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

Hydrolysis and initial polycondensation processes in the Si(OEt)(4)(TEOS)-RSi(OEt)(3)-EtOH-water-HC; systems (R=Me, Ph) [TEOS: RSi(OEt)(3):EtOH:water:HCl=1:1:24:x:y (x=12,2/3; y=2x10(-3), 4x10(-3), 8x10(-2))] have been investigated by using Si-29 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, ina 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.