Si-29 NMR study of distribution of oligomers in polycondensation of tetraethoxysilane

Si-29 NMR spectrometry has been used to determine the number molecular weight distribution of polymerization degrees of a system of oligomers formed in the sol-gel polycondensation of tetraethoxysilane (TEOS). On the basis of identification of the oligomers, monitoring of the concentration changes during the initial phase of the sol-gel polycondensation, and from the deviations observed between the experimentally determined number molecular weight distribution of polymerization degrees and the Flory-Stockmayer model it has been proved that cyclization represents a significant process in the acid-catalyzed polycondensation with substoichiometric amount of water (ratio of amounts TEOS/H2O = 1 : 1). The cyclization becomes important at the moment when the reaction mixture contains increased amounts of the linear tetramer and pentamer. The cyclization products form a basis for formation of polycyclic, highly condensed building units whose gradual aggregation produces the gel.