²⁹Si-NMR study of hydrolysis and initial polycondensation processes of organoalkoxysilanes. I. Dimethyldiethoxysilane

The hydrolysis and initial polycondensation processes of dimethyldiethoxysilane (DMDES) have been investigated by high-resolution ²⁹Si nuclear magnetic resonance spectroscopy (NMR) in a variety of acidic system (DMDES:ethyl alcohol:water:HCl = 1:4:x:y; x = 2 or 4, 1 × 10^-5 ≤ y ≤ 1 × 10^-2). On the basis of the behavior of signal intensities, those due to the following monomers and oligomers have been identified: Me₂Si(OEt)(OH), Me₂Si(OH)₂, Me₂(HO)SiOSi(OH)Me₂, Me₂(EtO)SiOSi(OH)Me₂, Me₂(EtO)SiOSiMe₂OSi(OH)Me₂, Me₂(HO)SiOSiMe₂OSi(OH)Me₂, (Me₂SiO)₃, Me₂(HO)SiO(SiMe₂O)₂Si(OH)Me₂, and (Me₂SiO)₄ (where Me = CH₃, Et = C₂H₅). It was shown that the substitution of the hydroxyl group for the ethoxy group resulted in an upfield shift of a signal, which was not consistent with the previous observations in tetra-functional alkoxysilane systems. In the course of the polycondensation process, cyclization reactions became predominant to form a considerable amount of the cyclic tetramer directly from the completely hydrolyzed dimer (Me₂(HO)SiOSi(OH)Me₂).