Abstract

The hydrolytic sol–gel polymerisation of molecular precursors (RO)₃Si-(CH₂)_n-(CHCH)₂-(CH₂)_n-Si(OR)₃, 1 (R = Et, n = 1), 2 (R = Et, n = 2), and 3 (R = ⁱPr, n = 2) was investigated under various conditions. In the case of the xerogels obtained from 1, a partial cleavage of the allylic Si–C bond occurred during the hydrolytic process, whereas starting from 2 and 3 no Si–C breaking was observed. The storage conditions of the solids appeared to be of great importance on the stability of the various xerogels. Under inert conditions (sealed tubes under vacuum) all the xerogels were very stable. In contrast, under atmospheric humidity after several months the solids undergo a total chemical oxidation of the CC bonds leading to the cleavage of CC bonds with formation of acid and allenic functionalities. This oxidation is followed by a drastic decrease of the specific surface area. It has been shown that H₂O vapour and oxygen are both necessary for oxidation. Each of these reagents alone does not lead to degradation. Gels obtained from 1 exhibited extensive cleavage of the allylic Si–C bond and an oxidative degradation of the organic fragment. The xerogels derived from 2 exhibited the same degradation of the organic unit, however only a few Si–C bonds were cleaved. It has been shown that this unexpected behaviour occurred only when the dienic moiety was included and bound to the silica network, suggesting a catalytic effect of the oxide matrix since the total oxidative cleavage of CC bonds in organic chemistry needs either powerful reagents like O₃ or catalytic conditions.