Hydrolysis behavior of a precursor for bridged polysilsesquioxane 1,4-bis(triethoxysilyl)benzene: A ²⁹Si NMR study

The hydrolysis behavior of 1,4-bis(triethoxysilyl)benzene (BTB), a precursor of bridged polysilsesquioxane, was investigated with high-resolution ²⁹Si nuclear magnetic resonance (²⁹Si NMR) spectroscopy at ambient temperature in a system with BTB:ethanol:water:HCl = 1:10:x:0.8 × 10^-4 (x = 3, 6 or 9). Signals due to hydrolyzed triethoxysilyl groups as well as unhydrolyzed triethoxysilyl groups [-Si(OEt)₃, -Si(OEt)₂(OH), -Si(OEt)(OH)₂ and -Si(OH)₃ (OEt = OCH₂CH₃)] formed four sub-regions based on the number of hydroxyl groups bound to a silicon atom. In addition, one silicon environment influenced the other silicon environment by an intra-molecular interaction between two silicon atoms, and each sub-region for monomeric species thus contained four signals. Based on the development of signal intensity, it is revealed that one of the two triethoxysilyl groups in BTB is hydrolyzed preferentially. Thus, when a triethoxysilyl group is hydrolyzed, the -Si(OH) _x (OEt)_3-x (x = 1, 2) groups formed undergo further hydrolysis, which is opposite to the tendency expected from the hydrolysis behavior of organotrialkoxysilanes under acidic conditions.