Photochemistry and Reaction Pathway of Oxime Esters as Radical Photoinitiators

In recent years, oxime esters are of growing interest as Type I photoiniators thanks to their high efficiency in radical photopolymerization. Several structures have been proposed by varying the chromophoric group or the oxime ester substituent in order to improve the initiation properties. In this paper, the relation between the structure, photophysical and photochemical properties, reactivity, and efficiency in photopolymerization processes of two commercial oxime esters has been studied. Their absorption properties were investigated by absorption spectroscopy and were correlated to the molecular structures and electronic transitions thanks to density functional theory (DFT) calculations. The understanding of the reaction pathway was supported by photolysis monitored by liquid chromatography (HPLC) and laser flash photolysis (LFP). Finally, the photoinitiator efficiency was studied in photopolymerization by real-time Fourier transform infrared (RT-FTIR) experiments and the results were related to the photochemical properties.