Atom Transfer Radical Polymerization under Electrochemical Generation of the Active Catalyst

Atom transfer radical polymerization (ATRP) is one of the most commonly employed controlled radical polymerization techniques, enabling the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and specific compositions and architectures. ATRP is often catalysed by a CuII/CuI system through a reversible activation/deactivation step in which CuIL+ reacts with dormant macromolecular species (RX) to produce propagating radicals R• and X-CuIIL+, which acts as a deactivator. The success of the process relies on the activation/deactivation equilibrium, which should be well shifted toward CuI so that radical-radical termination reactions become negligible and the process proceeds with almost constant [CuI]/[CuII] ratio. Although several complexes with low equilibrium constants in nonaqueous solvents are known, termination is never completely suppressed. Termination affects the optimal [CuI]/[CuII] ratio and in the long term may cause a complete stop of polymerization owing to lack of CuI. Therefore, unless high quantities of catalyst are used, which of course is undesirable, means of regenerating the active form of the catalyst should be found. The problem of ATRP control becomes more challenging in aqueous solutions where both the activation rate constant and the ATRP equilibrium constant are quite high. In this case, the conventional strategy of starting with a mixture of CuII and CuI does not give satisfactory results. In this communication, we describe an electrochemical approach to ATRP. The process starts with the deactivator form of the catalyst and the active CuI form is electrochemically generated. The polymerization rate and degree of control can be easily tuned by adjusting the electrolysis potential. In addition, the process can be stopped and, if necessary, restarted at any moment by simply switching the potential between values greater or smaller than E1/2 of the CuII/CuI couple. The efficacy of this method will be illustrated with examples in both aqueous and nonaqueous media.