Metal Acetylacetonate-Bidentate Ligand Interaction (MABLI) (Photo)activated Polymerization: Toward High Performance Amine-Free, Peroxide-Free Redox Radical (Photo)initiating Systems

Free radical polymerization (FRP) initiation from metal acetylacetonate-bidentate ligand interaction (MABLI) under mild conditions (room temperature, under air) is discussed here for different metal centers (Mn, V, and Cu). First, without light, in MABLI systems such as V(acac)(3)/2-diphenylphosphinobenzoic acid (2dppba), Mn(acac)(3)/2dppba, or Cu(acac)(2)/2dppba, electron-rich aryl (Ar-center dot) radicals were conveniently and efficiently produced by the reaction of electron-poor acac(center dot) radicals with the iodonium salt (Iod), leading to an enhancement of pure redox (no light) polymerizations. Second, it was found that V(III)*/Iod reaction is generating aryl (Ar-center dot) radicals at a high enough rate to initiate a photopolymerization process upon mild irradiation (LED@405 nm). This reaction can be implemented in redox photoactivated systems (e.g., using three-component V(acac)(3)/2dppba/Iod systems) in order to spectacularly enhance a slow redox process from 40 degrees C exothermicity in 800 s to 93 degrees C in less than 200 s. Third, an impressive chemical bleaching (without light) was reported for the Mn(acac)(3)/2dppba reaction. Photoactivation of the Mn(acac)(3)/2dppba/Iod system led to outstanding FRP initiation efficiencies (<20 s for more than 85% C=C conversion of a low-viscosity methacrylate resin). Light enhancement of surface curing was confirmed for all the redox photoactivated polymerizations using Raman confocal microscopy. Overall, amine-free peroxide-free MABLI radical initiating systems were highly improved for safer and even more efficient redox (photoactivated) polymerizations. This original combination of redox polymerization and photopolymerization will be highly worthwhile to combine in one approach the advantages of both techniques.