29Si NMR study on co-hydrolysis processes in Si(OEt)4–RSi(OEt)3 –EtOH–water–HCl systems (R=Me, Ph): effect of R groups

Abstract

Hydrolysis and initial polycondensation processes in the Si(OEt) ₄ (TEOS)–RSi(OEt) ₃ –EtOH–water–HCl systems (R=Me, Ph) [TEOS:RSi(OEt) ₃ :EtOH:water:HCl=1:1:24:x:y (x=12, 2/3; y=2×10 ^-3 , 4×10 ^-3 , 8×10 ^-2 )] have been investigated by using ²⁹ Si NMR spectroscopy. For comparison, the alkoxysilanes [TEOS, MeSi(OEt) ₃ (MTES), PhSi(OEt) ₃ (PTES)] were hydrolysed separately in a similar manner. In a water-rich TEOS–MTES system (water:Si=12:1), a silanol-terminated codimer [Me(HO) ₂ SiOSi(OH) ₃ ] was detected as well as (HO) ₃ SiOSi(OH) ₃ and Me(HO) ₂ SiOSi(OH) ₂ Me, suggesting that the hydrolysed monomers were condensed rather randomly. In contrast, in a water-rich TEOS–PTES system, hydrolysed monomers derived from TEOS and those from PTES were condensed independently; a silanol-terminated codimer [Ph(HO) ₂ SiOSi(OH) ₃ ] did not form, and only (HO) ₃ SiOSi(OH) ₃ and Ph(HO) ₂ SiOSi(OH) ₂ Ph were detected. These observations in the TEOS–PTES system suggest the presence of the association of the phenyl groups. In water-restricted systems (water:Si=1:3), monomers were only partially hydrolysed, and ethoxy-terminated codimers [R(EtO) ₂ SiOSi(OEt) ₃ (R=Me, Ph)] formed in both the TEOS–MTES and the TEOS–PTES systems.