Influence of the nature of the organic precursor on the textural and chemical properties of silsesquioxane materials

Abstract

The hydrolytic sol-gel polymerization of molecular organosilicon precursors with a rigid geometry C6H4[Si(OMe)3]2-1,4 and C6H3[Si(OMe)3]3-1,3,5 2 was investigated and compared to the results obtained with precursors having a more flexible structure C6H4RR-1,4 [R=R=CH2Si(OMe)3 3; R=R=CH2CH2Si(OMe)3 4]. Compounds 1-4 have been studied in the same conditions. They were hydrolyzed under nucleophilic catalysis (TBAF: tetrabutylammonium fluoride) in MeOH and in THF. The structure of the organic group was found to be a determining parameter for both the physical and chemical properties of the resulting silsesquioxane materials. The molecular precursors 1 and 2 containing a ‘rigid’ organic group led to hydrophilic solids with similar degrees of condensation. In all cases, high specific surface area (370-1018 m2 g1) and poor chemical reactivity towards Cr(CO)6 (11-33) were observed. By contrast, the precursors containing a ‘flexible’ organic group (3 and 4) led to hydrophobic solids; the texture, the degree of condensation and the reactivity towards Cr(CO)6 of these solids strongly depended on the solvent. For instance the solids prepared in MeOH had no significant specific surface area. The solids derived from the most flexible precursor (4) exhibited the highest chemical reactivity. The short range organization of the solid is a function of the geometry of the precursor and the experimental conditions.