Copper (Photo)redox Catalyst for Radical Photopolymerization in Shadowed Areas and Access to Thick and Filled Samples
journal contributionposted on 09.05.2017 by P. Garra, F. Dumur, D. Gigmes, A. Al Mousawi, F. Morlet-Savary, C. Dietlin, J. P. Fouassier, J. Lalevée
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The free radical polymerization of low viscosity methacrylate blends upon a LED irradiation at 405 nm under air is carried out using Cu(I)/iodonium salt/tin(II) organic derivative as photoinitiating systems. The system exhibits a high reactivity; where tin derivative plays a crucial role. It operates through a catalytic cycle in which Cu(I) is regenerated and can be used at low concentrations (0.1–0.3 wt %). Remarkable performances are achieved. At first, a final methacrylate conversion of 82% after 40 s in 1.4 mm thick samples is obtained for an irradiance of 35 mW/cm2 whereas such a conversion is only reached only when using a Cu(I)/iodonium salt system under a 200 mW/cm2 light exposure. Second, a 55% conversion is still obtained after 150 s under a very low irradiance (2.5 mW/cm2). Third, almost tack-free thick samples (1.4 mm) under air are produced upon sunlight exposure (65% of conversion for the 1.4 mm thick sample after 90 s of irradiation). Fourth, the photocuring of clear samples as thick as 9 cm (and presumably even more) with an impressive homogeneity through the entire polymerizable medium is feasible; the photopolymerization of 8.5 cm thick filled samples is also realized. Fifth and last, a lateral polymerization beyond the irradiated area is demonstrated with unprecedented extensions of 8 mm (tin(II) = 1.3%) and 28 mm (tin(II) = 8%), which allows polymerization reactions to occur in shadowed areas. The chemical mechanisms are studied by steady state photolysis and ESR-spin trapping experiments. The subsequent role of the hydroperoxides (ROOH) formed during the polymerization reaction is a key point i.e. for the polymerization in shadowed areas (thick and filled samples), these latent species (ROOH) will be generated from the oxygen inhibition and can diffuse for a full curing of the samples through a ROOH/Cu(I) redox initiation.