Direct determination of ATRP activation rate constant, k(act), is typically limited to slow or moderately fast reactions: k(act) < 10(2) Mo1(-1) dm(3) S-1 were determined by classical techniques, such as chromatography, spectrophotometry, NMR, etc. Therefore, these techniques are inadequate to study the fastest, and most useful, ATRP systems, such as the most frequently used Cu complexes with tris[2-(dimethylamino)ethyl] amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA) and substituted TPMA. In this study, electro-chemical methods have been used to investigate the kinetics of activation of a series of typical ATRP initiators (RX) by [(CuL)-L-I](+) (L= Me6TREN, TPMA or N,N,N',N",N"-pentamethyldiethylenetriamine, PMDETA) in MeCN. Both high (>10(4)mol(-1)dm(3) s(-1)) and low k(act) values have been measured. Analysis of the collected data showed that k(act) depends on E degrees of the catalyst, molecular structure of RX, and nature of the halogen atom. Although usually values of k(act) increase with decreasing standard reduction potential of [(CuL)-L-II](2+) there is no well-defined correlation between the two parameters. Instead, k(act) gives good and useful linear correlations with the standard reduction potential of [(XCuL)-L-II](+), and with K-ATRP or Gibbs free energy of C-X bond dissociation. (C) 2016 Elsevier Ltd. All rights reserved.