Inner sphere and outer sphere electron transfer reactions in atom transfer radical polymerization

The participation of electron transfer (ET) processes in atom transfer radical polymerization (ATRP) is discussed. Inner sphere electron transfer process (atom transfer) is the inherent part of ATRP. It includes the homolytic cleavage of C‐halogen bond in dormant macromolecular alkyl halides species by the Cu(I) activator with the rate constant kact≈︁100±1 M−1s−1. This process is reversible and the deactivation of growing radicals by X‐Cu(II) species is very fast with the rate constant of deactivation kact≈︁107±1 M−1s−1. However, in addition to the atom transfer process (inner sphere ET), outer sphere ET may also occur. Growing radicals can either be oxidized by Cu(II) species to carbocations or reduced by Cu(I) species to carbanions. These reactions were observed in the polymerization of p‐methoxystyrene and initiation with 2‐bromomalonates, respectively. In order to avoid these side reactions the catalyst structure should be fine tuned for the particular monomer and the range of targeted molecular weights. In addition, some side reactions can be avoided by slow addition of initiator (e.g. for benzhydryl halides), catalyst (for malonates and CCl4 as initiators) and by reducing polymerization temperature.