Visible-Light-Induced Atom-Transfer-Radical Polymerization with a ppm-Level Iron Catalyst

A low-ppm-level iron (Fe)-based photoinduced atom-transfer-radical polymerization (ATRP) under visible-light irradiation was developed. Various ligands, tris(4-methoxyphenyl)phosphine (TMPP), 4,4′-dinonyl-2,2′-dipyridyl (dNbpy), and tris[2-(dimethylamino)ethyl]amine (Me6TREN), were used to enhance the catalytic activity of Fe complexes. Activator FeII complexes were formed by the reduction of FeIII complexes with a monomer under visible-light irradiation. Linear semilogarithmic plots and low polydispersities (Mw/Mn < 1.4) were achieved, indicating that low-ppm-level Fe-based photoinduced ATRP was well-controlled. The effect of the catalyst concentration on polymerization was evaluated. A reduction of catalyst loadings resulted in increased polymerization rate and molecular weight. The order of Fe complex activity for the ligand was TMPP > dNbpy > Me6TREN. Additionally, this polymerization could be ceased and restarted, responding to light off and light on. Retention of the chain-end functionality was analyzed by 1H NMR and chain-extension experiments. Results showed that the partial chain-end functionality of the resulting polymers was lost. Thus, there is still room for improvement of the chain-end functionality and initiation efficiency of the resulting polymers.