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

The hydrolytic sol-gel polymerisation of a "rigid" molecular precursor 1,4-C6H4(Si(OMe)3)2 and a more "flexible" one 1,4-C6H4(CH2CH2Si(OMe)3)2 was studied under varied conditions. Two solvents, MeOH and THF, were employed. The influence of the catalyst was determined by using TBAF (tetrabutylammonium fluoride) or NH4F as nucleophilic catalysts, NH4OH and NaOH as basic catalysts and HCl as acid catalyst. The effect of precursor concentration was also studied. The rigid precursor always led 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 affected the porosity. By contrast, the flexible precursor led to hydrophobic solids, and the texture, degree of condensation and reactivity with Cr(CO)6 strongly depended on the solvent, catalyst and the concentration. In MeOH, no significant specific surface areas were observed, whereas, in THF, a high specific surface area was seen with TBAF catalysis at both precursor concentrations studied. Degrees of condensation were higher in THF. All the kinetic parameters involved in the polymerisation affected the properties of the resulting solids, the main factor being the geometry of the organic unit. 25 refs.