Polysilsesquioxane materials containing a dienic unit: unexpected reactivity of the organic unit

The hydrolytic sol-gel polymerisation of molecular precursors (RO)(3)Si-(CH 2)(n)-(CH=CH)(2)-(CH2)(n)-Si(OR)(3), 1 (R = Et, n= 1), 2 (R = Et, n = 2), a nd 3 (R = Pr-i, n = 2) was investigated under various conditions. In the ca se 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 appeare d to be of great importance on the stability of the various xerogels. Under inert conditions (sealed tubes under vacuum) all the xerogels were very st able. In contrast, under atmospheric humidity after several months the soli ds undergo a total chemical oxidation of the C=C bonds leading to the cleav age of C=C bonds with formation of acid and allenic functionalities. This o xidation 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 oxidati ve degradation of the organic fragment. The xerogels derived from 2 exhibit ed 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 onl y when the dienic moiety was included and bound to the silica network, sugg esting a catalytic effect of the oxide matrix since the total oxidative cle avage of C=C bonds in organic chemistry needs either powerful reagents like O-3 or catalytic conditions.