Influence of monomer concentration on structures of fully condensed polyphenylsilsesquioxanes

Fully condensed polyphenylsilsesquioxanes have been prepared via the hydrolysis and condensation of the same phenyltrimethoxysilane (PTMS) monomer in different concentrations. The effect of monomer concentration on molecular structure of the final products has been thoroughly investigated. Cage structured polyphenylsilsesquioxane (OPS) and ladder-like structured polyphenylsilsesquioxane (L-PPSQ) can be selectively synthesized from the same starting materials at lower and higher initial monomer concentrations. Meanwhile, online infrared detector, React IR TM¹⁵ was used to monitor the real time variation of the concentration and characteristic absorption peaks of any intermediate products that may exist. Then, React IR TM¹⁵ coupled with ²⁹Si nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) were used to monitor and identify the intermediate species during the reaction. Mechanism studies indicated that the condensation reaction of OPS and L-PPSQ both proceed through a T₁ structure dimer, which was formed by the quick hydrolysis of the PTMS monomer. As confirmed by ²⁹Si-NMR, GPC and MALDI-TOF-MS, the synthesis mechanisms of OPS and L-PPSQ during the hydrolysis-condensation reactions were delineated.