Recent Advances in Type I Photoinitiators for Visible Light Induced Photopolymerization

Photopolymerization and its application in the dental, three-dimensional (3D) printing, coating and electronic industry has become increasingly popular over the last decades. A huge variety of photoinitiators (PIs) and photoinitiating systems (PISs) have been developed that are able to generate reactive species, e.g. radicals, radical cations, and cations upon light absorption. In this Review, we focus on radical-generating Type I PIs which undergo homolytic cleavage after irradiation with visible light. The possibility to utilize electromagnetic irradiation above 400 nm for the initiation of polymerization reactions provides several advantages such as a lower energy demand and higher curing depths in pigmented reactive systems. Recent developments of PIs based on phosphorus and group 14 elements as well as other selected concepts for Type I visible light initiators are outlined and discussed within this review.