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Photoinduced organocatalyzed atom-transfer radical polymerization (O-ATRP) is a controlled radical polymerization technique that can be driven using low-energy, visible light and makes use of organic photocatalysts. Limitations of O-ATRP have traditionally included the need for high catalyst loadings (1000 ppm) and the narrow scope of monomers that can be controllably polymerized. Recent advances have shown that N , N -diaryl dihydrophenazine (DHP) organic photoredox catalysts (PCs) are capable of controlling O-ATRP at PC loadings as low as 10 ppm, a significant advancement in the field. In this work we synthesized five new DHP PCs and examined their efficacy in controlling O-ATRP at low ppm catalyst loadings. We found that we were able to polymerize methyl methacrylate at PC loadings as low as 10 ppm (relative to monomer) while producing polymers with dispersities as low as Đ = 1.33 and achieving initiator efficiencies ( I* ) near unity (102%). In addition to applying these PCs in O-ATRP, we carried out a thorough investigation into the structure–property relationships of the new DHP PCs reported herein and report new photophysical characterization data for previously reported DHPs. The insight into the DHP structure–property relationships that we discuss herein will aid in the elucidation of their ability to catalyze O-ATRP at low catalyst loadings. Additionally, this work sheds light on how structural modifications affect certain PC properties with the goal of bolstering our understanding of how to tune PC structures to overcome current limitations in O-ATRP such as the controlled polymerization of challenging monomers.