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)-1.5H(2)O-0.01/0.1/1.0 metal alkoxide [Si(OEt)(4), Sn(OBut)(4), Al(OBus)(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 C-13 and Si-29 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(OBut)(4), Al(OBus)(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- 1.5H(2)O sols with 10 mole% of metal alkoxides increases up to 80% after 7 h reaction time in the order: Si(OEt)(4) < Sn(OBut)(4) < Zr(OBun)(4) < Al(OBus)(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.