LAUSR

Rapid and precise acrylic polymer synthesis is essential for applications in drug delivery, programmable materials, and biosensors. However, achieving both speed and precision remains challenging, as reaction acceleration is typically coupled with increased radical concentration, leading to a trade-off between polymerization rate and molecular control. Photoiniferter RAFT polymerization, a catalyst-free, visible light-driven method, offers exceptional control but lacks a detailed mechanistic understanding of C–S bond photolysis. Here, we resolve this speed-control trade-off by leveraging a key photophysical feature of thiocarbonylthio compounds: C–S bond cleavage proceeds via an S1/S0 conical intersection (CI), enabling ultrafast, non-radiative relaxation and clean photolytic decomposition with minimal side reactions. Although quantum yield is low (0.3–0.5%), this mechanism inherently limits radical accumulation, even at elevated temperatures. As a result, propagation can be thermally accelerated without increasing termination, preserving excellent control. Coupled with flow chemistry, this strategy achieves 90% monomer conversion in 20 minutes with narrow dispersity (Đ = 1.02) and minimal dead chains (<2%). This work offers a scalable, energy-efficient route to precision polymers and advances the mechanistic understanding of controlled radical processes for next-generation materials.