Study on the photoinitiation mechanism of carbazole-based oxime esters (OXEs) as novel photoinitiators for free radical photopolymerization under near UV/visible-light irradiation exposure and the application of 3D printing

In this work, twenty-two oxime esters bearing different functional groups and based on carbazole as the chromophore were designed and synthesized as Type I photoinitiators. Interestingly, all structures mentioned in this work have never been reported before (even for their synthesis). These oxime esters were found to have good light absorption properties in the UV-visible range, and especially at 405 nm which is a reference wavelength in photopolymerization. Noticeably, several structures showed better photoinitiation abilities than the benchmark photoinitiator i.e. diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), evidencing the interest of these structures. In order to investigate the photoinitiation mechanism of the two series of oxime esters differing by the position of the photocleavable groups, electronic absorption spectra were determined by molecular modeling and density functional theory calculations. Photopolymerization abilities of the different oxime esters and release of CO2 during the polymerization experiments were determined by Real-Time Fourier Transformed Infrared (RTFTIR). Active free radicals were characterized by mean of Electron Spin Resonance Spin Trapping (ESR-ST) experiments. On the basis of these different complementary experiments, chemical mechanisms occurring during photopolymerization could be proposed. Experimental results showed that the decarboxylation reaction occurring subsequent to the photocleavage of oxime esters had a beneficial effect on the photoinitiation ability. Finally, 3D printed objects were successfully obtained by using oxime esters exhibiting the best photoinitiating abilities.