Homogeneous reduction of alkyl halides of relevance to atom transfer radical polymerization: estimation of standard reduction potentials of alkyl radicals

Atom transfer radical polymerization (ATRP) is a powerful method of polymerization of a wide range of monomers, yielding polymeric materials with pre-determined molecular weights, low polydispersities and desired molecular architectures [1]. Mechanistically, ATRP is based on the reversibile halogen atom transfer between a low oxidation state metal complex, [MtL]z+ and a dormant halogenated macromolecular species (Pn-X), resulting in the formation of a propagating radical (Pn•) and the metal complex in a higher oxidation state. The process is initiated by a radical generating reaction between [MtL]z+ and an alkyl halide, RX, resembling the monomer. The success of ATRP depends strongly on the appropriate equilibrium between activation/deactivation steps. In addition, electron transfer between the metal catalyst and the propagating radical should be very slow to avoid chain termination. Therefore, knowledge on the redox properties of alkyl radicals is highly desired. Another important issue, which has recently become a matter of some controversy [2], is the mechanism of the activation reaction in which reductive cleavage of a C-X bond occurs. Herein we report the results of a study on the mechanism of homogeneous dissociative ET to some alkyl halides (XCH2CN, XCH2CO2Et, XCH(CH3)CO2CH3, where X = Cl, Br, I), which are frequently used as initiators in ATRP. The kinetics of the homogeneous ET between electrogenerated aromatic or heteroaromatic radical anions, D•-, and RX has been analyzed in CH3CN. All compounds undergo a concerted dissociative electron transfer with formation of a fragment cluster in the solvent cage; that is the reaction dynamics is best described by the sticky dissciative ET model. The reaction between RX and D•- leads to a radical R•, which can react with D•- either by radical coupling (kc) or by electron transfer (ket). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q = ket/(ket + kc), as a function of EoD/D•- allows estimation of the reduction potentials of the intermediate radicals [3]. The standard reduction potentials obtained for the three radicals •CH2CN, •CH2CO2Et, •CH(CH3)CO2CH3 lie in the range from -0.54 to -0.71 V vs. SCE.