Scheme 1. [2, 5,7] 1,10-phenantrolines, [6] aryl/alkyl-2-pyridylmethanimines

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1 Macromol. Rapid Commun. 2000, 21, Communication: The effect of the nature of initiator and solvent in atom transfer radical polymerization of methyl methacrylate (MMA) using N-pentyl-2-pyridylmethanimine as the ligand in the presence of Cu(I)Br has been investigated. Under identical concentration and temperature conditions, p-toluenesulfonyl chloride (p-tscl) in diphenyl ether gives polymerization rates identical to ethyl 2-bromoisobutyrate in xylene. However, the former system induces better control of molecular weights and lower polydispersities. M n and M w/m n vs conversion for the ATRP of MMA initiated with p-tscl and 2-EBiB, catalyzed by CuBr/1; for conditions, see Fig. 1. Open symbols represent molecular weights, filled symbols represent polydispersities. The importance of the nature of initiator and solvent in atom transfer radical polymerization of methyl methacrylate catalyzed by copper(i) N-alkyl-2- pyridylmethanimine complexes a Mathias Destarac,* b Jérôme Alric, Bernard Boutevin Laboratoire de Chimie Macromoléculaire, UPRESA 50760, Ecole Nationale Supérieure de Chimie de Montpellier, 8, rue de l Ecole Normale, Montpellier, France Fax: ; boutevin@cit.enscm.fr a Cf. ref. [1] b Present address: Rhodia, Centre de Recherches d Auvervilliers, 52 rue de la Haie Coq, Aubervillers, France. Scheme 1. Introduction Since its discovery in 1995, [2, 3] atom transfer radical polymerization (ATRP) has been rapidly developed and can now be used to polymerize a wide range of monomers, including substituted styrenes, (meth)acrylates and acrylonitrile. [4] The chemistry of ATRP is based on the combination of an organic halide RX as the initiator with a metal/ligand catalytic system, which is capable of promoting a fast initiation compared to propagation and then reversibly activating halogenated chain ends during the polymerization (Scheme 1). Various transition metalbased catalytic systems (Cu, [1, 2, 5 10] Ru, [3, 11] [12, 13] Ni, [14, 15] Fe and Rh [16] ) in conjunction with organic halide initiators induce controlled polymerizations. Four main classes of nitrogen-based ligands are used as efficient ATRP catalysts when coordinated to Cu: 2,29-bipyridines, [2, 5,7] 1,10-phenantrolines, [6] aryl/alkyl-2-pyridylmethanimines [1, 8] and linear polyamines. [10] With N,N9-diimine ligands, polymerizations are poorly controlled. [9] The activity of the ATRP catalyst is a function of the redox potential of the copper/ligand complex. Within a certain range of redox potentials (where inner sphere electron transfer/atom transfer occurs exclusively), the higher the redox potential (best stabilization of the lower oxidation state), the poorer is the ATRP catalyst. From all the above mentioned Cu complexes, N,N9-diimines stabilize best the lower metal oxidation state. Comparatively, aromatic a-diimines (2,29-bipyridines) reveal poor p- Macromol. Rapid Commun. 2000, 21, No. 18 i WILEY-VCH Verlag GmbH, D Weinheim /2000/ $ /0

2 1338 M. Destarac, J. Alric, B. Boutevin acceptor quality and were originally described as bad ligands for metals in low oxidation states. [17] N-alkyl-2- pyridylmethanimine ligands, exhibiting intermediate structures (partially aromatic, partially aliphatic), have intermediate properties. The additional conjugation of phenanthrolines results in higher redox potentials compared with bipyridines. [18] Cu/linear polyamine complexes have the lowest redox potentials. [19] The polymerization of styrene can be used to illustrate the direct correlation between electrochemical properties of the ATRP catalyst and its activity. Rates of polymerization are inversely proportional to the redox potential of the catalyst. The former decrease in the following order: linear polyamines A 2,29-bipyridines, [10] 2,29-bipyridines A 1,10-phenanthrolines, [6] 1,10-phenanthrolines A alkyl-2- pyridylmethanimines. [20] erved initiation rate constant k i = k i K o eq compared to k p ) and/or possible side reactions. [21] Among numerous combinations tested, best results (i.e. best control over M n and M w/m n) were obtained with p-toluenesulfonyl chloride (p-tscl), in conjunction with CuBr. [21] We reconsidered this initiating system for the ATRP of MMA catalyzed by the complex CuBr/N-pentyl-2-pyridylmethanimine (CuBr/1) in two different oxygenated aromatic solvents (diphenyl ether (DPE) and 1,2-dimethoxybenzene (DMB), 50 vol.-%). Results were compared to those obtained using xylene as the solvent and ethyl 2-bromoisobutyrate (EBiB)/CuBr as the initiating system, conditions which are commonly used for this type of ATRP catalyst. [8] Following up a first communication, [1] this paper deals with the study of various reaction parameters including the nature of initiator and solvent with respect to obtaining a better compromise between the kinetics of polymerization and the level of control over the resulting PMMAs. Quantitative initiation which is fast compared to propagation one of the fundamental criteria to obtain good control of the polymerization is easily achieved for moderately reactive monomers such as styrene. For instance, with Cu/2,29-bipyridine and its derivatives as the catalyst, alkyl [2] and sulfonyl halides, [7] as well as 1,1,1-trichloroalkanes [5] are quite efficient initiators which lead to polystyrenes with well-controlled molecular weights and narrow polydispersities. In the case of MMA, the erved propagation rate constant (k p = k p K eq, according to the terminology defined by Matyjaszewski, [21] cf. Scheme 2) is higher than for styrene. [4] The choice of the ropriate initiator/cux pair (X = Cl, Br) is a key parameter in order to avoid slow initiation (low Experimental part Materials MMA was vacuum distilled from CaH 2 and stored under argon. DPE was dried over molecular sieves. Xylene and DMB were distilled before use. Copper(I) bromide was successively washed with glacial acetic acid, absolute ethanol and finally diethyl ether before use. All initiators were purchased from commercial sources and used without purification. N-pentyl-2-pyridylmethanimine (1) was synthesized according to the literature. [8e] Polymerization The general procedure was as follows. To a Schlenk flask, 10 ml of MMA, 8 ml of diphenyl ether and the ropriate amount of ligand were added. The solution was degassed by bubbling argon through it for 15 min. CuBr was then added. The flask was closed with a rubber septum and three freezepump-thaw cycles were performed. The solution was kept under argon and immersed in an oil bath thermostatted to 908C. The desired amount of initiator was dissolved in 2 ml of degassed DPE (kept at 308C). The initiator solution was then added dropwise to the reaction mixture by means of a degassed syringe. After an initial withdrawal (reference), solution samples were collected at given times and analyzed. Scheme 2. Characterization Monomer conversion was determined by GC using the polymerization solvent as an internal standard. Molecular weights and molecular weight distributions were measured by gel permeation chromatography (GPC) using a Spectra Physics instrument equipped with a Shodex RE-61 RI detector, and Phenogel columns (10 5, 10 4, 10 3, 500 Å, eluant: THF, 30 8C). Commercially available poly(methyl methacrylate) standards were used to calibrate the columns.

3 The importance of the nature of initiator and solvent in atom transfer Results and discussion Influence of the nature of the initiator Fig. 1 represents the evolution of ln[m] 0 /[M] with time when EBiB and p-tscl are used as initiators in 50 vol.-% DPE, at 90 8C. The two polymerizations exhibit an induction period whose length hardly varies with the initiator (about one hour for both experiments). Its presence remains unclear but may be due to some residual traces of oxygen left in the reactor despite of several pumpfreeze-thaw cycles. [22] In both cases, plots are linear indicating that the kinetics is first order with respect to monomer concentration. For initiation with p-tscl, the arent rate constant of propagation k p (= k p [M9]) calculated from Fig. 1 is 2.8 times lower than that for initiation with EBiB. In mixed halide systems (RX/CuY where X m Y), halogen atoms exchange rapidly between the carbon and copper center. [21, 23] In the case of substituted bipyridine-based systems, Matyjaszewski et al. [21] showed for the polymerization of MMA initiated with p-tscl/cubr that roximately 90% of the chains are terminated by a chlorine atom. Additionally, Percec et al. [24] reported detailed kinetic data showing a very fast initiation of sulfonyl halides compared to propagation in the ATRP of MMA. From these results, the difference in the rates of polymerization erved with the CuBr/1 complex can be mainly ascribed to differences in bond energies of the PMMA-Cl and the PMMA-Br chain ends. It is worth mentioning that polymerization rates calculated from Fig. 1 are much faster than initially reported. [1] We attribute these differences in rates to a partial oxidization of the CuBr used in our earlier experiments. Fig. 2. M n and M w/m n vs conversion for the ATRP of MMA initiated with p-tscl and 2-EBiB, catalyzed by CuBr/1; for conditions, see Fig. 1. Open symbols represent molecular weights, filled symbols represent polydispersities. Fig. 2 represents the evolution of number-average molecular weights (M n) and polydispersities (M w/m n) with conversion. In the case of EBiB, M n increases with conversion with slightly higher values than expected (Eq. (1)), but this effect is tending to be less pronounced after 80% conversion. M w/m n slightly decreases throughout the polymerization, reaching a value of 1.26 at 86% conversion. These results are nearly identical to those reported by Haddleton for the same initiating system used in xylene. [8e] The evolution profile of M n is presumably due to a relatively slow initiation compared to propagation. For initiation with p-tscl, M n increases linearly with conversion and fits perfectly the theoretical profile, which means that initiation is quite efficient (i. e. fast and nearly quantitative). This results in lower polydispersities than for an initiation by EBiB (1.14 at 90% conversion). M n th ˆ MŠ 0 InitiatorŠ 0 N Monomer conversion MW Initiator 1 Thus, the p-tscl/cubr pair ears to be the best compromise in order to prepare PMMAs with well-controlled molecular weights and narrow polydispersities. The use of a mixed halide system combines efficient fast initiation, rapid deactivation and induces no (or undetectable) side reactions without significantly affecting the rates of polymerization. Fig. 1. Evolution of ln[m] 0 /[M] with time for the ATRP of MMA in diphenyl ether (50 vol.-%) initiated with p-tscl and 2- EBiB, catalyzed by CuBr/1 at 908C; [RX] 0 /[CuBr] 0 /[1] 0 / [MMA] 0 = 1:1:3:100. Influence of the nature of the solvent The nature of the solvent is a parameter of first importance in ATRP. First, it can greatly affect the solubility of the catalyst. For instance, the CuX/2,29-bipyridine (X = Cl, Br) complex is only partially soluble in bulk MMA, styrene, and acrylates, as well as in aromatic solvents.

4 1340 M. Destarac, J. Alric, B. Boutevin Fig. 3. Effect of the nature of the solvent on the kinetics of the ATRP of MMA initiated with p-tscl, catalyzed by CuBr/1; [RX] 0 /[CuBr] 0 /[1] 0 /[MMA] 0 = 1:1:3:100, 50 vol.-% solvent, T = 908C. Fig. 4. M n vs conversion for the ATRP of MMA in various aromatic solvents; for conditions, see Fig. 3. Tab. 1. Apparent propagation rate constants k p (= k p [M9]) for the ATRP of MMA in various aromatic solvents with p-tscl/ CuBr/1 as the initiating system; p-tscl/cubr/1 = 1:1:3:100, 50 vol.-% solvent, T = 908C. Solvent 10 5 k p (s 1 ) Diphenyl ether ,2-Dimethoxybenzene 7.6 Xylene 4.9 Polar solvents, such as ethylene or propylene carbonate, [25] 2-propanol, [26] acetone [5b] or DMF [27] fully solubilize the catalyst under ropriate conditions. The increase in catalyst concentration available in solution compared to heterogeneous systems, leads to faster polymerization rates and lower polydispersities (because of the corresponding higher Cu(II) concentration). Chambard et al. [28] recently reported that besides its effect on catalyst solubility, the solvent affects activation rate constant k act (Scheme 1) in the case of styrene and butyl acrylate through its polarity and coordination ability. CuBr/1 is fully soluble in MMA under ATRP conditions. Keeping p-tscl/cubr as the initiating system, a series of three aromatic solvents was tested. It ears that the solvent has an important effect on the polymerization kinetics (Fig. 3). Tab. 1 gives arent propagation rate constants k p (= k p [M9]) calculated from Fig. 3. Compared with xylene as a reference, polymerizations in DMB and DPE are significantly faster by a factor of 1.5 and 3.3, respectively. As the catalyst is fully soluble in all three solvents, the aromatic oxygenated solvents affect the activity of the catalyst. Differences in the dielectric constants of the solvents (e values for o-xylene, 1,2- dimethoxybenzene and diphenyl ether equal 2.56, 4.45 and 3.73, respectively [29] ) and coordination ability of Fig. 5. M w/m n vs conversion for the ATRP of MMA in various aromatic solvents; for conditions, see Fig. 3. those solvents bearing phenoxy groups are presumably responsible for the differences in polymerization rates. This point is currently being investigated in our group. A similar accelerating effect has been erved with phenols as additives. [8c] The advantage of DMB and DPE over phenols as accelerators of the polymerization is their much higher tolerance with respect to the presence of oxygen in the reaction medium. The nature of the solvent has little effect on the evolution of molecular weights (Fig. 4) and polydispersities (Fig. 5). For all the solvents tested, M n increases linearly with conversion and polydispersities are low (M w/m n a 1.2 at high conversions). However, Fig. 4 and 5 clearly show that DPE gives a slightly better control over M n (closer to theoretical values) and M w/m n (at high conversions) than the other two solvents.

5 The importance of the nature of initiator and solvent in atom transfer Conclusions In the ATRP of MMA catalyzed by CuBr/N-alkyl-2-pyridylmethanimine complexes, the level of control over molecular weights, polydispersities and polymerization rates is markedly affected by the nature of initiator and solvent. p-toluenesulfonyl chloride combined with CuBr is a better initiating system than the conventionally used ethyl 2-bromoisobutyrate/CuBr pair. The former induces faster initiation which enables to control M n quite efficiently during the whole polymerization. Additionally, the use of a mixed halide system maintains a low radical concentration, which minimizes the contribution of chain termination. Therefore, low polydispersities ((M w/m n a 1.2) are maintained during the polymerization. Aromatic oxygenated solvents other than phenols, e. g. diphenyl ether or 1,2-dimethoxybenzene, accelerate ATRP while maintaining an excellent control of the polymerization. The decrease in rates erved for p-tscl is compensated by using an ropriate solvent, such as diphenyl ether. Under identical initial concentration conditions, the p- TsCl/CuBr system in diphenyl ether gives identical polymerization rates (k p = 1.63 s 1 ) than EBiB/CuBr in xylene (k p = 1.60 s 1 ) c. Acknowledgement: The authors wish to thank the CNRS/ELF Atochem France research group for financial support of this work. Received: September 17, 1999 Revised: September 1, 2000 c Cf. ref., [8 e] supporting information, p. 6; reactions were run using N-butyl-2-pyridylmethanimine as the ligand. [1] Preliminary results related to this work were presented at the 215 th National Meeting of the American Chemical Society, Dallas, TX, March 1998: M. Destarac, J. Alric, B. Boutevin, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1998, 39 (1), 308. [2] J.-S. Wang, K. Matyjaszewski, J. Am. Chem. Soc. 1995, 117, [3] M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura, Macromolecules 1995, 28, [4] Controlled Radical Polymerization, K. Matyjaszewski, Ed., ACS Symposium Series 685, American Chemical Society: Washington, DC [5] (a) M. Destarac, B. Boutevin, K. Matyjaszewski, in: Controlled Radical Polymerization, K. Matyjaszewski, Ed., ACS Symposium Series 768, American Chemical Society: Washington, DC 2000, p. 234; (b) M. Destarac, K. Matyjaszewski, E. Silverman, B. Ameduri, B. Boutevin, Macromolecules 2000, 33, 4613; (c) M. Destarac, K. Matyjaszewski, B. Boutevin, Macromol. Chem. Phys. 2000, 201, 265; (d) M. Destarac, B. Boutevin, Macromol. Rapid Commun. 1999, 20, 641; (e) M. Destarac, J.-M. Bessière, B. Boutevin, J. Polym. Sci, Part A: Polym. Chem. 1998, 36, [6] M. Destarac, J.-M. Bessière, B. Boutevin, Macromol. Rapid. Commun. 1997, 18, 967. [7] V. Percec, B. Barboiu, Macromolecules 1995, 28, [8] (a) D. M. Haddleton, A. J. Shooter, A. M. Heming, M. C. Crossman, D. J. Duncalf, S. R. Morsley, in: Controlled Radical Polymerization, K. Matyjaszewski, Ed., ACS Symposium Series 685, American Chemical Society, Washington, DC 1998, p. 284; (b) D. M. Haddleton, C. B. Jasieczek, M. J. Hannon, A. J. Shooter, Macromolecules 1997, 30, 2190; (c) D. M. Haddleton, A. J. Clark, M. C. Crossman, D. J. Duncalf, A. M. Heming, S. R. Morsley, A. J. Shooter, Chem. Commun. 1997, 13, 1173; (d) D. M. Haddleton, D. Kukulj, D. J. Duncalf, A. M. Heming, A. J. Shooter, Macromolecules 1998, 31, 5201; (e) D. M. Haddleton, M. C. Crossman, B. H. Dana, D. J. Duncalf, A. M. Heming, D. Kukulj, A. J. Shooter, Macromolecules 1999, 32, [9] G. M. Di Renzo, M. Messerschmidt, R. Mülhaupt, Macromol. Rapid Commun. 1998, 19, 381. [10] J. Xia, K. Matyjaszewski. Macromolecules 1997, 30,7697. [11] F. Simal, A. Demonceau, A. F. Noels, Angew. Chem., Int. Ed. Engl. 1999, 38, 4, 538. [12] C. Granel, Ph. Dubois, R. Jérôme, Ph. Teyssié, Macromolecules 1996, 29, [13] H. Uegaki, Y. Kotani, M. Kamigaito, M. Sawamoto, Macromolecules 1997, 30, [14] K. Matyjaszewski, M. Wei, J. Xia, N. E. McDermott, Macromolecules 1997, 30, [15] T. Ando, M. Kamigaito, M. Sawamoto, Macromolecules 1997, 30, [16] G. Moineau, C. Granel, Ph. Dubois, R. Jérôme, Ph. Teyssié, Macromolecules 1998, 31, 562. [17] H. tom Dieck, K.-D. Franz, F. Hohmann, Chem. Ber. 1975, 108,163. [18] B. R. James, R J. P. Williams, J. Chem. Soc. 1961, [19] P. V. Bernhardt, J. Am. Chem. Soc. 1997, 119, 771. [20] M. Destarac, B. Boutevin, unpublished results. [21] K. Matyjaszewski, J.-L. Wang, T. Grimaud, D. A. Shipp, Macromolecules 1998, 31, [22] K. Matyjaszewski, S. Coca, S. G. Gaynor, M. Wei, B. E. Woodworth, Macromolecules 1998, 31, [23] K. Matyjaszewski, D. A. Shipp, J.-L. Wang, T. Grimaud, T. E. Patten, Macromolecules 1998, 31, [24] V. Percec, B. Barboiu, H.-J. Kim, J. Am. Chem. Soc. 1998, 120, 305. [25] K. Matyjaszewski, Y. Nakagawa, C. B. Jasieczek, Macromolecules 1998, 31, [26] J. Xia, X. Zhang, K. Matyjaszewski, Macromolecules 1998, 32, [27] S. Pascual, B. Coutin, M. Tardi, A. 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Volume 31, Number 23 November 17, Copper Triflate as a Catalyst in Atom Transfer Radical Polymerization of Styrene and Methyl Acrylate

Volume 31, Number 23 November 17, 1998 Copyright 1998 by the American Chemical Society Copper Triflate as a Catalyst in Atom Transfer Radical Polymerization of Styrene and Methyl Acrylate Brian E. Woodworth,