SUMMARY:Samples of highly syndiotactic poly(propy1ene) (s-PP) crystallized in a C-pseudo-centered orthorhombic form are here characterized through high resolution solid state 13C NMR spectroscopy and wide angle X-ray diffraction. The 13C NMR CP MAS (cross-polarization, magic angle spinning) spectra of highly disordered quench-precipitated s-PP samples (with a structure very close to a C-pseudo-centered orthrohombic form) yield additional resonances beside those already reported in the literature for s-PP samples crystallized with the chains in a fully helical +TTGG j n conformation trans; G: gauche). From the correlation of 13C NMR CP MAS spectra with the X-ray diffraction profiles recorded at the same temperatures, it is possible to establish that at a given temperature (approximately 100°C, for our samples) a phase transition from the C-to the B-pseudo-centered orthorhombic form starts to occur. Correspondingly, the aforementioned additional resonances in the 13C NMR CP MAS spectra progressively disappear with the onset of the phase transition. These extra resonances are indeed completely absent in the 13C NMR CP MAS spectrum of samles crystallized in the B-pseudo-centered structure. NMR and X-ray diffraction data, according to differential scanning calorimetry, were interpreted with a recrystallization phenomenon; at temperatures below 140 "C for our samples, the variously sized Cpseudo-centered orthorhombic crystallites melt and readily re-crystallize in the B-pseudocentered orthorhombic form.

The synthesis and properties of two polycarbosilanes that have essentially a "SiH2CH2" composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of Cl3SiCH2Cl followed by LiAIH 4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC-and C-fiberreinforced composites. These polymers thermoset on heating at 200-400 oC (or at 100 °C with a catalyst) and give near stoichiometric SiC with low 0 content in ca. 80% yield on pyrolysis to 1000 oC.The second method involves ring-opening polymerization of 1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 oC, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec., solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-toceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described. BACKGROUND

The pyrolysis of poly(silylenemethylene) (PSE), [SiH 2 CH 2 ] n , a linear polycarbosilane with a regularly alternating Si-C backbone structure and a high-yield precursor to stoichiometric SiC, was investigated by using a combination of thermogravimetric analysis, evolved gas analysis, and solid-state NMR and IR spectroscopies. The observed evolution of D 2 from the deuterio-derivative of PSE, [SiD 2 CH 2 ] n , as the primary gaseous product in the range of ca. 250-400 °C, where cross-linking of the polymer occurs, suggests that loss of H 2 from the Si is a key step in the cross-linking process. A reaction pathway is postulated for the crosslinking and pyrolysis of PSE in which both 1,1-H 2 elimination and intramolecular H-transfer reactions lead to highly reactive silylene intermediates; these insert into Si-H bonds of neighboring polymer chains forming Si-Si bonds which rapidly rearrange to Si-C bonds at these temperatures to form Si-C interchain cross-links. The cross-links prevent extensive fragmentation of the polycarbosilane network as the temperature is increased further to the range (> ca. 420 °C) where homolytic bond cleavage occurs at an appreciable rate, leading to free radicals. These free radical processes are presumably the main mechanisms at higher temperatures (>475 °C) where extensive rearrangement of the Si/C network structure is evidenced by solid-state NMR spectroscopy. Further heating of the polymer to 1000 °C leads to the formation of silicon carbide (SiC) in high yield (ca. 85%).

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