X-ray crystal structures, packing behavior and thermal stability studies of a homologous series of n-alkyl-substituted polyhedral oligomeric silsesquioxanes

El Aziz, Youssef; Bassindale, Alan R.; Taylor, Peter G.; Stephenson, Richard A.; Hursthouse, Michael B.; Harrington, Ross W. and Clegg, William (2013). X-ray crystal structures, packing behavior and thermal stability studies of a homologous series of n-alkyl-substituted polyhedral oligomeric silsesquioxanes. Macromolecules, 46(3) pp. 988–1001.

DOI: https://doi.org/10.1021/ma302229v

Abstract

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-C_nH_2n+1)₈ (n = 11 and 18). Thermal cracking along the whole alkyl chain was observed rather than simple loss of the complete arm.