POLY(UREIDOSILAZANES) - PRECERAMIC POLYMERIC PRECURSORS FOR SILICON CARBONITRIDE AND SILICON-NITRIDE - SYNTHESIS, CHARACTERIZATION, AND PYROLYTIC CONVERSION TO SI3N4/SIC CERAMICS

The mixture of cyclo-[CH3Si(H)NH](n) oligomers obtained in the ammonolysis of CH3SiHCl2 has been converted to material of higher molecular weight by the reaction with urea (between 4:1 and 12:1 CH3Si(H)NH unit/urea ratio) in pyridine at 85 degrees C. The resulting poly(ureidosilazanes) (PUSZ) were obtained as white solids, gums or oils, depending on the CH3-Si(H)NH/urea ratio used. Their pyrolysis in a flow of argon to 1000 degrees C gave black silicon carbonitride in high yield. In a flow of ammonia, their pyrolysis to 1000 degrees C resulted in grayish-white silicon nitride ceramics with a carbon content of less than 2%. DRIFTS, TGA-MS, TGA-FTIR, cross-polarization, and single-pulse magic angle spinning solid state NMR as well as solution-state NMR spectroscopy were employed in the characterization of the PUSZs and in the investigation of their pyrolytic conversion to ceramic materials. Selective isotopic labeling using C-13- and N-15-labeled urea was particularly useful in NMR studies aimed at determining the constitution of the poly(ureidosilanes).