Substituent effects on the photoinitiation ability of coumarin-based oxime-ester photoinitiators for free radical photopolymerization

Abstract

In this study, a series of coumarin chromophore-based oxime-esters were designed and synthesized as visible light photoinitiators (PIs). Interestingly, upon exposure to irradiation using an LED at 405 nm, the investigated oxime-esters (OXEs) could undergo a direct photocleavage, followed by decarboxylation, generating active free radicals capable of effectively initiating acrylate polymerization. Seven of the 10 investigated OXEs have never been synthesized earlier according to the literature. Markedly, the newly proposed structures also exhibit thermal initiation ability and can be used as dual photo and thermal initiators. In addition, the oxime-ester system was also paired with an iodonium salt and exhibited a higher efficiency in free radical polymerization (FRP). The chemical mechanisms and the structure/reactivity/efficiency relationships were investigated using different techniques including real-time Fourier transform infrared (RT-FTIR) spectroscopy, UV-visible absorption spectroscopy, fluorescence (time-resolved or steady state), cyclic voltammetry and molecular modelling calculations. As a proof of their remarkable polymerization performance, the new OXEs were also used at 405 nm for direct laser write applications.