Tuning spectral response and polymerization control in nitroxide-mediated photopolymerization (NMP) initiators chromophore substitution

Three alkoxyamine-based photoinitiators bearing benzophenone (NMP22), naphthol (NMP23), and anthracene (NMP24) chromophores were synthesized to investigate the effect of chromophore identity on spectral response and polymerization control in nitroxide-mediated photopolymerization (NMP2). UV–Vis spectroscopy showed that chromophore selection tuned light absorption across the 200–450 nm range, with the anthracene-based initiator 4 exhibiting the broadest photo-response, extending to 450 nm. Photolysis half-life (t1/2) varied by nearly two orders of magnitude across the series (14 s for 2; 920 s for 3), reflecting chromophore-dependent nitroxide radical stability. All three NMP2 initiators photopolymerized poly(hexyl acrylate) polymers with molecular weights up to nine times greater than a 2,2-dimethoxy-2-phenylacetophenone (DMPA) control and low polydispersity indices, with linear molecular weight growth and pseudo-first-order kinetics confirming controlled, “living” character. Additionally, these NMP2 systems exhibited slower photopolymerization rates than conventional initiators, consistent with the reversible activation-deactivation mechanism inherent to nitroxide-mediated polymerization. This study demonstrates that chromophore substitution provides a practical and tunable route to control radical photopolymerization across a broad range of UV and visible wavelengths.