HYDROLYSIS AND CONDENSATION-REACTIONS OF TRANSITION-METAL ALKOXIDES -CALORIMETRIC STUDY AND EVALUATION OF THE EXTENT OF REACTION

The behavior of titanium and zirconium alkoxides towards complexation and water addition is analyzed through water titration and calorimetri c experiments, A simple model is presented, which allows evaluation of the mean hydrolysis and condensation constants, K-h and K-c, of both pure and complexed transition metal alkoxides in the sol state, throug h the analysis of consumed water versus initial hydrolysis ratio curve s. These constants allow comparison of the extents of the hydrolysis a nd condensation reactions for sols obtained from several alkoxide prec ursors. The complexation ratio only affects the condensation constant K-c, whereas the hydrolysis constant K-h remains unchanged. Analysis o f the K-h/K-c ratios has shown that in the sol state, silanol Si-OH gr oups are more stable than the Ti-OH or Zr-OH groups, Moreover, this st udy has shown that the proton concentration not only affects the kinet ics, but also the final composition of the system. Calorimetric studie s of the complexation and hydrolysis/condensation reactions have highl ighted the role of coordination unsaturation of the transition metal a lkoxides in the exothermicity of these reactions, clearly demonstratin g that coordination unsaturation is the driving force behind the react ivity of these alkoxides towards nucleophilic species (e.g. water, com plexing Ligands, polar solvents).