Silsesquioxanes derived from the bulk polycondensation of [3-(methacryloxy)propyl]trimethoxysilane with concentrated formic acid: Evolution of molar mass distributions and fraction of intramolecular cycles

The polycondensation of [3-(methacryloxy)propyl]trimethoxysilane was performed in bulk, using 98 wt % formic acid (molar ratio HCOOH/Si = 3 or 6), at 50 or 70 degreesC, fur periods of time of up to l month. The resulting silsesquioxanes (SSOs) were characterized by H-1 NMR, FTIR, SEC, UV-MALDI-TOF MS (linear, reflector, and post-source decay modes), and ESI-TOF MS. The residual concentrations of Si-OCH3 groups, and generated CH3OH and HCOOCH3, were quantitatively monitored by H-1 NMR during the initial stage of the synthesis. After 2 days of reaction at 50 degreesC, or about 6 h at 70 degreesC, the resulting SSO mainly contained residual Si-OH groups with only traces of Si-OCH3 groups, as was also confirmed by FTIR and UV-MALDI-TOF MS in the reflector mode, After this time, the polycondensation continued slowly during storage at room temperature, basically through Si-OH + Si-OH reactions. The average fraction of intramolecular cycles increased continuously during the polycondensation. An average value, f(av) = 0.81, was found for species containing 6-24 Si atoms in a typical reaction product. This evidenced the presence of species with relatively closed structures. In MS/MS spectra, fragmentation of one or more ester groups attached to Si atoms was observed, following a McLafferty rearrangement. The addition of water to the initial formulation (about 3:1 H2O/Si molar ratio) led to a slower polycondensation rate and to a decrease of the average fraction of intramolecular cycles (formation of more open structures). Therefore, the use of 98 wt % formic acid was useful to synthesize SSOs containing a higher fraction of intramolecular cycles than those resulting from conventional formulations.