Kinetics of Activation of Alkyl Halides by Copper(I) Complexes used as Catalysts in Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is a powerful polymerization technique for the synthesis of a vast range of polymeric materials with controlled molecular weights and well-defined architectures. In copper-catalyzed ATRP, a Cu(I)X/L complex (X = Br, Cl; L = polydentate amine ligand) is employed to activate a dormant species (PnX) to produce the propagating radical (Pn•) and a deactivator (XCuIIL). One of the most important parameters in ATRP is the activation rate constant (kact). Using simple alkyl halides as mimics of PnX, the kinetics of the activation reaction with several copper complexes has been studied by techniques such as NMR, UV-Vis, HPLC and gas chromatography. Most of these techniques, however, are suitable for the study of slow reactions and indeed the vast majority of the reported rate constants are smaller than 1.0 Lmol-1s-1. In this study we propose electrochemical methods for the determination of kact boh for slow and fast reactions. The kinetics of slow to moderately fast reactions can be easily analyzed by monitoring the decrease of the limiting current for the oxidation of Cu(I), either under pseudo-first-order conditions or under second-order conditions, depending on the value of kact. The rate constants of very fast reactions can be obtained by using cyclic voltammetry with digital simulation of voltammetric responses. These methods have been applied to a series of alkyl halides and some of the most widely employed Cu complexes. Careful analysis of the effect of the metal complex structure, especially factors that affect speciation of Cu(I), on the reaction rate has given valuable insights into the reaction mechanism, particularly the identity of the activating species.