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Atom-transfer radical polymerization (ATRP) is a reversible-deactivation radical polymerization in which an alkyl halide chain end and a redox-active catalyst establish a rapid equilibrium between dormant and propagating states. Activation transfers the halogen atom from the dormant polymer chain to a lower-oxidation-state metal complex, generating a carbon-centered radical and a higher-oxidation-state metal halide; deactivation reverses this step ^[1]. Because the equilibrium strongly favors dormant chains, the steady-state radical concentration is low, irreversible termination is suppressed, and most chains undergo comparable numbers of propagation events. The retained carbon–halogen terminus permits chain extension and block formation. Copper complexes are the most common catalysts, although other metals and photochemical or electrochemical modes can control the same activation–deactivation principle ^[2]. ATRP differs from RAFT and iodine-transfer polymerization because its reversible deactivation is mediated by catalyst-controlled halogen-atom transfer rather than degenerative transfer through thiocarbonylthio or iodide reagents.