Thermally activated delayed fluorescence (TADF) photosensitizers based on 9,10-dihydro-9,9-dimethylacridine/2,4,6-triphenylpyrimidine conjugates with strong visible absorption, large excited state reduction potentials, and long-lived triplet excited states were successfully employed in the organocatalyzed atom transfer radical polymerization (O-ATRP) of methacrylic monomers. A donor-modification strategy dramatically improved the stability of the photocatalyst radical cations, while retaining their high oxidizing strengths, a key requirement for controlled O-ATRP. Time-resolved...