SI-29-NMR STUDY OF HYDROLYSIS AND INITIAL POLYCONDENSATION PROCESSES OF ORGANOALKOXYSILANES .1. DIMETHYLDIETHOXYSILANE

The hydrolysis and initial polycondensation processes of dimethyldiethoxysilane (DMDES) have been investigated by high-resolution Si-29 nuclear magnetic resonance spectroscopy (NMR) in a variety of acidic systems (DMDES:ethyl alcohol:water:HCl = 1:4:x:y; x = 2 or 4, 1 x 10(-5) less-than-or-equal-to y less-than-or-equal-to 1 x 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).