Article pubs.acs.org/Macromolecules
Copper-Mediated CRP of Methyl Acrylate in the Presence of Metallic Copper: Effect of Ligand Structure on Reaction Kinetics Yaozhong Zhang,† Yu Wang,† Chi-how Peng,† Mingjiang Zhong,† Weipu Zhu,†,‡ Dominik Konkolewicz,† and Krzysztof Matyjaszewski*,† †
Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: The kinetics of copper-mediated controlled/ living radical polymerization (CRP) of methyl acrylate (MA) in the presence of Cu0 and two different ligands that form active catalyst complexes with copperTPMA (tris(2-pyridylmethyl)amine) and Me6TREN (tris(2-(dimethylamino)ethyl)amine) are compared. The critical difference between the ligands is that TPMA forms a CuI complex that undergoes essentially no disproportionation in a mixture of MA and dimethyl sulfoxide (DMSO), DMSO/MA (v/v = 1/2), while the complex with Me6TREN undergoes disproportionation to a limited extent. Parameters such as the surface area of Cu0 wire, the concentration of added CuIIX2/L, and ligand concentration were examined. Both the Me6TREN- and TPMA-based catalysts efficiently controlled the polymerization of MA. The TPMA-based system showed a power law order of 0.47 for the apparent propagation rate constant with the Cu0 surface area, very similar to the reported value for the Me6TREN-based system, which showed a power law of 0.44. These results demonstrate that the polymerization of MA in DMSO in the presence of metallic copper can be explained by a core atom-transfer radical polymerization (ATRP) process in which the Cu0 acts as a supplemental activator and reducing agent, rather than through the proposed single-electron-transfer living radical polymerization (SET-LRP) mechanism, which requires additional assumptions, such as complete and instantaneous disproportionation of CuI/L species.
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Scheme 1. ATRP Equilibriuma
INTRODUCTION Controlled/living radical polymerization (CRP) methods allow the synthesis of well-defined polymers with narrow distributions, predictable molecular weights, and complex architectures such as block copolymers, stars, and brushes.1 Atom transfer radical polymerization (ATRP)2 is one of the most widely used CRP techniques due to the range of architectures that can be synthesized using ATRP and its compatibility with various monomers and reaction conditions.3 In ATRP radicals are formed by activation of an alkyl halide by a transition metal catalyst in a low oxidation state, typically CuI/L, and control is provided via rapid deactivation of the macroradical by the X−CuII/L complex which is formed by the activation process. This process of activation/deactivation is shown in Scheme 1. In a well-controlled ATRP, the equilibrium is strongly shifted to the dormant alkyl halide by the X−CuII/L complex or the “persistent radical”. One limitation of normal ATRP is that a large amount of the CuI activator must be added to sustain an acceptable polymerization rate due to the buildup of CuII deactivator caused by termination reactions.2c,4 Recently, ATRP techniques were developed that reduce the amount of copper catalyst to ppm levels, while still allowing the reaction to occur in a reasonable time frame. A low copper concentration can be used by continuously regenerating the CuI/L species by reducing excess X−CuII/L deactivator © 2011 American Chemical Society
Pn−X is an alkyl halide (macro)initiator, CuI/L is the activator complex, Pn• is a (macro)radical and, X−CuII/L is the deactivator complex. a
complex. This can be achieved by directly applying a reducing potential as is done in electrochemically mediated ATRP (eATRP),5 using a reducing agent as in activators regenerated by electron transfer (ARGET),6 or by decomposition of free radical initiators as in initiators for continuous activator regeneration (ICAR) ATRP.7 These reaction mechanisms are depicted in Scheme 2. Various organic reducing agents such as glucose, ascorbic acid, hydrazine, amines, or excess of ligands,8 and inorganic reducing agents including tinII 2-ethylhexanoate Received: August 26, 2011 Revised: November 21, 2011 Published: December 12, 2011 78
dx.doi.org/10.1021/ma201963c | Macromolecules 2012, 45, 78−86
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Scheme 2. Mechanistic Description of ARGET ATRP and ICAR ATRPa
a
The eATRP mechanism is essentially the same as the ARGET mechanism, except a cathodic current reduces the CuII species.
Scheme 3. Elementary Reactions for SARA ATRP and SET-LRP Mechanisma
a
The ligands, propagation, and termination steps are omitted for clarity.
(Sn(EH)2),9 have been used in low copper ARGET ATRP systems. Cu0, which was first used in an ATRP in 1997,10 has also been employed as a reducing agent. Initially, Cu0 was used as the sole source of transition metal, where it acted as a supplemental activator in the presence of ligand to form the major activator, CuI/L. Cu0 was also used to reduce a certain fraction of the added X−CuII/L complex to form a more active CuI/L activator in situ.11 In this way, Cu0 plays a dual role of both a supplemental activator and a reducing agent (SARA), leading to the concept of SARA ATRP. Various reaction conditions,10,12 monomers,13 and ligands14 were used in SARA ATRP systems. Furthermore, other zerovalent metals including zinc, magnesium, and iron were used as in the SARA ATRP of methyl acrylate (MA) in dimethyl sulfoxide (DMSO), in addition to Cu0.15 In 2006, Cu0 was used in conjunction with tris(2(dimethylamino)ethyl)amine (Me6TREN) to polymerize MA in DMSO,12a leading to a controlled and relatively fast polymerization. Although all components, Cu0, Me6TREN, and polar solvents, had been used earlier in ATRP,16 this particular system was named single-electron-transfer living radical polymerization (SET-LRP). Cu0 was designated as the only activator in the proposed SET-LRP mechanism and X−CuII/L as the deactivator, with both the activator and deactivator being constantly regenerated by the instantaneous disproportionation of CuI/L. The key reactions in SARA ATRP and SET-LRP are illustrated schematically in Scheme 3. This paper reports the kinetics of the copper-mediated CRP of MA with Cu0 and (tris(2-pyridylmethyl)amine) (TPMA) ligand in DMSO and compares the kinetics of this specific
polymerization with an equivalent system that uses Me6TREN as the ligand. This study allows a direct comparison between the kinetics of a CRP conducted with a Me6TREN-based complex, which can undergo a limited extent of disproportionation, with a catalyst based on TPMA which does not disproportionate to any significant extent.17 The study will show that the kinetics and mechanism of an ATRP of MA with copper complexes formed with TPMA are essentially the same as those with Me6TREN ligand. Therefore, the assumptions made in the SETLRP mechanism, most notably the instantaneous disproportionation of CuI/L species, are neither necessary nor sufficient to explain the CRP of MA in DMSO in the presence of Cu0. A more accurate mechanism for this polymerization is SARA ATRP.
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EXPERIMENTAL SECTION
Materials. CuBr2 (99+%, Aldrich) was used as received. Cu0 (wire, diameter 1.0 mm, 99.9+%, Aldrich) was washed with MeOH/HCl first and then with fresh MeOH before use. Methyl 2-bromopropionate (MBP) (99.5+%, Aldrich), Me6TREN, and TPMA (99%, ATRP Solutions) were used as received. Methyl acrylate (MA) (99+%, Aldrich) was passed over a basic alumina column to remove antioxidant. Characterization. All spectroscopic measurements were performed on a Cary 5000 UV/vis/NIR spectrometer (Varian). Molecular weight and molecular weight distribution were determined by GPC, conducted with a Waters 515 pump and a Waters 2414 differential refractometer using PSS columns (Styrogel 105, 103, 102 Å) in THF as an eluent at 35 °C and at a flow rate of 1 mL/min. Linear PSt standards were used for calibration. Conversions of MA were determined from the area of the DRI response versus those of known concentrations of polymers in THF. 79
dx.doi.org/10.1021/ma201963c | Macromolecules 2012, 45, 78−86
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the Br− anions to CuI compared to CuII (log KCuBr2 = 4.3 and log KCuI Br2 = 9.6, respectively20). Ligands such as Me6TREN or TPMA have a similar effect on the disproportionation to the halogen anions outlined above. The degree of disproportionation depends on relative stability of CuI and CuII complexes in a manner identical to Br complexation. Although the solvent contributes to the degree of disproportionation, it is also important to consider the nature of the ligand itself. An excellent example of this seen in the comparison of disproportionation equilibrium constants for CuI in pure water, where log KD ≈ 6, whereas the disproportionation equilibrium constant for CuI/TPMA is 6.83 × 10−3 in water.22 This shows that the equilibrium for the TPMA-based system is strongly shifted toward the comproportionation side, despite the fact the disproportionation is almost complete in the absence of TPMA. This highlights the importance of considering both the nature of the solvent and also the presence of the ligand to properly understand the disproportionation of CuI. When considering the magnitude of the disproportionation equilibrium constant, it has been shown that this equilibrium constant is proportional to the ratio βII/(βI)2, where βI and βII are the complexation constants of CuI and CuII.17 In water, for TPMA βI = 7.94 × 1012 and βII = 3.89 × 1017,23 while for Me6TREN βI = 6.3 × 108 and βII = 2.69 × 1015.24 Therefore, the βII/(βI)2 ratio is 6 × 10−9 for TPMA but a million times larger for Me6TREN (i.e., 7 × 10−3). Although there is no such data available for TPMA and Me6TREN in DMSO, the extent of disproportionation of the TPMA-based complexes in DMSO is expected to be very small. This is due to the significantly lower polarity of DMSO compared to water, combined with the fact that the TPMA-based complexes already undergo only a limited extent of disproportionation in water.25 The high stability of CuBr species in DMSO further highlights this fact. In order to quantify the comproportionation/disproportionation beahavior, the evolution of concentration of CuII species was followed by UV−Vis for systems with TPMA and Me6TREN ligands in three different solvents: acetonitrile (MeCN), DMSO, and DMSO/MA = 1/2 (v/v). In all cases the initial conditions were [CuIIBr2]0 = 2.5 mM, [ligand]0 = 5.25 mM, and 4 cm Cu0 wire (d = 1 mm) in 4.5 mL solutions at 25 °C. Essentially complete comproportionation was observed (
