Synthesis and ceramic conversion reactions of pinacolborane- amd diethylborazine-modified poly(vinylsiloxane)s. The development of a processable single-source polymeric precursor to boron-modified silicon carbide
Ar. Brunner et al., Synthesis and ceramic conversion reactions of pinacolborane- amd diethylborazine-modified poly(vinylsiloxane)s. The development of a processable single-source polymeric precursor to boron-modified silicon carbide, CHEM MATER, 12(9), 2000, pp. 2770-2780
The design, synthesis, and properties of two new types of poly(borosiloxane
) preceramic polymers, diethylborazine- (PVS-DEB) and pinacolborane-modifie
d (PVS-PIN) poly(vinylsiloxane)s, are reported. The polymers are synthesize
d in excellent yields by the RhH(CO)(PPh3)(3)-catalyzed reactions of poly(v
inylsiloxane) (PVS) with either diethylborazine (DEB-H) or pinacolborane (P
IN-H). The spectroscopic data for the polymers, along with results of the R
hH(CO)(PPh3)(3)-catalyzed hydroboration reactions of diethylborazine and pi
nacolborane with the model compound vinyltris(trimethylsiloxy)silane, are c
onsistent 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 solub
le 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 hydrody
namic volume, molecular weight, radius of gyration, and conformation, which
indicate that the PVS-PIN and PVS-DEB polymers adopt a more branched and s
pherical structure compared to the unmodified PVS. Bulk pyrolysis of the mo
dified 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 signific
ant differences in the degree of grain growth and distribution with the bor
on-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-pr
ocessable, single-source precursors to small-diameter SiOCB fibers.