X‑ray Crystal Structures, Packing Behavior, and Thermal Stability Studies of a Homologous Series of n‑Alkyl-Substituted Polyhedral Oligomeric Silsesquioxanes

The homologous series of octa-n-alkylsilsesquioxanes (n-alkylPOSS) was prepared by hydrosilylation of octahydrido-octasilsesquioxane H8T8 with terminal n-alkenes in good yields (n = 2–18). The compounds were characterized by NMR spectroscopy (1H, 13C, 29Si), MALDI–TOF and ESI MS. Most of them have also been characterized using X-ray crystallography, including some polymorphs. The packing systems of the octa-n-alkylsilsesquioxanes are reported. Two types of packing have been observed, rod-like and disk-like (interdigitated). Some n-alkylPOSS crystals were formed in which the chains are tilted away from the axes of the cubic core. It was also observed that as alkyl-chains are flexible they can take up a variety of conformations. Melting points of the homologous series showed an odd–even alternating effect for alkyl chains longer than four CH2 units. The TGA of n-alkylPOSS under nitrogen showed that the thermal stability temperatures are between 200 and 400 °C. Pyrolysis-gas chromatography/mass spectrometry has been used to elucidate the thermal degradation process of T8(n-CnH2n+1)8 (n = 11 and 18). Thermal cracking along the whole alkyl chain was observed rather than simple loss of the complete arm.