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

Borazine readily dehydropolymerizes at moderate temperatures (70-110 degrees C) to give a soluble polymer, polyborazylene, in excellent yields 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 slowly adding ether solutions of the polymer to pentane. Elemental analysis indicates that compositions range from B3N3H3.4 to B3N3H3.9 for crude polymers and from B3N3H2.65 to B3N3H3.8 for precipitated samples, with average empirical. formulas of B3N3.1H3.6 and B3N3.1H3.4, respectively. Diffuse reflectance infrared Fourier transform (DRIFT) and B-11 NMR spectra indicate that the borazine ring structure is retained in the polymer. Powder X-ray diffraction suggests the presence of a layered 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 spectroscopic and molecular weight data indicate that the polymer appears to have a complex structure, having linear, branched-chain and fused-cyclic segments, related to those of the organic polyphenylenes. The isolation and X-ray structural characterizations of small amounts of the polycyclic boron-nitrogen compounds diborazine, 1:2'-(B3N3H5)(2), and borazanaphthalene, B5N5H8, supports this conclusion and provide models for the primary structural units of the polymer. Prolysis studies show that the polymer converts to boron nitride in excellent chemical, 89-99%, and ceramic yields, 84-93%. The quality of the resulting boron nitride was determined by elemental analysis, DRIFT spectra, powder XRD, density measurements, and TGA oxidation studies. Solutions of polyborazylene were also used to coat carbon and ceramic fiber yam bundles, which, when pyrolyzed under argon or ammonia, produced excellent boron nitride coatings as characterized by AES spectroscopy. Studies of the polymer/ceramic conversion process, as followed by TGA, TGA/MS, DRIFT, XRD, and microanalysis of materials produced at intermediate temperatures, suggest that the polymer converts to boron nitride by means of a two-dimensional cross-linking reaction. Alkyl-substituted polyborazylenes were produced by either the polymerization of B-alkylborazines or by the transition-metal catalyzed hydroboration of olefins by the parent polyborazylene. The alkylated polyborazylenes were found to have greatly increased solubilities in organic solvents. In addition, studies of their ceramic conversion reactions showed that polymer cross-linking was inhibited, with the initial weight losses occurring at slightly higher temperatures than in the parent polymers.