Imidazophenothiazine-based photocatalysts enable the organocatalyzed atom-transfer radical polymerization of styrene.

Organocatalyzed atom transfer radical polymerization (O-ATRP) represents a powerful, metal-free strategy for precision polymer synthesis. Despite being one of the most common commodity polymers produced worldwide, however, the controlled polymerization of styrene by O-ATRP remains a challenge due to its high activation barrier and low propagation rate. Furthermore, the details of the underlying mechanism of photoactivation in this system remain poorly understood. Here, we investigate ten photocatalysts based on an imidazophenothiazine scaffold with diverse excited-state characters to elucidate their roles in the O-ATRP of styrene. Guided by this initial screening, we also report the design of SMAT-Ph-IPTZ, a sulfur-bridged photocatalyst featuring a rigid donor-acceptor architecture that enhances molar absorptivity, increases triplet-state population, and improves redox reversibility. SMAT-Ph-IPTZ enables O-ATRP of styrene at ppm-level catalyst loadings at room temperature, providing competitive performance to leading organic photocatalysts, affording polymers with moderate dispersity (Đ = 1.5) and high molecular weight (M n = 14 kDa) within 24 hours. Our photophysical, mechanistic, and theoretical studies also reveal that singlet-state electron transfer dominates in the O-ATRP mechanism, in contrast to the established belief that triplet excited states should dominate in this system. Overall, this work provides a framework for the future development of next-generation organic photocatalysts for O-ATRP.