Reactions of Monofunctional Boranes with Hydridopolysilazane: Synthesis, Characterization, and Ceramic Conversion Reactions of New Processible Precursors to SiNCB Ceramic Materials Thomas Wideman, 1 Enriqueta Cortez, 2 Edward E. Remsen,* ,2 Gregg A. Zank,* ,3 Patrick J. Carroll, 1 and Larry G. Sneddon* ,1 Department of Chemistry and Laboratory for the Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323; Analytical Sciences Center, Monsanto Corporate Research, Monsanto Company, 800 North Lindbergh Blvd., St. Louis, Missouri 63167; and The Advanced Ceramics Program, Dow Corning Corporation, Midland, Michigan 48686-0995 Received May 23, 1997. Revised Manuscript Received August 4, 1997 X Three new series of processible polymeric precursors (PIN-HPZ, BCP-HPZ, DEB-HPZ) to SiNCB ceramic materials have been synthesized by reaction of hydridopolysilazane (HPZ) with the monofunctional boranes, pinacolborane (PIN-H), 1,3-dimethyl-1,3-diaza-2-bora- cyclopentane (BCP-H), and 2,4-diethylborazine (DEB-H). Polymers can be prepared with a controllable range of boron contents from ∼1 to 5%. Spectroscopic and chemical studies indicate the boranes are attached to the hydridopolysilazane backbone via B-N linkages that primarily result from dehydrocoupling reactions. The isolation of small amounts of trimethylsilane and Me 3 SiNH-substituted borane side products (i.e., PIN-NHSiMe 3 , BCP- NHSiMe 3 , DEB-NHSiMe 3 ) from the polymer reactions, as well as from model reactions of the boranes with hexamethyldisilazane, also suggest borane reactions at the Si-N bonds of the HPZ backbone lead to some polymer chain cleavage. Consistent with these observations, combined molecular weight/infrared spectroscopy studies show that although the polymers are modified throughout the molecular weight distribution, the modified polymers have lower molecular weights than the starting HPZ, with the highest borane concentrations in the lower molecular weight fractions. The glass transition temperatures (T g ) of the PIN-HPZ and BCP-HPZ polymers are in the 100-120 °C range, while those of the DEB-HPZ polymers decreased to as low as 25 °C with increasing modification. The polymers each showed regions of thermal stability, thus allowing the formation of PIN-HPZ, BCP-HPZ, and DEB-HPZ polymer fibers by melt spinning. Pyrolysis of these fibers to 1200 °C then yielded SiNCB ceramic fibers. Studies of the polymer to ceramic conversion reactions showed the modified polymers yield SiNCB ceramics containing ∼1-3% boron at 1400 °C, with the highest boron contents in the PIN-HPZ derived samples. At 1800 °C, the PIN-HPZ derived ceramic exhibited improved thermal stability with up to 23% nitrogen contents. In comparison, the ceramics obtained from unmodified HPZ, BCP-HPZ, and DEB-HPZ retained less than 4% nitrogen at this temperature. While the BCP-HPZ and DEB-HPZ derived ceramics showed crystallization properties similar to the ceramic obtained from unmodified HPZ, the PIN-HPZ derived ceramic was amorphous to 1600 °C and at 1800 °C showed only weak diffraction from -SiC. Introduction Because of their lightweight and excellent thermal and oxidative stabilities, silicon carbide (SiC) and silicon nitride (Si 3 N 4 ), as well as composite SiNC ceramics, are important structural materials. 4,5 Recent work, 6-21 has shown the addition of boron to these silicon-based materials can result in greatly enhanced ceramic prop- erties including reduced crystallinity and improved thermal and oxidative stabilities. These results have X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) University of Pennsylvania. (2) Monsanto Co. (3) Dow Corning Corp. (4) See, for example: (a) Narula, C. K. In Ceramic Precursor Technology and Its Applications; Marcel Dekker: New York, 1995; (b) Messier, D. R.; Croft, W. J. In Preparation and Properties of Solid State Materials; Wilcox, W. R.; Ed.; Marcel Dekker: New York, 1982; Vol. 7, Chapter 2. (c) Gmelin Handbook of Inorganic Chemistry; Springer-Verlag: Berlin, Silicon Supplement B2, 1984; B3, 1986 and references therein. (5) For recent reviews of polymer precursors to these materials, see: (a) Laine, R. M.; Babonneau, F. Chem. Mater. 1993, 5, 260-279. (b) Birot, M.; Pillot, J.-P.; Dunogue ´ s, J. Chem. Rev. (Washington, D.C.) 1995, 95, 1443-1477 and references therein. (6) Takamizawa, M.; Kobayashi, T.; Hayashida, A.; Takeda, Y. U.S. Patent No. 4,604,367, 1986. (7) (a) Funayama, O.; Kato, T.; Tashiro, Y.; Isoda, T. 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