Electrochemical approaches to the determination of rate constants for the activation step in atom transfer radical polymerization

• Determination of ATRP activation rate constants, kact, by electrochemical techniques.
• kact determined over a wide range of 8 orders of magnitude.
• Strong dependence of kact on RX structure, type of halogen atom and Cu catalyst.
• Good correlations between kinetic and equilibrium constants.
• Fairly linear correlations between kact and E° of [XCuIIL]+ or R-X bond energy.

Direct determination of ATRP activation rate constant, kact, is typically limited to slow or moderately fast reactions: kact < 102 mol−1 dm3 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, electrochemical methods have been used to investigate the kinetics of activation of a series of typical ATRP initiators (RX) by [CuIL]+ (L = Me6TREN, TPMA or N,N,N′,N′′,N′′-pentamethyldiethylenetriamine, PMDETA) in MeCN. Both high (>104 mol−1 dm3 s−1) and low kact values have been measured. Analysis of the collected data showed that kact depends on E° of the catalyst, molecular structure of RX, and nature of the halogen atom. Although usually values of kact increase with decreasing standard reduction potential of [CuIIL]2+, there is no well-defined correlation between the two parameters. Instead, kact gives good and useful linear correlations with the standard reduction potential of [XCuIIL]+, and with KATRP or Gibbs free energy of C-X bond dissociation.

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