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Iron complexes of bidentate nitrogen donor ligands as catalysts for Atom Transfer Radical Polymerization of styrene Rossella Ferro and Alfonso Grassi Dipartimento di Chimica, Università di Salerno, 84084 Fisciano (Salerno) The selective synthesis of polymeric materials with well designed properties and molecular architecture is a topic of increasing interest. Living polymerization is a process in which the rate of irreversible termination or chain transfer reactions is negligible in comparison to chain growth rate, permitting a control of the average molecular weights of the polymer synthesized. Among the polymerization techniques, the radical processes are of increasing interest because they allow homo and co- polymerization, in rather mild conditions, of a large variety of monomers ranging from unsaturated hydrocarbons to polar monomers. Recently atom transfer radical polymerization (ATRP) is becoming worthy of particular attention for the possibility of obtaining the living polymerization by means of a simple and reversible reaction of the growing radical polymeryl with a metal halide.1 A typical ATRP mechanism is depicted in Scheme 1. RX kact + Mtn Y / Ligand R + X kdeact Mtn+1 Y /Ligand Monomer (M) n P X + Mt Y / Ligand kact P kdeact + X Mtn+1 Y /Ligand +M kp Scheme 1 A metal mediated halide exchange process establishes a fast equilibrium between the growing (Pn•) and dormant (Pn-X) polymer chains. Typically this equilibrium is shifted in an ATRP process towards the dormant species: an extremely low radical concentration is thus obtained which slows down the rate of the termination reactions characteristic of a radical process, e.g. disproportionation and radical coupling. The fast halide exchange is controlled by the electrochemical redox potential between the two redox states of the metal centre, and ensures all polymer chains are growing at the same rate, furnishing excellent control over the radical polymerization. 1 A different approach, leading to the same result, is the so called reverse ATRP.2 This technique follows the same ATRP reactions of Scheme 1 but differs for the initiation reaction. In this case a typical radical initiator (e.g. 2,2’-azo-bisisobutyronitrile (AIBN), 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED)) can be used and left to react with the metal halide in high oxidation state, permitting the polymerization to proceed later on as in Scheme 1. Reverse ATRP exhibits two advantages: i) the radical initiator is generally less toxic and less expensive than the organic halide used in direct ATRP; ii) the metal catalyst is more soluble, thermally and air-stable because of the highest oxidation state. There is an increasing interest in the Iron mediated ATRP catalysis because of the biocompatibility and low cost of the metal complexes, and the easy accessibility to a wide number of complexes with different ligands.3 Previous studies by Gibson et al.4 showed that bidentate ligands containing immino nitrogen donors provided stable complexes suitable for polymerization of styrene and methyl methacrylate under ATRP protocol. At redox potential higher than nearly 200 mV (vs Ag/AgCl; E1/2= 177 mV, ∆E=130 mV for ferrocene used as external standard) a poor control over the radical polymerization was actually found. However the average molecular weight and molecular weight distribution of the polymer products were not found completely satisfactory: in particular the latter is far from the typical value of 1-1.1 characteristic of a living process. We investigated the performances in reverse ATRP of three Iron complexes, namely Fe(bpzm)Cl3 (1), Fe(bipy)Cl3 (2) and Fe(box)Cl3 (3) bearing the bidentate neutral nitrogen ligands, 3,5-dimethyl-bispirazolylmethane (bpzm), 2,2’-bipyridile (bipy), 1,1-bis[4,4dimethyl-1,3-oxazolin-2yl]ethane (box) respectively (Scheme 2). Aiming to assess the electronic properties which assure good performances in ATRP of these complexes, we choose the bpzm, bipy and box ligands as representative examples in which the steric and Lewis base properties can be tuned with simple chemical modifications. O O N N N N bpzm N N N N bipy box Scheme 2. The preliminary screening of 1-3 in ATRP of styrene in bulk showed that 3 is the best one in terms of monomer conversion and polydispersity index: actually a 64% monomer 2 conversion in 18 h at 120°C was observed and the corresponding polymer exhibited a Mw/Mn value of 1.30. Polymerization runs carried out in toluene solution to ensure the homogeneous conditions, produced polystyrene with Mw/Mn of 1.15, that is one of the lowest value for Iron based catalysts. Steric and electronic effects have been invoked to justify the different performances in ATRP of the Iron complexes bearing bi- and tri-dentate ligands with nitrogen donors. In the reaction of the radical polymeryl with 1-3 we consider negligible the steric effects because of the planar disposition of the ligand, and thus we focused our attention on the electronic properties of the complexes. The binding constant Kass of bpzm, bipy and box to FeCl3 were determined by UV-Vis measurements. The stoichiometry of the complexes (ligand/FeCl3 ) was determined by Job’s method5 and the Kass values were calculated using the Scatchard’s method:6 the values are given in Table 1. The highest affinity to the Fe(III) metal centre was observed for the box ligand which produces stable complexes with 1:1 stoichiometry in acetonitrile solution at room temperature. The bpzm, bipy ligands are weaker Lewis base and lead to an equilibrium reaction between the free and coordinated ligand. The electrochemical parameters ∆E and E1/2 of the Fe(II)/Fe(III) redox couple define the reversibility of the redox process and the corresponding potential: both parameters contribute in determining the molar concentration of the radical species in the ATRP protocol. The electrochemical parameters E1/2 and ∆E of 1-3, measured by Ciclic Voltammetry (VC), are reported in Table 1. Table 1. Binding constants and electrochemical parameters for complexes 1-3 Complex Stoichiometrya FeCl3/ligand Kassb E1/2 (V) ∆E (V) 1 1/1 1980 ± 20 -0.865 0.110 2 1/1 6100 ± 200 -0.651 0.114 3 1/1 10100 ± 300 -0.552 0.125 a Evaluated by Job’s method ([FeCl3]+[ligand]=1*10-4 M in CH3CN, T=25°C). b Calculated by Scatchard’s method. ([FeCl3]=4*10-5 M in CH3CN, T=25°C). 3 The box ligand produces the most stable complex 1 which is oxidized at higher potential than 2 and 3: the latter complexes are easily oxidized and exhibit lower stability and reversibility in agreement with the very modest performances in ATRP of styrene. Matyjaszewsky et al.7 proposed an ideal range of E1/2 (-0.5 ÷ +0.4 V vs Ag/AgCl) for the Copper catalysts to be used in ATRP. On the basis of Gibson and our results we are thus able to define a range of electrochemical potential E1/2 between -0.55 and ~ +0.2 V vs Ag/AgCl electrode (E1/2= 177 mV, ∆E=130 mV for ferrocene used as external standard) in which good performances of the Iron complexes can be expected in ATRP of styrene. To support the composition and chemical structure of the catalyst 1 we isolated and characterized the structure of the Fe(box)Cl2 complex by single crystal X-ray diffraction (see Figure 1). This complex, that is the reduced partner of 1, is still monomeric and exhibit a pseudotetrahedal geometry with a 1:1 metal to ligand stoichiometry. Figure 1. X-Ray crystal structure of Fe(box)Cl2. To show the potentiality of combining the ATRP of polar monomers with metal catalyzed polymerization of unsaturated hydrocarbons we synthesized diblock copolymer of syndiotactic polystyrene with polymethylmethacrylate using a two steps procedure. The first step is a typical Ziegler-Natta polymerization of styrene to yield syndiotactic polystyrene using the Cp*TiBn3/B(C6F6)3 catalyst under pseudoliving conditions. The polymerization run is terminated with 2-bromine-2-phenylethylisocyanate that produces a polystyryl macromonomer with a benzyl bromide chain ending. In the second step methyl methacrylate is polymerized using the living ATRP protocol with the CuBr/PMDETA8 catalyst in the presence of the brominated macroinitiator (Scheme 3). sPS-PMMA diblock samples with different monomer block lengths were obtained and characterized by NMR spectroscopy, thermal analysis and X-ray powder diffraction. By slow cooling of hot THF solutions of the diblock copolymer, with block lengths of Mn (PS) = 1.3x104 Da and Mn (PMMA)= 1.2x104 Da a gel was obtained at room temperature. 4 The treatment of this gel with supercritical CO2 removes the clathrated solvent leading to a white solid sponge. A THF solution 0.04% of this sample was deposited by spin coating on silica surface. The AFM image reveals the presence in the thin film of micelles of about 30 nm of diameters (see Figure 2). Br N C O O + Br + N H Ti Toluene T = 0°C, t = 30 min Bn Ph Bn O H 3 CO Ph Ph CuBr / PMDETA Anisolo T = 90°C, t = 17h O N H Ph Ph Ph Ph R R R = COOCH 3 Scheme 3 The thermal analysis showed that the micella’s core consists of crystalline syndiotactic polystyrene and the outer sphere of amorphous PMMA. Interestingly the Xray powder diffraction analysis suggested that the polystyrene core is in the nanoporose δ crystalline form containing empty cavities in which small molecules can be hosted.9 Thus the main challenge of this project is the possibility of using these micelles as shuttle of small molecule to be used in drug delivery and analytical application. Figure 2. AFM image of sPS-PMMA diblock copolymer. 5 References 1. 2. 3. 4. 5. 6. 7. 8. 9. (a) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (b) Kamigaito, M; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689. (a) Moineau, G.; Dubois, P.; Jérôme, R.; Senninger, T.; Teyssié, P. Macromolecules 1998, 31, 545. (b) Wang, J.-S.; Matyjaszewski, K. Macromolecules 1995, 28, 7572. (a) Chen, X.-P.; Qiu, K.-Y. J. Appl. Polym. Sci. 2000, 77, 1607. (b) Zhu, S. M.; Yan, D. Y.; Zhang, G. S. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 765. (c) Wang, G.; Zhu, X.; Cheng, Z.; Zhu, J. J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 2912. (a) Gibson, V. C.; O’Reilly, R. K.; Reed, W.; Wass, D. F.; White, A. J. P.; Williams, D. J. Chem. Comm. 2002, 1850. (b) Gibson, V. C.; O’Reilly, R. K. ; Wass, D. F. ; White, A. J. P.; Williams, D. J. Macromolecules 2003, 36, 2591. (c) O’Reilly, R. K.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2003, 125, 8450. (a) Job, P. Ann. Chim. 1928, 9, 113. (b) Gil, V. M. S.; Oliveira, N. C. J. Chem. Ed. 1990, 473. Scatchard, G. Ann N. Y. Acad. Sci. 1949, 51, 660. Qiu, J.; Matyjaszewski, K.; Thouin, L.; Amatore, C. Macromol. Chem. Phys. 2000, 201, 1625. Liu, S.; Sen, A. Macromolecules 2000, 33, 5106. Milano, G.; Venditto, V.; Guerra, G.; Cavallo, L.; Ciambelli, P.; Sannino, D. Chem. Mater. 2001; 13(5); 1506. 6