Photoinitiator or photosensitizer? Dual behaviour of m-terphenyls in photopolymerization processes

Abstract

A series of eight m-terphenyl-based compounds functionalized with amino, diethylamino, piperidine, and pyrrolidine moieties were synthesized and investigated for the first time as one-component free-radical photoinitiators for acrylate and methacrylate monomers. Developed compounds were also applied as photosensitizers of iodonium salt in two-component initiating systems for free radical and cationic photopolymerization. That is why they are excellent for hybrid polymerization e.g. synthesis of interpenetrating polymer networks (IPN). The mechanical properties of produced IPN samples were measured. We investigated the effect of the amine group modification and the presence of one or two bromine atoms on the photophysical and electrochemical properties of the m-terphenyls and their consequent efficiency in polymerization processes. Two bromine substituents in the molecule slightly shift the absorption spectrum towards longer wavelengths in comparison to a compound with one bromine atom. Functionalization of amine group by the introduction of alkyl moieties strongly affects absorption properties, the wavelength corresponding with the maximum absorption values increases in the following order: amine group > diethylamine group > piperidine group > pyrrolidine group. Investigated compounds were found to be efficient in photoinitiating systems for free-radical, cationic, and hybrid photopolymerization, kinetic of polymerization processes with the use of different monomers were followed using real-time FT IR methodology. Moreover, the chemical mechanism was investigated using photolysis and ESR experiments. Developed photoinitiators were also applied for laser writing experiments. In the next step, the key printing parameters were settled, the critical energy necessary to initiate polymerization (Ec) and penetration depth of curing light (Dp), and studied systems were applied for resin used for LCD-based 3d printing.