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)(PPh 3 ) 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)(PPh 3 ) 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 (MeSiO 1.5 ) 0.42 (PhSiO 1.5 ) 0.37 (ViMe 2 SiO 0.5 ) 0.17 (DEB(CH 2 ) 2 Me 2 SiO 0 .5 ) 0.04 to (MeSiO 1.5 ) 0.42 (PhSiO 1.5 ) 0.37 (DEB(CH 2 ) 2 Me 2 SiO 0.5 ) 0.21 and those of the PVS−PIN polymers from (MeSiO 1.5 ) 0.42 (PhSiO 1.5 ) 0.37 (ViMe 2 SiO 0.5 ) 0.18 (PIN(CH 2 ) 2 Me 2 SiO 0. 5 ) 0.03 to (MeSiO 1.5 ) 0.42 (PhSiO 1.5 ) 0.37 (PIN(CH 2 ) 2 Me 2 SiO 0.5 ) 0.21 . The borane-modified polymers are soluble and stable in ethers, acetone, methylene chloride, benzene, and pentane. Molecular weight analyses using SEC 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 °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.