| Abstract
| - Recent investigations have implicated cagelike precursors in the unusually high gelationconversion (∼82%) of acid-catalyzed tetraethoxysilane. However, the statistical models used so far cannotcapture kinetic or composition-dependent features of alkoxysilane polycondensation. Here we take a firststep toward unified modeling of the kinetics and structure of silica gelation. Dynamic Monte Carlosimulations [Šomvársky, J.; Dušek, K. Polym. Bull.1994, 33, 369] are developed which permit competitionbetween extensive cyclization and growth. The model includes well-established kinetic trends (hydrolysispreequilibrium and first-shell substitution effects). As a first approximation, unimolecular-like terms forcyclization reactivity follow the experimental pattern of bimolecular rate coefficients. The presentsimulations allow unlimited formation of three-site rings, giving rise to many structures that are notthose of real silicates (where four-site rings dominate). However, the level of cyclization (both cycles permolecule and per site) is consistent with that of real silicates and is enough to delay gelation to 82%conversion or higher. These simulations also display a broader range of gelation behavior than priorkinetic models. At high to moderate monomer concentrations, competition between cyclization and growthcauses the expected delay of gelation. Upon further dilution, we discover a third regime, absent fromprior kinetic gelation models but important for siloxanes: formation of a distribution of polycyclicprecursors that still retain enough functionality to gel.
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