Double‐external‐field enables bulk controlled radical polymerization with narrow molecular weight distribution at high conversion

To control over molecular weight and its distribution in bulk controlled radical polymerization (CRP) at high conversion remains a challenge. Currently, there are few reports about bulk CRP regulated by external field. In this work, a new strategy combining external fields of light and ultrasound, namely double‐external‐field, is reported to overcome the challenge. Light irradiation directly reduces the deactivator Cu II Br 2 /L in the presence of free amine ligand, while ultrasonic agitation improves the homogeneity of the system and moderates the diffusional limitations on activation‐deactivation and termination processes. Bulk polymerizations of methyl acrylate (MA) were conducted in a controlled manner at conversion over 90%, producing PMA with low dispersities ( Đ = 1.05–1.07) and good retention of chain‐end functionality (77%). In addition, good control over the polymerizations for methyl methacrylate (MMA) and styrene was obtained, although the chain‐end functionality of PMMA‐Br requires further improvement. It is believed that this as‐developed double‐external‐field regulation strategy is also applicable to other light induced bulk RDRP systems to improve the controllability.