Wavelength-Resolved Oxime Ester Photoinitiator Decay in Radical Polymerization

We examine the wavelength-dependent efficiency of a set of oxime ester based photoinitiators via photopolymerization action plots in methyl methacrylate (MMA), assessing the polymer yield wavelength-by-wavelength after irradiation between 325 and 460 nm with a constant number of photons at each wavelength. We systematically vary the structural elements within three carbazole-based oxime esters, i.e., 1, 1-(9-dodecyl-6-nitro-9H-carbazol-3-yl)­ethan-1-one O-acetyl oxime, 2, 1-(9-dodecyl-6-nitro-9H-carbazol-3-yl)­ethan-1-one O-(4-methoxybenzoyl) oxime, and 3, 1-(9-dodecyl-6-nitro-9H-carbazol-3-yl)­ethan-1-one O-(4-nitrobenzoyl) oxime, changing their substitution pattern on the carboxyl group from alkyl to two substituted aromatic functionalities. The resulting photopolymerization action plots are strongly mismatched with the extinction spectra of each oxime ester photoinitiator by close to 75 nm to the red edge of the main absorption maximum. The strongly red-shifted reactivity confirms thatat least under the examined conditionsextinction spectra constitute no valid guide for predicting maximum photopolymerization yields. We subsequently examine the wavelength-resolved dependence of the initiator decay in two solvents in the absence of MMA yet identical initiator concentrations by following the photofragmentation reaction via 1H NMR spectroscopy. In both examined solvents, i.e., dimethyl sulfoxide (DMSO) and methyl isobutyrate (MIB), the obtained photoinitiator decay action plots also display a significant mismatch between the extinction spectrum and the wavelength-resolved photochemical action. The maximum reactivity in MIB and DMSO is red-shifted by close to 50 and 65 nm, respectively, and the photochemical action plot in DMSO exhibits a secondary local maximum. Notably, the decay action plots also do not display a reactivity maximum at the absorption maximum. We submit that the red-shifted maximum of the initiator decay is correlated with the enhanced photopolymerization activity on the red-edge of the absorption spectrum.