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 composi tions that yield homogeneous gels. We find in these systems the preval ence of extensive nonrandom cyclization over a wide composition range. We unambiguously monitor the formation of various well-defined silica te oligomers early in the reaction to quantify the competing processes of cyclization and bimolecular condensation. We introduce a new kinet ic model that is substantially modified from previously used random br anching models in order to account for cyclization reactions, As do pr evious models, this model allows first-shell substitution effects on c ondensation rate constants, but it does not rely on mean field site ki netics. The dimerization rate constant varies strongly with pH in a ma nner consistent with a reaction mechanism involving ionized intermedia tes. it also decreases mildly with the water concentration experienced during reaction. Higher condensation rate constants vary among each o ther and with pH in a manner consistent with the expected ionization b ehavior. The cyclization rate constants are comparable in magnitude to the dimerization rate constant. Near the expected isoelectric point o f the end-group, cyclization is favored over end-group/end-group bimol ecular condensations. At higher pH though, bimolecular reactions are f avored and there is no longer a negative first-shell-substitution effe ct. Dilution with ethanol increases the cyclization rate constants and tends to lower bimolecular rate constants, These results suggest a co rrelation between the compositions that favor homogeneous gelation and those that exhibit selective cyclization early in the reaction.