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Photoinduced atom transfer radical polymerization (photo-ATRP) has risen to the forefront of modern polymer chemistry as a powerful tool giving access to well-defined materials with complex architecture. However, most photo-ATRP systems can only generate radicals under biocidal UV light and are oxygen-sensitive, hindering their practical use in the synthesis of polymer biohybrids. Herein, inspired by the photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, we demonstrate a dual photoredox/copper catalysis that allows open-air ATRP under green light irradiation. Eosin Y was used as an organic photoredox catalyst (PC) in combination with a copper complex (X–Cu II /L). The role of PC was to trigger and drive the polymerization, while X–Cu II /L acted as a deactivator, providing a well-controlled polymerization. The excited PC was oxidatively quenched by X–Cu II /L, generating Cu I /L activator and PC˙ + . The ATRP ligand (L) used in excess then reduced the PC˙ + , closing the photocatalytic cycle. The continuous reduction of X–Cu II /L back to Cu I /L by excited PC provided high oxygen tolerance. As a result, a well-controlled and rapid ATRP could proceed even in an open vessel despite continuous oxygen diffusion. This method allowed the synthesis of polymers with narrow molecular weight distributions and controlled molecular weights using Cu catalyst and PC at ppm levels in both aqueous and organic media. A detailed comparison of photo-ATRP with PET-RAFT polymerization revealed the superiority of dual photoredox/copper catalysis under biologically relevant conditions. The kinetic studies and fluorescence measurements indicated that in the absence of the X–Cu II /L complex, green light irradiation caused faster photobleaching of eosin Y, leading to inhibition of PET-RAFT polymerization. Importantly, PET-RAFT polymerizations showed significantly higher dispersity values (1.14 ≤ Đ ≤ 4.01) in contrast to photo-ATRP (1.15 ≤ Đ ≤ 1.22) under identical conditions. 

Efficient transfer of halogen atoms is essential for controlling the growth of polymers in atom transfer radical polymerization (ATRP). The nature of halogens may influence the efficiency of the halogen atom transfer during the activation and deactivation processes. The effect of halogens can be associated with the C–X bond dissociation energy and the affinity of the halogens/halides to the transition metal catalyst. In this paper, we study the effect of halogens (Br vs. Cl) and reaction media in iron-catalyzed ATRP in the presence of halide anions as ligands. In Br-based initiating systems, polymerization of methacrylate monomers was well-controlled whereas Cl-based initiating systems provided limited control over the polymerization. The high affinity of the Cl atom to the iron catalyst renders it less efficient for fast deactivation of growing chains, resulting in polymers with molecular weights higher than predetermined by Δ[M]/[RX] o and with high dispersities. Conversely, Br can be exchanged with higher efficiency and hence provided good control over polymerization. Decreasing the polarity of the reaction medium improved the polymerization control. Polymerizations using ppm levels of the iron catalyst in acetonitrile (a more polar solvent) yielded polymers with larger dispersity values due to the slow rate of deactivation as opposed to the less polar solvent anisole, which afforded well-controlled polymers with dispersity <1.2.