FORMATION OF CAGE-LIKE INTERMEDIATES FROM NONRANDOM CYCLIZATION DURING ACID-CATALYZED SOL-GEL POLYMERIZATION OF TETRAETHYL ORTHOSILICATE

In the sol-gel synthesis of silica by the acid-catalyzed hydrolysis an d condensation of tetraethyl orthosilicate (TEOS), the fractal dimensi on, density, and homogeneity of the gel depend strongly on reaction co nditions such as the acid and water concentrations. However, Si-29 nuc lear magnetic resonance shows that gel conversions of at least 80% are observed over a wide range of initial acid (10(-5)-10(-1) mol/L), wat er (4-22 mol/L), and TEOS (1.3-2.5 mol/L) concentrations. This is very high compared to the value predicted by random branching theory even under the worst possible negative first-shell substitution effect. Mor eover, nonrandom cyclization is commonly evidenced in these systems by Si-29 NMR, Raman spectroscopy, and gas chromatography with mass spect roscopy. The gel conversions suggest that nonrandom cyclization reacti ons are strongly favored, and the development of conversions with time is consistent with the formation of compact three-dimensional cagelik e intermediates. These are probably the same nanometer-scale objects s hown by recent scattering and microscopy observations.