Cage-like structure formation during sol-gel polymerization of glycidyloxypropyltrimethoxysilane

The sol-gel polymerization of 3-glycidyloxypropyltrimethoxysilane (GTMS) was followed by size exclusion chromatography and Si-29 NMR. Extensive non-random cyclization under formation of polyhedral cycles - cubic cages predominates at the beginning of the reaction. Structure growth of polysilsesquioxanes proceeds by combining the incompletely condensed cage frameworks. The extent of the cage formation increases with dilution and the amount of water and depends appreciably on a catalyst. The cage fraction was isolated from a reaction mixture using preparative size exclusion chromatography and identified by Si-29 NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). High contest of polyhedral cages prevents gelation of the trifunctional GTMS monomer. Reaction of pendant epoxy groups is much slower; however, at a late reaction stage the: epoxy hydrolysis can be significant. Under some conditions, like base catalysis, polysilsesquioxane clusters an crosslinked by intermolecular condensation of SiOH with hydrolyzed epoxy groups and the system gels. The cage with epoxide functionalities may serve as a rigid precursor of crosslinking. (C) 2000 Elsevier Science B.V. All rights reserved.