UV-initiated iodine-mediated polymerization of n-butyl methacrylate employing an iodo chain-transfer agent and a photoinitiator was studied with respect to its kinetics and mechanism. UV irradiation was found to be a significant boost for molar-mass control compared to thermally initiated systems, drastically lowering dispersities from. above 1.5 down to 1.1. The reason for this, as proven by extensive kinetic simulations, is the incessant C I-bond cleavage and formation of free iodine on the nanomolar scale, which leads to a reversible termination mechanism coexisting to degenerative chain-transfer. In experiments with intermittent irradiation, the system was perfectly dormant with no polymerization occurring during the dark time, illustrating the potential as photoswitchable controlled polymerization system. Photopolymerization at elevated temperatures is presented as a tool for increasing polymerization rate while fully retaining molar-mass control. The presented system is highly robust and proceeds without the use of smelly or toxic additives, offering a cheap and green way to produce externally triggered well-defined polymer and extending the scope of iodine-mediated polymerizations.