Toward Nitroxide-Mediated Photopolymerization

A new alkoxyamine (methyl 2-((4-benzoylphenyl)((1-methoxy-2-methyl-1-oxopropan-2-yl)oxy)amino)-2-methylpropanoate 4) bearing a chromophore group directly linked to the aminoxyl function is proposed as a photoiniferter. This original compound decomposes under UV irradiation to generate the corresponding alkyl and nitroxide radicals. Drastic changes of the photophysical or photochemical properties of the starting chromophore are noted. Laser flash photolysis experiments showed both a singlet state cleavage and an efficient shortening of the triplet state lifetime of 4. MO calculations suggest a C−O bond homolytic dissociation under UV irradiation through both S1 and T1 pathways. However, an ESR study evidences that both N−O and C−O homolysis occur under UV irradiation. The efficiency of 4 as a conventional photoinitiator is close to that of 2,2′-dimethoxyphenyl acetophenone. When 4 was used as a photoiniferter in nitroxide-mediated photopolymerization (NMP2), a linear growth of the poly(n-butyl acrylate) chain, reaching 80% of conversion in <500 s, is observed combined with a reinitiation of the photopolymerization after the end of the irradiation: this is the first report showing a at least partial NMP2 process.