Synthesis of polymeric inorganic-organic hybrid materials. A surface-initiated grafting from approach based on irradiation of the inorganic substrate

Demanding applications such as catalysis, energy transfer, drug delivery, separation science, and sensors, drives current research in the field of hybrid inorganic-organic functional materials towards novel molecular architectures, fine control of composition, molecular weight and morphology. Covalent hybrid inorganic-organic materials offer advantages in terms of physical and chemical stability, surface coverage, and favorable electronic properties. Controlled radical (NMP, ATP), chain transfer (RAFT), and catalyst-transfer (KCTP) polymerization methods have been employed in the field of hybrid materials. These methods require specific chemical modification of the inorganic substrate. An attractive alternative is to resort to physical means of activation by making use of ionizing radiations or light of appropriate wavelength. This approach can be applied, for instance, to metal oxides (TiO2, ZnO or silica) by exploiting the reactivity of engineered defects. In our group, we have undertaken a mechanistically-aware approach to grafted polymerization of polyvinyls directly from ?-irradiation-induced silica defects. In the case of styrene, we have confirmed that both a cationic, and a radical polymerization mechanism operate in parallel, although within different time frames, leading to two polymer fractions of distinct molecular weight distributions. This holds true both in the bulk, and in the grafted polymer. We have sketched a mechanistic framework that enables us to direct the polymerization reaction towards either the radical or the cationic mechanism, thus resulting in a simple rather than bimodal molecular weight distribution. Particularly, we have achieved the growth of polystyrene from ?-irradiated silica through a cationic mechanism affording good polymer loading of the inorganic support with a satisfactory molecular weight and molecular weight distribution in short reaction times and mild reaction conditions, by resorting to electron or radical scavengers, and to a carbocation stabilizer. The value of these results is in their proving that stable, covalently-bound hybrid polymeric-inorganic materials can be obtained by taking advantage of the reactivity of defects, and that the chemical arsenal that was developed in the last decades for controlled polymerizations can be applied to such systems. Our current focus is the application of this approach to different metal oxides (TiO2, ZnO) and different monomers (vinyl ethers, thiophenes, pyrroles, tetrahydrofuran).