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Diblock Copolymers Via Atom Transfer Radical Polymerization Utilizing Halide Exchange Technique

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Diblock Copolymers Via Atom Transfer Radical Polymerization Utilizing Halide Exchange Technique Polymer Journal, Vol. 37, No. 2, pp. 102–108 (2005) Synthesis of Amphiphilic Poly(ethylene oxide)-b-Poly(methyl methacrylate) Diblock Copolymers via Atom Transfer Radical Polymerization Utilizing Halide Exchange Technique y Xiaoyi SUN,1 Hailiang ZHANG,1; Lingjun ZHANG,1 Xiayu WANG,1 and Qi-Feng ZHOU2 1Institute of Polymer Science, Xiangtan University, Xiangtan 411105, Hunan Province, China 2Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China (Received September 14, 2004; Accepted November 17, 2004; Published February 15, 2005) ABSTRACT: Amphiphilic diblock copolymers with different molecular weights and low polydispersities were synthesized by atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) using PEO–Br as an ini- tiator, which was obtained by the esterification of poly(ethylene oxide) (PEO) with 2-bromoisobutyryl bromide. The polymerization proceeded in solution using halide exchange technique to control ATRP. Fourier transform infrared spectroscopy (FT-IR) and 1H NMR studies confirm the composition of PEO–Br macroinitiator and related diblock co- polymers. The results obtained by GPC analysis show that the number average molecular weight was increased versus monomer conversion and the polydispersities were quite low (<1:10), which is the character of a controlled/‘‘living’’ polymerization. Moreover, the crystallization behavior of PEO-b-PMMA block copolymers was studied by means of differential scanning calorimetry (DSC). It was found that the crystallization behavior of the block copolymers exhib- ited considerable differences in comparison to the neat PEO. The crystallization rate and the degree of crystallinity de- creased by an increase of PMMA content. [DOI 10.1295/polymj.37.102] KEY WORDS Atom Transfer Radical Polymerization /Diblock Copolymer/Poly(ethylene oxide)/ Poly(methyl methacrylate) / Halide Exchange / Amphiphilic block copolymers, consisting of a hy- block copolymers containing PEO and PS by ATRP drophobic polymer covalently linked to a hydrophilic using a PEO macroinitiator obtained by functionaliz- polymer, have attracted special attention in fundamen- ing the HO–PEO–OH with 2-bromopropionyl or 2- tal and applied research for their unique chain archi- chloropropionyl ester end group. The polymerization tecture and physical properties.1–5 In particular, block proceeded in a living manner to yield well-defined copolymers composed of hydrophilic poly(ethylene block copolymers, which are difficult to synthesize oxide) (PEO) have been extensively studied,4–6 owing by the sequential anionic method. Thus, esterification to its unique properties such as solubility, crystallini- of a hydroxyl group of a preformed macromolecule ty, flexibility, etc. with halogenated acyl halide proved to be an excellent Recent developments in controlled/‘‘living’’ radical method for producing macroinitiators suitable for polymerization have made it possible to control the ATRP.13,14 With this approach, a number of block polymerization to obtain polymers with predeter- copolymers were synthesized from PEO macroinitia- mined molecular weights, narrow molecular weight tor.4,15–23 For example, Ho¨cker and coworkers4,16 distributions, well-defined compositions, architec- prepared poly(ethylene oxide)-b-poly(hydroxyethyl tures, and functionalities. The applicability of control- methacrylate) (PEO-b-PHEMA) and PEO-b-PS block led/‘‘living’’ radical polymerization techniques in the copolymers. Lim et al.17 reported the synthesis of synthesis of block polymers is well documented in the block copolymers comprising PEO and poly(fluorooc- literature of those that use the nitroxide-mediated rad- tyl methacrylates). The ATRP of MMA in the synthe- ical polymerization (NMRP),7,8 atom transfer radical sis of triblock copolymers from a difunctional PEO polymerization (ATRP)9,10 and reversible addition- macroinitiator was also reported.20 Moreover, Chen fragmentation chain transfer (RAFT) polymeriza- et al.21–23 reported the synthesis of poly(ethylene tion.11 Among them, ATRP is a convenient method oxide)-b-poly(3-(trimethoxysilyl) propyl methacry- for the preparation of block copolymers where a tran- late) (PEO-b-PTMSPMA) diblock copolymers by sition metal compound acts as a carrier of a halogen ATRP using PEO–Br as a macroinitiator, and pre- atom in a reversible redox process. pared soluble nanocapsules through self-assembly of Recently, Kops et al.12,13 reported the synthesis of these block copolymers. In these works, the block co- yTo whom correspondence should be addressed (E-mail: [email protected]). 102 Synthesis of Amphiphilic Diblock Copolymers via ATRP CH3 O CH2Cl2 H3C O CH2 CH2 OH + Br C C Br n DMAP TEA CH3 O CH3 MMA H3COCH2 CH2 O C C Br n ° CH CuX/bipy 50 C 1 3 CH O 3 CH3 H COCH CH O C C CH C X 3 2 2 n 2 m ( X= Br or Cl ) C O 2 CH3 O CH3 Scheme 1. Synthetic route of PEO-b-PMMA diblock copolymer. polymers were prepared by ATRP using PEO–Br/ with water, once with 5% sodium carbonate solution, CuBr or PEO–Cl/CuCl initiator/catalyst system. and again with water before dried with anhydrous cal- However, the mixed halogen system PEO–Br/CuCl cium chloride and distilled. CuCl was prepared from has hardly been used in the synthesis of block copoly- CuCl2.2H2O, then stirred in glacial acetic acid and mers, which is expected to improve the molecular washed with methanol, and then dried in vacuum. weight distribution of the resulting polymer. Methyl methacrylate (MMA) was passed through a In this paper we describe how well-defined PEO-b- column with activated Al2O3 (120–160 mesh) to re- PMMA diblock copolymers with low polydispersities move the inhibitor, stored over CaH2, and then vacu- may be synthesized by the ATRP route using halide um distilled before polymerization. All other reagents exchange technique. The synthesis is outlined in (p-toluenesulfonyl chloride (p-TSCl), tetrahydrofuran Scheme 1 and first a monofunctional hydrophilic 2- (THF), 2,20-bipyridine (bipy), ether, ethanol, toluene) bromo isobutyrate PEO macroinitiator (1) is synthe- were purchased from commercial sources and used as sized, which is subsequently used in the preparation received without purification. of diblock copolymers PEO-b-PMMA (2) by being heated with methyl methacrylate in solution under Preparation of PEO–Br Macroinitiator ATRP conditions. The structures and molecular char- A 0.915 g (7.5 mmol) sample of DMAP in 20 mL of acteristics of the PEO-b-PMMA diblock copolymers dry methylene dichloride was mixed with 0.505 g were studied by GPC, FT-IR and 1H NMR. The crys- (5.0 mmol) of TEA. The solution was transferred into tallization behavior of the copolymers was investigat- a 250 mL three-neck round-bottom flask equipped ed using differential scanning calorimetry (DSC). with condenser, dropping funnel, gas inlet/outlet, and a magnetic stirrer. After cooling to 0 C, 2.875 g EXPERIMENTAL (12.5 mmol) of 2-bromoisobutyryl bromide in 20 mL of CH2Cl2 was added. To the formed yellow disper- Materials sion was added 25 g (5 mmol) of PEO in 100 mL of 2-Bromoisobutyryl bromide (98%, Alfa) was fresh- dry CH2Cl2 dropwise during 1 h under dry nitrogen; ly distilled by room temperature under vacuum. Tri- subsequently the temperature was allowed to rise to ethylamine (TEA) was refluxed with p-toluenesulfon- room temperature. The reaction was continued under yl chloride and then distilled. The resulting amine free stirring for 18 h. The solution was filtered, half of TEA was stored over CaH2. Before use, it was re- the solvent was evaporated, and the PEO–Br macroi- fluxed and distilled again. 4-(Dimethylamino)pyridine nitiator was precipitated in cold diethyl ether. After (DMAP) was recrystallized from toluene. Methylene dissolution in absolute ethanol, the solution was stored dichloride (CH2Cl2) was shaken with portions of con- overnight to recrystallize the product. The macroini- centrated H2SO4 until the acid layer remained color- tiator was filtered, washed with cold diethyl ether, less, then washed with water, aqueous 5% NaHCO3 and dried in vacuum. and with water again, and finally distilled from CaH2. PEO (Mn ¼ 5000, Alfa) was dried by azeotropic dis- ATRP of MMA Using PEO–Br Macroinitiator tillation with toluene before use. Traces of residual PEO-b-PMMA diblock copolymers were synthe- toluene were removed under vacuum. Chlorobenzene sized by solution polymerization in chlorobenzene. (99%, Acros) was washed with concentrated sulphuric In a typical run, a glass tube was charged with acid to remove thiophenes, followed by washed twice 0.25 g (0.05 mmol) of PEO–Br macroinitiator, 0.50 g Polym. J., Vol. 37, No. 2, 2005 103 X. SUN et al. Mp=22600 Mp=20400 (5 mmol) of MMA, 0.0050 g (0.05 mmol) of CuCl or PEO-Br 0.0072 g (0.05 mmol) of CuBr, 0.0234 g (0.15 mmol) of bipy and 1.125 g of chlorobenzene. After degassed with three freeze-pump-thaw cycles, the tube was sealed under vacuum and then immersed in a thermo- stated oil bath at 50 C. At a certain time, the tube was withdrawn and cooled to room temperature. The reac- tion mixture was diluted with THF. After passing the solution through a column with activated Al2O3 to re- move the catalyst, and precipitating into ether, PEO-b- PMMA diblock copolymer was obtained and dried in a vacuum oven overnight at room temperature. Characterization 16 17 18 19 20 21 22 23 The yield of the polymerization was determined Elution time (min) gravimetrically. FT-IR spectra of the macroinitiator Figure 1. GPC traces of PEO–Br and related PEO-b-PMMA and block copolymers were recorded on a PerkinElmer diblock copolymers catalized by CuBr/bipy. Spectrum One Fourier Transform Infrared Spectrome- ter using the KBr pellet technique. The number aver- age molecular weights Mn and polydispersity Mw=Mn and CuBr/bipy complex as a catalyst under ATRP were measured with a Waters 1515 gel permeation condition.
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