PhotoATRP Catalyzed by Iron/Phosphine Complexes under Green Light

Abstract

In thermal iron-catalyzed atom transfer radical polymerization, arylphosphines primarily serve as sacrificial reducing agents consumed during activator generation, thereby requiring excess phosphine (≥1.5 equiv) and high catalyst loadings (≥5000 ppm) at elevated temperatures. Here, we report that green-light irradiation (λ ≈ 525 nm) transforms phosphines from consumable reagents into regenerable co-catalysts within a ligand-to-metal charge-transfer (LMCT) redox cycle. This regeneration enabled the controlled polymerization of methacrylates at ambient temperature using FeBr3/phosphine loadings as low as 50 ppm at an equimolar ratio. The catalytic efficiency of various triarylphosphines correlated with their electron donor strength, with the most electron-rich tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP) yielding well-controlled polymers with low dispersity (Đ < 1.3). UV−vis spectroscopy, 31P NMR, transient absorption spectroscopy, and cyclic voltammetry provided evidence for LMCT in the photoexcited FeBr3/TTMPP complex. TTMPP was regenerated, thereby closing the catalytic cycle and enabling the use of an equimolar amount of phosphine under irradiation. The system exhibited temporal control, chain-end fidelity, and oxygen tolerance in sealed vials without prior deoxygenation.