Colloidal and Supported TiO2: Toward Nonextractable and Recyclable Photocatalysts for Radical Polymerizations in Aqueous Dispersed Media

Two high surface area titania forms, dispersed as stable nanoparticles or coated on fused silica microfiber, are used as nonleachable photocatalysts for the radical photopolymerization of methyl methacrylate miniemulsion in water. At low loading (0.17 wt%/wt(monomer)), both nanoscale TiO2 yield approximate to 50% conversion after 10 min UV irradiation, compared to 63% with a conventional type I photoinitiatior (hydroxyacetophenone). High-molecular-weight values (>180 kDa) and a polydispersity index of about 1.5 are achieved, indicating that undesirable degradation is negligible. In the proposed mechanism, a surface initiation takes place through the generation of hydroxyl radicals from H2O oxidation. It is shown that irradiance and TiO2 content are two key parameters for controlling molecular weight and conversion values. The supported TiO2 form can be easily recovered and reused up to four times, despite a gradual reduction in conversion. Deposited on the wall of the annular photochemical reactor, it enables reaction scaling-up.