D. Hoebbel et al., On the influence of metal alkoxides on the epoxide ring-opening and condensation reactions of 3-glycidoxypropyltrimethoxysilane, J SOL-GEL S, 21(3), 2001, pp. 177-187
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-glycidoxypropyltrimet
hoxysilane (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 e
thanol 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 effec
t 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 concentrat
ion (1 mole%). The ring-opening rate increases in sols with higher metal al
koxide 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 react
ion 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 t
hermal sol-gel transformation. Generally, maximum activity of metal alkoxid
es in ring-opening and condensation reactions was found in sols and gels wi
th 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 compa
red with corresponding nanoscaled metal oxides.