Synthesis of awell-defined Epoxy Copolymer by Atom Transfer Radical Polymerization

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1 RAPID COMMUNICATION Synthesis of awell-defined Epoxy Copolymer by Atom Transfer Radical Polymerization ZHIMING CHEN, HUILING BAO, JUZHENG LIU Department of Chemistry and Chemical Engineering, Southeast University, Nanjing , China Received 26 March 2001; accepted 19 July 2001 Keywords: ATRP; amphiphilic; epoxy copolymer INTRODUCTION Correspondence to: Z. Chen ( JournalofPolymerScience:PartA:PolymerChemistry,Vol.39, (2001) 2001 John Wiley &Sons, Inc. Nature has taken advantage of the rich liquid crystalline behavior of amphiphilic liquids to create ordered yet fluid biomembrane structures and to modulate the dynamic process in cells. 1 During the last decade, a vast amount of work has been done on the organization of colloidal metal and semiconductor particles such as Cd 1 y Mn y S, 2(a) Ag 2 S, 2(b) and CdSe 2(c) into nanosized assemblies with diameters ranging from 2to 10 nm. Various colloidal methods including Langmuir Blodgett film, 3(a) organometallic techniques, 3(b) twophase liquid liquid systems, 3(c) and both normal 3(d) and reverse micelles 3(e) were employed. However, organization of amphiphilic polymer assemblies has not been extensively reported despite their typical applications in schemes of nanoscale arrays for electronic and electro-optic devices, membrane filters with controllable pore sizes, and waveguides. 4 Amphiphilic polymers are traditionally prepared from living polymerization methodologies (e.g., anionic, 5(a) cationic, 5(b) and grouptransfer polymerization 5(c) ) that allow access to the synthesis of copolymers with well-defined molecular weights, well-controlled compositions, and functional architectures. However, difficulties emerge when polymerization employing low-purity monomers and solventsaswellasmonomerswithpolarand/orfunctional groups is carried out. As aresult, their industrial applications might be limited, whereas free-radical polymerization, one of the most important commercial polymerization processes, is absent these drawbacks. Nevertheless,theconventionalfree-radicalpolymerization method often disallowed access to synthesis of the well-defined copolymers for transfer and termination reactions. Until very recently, anovel controlled/ living radical polymerization method, termed atom transfer radical polymerization (ATRP), has received considerable attention because it is able to control the polymerization very well. This process involves areversibleequilibriumbetweenanalkylhalide(rox),as aninitiator,andthetransitionmetalcomplexactingas a carrier of the halogen atom, capable of effectively maintaining alow, stationary concentration of the active species, thus resulting in less significant termination reactions between radicals. Thus fast and dynamic equilibrium is established between the active and the dormant species, and apossible controlled/ living radical polymerization may occur. 6 Significant progress has been made in ATRP since it was first developed by several research groups. 7 Matyjaszewski et al. 7(a),(b) reported ATRP of styrene initiated by an alkyl chloride and catalyzed by the heterogeneous CuCl/2,2 -bipyridine complex. In addition, they improved the systems by employing more solubi- lizing4,4 -diheptyl-2,2 -bipyridine(dhbpy)or4,4 -di(5- nonyl)-2,2 -bipyridine (dnbpy) ligands. 8 With the use of multidentate aliphatic amines 9(a),(b) and piocolyl structure based amines 9(c) as ligands, well-controlled, copper-mediated ATRP was described by this group. Ruthenium-mediated ATRP was promoted by Sawamoto et al. 7(c),10(a) (e) More recently, they exam- 3726

2 RAPID COMMUNICATION 3727 ined the influences of structures of the initiating radical species and terminal halogens in the RuCl 2 (PPh 3 ) 3 - mediated ATRP of methyl methacrylate. 10(f) Arylsulfonyl halides 7(d),11(a) (c) and alkylsulfonyl halides, 11(c) an alternative type of initiators for ATRP, were discovered by Percec s group. With the use of these novel initiators, Percec and coworkers successfully carried out heterogeneous 7(d) and homogeneous 11(a),(d) ATRP of styrene(s), 7(d),11(a) (e) methacrylates, 11(b) (e) and acrylates. 11(b),(d),(e) In their later study, living radical polymerization catalyzed by Cu 2 O, Cu(0), and combinations of both in conjunction with bpy in the presence of phase-transfer catalysts (PTC) was discussed. 11(f) Their investigation showed that this process provided polymers with narrower M w /M n values than those of the homogeneous process and higher rates than those of the heterogeneous process. 11(f) In the past few years, ATRP has displayed a wide versatility in the range of transition metals, ligands, and initiators. Besides copper and ruthenium complexes, Ni-, 12 Fe-, 13 Pd-, 14 Re-, 15 Mo-, 16 Rh-, 11(a),17 and Cu(0) 18(a) -mediated ATRP were found to be very efficient. Accordingly, more ligands were also known to be suitable such as 1,10-phenanthroline or 4,7-diphenyl-1,10-phenanthroline, 18 substituted phosphines, 12(c),(e),13(b),(c),19 tetradentate Schiff base, 20 carbon monoxide, and cyclopentadienyl. 13(a),21 Meanwhile, various types of initiating systems, such as vinyl-containing, 22(a) typical radical, 22(b) hexafunctional iron tris(bipyridine), 4 and surface-bound initiators 22(c),(d) have been reported. In addition, the improved ATRP was provided by adding certain amounts of zerovalent metal. 23 Following the above-cited studies, the applicability of ATRP toward the synthesis of well-defined polymers with block, graft, star, polymer brushes, branched or hyperbranched structure has been demonstrated. 4,12(e),24 In particular, interest in the synthesis of star polymers has arisen 25 because of their unique shapes and characters. Arm-first 25(i),(j) and core-first 25(b),(d),(e) approaches were both used to make star polymers. For example, Sawamoto et al. 25(e) investigated the synthesis of star polymers from dichloroacetate-functionalized calixarenes with four, six, and eight initiating sites per molecule. The nodule method was employed by Matyjaszewski et al. 25(h),(j) in the synthesis of PSt and PtBA star polymers. So far, ATRP has not only succeeded in styrene, methacrylates, acrylates 26(a) and acrylonitrile 26(b) but also in a number of functional monomers, including 2-hydroxyethyl (meth)acrylate, 27 4-vinyl pyridine, 28 oxazoline monomer, 4 (dimethylamino)ethyl methacrylate (DMAEMA), 29 p-(chloromethyl)styrene (CMS), 30 2-((2-bromopropionyl)oxy)ethyl acrylate (BPEA), 31 and 3-O-methacryloyl-1,2:5,6-diO-isopropylidene-D-glucofuranose (MAIpGlc). 32 However, the ATRP of monomers containing oxirane groups, which has potential applications in advanced biotechnology [such as DNA separations, target medicines, enzyme immobilizations, and immunology determinations by the easy conversion of oxirane groups into a series of functional groups (e.g., OH, NH 2, and COOH) as shown in Scheme 1], has not been the subject of much attention. 33 Consequently, our study was directed toward the synthesis of poly(n-butyl methacrylate-b-glycidyl methacrylate) by ATRP based on the initiation system of (CuCl 2 /Cu/ethyl-2-bromopropionate/phenanthroline). Also discussed is the hydrolysis of block copolymers leading to formation of the amphiphilic block copolymers with a regular surface morphology observed in the TEM analysis. EXPERIMENTAL Materials n-butyl methacrylate (BMA), glycidyl methacrylate (GMA), and 4-methyl-2-pentanone (Shanghai Zhenxin, Co.) were distilled over MgSO 4 under vacuum just before use. Phenanthroline (Phen; Shanghai Reagent, Co.) was recrystallized twice from acetone and dried under argon. Ethyl-2-bromopropionate (EPN Br) (Acros organics) and CuCl 2 were commercially available and used without further treatment. Polymerization Scheme 1 Polymerization was carried out using Schlenk techniques under argon atmosphere. To a dry, 10-mL round-bottom Schlenk flask, with a magnetic stir bar, ligand (4 equiv for Phen), Cu (4 equiv), and CuCl 2 (2 equiv) were added. The flask was closed with a stopcock. The contents of the flask were then placed under vacuum and the flask was backfilled with argon three times to remove oxygen. The degassed monomer and solvent were then added by syringe technique. After

3 3728 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 39 (2001) the mixture was stirred at room temperature until it was homogeneous, the initiator was added and the flask was immersed in an oil bath kept at the desired temperature with rigorous stirring. At timed intervals, samples, which were used for the determination of percentage monomer conversion and molecular weight by GC and GPC, respectively, were withdrawn via syringe and diluted with THF. After a certain time, the flask was removed from the oil bath and the mixture was diluted with THF. The solution was passed over a column with neutral alumina to remove the catalyst, after which the rest of the solution was concentrated by rotary evaporation; the product was then dried at 60 C under vacuum. The block copolymerization was carried out by the following two procedures. By the macroinitiator technique, the first monomer was prepared and purified as described earlier, and then the block copolymer was obtained by the addition of the catalyst, second monomer, and macroinitiator, whereas by the successive addition technique, the second monomer was directly added until the first monomer had reached the desired conversion. Hydrolysis of the P(BMA-b-GMA) A g sample of P(BMA-b-GMA) was dissolved with 10 ml of THF, and 3 ml of H 2 O containing 10 drops of HCl (conc.) was added dropwise with magnetic stirring. After 30 min, the hydrolyzed copolymer was produced. The copolymer was finally isolated by rotary evaporation. Characterization The monomer conversion was determined by measuring the concentration of residual monomer on a Shimadzu GC-14 gas chromatograph (Shimadzu, Japan) equipped with a capillary column (DB-WAX; J&W Scientific, Folsom, CA). Molecular weight and its distribution were measured using a size-exclusion chromatograph (SEC) equipped with a mix-bed 5- mplgel column (Waters GPCV 2000, USA). Polystyrene standards were used for calibration. The functional group and surface morphology of the block copolymer were obtained by FTIR (Nicolet M7-JR 750, USA) and JEOL- 2000EX transmission electron microscope (JEOL, Peabody, MA) operated at 120 kv, respectively. RESULTS AND DISCUSSION In most solvents such as biphenyl ether, 6(b) xylene, 19 and toluene 34 as well as in the case of bulk, ATRP systems with the ligand of 2,2 -bipyridine are generally heterogeneous. To obtain homogeneous systems, substituted bipyridines 8 or picolyl structure based amines 9(c) were used. However, neither of them is commercially available. It is worth noting that by using suitable solvents, some authors carried out polymerization of MMA and styrene based on the commercially available bipyridine ligands, respectively. 35 Our investigation showed that 4-methyl-2-pentanone could also generate a homogeneous ATRP medium for polymerizations of BMA by using the more available and less expensive phen ligands. Hence, we decided to investigate synthesis of the amphiphilic block epoxy copolymer based on this system. It was previously reported 6(b) that Cu(I) complexes with bipy or phenanthroline ligands fall into two structural types, depending on the ligand-to-copper stoichiometry. Complexes of a 1:1 stoichiometry between bipy or phenanthroline and Cu(I) halides are either halogenbridged dimers, LCu( -X) 2 CuL, or 2:1 ligand-to-copper cations with a dihalocuprate counteranion, L 2 Cu CuX 2. Complexes of a 2:1 stoichiometry between bipy or phenanthroline and Cu(I) are monomeric complexes of the formula L 2 Cu X. The optimum ratio of the EPN Br/CuCl 2 /Cu/Phen initiation system employed here is 1/2/4/4 known in our study, indicating that the possible structure for the active species is (phen)cu( - X) 2 Cu(phen) or (phen) 2 Cu CuX 2 because reduction of 1 equivalent Cu(II) halide by Cu(0) regenerates 2 equivalent Cu(I) halides (i.e., L phen). Synthesis of P(BMA-b-GMA) Using Successive Addition of Monomers n-butyl methacrylate was polymerized first and GMA second, as described earlier. The polymerization process of PBMA to the block copolymer is shown in Table I. The MWDs of PBMA precursor and copolymer are narrow, and the molecular weights agreed well with the predicted values, that is, M n th W 0 / ROX 0 Conv. (1) where W 0 and [ROX] 0 represent the initial mass of the monomer in grams and the initial concentration in moles, respectively. The phenomenon demonstrated an efficient initiation process. As depicted in Figure 1, addition of GMA to PBMA precursor led to a shift to higher molecular weight of the peak position, except for the absence of any shoulder peak at the elution position of the precursor. All of these results were characteristic of a controlled/ living polymerization, despite the possible existence of a statistical copolymer between GMA and BMA, because of the existence of residual BMA monomers after the first step. The reason is that this segment is so small relative to the block copolymers

4 RAPID COMMUNICATION 3729 Table I. at 90 C Successive ATRP Copolymerization of GMA Initiated with PBMA Precursor in 4-Methyl-2-pentanone Number [PBMA]/ [PGMA] Time First Monomer (h) Time Second Monomer (h) M n (cal) M n (sec) M w /M n BR-1 88/ ,260 12, BR-2 69/ ,520 13, BR-3 24/ ,870 12, BR-4 15/ ,810 13, with a clean GMA sequence that it has almost no effect on the properties of the block copolymers. Synthesis of P(BMA-b-GMA) Using PBMA as Macroinitiator PBMA with M n (sec) 15,280 and M w /M n 1.28 was used as the macroinitiator to produce block copolymer with GMA by ATRP technique. Assuming a fast initiation, K eq k act P Cu II X k deact Cu I PX insignificant termination reactions, and a steady concentration of propagating radicals, the following rate laws were derived [eqs. (2) (4)]. 6(a),(b),7(b) A fast equilibrium is a necessary condition to observe the low polydispersity in the controlled/ living free-radical polymerization. 6(a) (2) number of active species remains constant throughout the reaction (i.e., kp[p n ] constant). The slope of the kinetic plot, equal to the value of k app according to eq. (4), is s 1, which means the initiation was fast and there was no destruction of the active species. A small deviation was observed during the process, however, presumably as a result of certain amounts of chain transfer or termination. Figure 3 shows that the number-average molecular weight (M n ) increases linearly with the monomer conversion, indicating a constant concentration of growing chains throughout the reaction. Moreover, the MWD was very narrow (MWD 1.2), displaying a very effective deactive process for this initiation system, although the polydispersity increased slightly toward the end of the reaction, again presumably the result of some side reactions. It can be clearly found that the polymerization was well controlled by the efficient initiation system, despite some insignificant deviations, according to Figures 2 and 3. Moreover, as reported, 6(a) for pure bromide-based systems, more side reactions may occur, whereas for the mixed-halide systems (ROX, CuOY, where X Y), CuOBr predominates over CuOCl at equilibrium concentrations and, conversely, COCl is formed in preference to COBr, which resulted in faster deactivation of the propagating radicals. Thus the CuCl 2 /Cu/EPN Br mixed-halide system, which proved Cu I R p k app P M k p K eq In M (3) Cu II X As shown in Figure 2, the ATRP of GMA proceeds through approximately first-order kinetics with respect to monomer concentration, which suggested that the ln M 0 / M k app t (4) Figure 1. Stack plot of SEC traces of P(BMA-b-GMA) block copolymer and PBMA precursor.

5 3730 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 39 (2001) completely after the hydrolysis. Instead, a broad and strong peak located at cm 1, characteristic of the hydroxy group, appeared; thus it can be confirmed that the oxirane group was completely hydrolyzed to the hydroxy group. TEM Analysis of P(BMA-b-GMA) Surface Morphology Figure 2. Kinetic plot of the ATRP of GMA using PBMA as the macroinitiator at 90 C. Compositions: [GMA]/[Macroinitiator]/[CuCl 2 ]/[Cu]/[Phen] 140/1/2/ 4/4 in 4-methyl-2-pentanone. to give better control over the polymerization, was employed for preparation of the macroinitiator. Hydrolysis of P(BMA-b-GMA) Block Copolymers The typical FTIR spectra of the block copolymers before and after the hydrolysis show that the peaks characteristic of the oxirane group located at cm 1 (symmetric stretching vibration of the ring), cm 1 (asymmetric stretching vibration of the ring), and cm 1 (COH bending vibration) disappeared In the bulk, microphase separation of block copolymers usually forms materials with discrete morphologies that are dependent on the temperature, the degree of polymerization, the interaction parameter, and the polymer structure. 4 In addition, surface properties of polymer materials are of particular importance in many areas, ranging from biotechnology to advanced microelectronics. 4 Therefore, the block copolymer P(BMA-b-GMA) was expected to exhibit microphase separation because of its amphiphilic properties. Figure 4 shows a TEM micrograph of the surface morphology of the hydrolyzed P(BMA-b-GMA) film prepared by casting from the THF solution. In this micrograph, the dark and light regions correspond to the hydrolyzed PGMA and PBMA domains, respectively, because phosphotungstic acid selectively stains hydrophilic segments. Regular and spherical PBMA domains with a diameter of about 8 nm embedded in the continuous phase of the hydrolyzed PGMA domains can be observed clearly, which indicates the formation of a welldefined amphiphilic molecular structure. Figure 3. Dependency of M n and MWD on monomer conversion for the polymerization of P(BMA-b-GMA) block copolymer using PBMA as the macroinitiator at 90 C. Compositions: [GMA]/[Macroinitiator]/[CuCl 2 ]/[Cu]/[Phen] 140/1/2/4/4 in 4-methyl-2- pentanone.

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