A reversible-deactivation radical polymerization in which an iodo chain end mediates degenerative transfer is iodine transfer polymerization (ITP). A propagating radical exchanges iodine with a dormant alkyl iodide (the transfer agent or a polymer–iodide), so that most chains rest as alkyl iodides and only a small fraction are active radicals at any instant. Molecular weight is set by the monomer-to-iodide ratio, and the iodine terminus remains available for block extension. ITP of ethylene and its copolymerization with vinyl acetate demonstrate that the same degenerative iodine transfer operates for unactivated and activated alkenes ^[1]. Chlorotrifluoroethylene-based block copolymers are likewise prepared by ITP, exploiting the stability of fluoroalkyl iodides as dormant species ^[2]. Reverse iodine transfer polymerization (RITP) generates the alkyl iodide in situ from molecular iodine and a conventional radical initiator, then proceeds by the same transfer equilibrium; emulsifier-free acrylate emulsions have been prepared on that basis ^[3]. ITP is distinct from ATRP, which requires a redox-active metal for atom transfer, and from RAFT, which uses a thiocarbonylthio rather than an iodide transfer agent.