Living Cationic Polymerization of p‐Methoxystyrene Catalyzed by Benzoselenium Salt Under Ambient Conditions

A benzoselenium selenonium salt (BzSe) was synthesized and investigated as a bench‐stable and water‐tolerant Lewis acid catalyst for the controlled cationic polymerization of p ‐methoxystyrene ( p MOS) under ambient conditions. Compared to previously studied selenonium salts, BzSe demonstrated stronger Lewis acidity, as confirmed by proton nuclear magnetic resonance ( 1 H NMR) analysis. Using a cationic reversible addition‐fragmentation chain transfer (RAFT) agent, DTCB, the polymerization exhibited excellent living characteristics, including controlled molecular weights, narrow molecular weight distributions (MWDs, Ð < 1.30), and rapid polymerization rates (> 99% monomer conversion within 1 min). The molecular weight could be precisely tuned by varying the monomer‐to‐DTCB molar ratio. Importantly, this system demonstrated tolerance to water, with slight decreases in molecular weight attributed to controlled termination events, which could be reactivated for further chain propagation. The versatility of this catalytic system was highlighted by its applicability to various vinyl ethers and styrene derivatives, as well as the synthesis of block copolymers. This study provides a significant advance in living cationic polymerization by addressing long‐standing challenges in mild and efficient polymerization systems.