Sol-gel polymerization process

The first sol-gel polymerization was carried out by Ebelmen, who described as early as 1845 "the conversion of silicic acid exposed to moist air into solid glass" [1] . However, the beginnings of sol-gel polymerization date back to the 1930s, when the German firm Schott first used a sol-gel process to manufacture glass containers.

The principle of the sol-gel process, formerly known as "soft chemistry", is based on the use of a succession of hydrolysis-condensation reactions, at moderate temperature, to prepare oxide networks, which can in turn be heat-treated. The oxide network is formed in solution at near-ambient temperature. The process involves the conversion of metal alkoxides, such as silicon, zirconium, aluminum and titanium alkoxides, into solution. Although derivatives other than alkoxides (e.g. chlorides) can be used, these are by far the most widely employed, thanks to their moderate reactivity and modularity. Indeed, the choice of alkyl group enables the alkoxide's reactivity to be modulated according to the final properties envisaged for the material.

As we shall see in this article, alkoxide reactivity is also highly dependent on the nature of the metal (or metalloid), and it may be necessary to use additives to reduce or increase this reactivity depending on the case. In particular, we will detail the different modes of catalysis and inhibition encountered in sol-gel processes.