Synthesis and Ceramic Conversion Reactions of Pinacolborane- and Diethylborazine-Modified Poly(vinylsiloxane)s. the Development of a Processable Single-Source Polymeric Precursor to Boron-Modified Silicon Carbide

The design, synthesis, and properties of two new types of poly(borosiloxane) preceramic polymers, diethylborazine- (PVS-DEB) and pinacolborane-modified (PVS-PIN) poly(vinylsiloxane)s, are reported. The polymers are synthesized in excellent yields by the RhH(CO)(PPh3)(3)-catalyzed reactions of poly(vinylsiloxane) (PVS) with either diethylborazine (DEB-H) or pinacolborane (PIN-H). The spectroscopic data for the polymers, along with results of the RhH(CO)(PPh3)(3)-catalyzed hydroboration reactions of diethylborazine and pinacolborane with the model compound vinyltris(trimethylsiloxy)silane, are consistent with predominately anti-Markovnikov hydroboration of the polymer vinyl groups by the boranes. The degree of polymer hydroboration is simply controlled by varying the reactant ratios with the compositions of the PVS-DEB polymer ranging from (MeSiO1.5)(0.42)(PhSiO1.5)(0.37)(ViMe(2)-SiO0.5)(0.17)(DEB(CH2)(2)Me2SiO0.5)(2) to (MeSiO1.5)(0.42)(PhSiO1.5)(0.37)(DEB(CH2)(2)Me2SiO0.5)(0.21) and those of the PVS-PIN polymers from (MeSiO1.5)(0.42)(PhSiO1.5)(0.37)(ViMe(2)SiO(0.5)) Me2SiO0.5)0.03 to (MeSiO1.5)(0.42)(PhSiO1.5)(0.37)(PIN(CH2)(2)Me2SiO0.5)(0.21) The borane-modified polymers are soluble and stable in ethers, acetone, methylene chloride, benzene, and pentane. Molecular weight analyses using SEG with refractive index, viscometry, and light-scattering triple detection show significant; differences in hydrodynamic volume, molecular weight, radius of gyration, and conformation, which indicate that the PVS-PIN and PVS-DEB polymers adopt a more branched and spherical structure compared to the unmodified PVS. Bulk pyrolysis of the modified resins produced SiOCB ceramic chars with boron contents ranging from 0.3 to 0.7% depending upon the percentage of polymer modification. XRD and TEM studies of the 1800 degrees C chars of PVS and PVS-PIN showed significant differences in the degree of grain growth and distribution with the boron-modified char showing both a smaller average crystallite size and a more narrow size range. The PVS-PIN polymers were found to be excellent melt-processable, single-source precursors to small-diameter SiOCB fibers.