STUDY OF POLYCONDENSATION OF TETRAETHOXYSILANE CATALYZED WITH DIBUTYLBIS[1-OXO(DODECYL)OXY]STANNANE USING H-1-NMR AND SI-29 NMR SPECTROMETRY AND QUASI-ELASTIC LIGHT-SCATTERING

The polycondensation of tetraethoxysilane (TEOS) catalyzed with dibuty lbis[1-oxo(dodecyl)oxy]stannane (dibutyltin dilaurate, DBTDL) has been investigated with the help of H-1 and Si-29 NMR spectrometry and quas i elastic light scattering (QELS). It has been found that in contrast to the acid catalyzed polycondensation the siloxane chain grows in lin ear and branched way, the proportion of cyclization reactions being sl ight. The rate of chain growth is relatively high, which is indicated by the impossibility to detect the presence of shorter oligomers in th e reaction mixture by means of Si-29 NMR. The found rate constant of p olycondensation of TEOS catalyzed with DBTDL is k = 0.052 dm(6) min(-1 ) mol(-1). From the determined condensation degree at the gelation poi nt (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 bran ched macromolecules, the reaction mixture containing no distinct propo rtions of polycyclic and highly condensed structural units. The densit y of network of the gel formed is relatively low, which was proved by Si-29 CP/MAS NMR spectra of the solid gels. Investigation of QELS of t he reaction mixture showed that the relatively large particles, microg els, were formed practically immediately after the reaction had starte d. The presence of such particles in the reaction mixture of acid cata lyzed polycondensation of TEOS was not proved.