ELECTROCHEMICAL INVESTIGATIONS ON LIQUID-STATE POLYMERIZING SYSTEMS -CASE OF SOL-GEL POLYMERIZATION OF TRANSITION-METAL ALKOXIDES

The polymerization kinetics of zirconium propoxide (and in a few cases of titanium butoxide) were studied in various conditions by following the diffusion of an electroactive probe attached to the metal center. Two different probes were considered, implying the complexation of th e metal alkoxides with bidentate ligands functionalized by ferrocene m oeties, respectively, a substituted acetylacetone (acac) ligand, and a stronger complexing ligand as a salicylate ligand. The polymerization kinetics can be followed with good accuracy on a real time scale by i nvestigating the variation of the diffusion coefficients of the bounde d ferrocenes. This variation can be related to the mass increase of th e oligomers formed during the hydrolysis/condensation process. Electro chemical probing with the functionalized acac ligand shows that some d ecomplexation of the ligand occurs during the polymerization process. On the contrary, the salicylate ligand was irreversibly linked to the metal centers for the alkoxide precursor and also for the larger forme d oxopolymers, allowing an accurate measurement of the mass variation of the electroactive species. The electrochemical results have been co mpared with information provided by other techniques. The influence of the electron self-exchange reaction between species with different di ffusion coefficients on the measured diffusion coefficient was conside red in the global analysis.