Hydrolytic polycondensation reactions have been at the heart of the development of the school of thought associated with soft chemistry methods for producing new materials. Soft chemistry was born in the 1970s under the impetus of glassmakers and ceramists who were looking for methods or processes to obtain materials, mainly oxides (silica SiO2 for glasses, titanium oxide TiO2 for pigments, barium titanate, BaTiO3 for capacitors, etc.) at lower temperatures than those used in conventional solid state chemistry synthesis modes. Hydrolytic polycondensation reactions enable metal oxides or oxo-polymers to be generated in water or a hydroalcoholic medium, over a temperature range generally extending from room temperature to 100°C. These conditions therefore make it easy to marry polymers or organic supramolecular architectures with the oxide-based components forming many mineral solids.
In the first lesson, we reviewed the history of research in this field, presented and compared the two main routes to synthesizing oxide-based materials, and finally discussed the main mechanisms for forming the solids obtained. Via hydrolysis and condensation reactions, the two main synthetic routes (aqueous or organic) can generate solids of widely varying compositions in the form of nanoparticles, gels or powders that are easily compatible with organic compounds. The aqueous route uses metal salts as precursors, with the pH and oxidation state of the metal cation driving hydroxylation reactions, themselves precursors to polycondensation reactions known as "oxolation" (creation of M-O-M bridges) or "olation" (creation of M-OH-M bridges). Depending on cation charge and pH, the anionic or cationic polycondensates generated are transformed into zero-charge precursors, leading to solid formation. The organic route is usually carried out in a water-miscible solvent such as alcohol. It uses metal alkoxides as precursors, with water driving the hydroxylation reactions that are precursors to the polycondensation reactions leading to the formation of oxometallic polymers, which are generally neutral or only slightly charged. After many years of study, the field of research associated with the formation mechanisms of compounds resulting from hydrolytic polycondensations still remains a topical area. In conclusion, we have discussed the elements that make these studies difficult. In particular, a number of experimental reasons have been put forward. Spectroscopic speciation of many elements is particularly delicate in the concentration regime associated with solid formation, the zero-charge precursor is rarely isolated, and the separative chemistry of the polymeric species formed is difficult because equilibria are often rapid and the system can be modified by measurement. It is therefore difficult to access mass distribution diagrams of the mineral oxo-polymers formed. Efforts to understand the formation mechanisms must not be slackened. Modeling can be a great help in channelling or directing experimental research in this field. This understanding is the prerequisite for controlled access to high-performance nanomaterials.
