Coumarin Derivatives as Photoinitiators in Photo-Oxidation and Photo-Reduction Processes and a Kinetic Model for Simulations of the Associated Polymerization Profiles

In this work, the design of an original series of coumarin derivatives (CoumC1–CoumC13) is presented. Chemical structures of these coumarins were selected by molecular modeling to ensure both excellent light absorption properties and high potential photoreactivity (high excited-state energy). As a striking result of this in silico design, eight of the 13 proposed coumarins were never presented and were specifically proposed for this work, indicating the interest in their structures. New multicomponent photoinitiating systems (PISs) based on these coumarins are proposed for the free radical polymerization (FRP) of (meth)acrylates and examined upon visible light irradiation using a light-emitting diode (LED) as a safe and economical irradiation source. The proposed systems are based on coumarins that act as remarkable photoinitiators/photosensitizers (PIs/PSs) combined with N-phenylglycine (NPG) and/or iodonium salt (Iod) as co-initiators to generate the reactive species (radicals) that will initiate the photopolymerization reaction. Investigations of the chemical mechanisms suggested by the different interactions that take place as well as the photophysical and photochemical properties of coumarins are provided. It is possible to obtain high polymerization rates (Rp), excellent photoinitiating abilities, and great reactive function conversions (FC) with these structures based on the coumarin core. Coumarins show high efficiency for the FRP of (meth)acrylates by reaction with iodonium salt (a photo-oxidation process) or with amine (a photo-reduction process). These compounds were used in three-dimensional (3D) printing resins but also for the preparation of photocomposites. In this latter case, excellent depth of cure was noted as remarkable behavior. To rationalize the experimental results, a kinetic model for a three-component system has been established to analyze the role of oxygen in the monomer conversions and the conversion enhancement observed while using an amine as a co-initiator.