On the Influence of Metal Alkoxides on the Epoxide Ring-Opening and Condensation Reactions of 3-Glycidoxypropyltrimethoxysilane

The extent of the epoxide ring-opening, the formation of ethyl ether groups as one reaction product of the ring-opening and the condensation degree of RSi(O_0.5)₃ units in sols and gels of the system 3-glycidoxypropyltrimethoxysilane (GPTS)-1.5H₂O-0.01/0.1/1.0 metal alkoxide Si(OEt)₄, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃, Ta(OEt)₅, Ti(OEt)₄, Zr(OBuⁿ)₄] in ethanol has been examined by means of liquid- and solid-state ₁₃C and ₂₉Si NMR spectroscopies. The results reveal a strong epoxide ring-opening effect of Al-alkoxides in hybrid sols after 24 h reaction time and of Zr-, Ta-, Al- and Sn-alkoxides in corresponding hybrid gels already at low concentration (1 mole%). The ring-opening rate increases in sols with higher metal alkoxide concentration (10 mole%) but decreases at 50 mole% concentration of Al-, Ti- and Zr-alkoxides. The ring-opening activity of metal alkoxides in 10 mole% hybrid sols increases after 24 h reaction time in the order Si(OEt)₄ < Ti(OEt)₄ < Zr(OBuⁿ)₄ < Ta(OEt)₅, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃. The 24 h hybrid sols and gels contain considerable amounts (up to 90%) of ethyl ether groups as reaction product of the ring-opening reaction which lowers the formation of polyether bonds. The condensation degree (c.d.) of RSi(O_0.5)₃ units of GPTS-1.5H₂O sols with 10 mole% of metal alkoxides increases up to 80% after 7 h reaction time in the order: Si(OEt)₄ Sn(OBu^t)₄ < Zr(OBuⁿ)₄ < Al(OBu^s)₃ < Al(OEtOBu)₃ < Ta(OEt)₅ < Ti(OEt)₄. An additional increase in c.d. up to 90% follows after the thermal sol-gel transformation. Generally, maximum activity of metal alkoxides in ring-opening and condensation reactions was found in sols and gels with 10 mole% additives. The effect of water on the epoxide ring-opening and on c.d. is discussed. Furthermore, the activity of metal alkoxides is compared with corresponding nanoscaled metal oxides.