Thermal rearrangements during liquid–vapor phase pyrolysis polycondensation of polysilane to high-functional polycarbosilane: spectral and thermal studies

Thermally induced Kumada rearrangement of –Si–Si– linear chains in polysilanes to –Si–C– chain was conducted in the liquid–vapor phase, followed by isothermal treatments at 410, 460 °C and pressure of 15 kgf cm−2 leading to polycarbosilane(PCS) with higher silane (–Si–H) content in the range 0.46–0.58 mass%. Molecular and thermal changes during the oligomer to polymer transformation were investigated, applying spectral and thermal techniques. FTIR, Raman, and 1H, 13C, and 29Si-NMR analytical results established the chemical structural formula, –Si–Si–, –Si–C–, –Si–H bonding networks, and evolution of –Si–H functionality in as-synthesized polycarbosilane during the thermal transformations. FTIR and 29Si-NMR studies followed the increase in silane content (–Si–H). Raman data revealed the formation and disappearance of –Si–Si– functional group as the transformation progresses. Average molecular mass increased proportionally with polymerization reaction time. Thermogravimetric studies at 1400 °C confirmed a polymer to ceramic conversion (ceramic yield) of as-synthesized PCS increased with the increase in mass average molecular mass and found to be as high as 88% mass. The formation of a high purity green β-SiC powder on heat treatment at 1500 °C confirmed the high molecular polycarbosilane.