Revisiting Iodine: From Chain Transfer Agent to Halogen Bonding and Nonmetallic Lewis Acid Catalysis

Iodine has long been recognized as a chain transfer agent in radical polymerizations, enabling controlled synthesis through reversible C-I bond exchange. Recent advancements have expanded its role as a nonmetallic Lewis acid, driven by halogen bonding and three-center-four-electron (3c4e) interactions. This Viewpoint examines the evolution of iodine-mediated radical polymerization and highlights the role of 3c4e interactions in supramolecular chemistry and polymer synthesis, with a particular focus on ring-opening polymerizations (ROP) and CO₂-based copolymerizations. These developments bridge radical chemistry with halogen bonding, offering environmentally friendly, metal-free alternatives to traditional methods. We explore how iodine catalysis contributes to sustainable polymer synthesis and propose strategies for advancing high-performance, sustainable polymerization systems.