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Catalytic Synthesis of Polymethylsilsesquioxanes Richard M. Laine , Jeffrey A. Rahn , Kay Youngdahl Blohowiak , and John F. Harrod 1,3
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Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195 Department of Chemistry, McGill University, Montreal H3A 2A7, Canada 1
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Dimethyltitanocene, Cp TiMe , will promote the redistribution of -[MeHSiO] - oligomers to produce a polymethylsilsesquioxane copolymer of the type -[MeHSiO] [MeSi(O) ] -. The mecha nism for this redistribution is suggested to involve σ-bond meta thesis promoted by a Ti(IV) species. The structural evolution of the copolymer on heating to 1000 °C is followed by using solid-state Si NMR spectroscopy. Thermogravimetric analysis (TGA) experiments and chemical analysis are used to support the NMR results. The Cp TiMe -derived catalyst also promotes alcoholysis of the Si-H bonds in the copolymer. This reaction produces alkoxy derivates, -[Me(RO)SiO] [MeSi(O) ] -(R is Me, Et, n-Pr, or n-Bu). Fur thermore, this catalyst will also polymerize the oligosilazanes -[MeHSiNH] - or -[H SiNMe] -, but only if some -[MeHSiO] -is present. The ceramic yields of the copolymers -[Me HSiO] -:-[MeHSiNH] -(with 1:1, 1:3, 1:9, and 1:18 ratios) are much higher than yields found for the pure polysilazane. 2
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SILSESQUIOXANES R E P R E S E N T A U N I Q U E and poorly studied subset of polyalkylsiloxanes. This statement is true despite the fact that they offer many exceptional properties.
For example, silsesquioxanes,
R S i ( 0 ) , be 15
cause of their need to form three S i - O - S i bonds, assume regular polyhedral shapes such as that shown in structure 1 for octamethyloctasilsesquioxane
α). Corresponding author: Department of Materials Science and Engineering,
University of Michigan, Ann Arbor, MI 48109-2136
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Current address: Department of Chemistry, Eastern Washington University, Cheney, WA 99004
0065-2393/92/0230-()553$06.00/0 © 1992 American Chemical Society
In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch038
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H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS
These polyhedral shapes have geometries very similar to those found for silica and its derivatives. Feher and co-workers (2, 3) used this similarity as the basis for developing molecular models of silica surfaces. The regular geometry also contributes to such properties as high-temperature stability (I) and high hardness (4). Octamethyloctasilsesquioxane is stable in air to 415 °C, at which temperature it sublimes. Polysilsesquioxanes appear to have cage rather rather than ladder structures (5). The polymer is formed by the opening of a polyhedral edge, as suggested for polymethylsilsesquioxane (see structure 2).
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Polymethylsilsesquioxane, -[MeSi(0)i ] -, is stable to temperatures above 500 °C in air and to at least 600 °C in nitrogen. The phenyl derivative is reported to be stable to 800 °C (J). The cage structure is useful for making microporous materials (6). 5
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Polymethylsilsesquioxanes have been used as protective polymer coatings in the electronics industry (7) and as precursors to silica and S i O ^ C * glasses (Scheme I) (8-10). From an engineering standpoint, these materials offer exceptional properties that should lead to widespread applications. Unfortunately, their propensity for forming gels makes it extremely difficult to prepare useful, processable quantities of any given material. Until recently, the only method of preparing silsesquioxanes was via hydrolysis of the alkyltrichloro- or trialkoxysilane (Scheme II) (I, 2, 6). Separation of the polyhedral- or polyalkylsilsesquioxane from the reaction mixture is extremely difficult. Isolable yields of these compounds are quite poor, typically ranging from 15 to 30%. Consequently, the discovery (II) that titanium will catalyze the redistribution of alkoxysilanes under extremely mild conditions (reaction 1) suggested that it might be possible to synthesize polysilsesquioxanes via a similar route that would eliminate the need for a hydrolytic synthesis (reaction 2).
3MeHSi(OEt)
