Electrochemical investigations on the sol-gel polymerization of transition-metal alkoxides

The kinetics of polymerization of zirconium propoxide has been studied under various conditions using an original electrochemical method. The alkoxides were complexed with bidentate ligands some of which were functionalized by ferrocene electroactive moieties. The bound electroactive ferrocene component diffusion is used as a probe for determining the mass variation of the oligomers formed through the hydrolysis-condensation process. Chronoamperometry provided a means for studying diffusion kinetics. In this way polymerization kinetics can be followed with a fair precision on a real timescale, provided that the electroactive probe binds irreversibly to the polymerizing zirconium species. Two organic complexants, acetylacetone and ethyl acetoacetate, have been used in this study, in order to obtain gels from the highly reactive zirconium n-propoxide. The binding part of the electroactive probe was the strongly complexant salicylate ligand. The electrochemical results reveal the kinetics of polymerization, and give in addition an estimation of the mobile species present once the system has reached a stable state. The results have been confirmed by several other techniques including SEM, elemental analysis, light diffusion and BET specific surface determination on the resulting xerogels.