Synthesis and Characterization of Amphiphilic AB2 Star Copolymers Comprised of Poly(ethylene oxide) and Polystyrene Blocks by ATRPDairen Lu, Kui Xu, Yun Wang, Kang Tao , and Ruke Bai*Department of Polymer Science and Engineering, University of Science andTechnology of China, Hefei, 230026, P. R. ChinaE-mail: bairk@; Tel: 0086-551-3600722, Fax: 0086-551-3631760AbstractA new macroinitiator MPEO-Br2 was synthesized by the reaction of monomethoxy end-capped poly(ethylene oxide) with 3, 5-bis(bromomethyl)benzoic chloride, and then used to initiate the polymerization of styrene at 110o C with CuBr/bpy as catalyst system. The results of polymerization show the characteristics of living free radical polymerization. There exists a linear relationship between ln([M]0/[M]) and polymerization time, and the molecular weights increase linearly as the increase of monomer conversion with low polydispersity. Furthermore, well-defined amphiphilic AB2 star copolymers were prepared and confirmed from the structure1and molecular weight of the copolymers characterized by H-NMR and GPC.Keywords: AB2 star copolymer; Atom transfer radical polymerization; Polystyrene1. IntroductionAmphiphilic block copolymers are widely explored for their ability to form organized micellar aggregates with different morphologies in polymer solution, in bulk state, and in thin polymer layers at surfaces and interfaces [1]-[3]. One of the most-exploited amphiphilic block copolymers is composed of poly(ethylene oxide) and polystyrene blocks. Many reports are available on the micellar structures of these PEO-PS block copolymers in water [4] and organic solvents [5]-[7] with variable molecular weight and chemical composition.PEO-PS copolymers were originally prepared by sequential living anionic polymerization of styrene and ethylene oxide [8]. However, the procedure had to be carried out under anhydrous conditions and extensive purification of monomers, initiator and solvent was also required. Fortunately, over the past decades, great progress has been made on living free radical polymerization, such as nitroxide-mediated stable free-radical polymerization (NMP) [9], atom transfer radical polymerization (ATRP) [10], reversible addition-fragmentation chain transfer (RAFT) [11]. Compared to anionic polymerization, living free radical polymerization does not require strict purification of monomers and solvent and are suitable of a variety of vinyl monomers, tolerant the presence of many functional groups. This allows polymer scientists to design and obtain well-defined (co)polymers conveniently. Since then, many types of PEO-PS copolymers were prepared, especially with ATRP route. The strategy is mainly based on end-functionalized PEO as macroinitiator to initiate the polymerization of styrene to form well-defined PEO-PS copolymers. For example, PS-b-PEO-b-PS triblock copolymer was reported by Kops group [12]. Difunctional macroinitiators were synthesized by reacting dihydroxyl PEO with 2-bromopropionyl chloride or 2-chloropropionyl chloride and subsequently used as initiatorsfor the polymerization of styrene. In a similar way, monofunctional macroinitiator was prepared by monomethoxy poly(ethylene oxide) (MPEO) and 2-chloro-2-phenylacetyl chloride and used as initiator for the syntheses of PEO-b-PS or PEO-b-PMMA diblock copolymers [13]. Pan et al. [14] prepared H-shaped PEO-PS copolymers choosing PEO as the middle segment from tetrafunctional macroinitiator which was achieved by reacting dihydroxy PEO with 2, 2-bis(methyl-2- bromopropionate) propionyl chloride. Moreover, star-shaped and dendrimer-like copolymers made of PEO and PS were synthesized by sequential anionic polymerization of ethylene oxide and atom transfer radical polymerization (ATRP) of styrene [15].In this paper, we synthesized a new macroinitiator containing 3, 5-dibromomethyl benzoate at the end of MPEO and studied the polymerization behavior of styrene from the macroinitiator under ATRP condition. The results indicate that well-difined amphiphilic AB 2 star copolymers comprised of PEO and PS have been synthesized successfully with the new macroinitiator.H 3COOH n H 3H 3COO n CH 2CH 2C PS BrOBrm11Scheme 1. Synthetic route of the PEO-PS AB 2 star copolymers2. Results and DiscussionIn order to prepare AB 2 star block copolymers, we designed and synthesized a new macroinitiator. The synthetic route is outlined in Scheme 1. Monomethoxy poly(ethylene oxide) (MPEO) with molecular weight 1900 reacted in benzene for 24 hours with excess 3,5-bis(bromo- methyl)benzoic chloride which was premade according to literature [16]. After concentrating the reaction solution and precipitating the polymer carefully in diethyl ether, MPEO-Br 2 was obtained and characterized by means of 1H-NMR and GPC. The spectrum of MPEO-Br 2 was shown in Figure 1, the signals at 3.61 ppm and 3.49 ppm are ascribed to (CH 2CH 2O) and OCH 3 in MPEO respectively, the aromatic protons assign at 7.98 ppm and 7.60 ppm and methylene protons connected to bromine and oxygen of the ester group appear at 4.48 ppm. The result of the NMR spectrum demonstrates the successful synthesis of the target product. GPC trace shows only one peak with molecular weight 2508 and polydispersity (PDI) 1.02.8642Chemical shiftsFigure 1. 1H-NMR spectrum of macroinitiatorTable 1. Polymerization results of styrene using PEO-macroinitiatorNo. Time(h) Conv.(%)a)M n,theo b)M n,NMR c)M n,GPC d)M w /M n d)f e)1 2 50 6.66×1037.80×103 1.03×104 1.20 0.80 2 4 65 8.00×1039.09×103 1.24×104 1.10 0.83 3 6 78 9.16×103 1.02×104 1.40×104 1.13 0.89 4 8839.61×103 1.42×104 1.14 5 10 87 9.97×103 1.46×104 1.16 6 12 891.02×1041.13×1041.47×1041.19 0.87a Conversion of styrene was calculated on the basis of gravimetric method.bM n,theo = ([St]0/[I]0)×Conv.×104 + M n,PEO , where [St]0 and [I]0 are initial molar concentrations of styrene andmacroinitiator, 104 and M n,PEO are molecular weight of styrene and macroinitiator respectively.cCalculated according to Equ. (1).d Obtained from gel permeation chromatography (GPC).eInitiation efficiency of the polymerization of styrene from macroinitiator.Polymerization of styrene was performed under ATRP condition in the presence of themacroinitiator, MPEO-Br 2. Since PEO is soluble in styrene, the reaction was conducted in bulk in polymerization tube at 110o C with bpy/CuBr as catalyst system. The molecular weights of the obtained copolymers were determined by 1H-NMR and GPC, and results are listed in Table 1. It is obviously that molecular weights increase with monomer conversion, and polydispersitis (PDI) are relative low (M w /M n ≤1.20). It also can be found that initiation efficiencies are acceptable in the case of ATRP process (f ≥0.80). Figure 2 shows the GPC traces of the star copolymers, which are symmetrical and move towards higher molecular weight with increasing conversion.987654321Chemical shiftsFigure 3.1H-NMR spectrum of the obtained AB2 star copolymersThe result of the kinetic investigation for the polymerization of styrene is shown in Figure 4. It can be observed that the relationship between ln([M]0/[M]) and the polymerization time is a straight line, which indicates that the polymerization is a first order reaction with respect to monomer concentration and that the number of free radicals keeps constant during the polymerization and termination is neglectable. Furthermore, in Figure 5, we can see that the molecular weights increase linearly as the increase of monomer conversion, which is generally observed in controlled/living free radical polymerization processes. In addition, the molecular weight distribution is narrow even at high conversion of the polymerization. All of these evidences discussed above demonstrate that the polymerization process has the characteristics of living radical polymerization and well-defined PEO-(PS)2 copolymers have been prepared successfully with the macroinitiator.l n ([M ]0/[M ])C o n v e r s i o nPolymerization time (h)Figure 4. Semilogarithmic kinetic plot of styrene conversion versus polymerization timeM w /M nM nconversion (%)Figure 5. The relationship of molecular weights and polydipersities with styrene conversionConclusionIn summary, a new macroinitiator MPEO-Br 2 has been prepared and demonstrated to be effective for atom transfer radical polymerization of styrene in bulk. The results show that the molecular weights increase linearly as the increase of monomer conversion with the narrowmolecular weight distribution even at high conversion. Moreover, the linearity of the plot of ln([M0]/[M]) vs. time indicates that the polymerization follows first order kinetics with respect to the monomer concentration and the concentration of growing chain species remains constant.. All of these evidences imply that the process proceed by a living free radical polymerization mechanism. In addition, amphiphilic AB2 star copolymers, PEO-(PS)2, have been obtained via ATRP from the macroinitiator, which are expected to be interesting precursors for supramolecular self assembly.Experimental partMaterialsMonomethoxy poly(ethylene oxide) with M n = 1900 was dried by azeotropic distillation of its toluene solution. Styrene was passed through a column of neutral alumina to remove inhibitor. All other chemical reagents were analytical grade and used as received without further purification.Synthesis of PEO-Br2 macroinitiator3, 5-bis(bromomethyl)benzoic chloride was prepared according to literature[16]. A typical synthetic procedure for PEO-macroinitiator is described as follows. To anhydrous PEO (1.9g, 1.0mmol) in benzene (20ml), 3, 5-bis(bromomethyl)benzoic chloride (2.0g, 6.0mmol) in benzene (10ml) was added dropwise at 0o C and then stirred at 50o C for 24h. The solution was concentrated under reduced pressure and precipitated thrice in diethyl ether. The precipitate was dried at 30o C for 24h to provide MPEO-Br2 (1.84g, yield: 84%).Polymerization of styreneThe polymerization of styrene was performed in bulk with polymerization tubes under ATRP condition. A polymerization tube was charged with CuBr, 2, 2’-bipyridine, MPEO-Br2 and styrene, molar ratio: [St]/[MPEO-Br2]/[CuBr]/[bpy] = 86:1:2:6, and then degassed by three freeze- vacuum-thaw cycles. After being sealed under vacuum, the tube was immersed into an oil bath and the reaction mixture was thermostated at 110o C for a predetermined time. The tube was rapidly cooled by ice water and polymer was dissolved in THF when the polymerization finished. After passing the solution through neutral alumina and precipitating in methanol, the precipitated polymer was dried in a vacuum oven at 40o C for 24h.Characterization1H nuclear magnetic resonance (1H-NMR) spectra were recorded on a Bruker DMX-300 instrument with CDCl3as the solvent and tetramethylsilane (TMS) as an internal standard. Molecular weight and molecular weight distribution were measured by means of a waters 150C gel permeation chromatography (GPC) equipped with 103, 104, 105 Å columns, THF(1.0ml/min) is used as the eluent, and the calibration was carried out based on polystyrene standard.AcknowledgementsThe authors are grateful for financial support from the National Natural Science Foundation of China (No. 20474059) and Specialized Research Fund for the Doctoral Program of Higher Education (No.20040358017).References[1] (a) Green, P. F.; Christensen, T. M.; Russell,T. P.; Jerome, R. Macromolecules 1989, 22, 2189. (b) Xu, T.;Goldbach, J. T.; Russell, T. P. Macromolecules 2003, 36, 7296. (c) Kim, S. H.; Misner, M. J.; Xu, T.;Kimura, M.; Russell, T. P. AdV. Mater.2004, 16, 226-231.[2] (a) Tsukruk V. V. Adv. Mater. 2001, 13, 95. (b) Ornatska, M.; Jones, S. E.; Naik, R. R.; Stone, M.; Tsukruk,V. V. J. Am. Chem. Soc. 2003, 125, 12722.(c)Tsukruk, V. V.; Ornatska, M.; Sidorenko, A. Prog. Org. 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