Si-29 NMR investigation of condensation reactions of diphenylsilanediol in presence of Ti-, Zr-, Al-, Sn- and Si-alkoxides

The investigation of the effects of small amounts of Ti-, Zr-, Al-, Sn- and Si-alkoxides and those of triethylamine and hydrochloric acid on the condensation reaction of diphenylsiloxanediol (DPSD) in molar ratio Si:additive = 1:0.01 and 1:0.04 by Si-29 NMR shows a decrease in their activity in the order Ti approximate to HCl > Al > Sn approximate to N(Et)(3) approximate to Zr > Si after 6 h reaction time. The influence of the type of alkoxide ligands attached to the metal on the condensation rate of DPSD was found relatively low compared to the different metals. In presence of Ti-alkoxide the condensation reaction of DPSD leads to di-, tri- and tetrasiloxanediols, the latter of which dominates and crystallizes from the solution as octaphenyltetrasiloxanediol. The catalysis of DPSD solutions by Al-alkoxide and HCl results preferably in tetraphenyldisiloxanediol species even after a few days. The complexity of metal alkoxides and the hydrolytic stability of heterometallic Si-O-M bonds have been discussed as possible reasons for the differences in the catalytic activity of the metal alkoxide.