3580 Chem. Commun., 2011, 47, 3580–3582 This journal is c The Royal Society of Chemistry 2011 Cite this: Chem. Commun., 2011, 47, 3580–3582 New insights into the mechanism of activation of atom transfer radical polymerization by Cu(I) complexesw Patrizia De Paoli, Abdirisak A. Isse,* Nicola Bortolamei and Armando Gennaro Received 11th January 2011, Accepted 31st January 2011 DOI: 10.1039/c1cc10195a The kinetics of activation of RX by a Cu I complex has been investigated in MeCN both in the absence and presence of halide ions. The system Cu I /L/X  (L = Me 6 TREN) is mainly composed of Cu I L + , XCu I L and Cu I X 2  , but only Cu I L + is found to be an active catalyst reacting with RX. Atom transfer radical polymerization (ATRP) is one of the most used methods of controlled/living radical polymerization for the synthesis of a vast range of well-defined, low-poly- dispersity polymeric materials. 1 The process is initiated by a reversible reaction between a transition metal complex (mainly Cu(I) with an amine ligand, Cu I L + ) and an activated alkyl halide to produce the propagating radical. RX + Cu I L + $ R  + XCu II L + (1) This reaction is considered to involve a transfer of a halogen atom from the alkyl halide to the metal center. 2 Both the equilibrium constant (K ATRP ) and the activation rate constant (k act ) play a fundamental role in the process and, indeed, much effort has been devoted to the determination of K ATRP and k act , especially for Cu complexes. 3–6 Most of these studies were carried out in the presence of halide ions, which are known to form stable Cu I and Cu II complexes. 7 Indeed, we have recently shown that CuL complexes with very high stability constants (log b = 23–27 for Cu II and 6.5–7.3 for Cu I ) get involved in competitive equilibria with bromide or chloride ions in MeCN. 8 In particular, Cu I always exists in solution, under ATRP conditions, as a multiplicity of species including binary and ternary complexes as well as mononuclear and dinuclear species. The most important species, which constitute ca. 95% of Cu I under various conditions, are Cu I L + , XCu I L and Cu I X 2  . These findings pose some questions on the significance of the literature values of K ATRP and k act . In particular, a fundamental issue to be urgently elucidated regards the nature of the active Cu I species. Herein, we report the results of a study aimed to identify the real catalyst in Cu I -catalyzed ATRP. We measured k act for the activation of benzyl chloride (BC), chloroacetonitrile (CAN), ethylbromoacetate (EBA) and allyl bromide (ALB) by Cu I L + (L = Me 6 TREN = tris(2-dimethyl- aminoethyl)amine) in MeCN both in the absence and presence of halide ions (X  ) and the effect of X  on k act was analysed with the aid of recently reported Cu I speciation data. Several methods mainly based on gas chromatography, NMR, UV-vis and HPLC have previously been used to measure ATRP activation rate constants. 4–6 Most of these techniques, however, are suitable to the study of slow reactions and indeed the vast majority of the reported rate constants are smaller than 1.0 L mol 1 s 1 . 4 Herein, we propose a new method based on electrochemical monitoring of Cu I or Cu II for the measurement of k act not only for slow reactions but also for moderately fast reactions. The equilibrium in reaction (1) is highly shifted to the left, which makes it difficult to obtain kinetic information by simply mixing RX and Cu I L. To make the kinetics irreversible, all experiments were carried out in the presence of 2,2,6,6- tetramethyl-1-piperidinyloxy (TEMPO) in a large excess with respect to Cu I L + . It is known that this nitroxide radical quantitatively scavenges the alkyl radicals originating from reaction (1). 9 This is because the rate constant (k c ) of R  coupling with TEMPO is much higher than the deactivation rate constant so that back transformation of the alkyl radical to the dormant species is avoided. Termination (k t ) via R  dimerization is also negligible since k c [TEMPO] c k t [R  ]. 5a Thus, the overall process can be considered to be irreversible and kinetically controlled by the initiation reaction. The rate constant of reaction (1) in the presence of excess TEMPO can be measured by monitoring either the decrease of [Cu I L + ] or the increase of [XCu II L + ] with time. Both these species exhibit well behaved voltammetric waves and are therefore suitable as redox probes for kinetic analysis. Fig. 1 shows an example of the voltammetric behavior of Cu I L + and XCu II L + at a glassy carbon rotating disc electrode (RDE) in MeCN. Chronoamperometry under steady state conditions was used to monitor the decay of Cu I L + (or formation of XCu II L + ). This consists in an instantaneous shift of the potential of the RDE (at a fixed angular velocity) from an initial value (E i ) before the wave, where no redox process occurs at the electrode, to a value (E f ) in the plateau region where the electrode process has the maximum rate. The output of a typical chronoamperometry is shown in Fig. 1b for benzyl chloride (BC). In the absence of RX, there is no homogeneous Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131, Padova, Italy. E-mail: abdirisak.ahmedisse@unipd.it; Fax: +39 049 8275239; Tel: +39 049 8275677 w Electronic supplementary information (ESI) available: Detailed description of electrochemical instrumentation and measurements, and data analysis. See DOI: 10.1039/c1cc10195a ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Downloaded by Universita di Padova on 11 March 2011 Published on 16 February 2011 on http://pubs.rsc.org | doi:10.1039/C1CC10195A View Online