Influence of kinetic parameters on the textural and chemical properties of silsesquioxane materials obtained by sol-gel process

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Abstract
The hydrolytic sol-gel polymerization of a ‘rigid’ molecular precursor 1,4-C 6 H 4 [Si(OMe) 3 ] 2 1 and a more ‘flexible’ one 1,4-C 6 H 4 [CH 2 CH 2 Si(OMe) 3 ] 2 2 was investigated varying the experimental conditions. Two solvents, MeOH and THF, were employed. The influence of the catalyst has been determined by using TBAF (tetrabutylammonium fluoride) or NH 4 F as nucleophilic catalysts, NH 4 OH and NaOH as basic catalysts and HCl as acid catalyst. The effect of concentration of the precursor was also studied. Molecular precursor 1 led always to hydrophilic solids with similar degrees of condensation (63-67%). In all cases high specific surface areas and poor chemical reactivity towards Cr(CO) 6 were observed and the solvent had an influence on the porosity. By contrast, the precursor 2 led to hydrophobic solids and the texture, the degree of condensation and the reactivity with Cr(CO) 6 were strongly dependent on the solvent, the catalyst and the concentration. In MeOH no significant specific surface areas were observed, whereas in THF a high specific surface area was observed with TBAF catalysis at both precursor concentrations studied. Degrees of condensation were higher in THF. All the kinetic parameters involved in the hydrolytic sol-gel polymerization of molecular organosilicon precursors were of importance on the properties of the resulting solids, the main factor being the geometry of the organic unit.

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