Photo-induced organo-manganese-mediated radical polymerization of acrylates under LED irradiation

The role of manganese in photo-induced organometallic-mediated radical polymerization (photo-OMRP) under LED (light-emitting diode) irradiation has not been previously explored. In this study, four manganese (II) complexes, namely [Mn(6-tert-butyl-N,N '-propanebis [salicylimine])] (Mn-Bu), [Mn(6,3-di-tert-butyl-N,N '-propanebis [salicylimine])] (Mn-Bu-2), [Mn(6-ethoxy-N,N '-propanebis [salicylimine])] (Mn-Ethoxy), and [Mn(6,3-di-ethoxy-N,N '-propanebis [salicylimine])] (Mn-Ethoxy(2)), were synthesized using both symmetrical and non-symmetrical tetradentate N2O2 Schiff base ligands. The Schiff base ligands were characterized by Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and nuclear magnetic resonance spectroscopy (NMR). The Mn (II) complexes were characterized by FTIR, UV-Vis, elemental analysis, molar conductivity, cyclic voltammetry, MALDI-TOF mass spectrometry, electron paramagnetic resonance, and rationalized with density functional theory (DFT) studies. All complexes were evaluated as photo-OMRP mediators under LED irradiation at 390 nm and in the 400-800 nm range. The photopolymerization utilized methyl acrylate (MA) and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) in varying molar ratios of [MA]/[Mn-II]/[TPO] (900/1/1, 600/1/1, and 600/1/0.5). The photo-OMRP of MA mediated by Mn-Bu-2 showed the best control over molecular weight (M-n matching M-n,M-th) and dispersity (& ETH; < 1.30) at a [MA]/[Mn-II]/[TPO] = 600/1/0.5 M ratio. The polymerization system demonstrated a living character, as evidenced by reversible "on/off" light source switching. Furthermore, a block copolymer of poly (methyl acrylate)-b-poly (butyl acrylate) was successfully synthesized employing a sequential visible-light-induced process, with the Mn-Bu-2 system showing the best results.