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 the metal alkoxides with bidentate ligands functionalized by ferrocene moeties, 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 investigating the variation of the diffusion coefficients of the bounded ferrocenes. This variation can be related to the mass increase of the oligomers formed during the hydrolysis/condensation process. Electrochemical probing with the functionalized acac ligand shows that some decomplexation 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 formed oxopolymers, allowing an accurate measurement of the mass variation of the electroactive species. The electrochemical results have been compared with information provided by other techniques. The influence of the electron self-exchange reaction between species with different diffusion coefficients on the measured diffusion coefficient was considered in the global analysis.