Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

ATRP is a powerful method of synthesis of a variety of polymeric materials.1 Mechanistically, it is based on the reversibile halogen atom transfer between a low oxidation state metal complex, [MtL]z+ and a dormant species, resulting in the formation of a propagating radical (Pn•). The process is initiated by a radical generating reaction between [MtL]z+ and an alkyl halide 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 Pn• should be very slow to avoid chain termination. Therefore, knowledge on the redox properties of alkyl radicals is highly desired. Herein we report the standard reduction potentials, SRPs, of some alkyl radicals of relevance in ATRP. The method of estimation of SRP is based on the homogeneous reduction of a precursor such as an alkyl halide and the competition between further reduction of the radical, R•, and radical-radical coupling between R• and the electrogenerated homogeneous electron donor.2 Application of this technique to the radicals •CH2CN, •CH2CO2Et and •CH(CH3)CO2CH3 in acetonitrile gave SRP values in the range from -0.54 to -0.71 V vs. SCE.