Photocatalysts as Photoinitiators

During the past decade, photopolymerization has been the focus of intense research efforts owing to the constant growth of industrial applications and the development of new photoinitiators and monomers. The applicability of photopolymerization extends from conventional areas, such as coatings, inks, and adhesives, to high-tech domains, such as optoelectronics, laser imaging, stereolithography, 3D-printing and nanotechnology. From a mechanistic point of view, a photopolymerization reaction is initiated by a photoinitiator or a photoinitiating system, which, under exposure to a light source, generates active species, inducing the polymerization process. Over the years, various photoinitiating systems have been developed, expanding the scope of applicability. Recently, the field of photopolymerization has undergone a renaissance with the development of photocatalytic systems enabling the content of photoinitiators in the photocurable resins to be drastically reduced, allowing the polymerization speed to increase and higher conversion yields. In this chapter, we propose an overview of the principal visible light photoredox catalysts used in photopolymerization reactions. Especially, a special emphasis will be devoted to photocatalysts operating under low light intensity exposure.