Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Patent
1986-12-01
1991-04-16
Shaver, Paul F.
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
586402, 586410, C07F 710
Patent
active
050084224
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The invention relates to the synthesis of compounds (by which it is intended to include monomers, oligomers and polymers) containing the structure Si--N in the molecule. The invention concerns primarily lazanes and their derivatives, which may be pyrolyzed to yield a variety of ceramic products, and also relates to siloxazanes (by which it is meant monomers, oligomers and polymers containing the O--Si--N unit) and other compounds containing one or more Si--N bonds. The invention additionally relates to the synthesis of novel, high molecular weight polysilazanes and their precursors, and the use of these unique compounds for the fabrication of ceramic coatings, fibers, binders, and injection-molded articles. The invention also relates to the use of polysiloxazanes and polyhydridosiloxanes as ceramic precursors.
2. Background
Polysilazanes and their derivatives are useful among other things, for the preparation of silicon nitride (Si.sub.3 N.sub.4, silicon carbide (SiC), Si.sub.3 N.sub.4 /SiC alloys, Si.sub.3 N.sub.4 /carbon alloys, Si.sub.3 N.sub.4 /boron nitride alloys, and mixtures thereof. These ceramic materials can be used as structural materials, protective coatings, and electronic materials because of their hardness, strength, structural stability under extreme environmental conditions and their wide variety of electronic properties. In particular, these materials can be formed into ceramic fibers of value for reinforcement of composite materials. See, for example, (a) Department of Defense Proceedings, Fourth Metal Matrix Composites Technical Conference, May 19-21, 1981, prepared for DOD Metal Matrix Composites Information Analysis Center; and (b) J. J. Brennan, "Program to Study SiC Fiber-Reinforced Matrix Composites", Annual Report to Dept. of Navy (Nov. 1980), Contract No. N00014-78-C-0503.
Historically, polysilazanes were first synthesized by Stock et al almost 60 years ago (see, e.g., Stock, A. and K. Somieski, Ber. Dtsch. Chem. Ges. 54:740 (1921)) via a simple ammonolysis technique (Scheme I). However, this ##STR1## approach usually produces mixtures of cyclomers where x is 3 to 5 that are obtained as the major products and small amounts of linear oligomers where y is less than or equal to about 10. Because of their low molecular weight, however, these linear oligosilazanes are too volatile to be used as preceramic materials.
In order to obtain higher molecular weight, nonvolatile materials, it was necessary to promote cross-linking reactions. In this manner, moderate molecular weight polysilazanes have been synthesized using a variety of techniques. See, e.g., Kruger, C. R. and E. G. Rochow, J. Polymer Sci. 2A:3179-3189 (1964). Rochow et al. discovered that ammonium chloride catalyzes cross-linking in simple oligodimethylsilazanes to form polysilazanes (Scheme II) which ##STR2## were proposed to contain cyclic monomer units cross-linked through nitrogen as suggested by the structure of Formula 1. ##STR3##
The Penn et al. work follows up on U.S. Pat. Nos. 3,853,567 to Verbeek and 3,892,583 to Winter et al., wherein a high temperature elimination/condensation reaction was shown to lead to soluble, highly cross-linked polymers as shown in Scheme III. Pyrolysis at high temperatures provides ceramic ##STR4## yields of 60% with a mixture of Si.sub.3 N.sub.4 and SiC ceramic materials.
A related cross-linking approach described, inter alia, in U.S. Pat. Nos. 4,312,970, 4,340,619, 4,535,007 and 4,543,344 begins with the preparation of tractable polysilazanes having Me.sub.3 Si groups linked to the polymer backbone (Scheme IV) with the highest molecular weights reported in the available literature, i.e. about Mw.about.15,000 D and Mz.about.39,000 D: ##STR5## Ceramic yields obtained from pyrolysis fibers formed from this polymer are on the order of 45-55% with compositions of 96% Si.sub.3 N.sub.4, 2% carbon and 2% oxygen after curing.
U.S. Pat. No. 4,482,669 to Seyferth et al. discloses that it is possible to cross-link low molecular weight cyclic oligomers containing Si-
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Blum Yigal D.
Laine Richard M.
Shaver Paul F.
SRI - International
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