Recent advances on iron-based photoinitiators of polymerization

With regards to the environmental and safety concerns raised nowadays by the polymer industry, polymerization processes requiring light as the activation source are currently knowing a revival of interest after decades of disinterest. Recent progresses in photopolymerization are notably sustained by the development of new polymerization application namely 3D and 4D printing enabling an unprecedented access to structures of incredible complexity. Parallel to the manufacturing process, the polymer material itself and its composition are also facing numerous questions concerning their environmental and human health impacts. Beyond the monomers, the different additives introduced in the resin can also be potentially toxic so that a great deal of efforts is currently done to develop new photoinitiators with a toxicity assessed as being insignificant. In this field, iron-based metal complexes are candidates of choice due to the clinical demonstration of their very low toxicities. Among iron complexes, ferrocene is among the most widely studied iron-based metallocene. Due to its unique and reversible electrochemical properties as well as their absorption centred in the visible range, numerous photoinitiators have been developed with ferrocene or its oxidized form i.e. ferrocenium. Parallel to this, the remarkable electrochemical properties of ferrocenium made it an excellent candidate for its incorporation in photoinitiating systems. However, interest for iron-based photoinitiators was not limited to ferrocene and ferrocenium, and Schiff base complexes were also successfully used as initiators of polymerization. In this review, an overview of the different iron-based photoinitiating systems reported to date is presented. More precisely, if iron-based photoinitiators are developed since 50 years, a special focus will be done on photoinitiators designed for recent polymerization conditions, namely, under visible light and low light intensity.