A NMR study on the hydrolysis, condensation and epoxide ring-opening reaction in sols and gels of the system glycidoxypropyltrimethoxysilane-water-titaniumtetraethoxide

Abstract

The examination of hydrolysis, homo- and hetero-condensation reactions, of the condensation degree and the extent of epoxide ring opening in the course of sol-gel process was carried out by means of liquid- and solid-state 29Si and 13C NMR in the system 3-glycidoxypropyltrimethoxysilane (GPTS)-titaniumtetraethoxide-water (molar ratio 1:1:1.5-14) which is frequently used for the synthesis of heterometal hybrid polymers. The monomeric silanol groups in the GPTS-prehydrolysate immediately co-condense with the Ti-tetraethoxide to Si-O-Ti bonds to an extent of about 50-60% which remain stable in sols and also in the corresponding gels at low amounts of free water (0.02 H2O/OR) in the sol. An increasing amount of free water in the sol (≥ 0.12 H2O/OR) leads to an increased hydrolytic cleavage of the heterometal bonds and to the formation of homo-condensed polysiloxanes. The condensation degree of RSiO1.5 units in the Ti-containing sols is with 30-60% relatively high in comparison to Ti-free GPTS sols (ca. 5%) whereas the condensation degree of GPTS derived gels (81%) was found to be similar to that of the Ti-containing gels (60-80%). Ti-tetraethoxide accelerates the ring opening reaction of the epoxide group in the sols in dependence on the water content. Up to 78% of the epoxide rings are opened after 24 h in the sol with the highest water content (2 H 2O/OR). No epoxide rings can be detected in Ti-containing gels which derive from sols with an amount of free water of ≥ 0.12 H2O/OR. The results give a first insight into the different parallel reactions in this system and can contribute to more structure controlled syntheses of heterometal hybrid polymers.