Recent Advances of Transition Metal Complexes for Photopolymerization and 3D Printing under Visible Light

The possibility of exploiting visible light to induce polymerizations is extremely appealing from a technological point of view as it improves the sustainability of the overall process. To achieve this objective, it is necessary to employ single- or multicomponent systems containing a photoinitiator, and in some cases, a photosensitizer in combination. Due to their long-lived excited states and reversible redox properties, transition metal complexes are a valid choice to be applied in photoinitiating systems, often exhibiting enhanced conversions compared to purely organic compounds. This review presents an overview of the transition metal complexes exploited in photopolymerization reactions. Particular attention will be devoted to recent applications in 3D printing, highlighting the possible challenges that need to be faced to achieve highly efficient and more sustainable processes.