligomethylhydridosiloxanes can also be catalyzed by tetrabutylammonium hydroxide to yield polymethylsilsesquioxanes (scheme 1) [5]. This replaces the step-growth solgel polymerization process of organotrialkoxysilanes, which requires solvent, stoichiometric water and produces alcohol and water condensation by-products. Tetraalkylammonium hydroxides, as catalysts, are also attractive because they readily decompose by heating above 150 ˚C; thus, they can be easily removed from the final materials. Scheme 1
-[MeHSiO]n-
Bu4NOH
0.33n MeSiH3 + -[MeSiO1.5 ]n-
In this paper we report on both the catalytic and stoichiometric redistribution of organohydridosiloxanes to produce polysilsesquioxane foams and gels of the formula (RSiO1.5)n, which otherwise cannot be obtained easily, if at all, through traditional sol-gel means. The formation of alkylhydridosilanes (RSiH3) suggest that this reaction occurs via a different route than previously reported studies of reaction of trialkoxysilanes, (RO)3SiH, with nucleophilic catalysts which forms hydrogen gas [6, 7]. EXPERIMENTAL DETAILS Oligohydridosiloxanes, [RHSiO]n (R = Me, Et, Ph) and [MeHSiO]n[Me2SiO]n were purchased from Gelest and used as received. In a typical reaction, the oligohydridosiloxane was weighed out in a flask and catalytic amounts of tetrabutylammonium hydroxide (TBAH) (0.1N solution in MeOH/toluene, purchased from Acros and used as received) were added. The reaction, performed both under argon atmosphere and repeated in air, is exothermic and
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immediate generation of RSiH3 was observed; for R = Me, Et, the silane is gaseous and is rele
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