Influence of synthesis conditions on the cross-link architecture of silsesquioxanes prepared by in situ water production route

The esterification reaction between carboxylic acids and alcohols has been used as the source of water for the hydrolysis-condensation reactions of difunctional and trifunctional organosilanes. Diphenylsilanediol (DPDO) has been reacted with methacryloxypropyltrimethoxysilane (MPTMS) and glycidoxypropyltrimethoxysilane (GPTMS) and the obtained products have been characterized by vibrational spectroscopy (FT-IR, FT-Raman) nuclear magnetic resonance (NMR) and gel permeation chromatography. The relation between water availability from the in situ water production process (ISWP) and silsesquioxanes morphology has been evaluated in the case of DPDO/MPTMS mixtures, changing molecular features of acids and alcohols. These measurements have shown that the pK of the carboxylic acid used in the esterification reaction has a valuable influence on the silanes cross-linking ability. Acids with low pK values and heteroatoms substituents favor the silane hydrolysis and allow the growth of high molecular weight species. Using acetic acid/ethanol mixture leads to the best results for DPDO/MPTMS reaction, with a narrow distribution of silsesquioxane species. Under the same conditions, the reaction of DPDO with GPTMS produces polymeric species and only avoiding the presence of the difunctional precursor allows to limiting the silsesquioxanes species growth.