Publications : 2010

The photochemical properties of two new photocleavable alkoxyamines bearing a benzophenone-derived chromophore were studied by electron spin resonance (ESR). The C-O bond cleavage has been demonstrated and the photolysis rate constants (k(d)) determined over a large range of light intensity through the monitoring of the nitroxide concentration in aerated conditions. The obtained kinetic data highlight for the first time the linear dependence of kd on the light intensity for alkoxyamines: this should be a driving factor for nitroxide mediated radical photo-polymerization (NMP2).

New photosensitive alkoxyamines have been designed using molecular orbital calculations to improve the selective C-O versus N-O cleavage. The targeted light-sensitive alkoxyamine is synthesized in one step and its reactivity under UV has been investigated using both ESR and LFP. The ability of this alkoxyamine to control the photopolymerization of n-butyl acrylate is evidenced through a process called nitroxide-mediated photopolymerization NMP(2). The selected alkoxyamine is finally used to prepare covalently bonded multilayered micropatterns. This original application highlights the high potential of this technique for some specific applications that require spatial control.

The photopolymerization ability of photosensitized alkoxyamines has been investigated. These compounds behave as interesting two-component photoinitiators. Laser flash photolysis, electron spin resonance, and density functional theory allow to support the interactions encountered between the photosensitizer (benzophenone and isopropylthioxanthone) and the alkoxyamines (C-O bond breaking and hydrogen transfer) and the side reactions of the nitroxide radical with photosensitizer (electron transfer). (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2910-2915, 2010

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.