The synthesis of hyperbranched polymers with alkyl chloride‐type chain ends was accomplished by the copolymerization of methyl methacrylate and a crosslinker, ethylene glycol dimethacrylate, in the presence of 1‐chloro‐1,2‐benziodoxol‐3(1H)‐one (BIO‐Cl), which served as both a radical initiator and an efficient chain transfer agent. The polymerizations were carried out under thermal (80 °C) and photochemical (irradiation with visible light) conditions. As a result of the transfer of Cl atoms from BIO‐Cl to the propagating radicals, gelation was delayed to high monomer conversions and, until that point, soluble branched polymers were formed. Even pure crosslinker could be polymerized to moderate conversions (>15%, depending on the amount of added BIO‐Cl), yielding branched polymers prior to formation of networks. The effect of the concentrations of crosslinker and hypervalent iodine(III) compound on the outcome of the (co)polymerizations as well as the kinetic features of Cl transfer and decomposition of BIO‐Cl were studied in detail. © 2022 Society of Industrial Chemistry.