SYNTHESIS, PROPERTIES, AND CERAMIC CONVERSION REACTIONS OF POLYBORAZYLENE - A HIGH-YIELD POLYMERIC PRECURSOR TO BORON-NITRIDE

Borazine readily dehydropolymerizes at moderate temperatures (70-110 d egrees C) to give a soluble polymer, polyborazylene, in excellent yiel ds of 81-91%. The polymer is isolated as a white solid that is soluble in ethers such as glyme and THF. The polymer can be precipitated by s lowly adding ether solutions of the polymer to pentane. Elemental anal ysis indicates that compositions range from B3N3H3.4 to B3N3H3.9 for c rude polymers and from B3N3H2.65 to B3N3H3.8 for precipitated samples, with average empirical. formulas of B3N3.1H3.6 and B3N3.1H3.4, respec tively. Diffuse reflectance infrared Fourier transform (DRIFT) and B-1 1 NMR spectra indicate that the borazine ring structure is retained in the polymer. Powder X-ray diffraction suggests the presence of a laye red structure for the polymer in the solid state, which is consistent with its observed density of 1.5-1.6 g/cm(3). Molecular weight studies using size exclusion chromatography (SEC)/viscometric detection (VISC ) give accurate determinations of a lower value of M(n), 500-900 g/mol . SEC/low-angle laser Light scattering (LALLS/ultraviolet absorbance ( UV) give determinations of M(w) biased toward high molecular weights, with values usually between 3000 and 8000 g/mol. The combined spectros copic and molecular weight data indicate that the polymer appears to h ave a complex structure, having linear, branched-chain and fused-cycli c segments, related to those of the organic polyphenylenes. The isolat ion and X-ray structural characterizations of small amounts of the pol ycyclic boron-nitrogen compounds diborazine, 1:2'-(B3N3H5)(2), and bor azanaphthalene, B5N5H8, supports this conclusion and provide models fo r the primary structural units of the polymer. Prolysis studies show t hat the polymer converts to boron nitride in excellent chemical, 89-99 %, and ceramic yields, 84-93%. The quality of the resulting boron nitr ide was determined by elemental analysis, DRIFT spectra, powder XRD, d ensity measurements, and TGA oxidation studies. Solutions of polyboraz ylene were also used to coat carbon and ceramic fiber yam bundles, whi ch, when pyrolyzed under argon or ammonia, produced excellent boron ni tride coatings as characterized by AES spectroscopy. Studies of the po lymer/ceramic conversion process, as followed by TGA, TGA/MS, DRIFT, X RD, and microanalysis of materials produced at intermediate temperatur es, suggest that the polymer converts to boron nitride by means of a t wo-dimensional cross-linking reaction. Alkyl-substituted polyborazylen es were produced by either the polymerization of B-alkylborazines or b y the transition-metal catalyzed hydroboration of olefins by the paren t polyborazylene. The alkylated polyborazylenes were found to have gre atly increased solubilities in organic solvents. In addition, studies of their ceramic conversion reactions showed that polymer cross-linkin g was inhibited, with the initial weight losses occurring at slightly higher temperatures than in the parent polymers.