Synthesis of (PhSiO3/2)0.35(MeSiO3/2)0.40(Me2ViSiO1/2)0.25 Resins

Abstract

Silsesquioxanes are generally prepared by hydrolysis and condensation of a trichloro- or trialkoxysilane with a general formula RSiX3, where X is Cl or an alkoxy group. Initial hydrolysis often resulted in silanol functional oligomers, and these oligomers are then condensed with a catalyst to form silsesquioxanes. Typical condensation catalysts also catalyze Si−O−Si bond rearrangement, so in many cases the structure of the oligomers has little bearing on that of the resins. In the process of synthesizing a silsesquioxane with a formula of (PhSiO3/2)0.35(MeSiO3/2)0.40 (Me2ViSiO1/2)0.25 (Ph = phenyl; Me = methyl; Vi = vinyl), we found the reaction conditions during the hydrolysis of MeSi(OMe)3, PhSi(OMe)3, and (Me2ViSi)2O (Vi = vinyl) monomers were critical in achieving a high molecular weight resin. To understand this relationship between hydrolysis conditions and final product properties, reaction intermediates at various stages of hydrolysis were characterized using multinuclear nuclear magnetic resonance, size exclusion chromatography, and electrospray ionization Fourier transform mass spectrometry techniques. The analytical data indicated that the cohydrolyzate or oligomers prepared in the presence of toluene contained a large amount of silanol and methoxy groups which undergo facile condensation in the presence of a base catalyst to form a high molecular weight resin. However, when the hydrolysis was conducted by the addition of water to the alkoxysilanes without an organic solvent, the cohydrolyzate formed was of similar size but was more condensed and had fewer condensable groups. Such a reaction intermediate condensed in the presence of a base catalyst produced a relatively low molecular weight resin with a narrow molecular weight distribution. The difference in the molecular weight of the two silsesquioxane products can be explained by preference of intramolecular or intermolecular condensation for the cohydrolyzate, possibly resulting from the difference in interfacial interaction between the alkoxy monomers and water.