A Non-Hydrolytic Sol-Gel Route to Organic-Inorganic Hybrid Polymers: Linearly Expanded Silica and Silsesquioxanes

Condensation reactions of chlorosilanes (SiCl₄ and CH₃SiCl₃) and bis(trimethylsilyl)ethers of rigid, quasi-linear diols (CH₃)₃SiO-AR-OSi(CH₃)₃ (AR = 4,4'-biphenylene (1) and 2,6-naphthylene (2)), with release of (CH₃)₃SiCl as a volatile byproduct, afforded novel hybrid materials that feature Si-O-C bridges. The precursors 1 and 2 were characterized using FTIR and multinuclear (¹H, ¹³C, ²⁹Si) NMR spectroscopy as well as single-crystal X-ray diffraction analysis in case of 2. Pyridine-catalyzed and non-catalyzed transformations were performed in THF at room temperature and at 60 °C. In most cases, soluble oligomers were obtained. The progress of these transsilylations was monitored in solution with ²⁹Si NMR spectroscopy. Pyridine-catalyzed reactions with CH₃SiCl₃ proceeded until complete substitution of all chlorine atoms; however, no gelation or precipitation was found. In case of pyridine-catalyzed reactions of 1 and 2 with SiCl₄, a Sol-Gel transition was observed. Ageing and syneresis yielded xerogels 1A and 2A, which exhibited large linear shrinkage of 57-59% and consequently low BET surface area of 10 m²⋅g^-1. The xerogels were analyzed using powder-XRD, solid state ²⁹Si NMR and FTIR spectroscopy, SEM/EDX, elemental analysis, and thermal gravimetric analysis. The SiCl₄-derived amorphous xerogels consist of hydrolytically sensitive three-dimensional networks of SiO₄-units linked by the arylene groups. The non-hydrolytic approach to hybrid materials may be applied to other silylated precursors, if the reactivity of the corresponding chlorine compound is sufficient.