Study of Polycondensation of Tetraethoxysilane Catalyzed with Dibutylbis[1-oxo(dodecyl)oxy]stannane Using 1H NMR and 29Si NMR Spectrometry and Quasi Elastic Light Scattering

The polycondensation of tetraethoxysilane (TEOS) catalyzed with dibutylbis[1-oxo(dodecyl)oxy]stannane (dibutyltin dilaurate, DBTDL) has been investigated with the help of 1 H and 29 Si NMR spectrometry and quasi elastic light scattering (QELS). It has been found that in contrast to the acid catalyzed polycondensation the siloxane chain grows in linear and branched way, the proportion of cyclization reactions being slight. The rate of chain growth is relatively high, which is indicated by the impossibility to detect the presence of shorter oligomers in the reaction mixture by means of 29 Si NMR. The found rate constant of polycondensation of TEOS catalyzed with DBTDL is k = 0.052 dm 6 min -1 mol -1 . From the determined condensation degree at the gelation point (0.20-0.35) and the ratio of consumed amount of ethoxysilane units to the consumed amount of monomer at the gelation point (0.49-0.69) it follows that the gel is predominantly formed by cross-linking of branched macromolecules, the reaction mixture containing no distinct proportions of polycyclic and highly condensed structural units. The density of network of the gel formed is relatively low, which was proved by 29 Si CP/MAS NMR spectra of the solid gels. Investigation of QELS of the reaction mixture showed that the relatively large particles, microgels, were formed practically immediately after the reaction had started. The presence of such particles in the reaction mixture of acid catalyzed polycondensation of TEOS was not proved.