In the (Very) Long Run We Are All Dead: Activation and Termination in SET-LRP/SARA-ATRP Simon Harrisson* ,† and Julien Nicolas ‡ † IMRCP, UMR CNRS 5623, Universite ́ de Toulouse, 118 route de Narbonne, 31062 Toulouse Cedex 9, France ‡ Institut Galien Paris-Sud, Univ Paris-Sud, UMR CNRS 8612, Faculte ́ de Pharmacie, 5 rue Jean-Baptiste Cle ́ ment, F-92296 Châ tenay-Malabry Cedex, France * S Supporting Information ABSTRACT: The rate constants of activation and termination were determined for SET-LRP/SARA-ATRP polymerizations of methyl acrylate. Measurement of the rate of generation of CuBr 2 throughout the reaction (using data from Levere et al., Macro- molecules 2012, 45, 8267−8274) allowed evaluation of the chain length dependence of the two rate constants, which were found to be 1.25(9) × 10 −4 DP n −0.51(3) cm·s −1 (activation) and 3.1(1) × 10 9 DP n −0.49(2) L·mol −1 ·s −1 (termination). Addition of the CuBr 2 deactivator at the beginning of the reaction is found to result in a higher proportion of dead chains due to rapid termination of short chains. I n a recent publication, 1 Percec and co-workers used UV−vis spectroscopy to measure the generation of CuBr 2 during polymerization of methyl acrylate (MA) initiated by methyl bromopropionate (MBP) and catalyzed by activated copper wire. Four polymerizations were carried out, using low (60/1) and high (222/1) ratios of MA/MBP and in the presence or absence of initial added CuBr 2 . A monotonic increase in CuBr 2 concentration was observed, from which it was deduced that CuBr 2 is not reduced to CuBr. These experiments generated accurate kinetic data which we will make use of in this communication to shed light on activation and termination processes in reversible deactivation radical polymerization 2 in the presence of copper(0), known variously as SET-LRP 3,4 or SARA-ATRP. 5 Introduced by Percec et al. in 2002, 6 and expanded to acrylates and methacrylates in 2006, 7 SET-LRP is a type of reversible deactivation living radical polymerization in which radicals are initially generated by the reaction of an activated alkyl halide (typically an alkyl bromide) with copper metal in the form of wire, powder, or a colloidal suspension generated by in situ disproportionation of CuBr. Deactivation occurs, as in atom transfer radical polymerization (ATRP), by reaction of the propagating radicals with CuBr 2 , forming a dormant polymer chain and CuBr. The fate of the CuBr generated in the activation and deactivation steps has been the subject of controversy. According to one interpretation (SET-LRP), 3 the CuBr instantaneously disproportionates to form CuBr 2 and nano- particles of copper(0). The nanoparticulate copper is a highly reactive activator, while disproportionation of CuBr provides a source of deactivator which does not involve bimolecular termination or the persistent radical effect, the main source of CuBr 2 in classical ATRP polymerizations. This has led to claims that SET-LRP polymerizations proceed without any bimolec- ular termination, 8,9 supported by kinetic data showing that a polymerization which has been interrupted by removal of the copper wire recommences at an identical rate when the copper wire is reintroduced 10,11 and by NMR and MALDI data showing complete 1,4,9,12−14 or nearly complete 15−19 retention of chain end functionality even at high conversions. The latter feature has allowed the preparation of complex multiblock architectures with high fidelity, either using sequential SET- LRP polymerizations 15,16,18 or by SET-nitroxide radical coupling (SET-NRC). 20−26 A second interpretation (SARA-ATRP) 5,27−37 is that the copper acts as a supplemental activator and reducing agent (SARA). In this scheme, Cu I (specifically [Cu I L] + , where L represents the ligand), 38 not Cu 0 , is the major activating species, and Cu 0 reduces Cu II to Cu I through comproportio- nation. Activation of dormant polymer chains by Cu 0 occurs at a slow rate throughout the polymerization, and disproportio- nation is negligible. The key reactions of both mechanisms are summarized in Scheme 1. In our own work on this polymerization, 39−42 we have assumed that these interpretations are not totally incompatible: that both CuBr and Cu are activating species and that disproportionation and comproportionation take place con- currently, with one or the other predominating depending on Received: May 20, 2014 Accepted: June 11, 2014 Published: June 16, 2014 Letter pubs.acs.org/macroletters © 2014 American Chemical Society 643 dx.doi.org/10.1021/mz500305j | ACS Macro Lett. 2014, 3, 643−647