Mechanism of Activation of Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is one of the most studied techniques of controlled living radical polymerization. It can be applied to a wide range of monomers for the preparation of many polymers with controlled molecular weights and well-defined architectures. ATRP is catalyzed by transition metal complexes, especially Cu(I) complexes with nitrogen ligands (L). The success of the process mainly relies on the establishment of an equilibrium in which a dormant alkyl halide (RX) reversibly reacts with an activator (CuIL) to produce the propagating radical (R\u2022) and a deactivator (XCuIIL). The mechanism of this activation step has recently been a subject of debate. The reaction is often considered to occur by halogen atom abstraction (ISET-AT), but in principle a reaction route involving a dissociative electron transfer (DET), either stepwise (OSET-SW) or concerted (OSET-C), is also possible. In this communication we wish to analyze the results of a wide study on the activation mechanism of activated alkyl halides used as initiators in ATRP. We used several approaches to study the process: (i) analysis of the dynamics of DET to RX at a glassy carbon electrode; (ii) homogeneous reduction of RX by aromatic radical anions acting as outer-sphere donors; (iii) comparative kinetic analysis involving Cu(I) complexes and outer-sphere donors; (iv) analysis of activation thermodynamics; (v) solvent effects on the activation rate. These investigations have produced several lines of evidence clearly showing the impossibility of stepwise DET involving the intermediate formation of RX\u2022-. The data also unequivocally exclude the possibility OSET-C, leaving ISET-AT as the only feasible reaction route