Acidic sol-gel polymerization of TEOS: Effect of solution composition on cyclization and bimolecular condensation rates

For batch sol-gel polymerization of TEOS in acidic ethanol solutions, we report kinetic trends over a wide range of initial solution compositions that yield homogeneous gels. We find in these systems the prevalence of extensive nonrandom cyclization over a wide composition range. We unambiguously monitor the formation of various well-defined silicate oligomers early in the reaction to quantify the competing processes of cyclization and bimolecular condensation. We introduce a new kinetic model that is substantially modified from previously used random branching models in order to account for cyclization reactions. As do previous models, this model allows first-shell substitution effects on condensation rate constants, but it does not rely on mean field site kinetics. The dimerization rate constant varies strongly with pH in a manner consistent with a reaction mechanism involving ionized intermediates. It also decreases mildly with the water concentration experienced during reaction. Higher condensation rate constants vary among each other and with pH in a manner consistent with the expected ionization behavior. The cyclization rate constants are comparable in magnitude to the dimerization rate constant. Near the expected isoelectric point of the end-group, cyclization is favored over end-group/end-group bimolecular condensations. At higher pH though, bimolecular reactions are favored and there is no longer a negative first-shell-substitution effect. Dilution with ethanol increases the cyclization rate constants and tends to lower bimolecular rate constants. These results suggest a correlation between the compositions that favor homogeneous gelation and those that exhibit selective cyclization early in the reaction.