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

• Published in: Macromolecules 2002-02, Vol.35 (4), p.1160-1174
• Main Authors: Eisenberg, Patricia, Erra-Balsells, Rosa, Ishikawa, Yoko, Lucas, Juan C, Nonami, Hiroshi, Williams, Roberto J. J
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Inorganic and organomineral polymers; Physicochemistry of polymers; Preparation
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma011305g

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 °C, for periods of time of up to 1 month. The resulting silsesquioxanes (SSOs) were characterized by 1H 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 1H NMR during the initial stage of the synthesis. After 2 days of reaction at 50 °C, or about 6 h at 70 °C, 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.