Collect. Czech. Chem. Commun. 1996, 61, 691-703
29Si NMR Study of Distribution of Oligomers in Polycondensation of TetraethoxysilaneJiří Brusa, Jiří Karhanb and Petr Kotlíka
a Department of Chemical Technology of Monuments Conservation, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic
b Central NMR Laboratory, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic
29Si 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.