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Amphiphilic ABA triblock copolymers via combination of ROMP and ATRP in ionic liquid: Synthesis, characterization, and self-assembly
Reactive & Functional Polymers 68 (2008) 1601–1608
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Amphiphilic ABA triblock copolymers via combination of ROMP and ATRP in ionic liquid: Synthesis, characterization, and self-assembly Meiran Xie *, Yi Kong, Huijing Han, Jiaxin Shi, Liang Ding, Chunmei Song, Yiqun Zhang Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 15 June 2008 Received in revised form 17 September 2008 Accepted 18 September 2008 Available online 25 September 2008
Keywords: Ring-opening metathesis polymerization (ROMP) Atom transfer radical polymerization (ATRP) Block copolymers Ionic liquid Micelles
a b s t r a c t A new amphiphilic ABA triblock copolymer was synthesized by ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP) in ionic liquid (IL). The bromo-terminated macroinitiator was prepared via ROMP of norbornene derivative, which is compatible with ionic liquid [bmim][BF4], in conjunction with a chain transfer agent (CTA) in [bmim][BF4]. ROMP with CTA in [bmim][BF4] exhibited a different performance by changing the reaction conditions, and importantly there has been a good control over polymerization process and relatively low polydispersity index of polymer in [bmim][BF4]. Based on the telechelic macroinitiator, the amphiphilic ABA triblock copolymer was synthesized by ATRP in [bmim][BF4]. The prepared PDMAEMA–PNBEDE– PDMAEMA triblock copolymer can self-assemble spontaneously in H2O to form polymeric micelles, which was characterized by DLS, AFM, and TEM. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, amphiphilic block copolymers have attracted much attention due to their applications as stabilizers in emulsions or dispersions and drug delivery carriers, etc. [1–4]. The amphiphilic block copolymers containing hydrophilic poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA) block are of particular scientific interest, since they are sensitive to temperature and pH. The PDMAEMA block exhibits a low critical solution temperature (LCST) in the range of 38–40 °C and pH sensitivity characterized by a critical point at pH = 5.4 [5]. Atom transfer radical polymerization (ATRP) is commonly used to produce amphiphilic diblock or triblock copolymers with PDMAEMA as a hydrophilic block, such as PS-b-PDMAEMA [6], PSMA-b-PDMAEMA [7], PtBMA-b-PDMAEMA [8], PCLb-PDMAEMA [9], and PDMAEMA-b-PMMA-b-PDMAEMA [10]. However, as in any single polymerization mechanism,
* Corresponding author. Tel.: +86 21 62233493; fax: +86 21 62232414. E-mail address: [email protected] (M. Xie). 1381-5148/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2008.09.001
the range of suitable monomers and therefore resultant polymer blocks is somewhat limited. Many studies have been published on the synthesis of block copolymers through different polymerization mechanisms [11–16]. For example, ring-opening metathesis polymerization (ROMP), which is a well-known polymerization technique for the synthesis of highly functionalized block copolymers because the development of a well-defined ruthenium catalyst with improved control features and functional group tolerance [17], has been paired with anionic polymerization [11], nitroxide-mediated radical polymerization (NMP) [12], reversible addition–fragmentation chain transfer polymerization (RAFT) [13], and ATRP [14–16]. Hillmyer and coworkers synthesized ABA triblock copolymers of mechanistically incompatible monomer pairs such as 1,5-cyclooctadiene with styrene and acrylate monomers by a tandem ROMP and RAFT [13]. Grubbs and coworkers prepared difunctional macroinitiators by Ru-catalyzed ROMP of 1,5-cyclooctadiene in the presence of chain transfer agent (CTA) containing latent ATRP initiating sites [14]. Until now, however, few PDMAEMA-containing block copolymers have been synthesized by ROMP and ATRP.
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Ionic liquids (ILs) have received substantial interest as prospective environmentally friendly reaction media for a wide range of organic reactions and polymer synthesis [18–20]. The merits in the application of ILs as solvents in polymerization processes have been demonstrated. For example, in ATRP, the use of ILs as solvents facilitates separation of the polymer from residual catalyst and reduces the extent of side reactions [21–23], whereas scarce reports have been published for ROMP processes on the use of ILs as the reaction medium [24–26]. Our strategy for preparing amphiphilic ABA triblock copolymers is based on the combination of ROMP and ATRP in IL medium. The work here involves ROMP of norbornenyl derivative with the pendant of polyethylene glycol monoethyl ether in the presence of 1,2-bis(bromoisobutyryloxy)- 2-butene (1) as CTA to produce the bromo-ended telechelic polymers via the ruthenium-based catalyst in IL, namely 1-butyl-3-methylimidazoliumtetrafluoroborate ([bmim][BF4]) at different reaction conditions, and the synthesis of amphiphilic ABA triblock copolymers containing PDMAEMA block by ATRP using the bromoended telechelic ROMP polymers as macroinitiators in [bmim][BF4]. We have chosen the low-viscous [bmim][BF4], which is miscible with the norbornenyl derivative monomer, offering the possibility to control over ROMP reaction, but is immiscible with the ROMP product, facilitating the product recovery. To our knowledge, this is the first report on the synthesis of amphiphilic ABA triblock copolymers by ROMP and ATRP in [bmim][BF4]. In addition, the micellar characteristics of the amphiphilic block copolymers were investigated by using dynamic light scattering (DLS), atom force microscopy (AFM), and transmission electron microscopy (TEM) measurements. 2. Experimental 2.1. Materials 5-Norbornene-endo,endo-2,3-dicarboxylicanhydride, pentamethyldiethylenetriamine (PMDTA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDC HCl), triethylamine, a-bromoisobutyryl bromide, 2(dimethylamino)ethylmethacrylate (DMAEMA), cis-1,4butenediol, 2-(2-(2-(2-ethoxy)ethoxy)ethoxy)-ethanol, CuBr, benzylidene [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(tricyclohexylphosphine) ruthenium [(SIMes)(PCy3)Ru(CHPh)Cl2], and 4-dimethylaminopyridine (DMAP) were purchased from Aldrich, Fluka, or Alfa Aesar and used without further purification unless otherwise noted. Solvents were distilled over drying agents under nitrogen prior to use: chloroform from calcium hydride, tetrahydrofuran from sodium. 1,2-Bis(bromoisobutyryloxy)-2-butene (1) was prepared using a modified literature procedure [27]. 2.2. Characterization 1
H, 13C NMR spectra were recorded using tetramethylsilane (TMS) as an internal standard on a BRUKER AVANCE-500 spectrometer. Relative molecular weights
and molecular weight distributions were measured by gel permeation chromatography (GPC) system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR3, HR4, and HR5; 7.8 300 mm). GPC measurements were carried out at 35 °C using THF as the eluent with a flow rate of 1.0 ml/min. The system was calibrated with polystyrene standards. The size distribution was determined with a Malvern Zetasizer, A Zetasizer Nano ZS light scattering apparatus (Malvern Instruments, U.K.) with a He–Ne laser (633 nm, 4 mW) was used for DLS measurements. The Nano ZS instrument incorporates noninvasive backscatter (NIBS) optics with a detection angle of 173°. The z-average diameter and the polydispersity of the sample were automatically provided by the instrument using cumulant analysis. AFM observations were performed on SPM AJ-III atomic force microscope at a measure rate of 1.0005 Hz in the tapping mode, and the AFM images were obtained at room temperature in air. TEM was performed on a JEM-2100 microscope operating at an acceleration voltage of 200 kV. Elemental analysis was carried out with an Elementar vario EL 4578. 2.3. Synthesis of 1,2-bis (bromoisobutyryloxy)-2-butene (1) To a solution of a-bromoisobutyryl bromide (5.5 mL, 44.0 mmol) in CH2Cl2 (25 mL), which was cooled to 0 °C, was added dropwise a mixture of cis-1,4-butenediol (1.64 mL, 20.0 mmol), triethylamine (6.7 mL, 60.0 mmol), and CH2Cl2 (25 mL) under a nitrogen atmosphere over 20 min. The reaction was allowed to warm to room temperature and stirred for an additional 24 h. During this time a white precipitate formed. The precipitate was filtered off and the filtrate was subsequently washed with 1 M HCl, saturated NaHCO3 aq., and water. The organic layers were dried over Na2SO4, and concentrated to yield 6.72 g of yellow liquid (87%). 1H NMR (CDCl3): d (ppm) = 5.83 (m, 2H, CH=CH), 4.82 (d, 4H, 2 CH2CH=CH), 1.95 (m, 12H, 4 CH3). 13C NMR (CDCl3): d (ppm) = 171.34, 127.92, 61.46, 55.49, 30.76. 2.4. Synthesis of 5-norbornene-endo,endo-2,3-dicarboxylic acid bis-2-(2-(2-ethoxy ethoxy) ethoxy) ethyl ester (NBEDE, 2) 5-Norbornene-endo,endo-2,3-dicarboxylic anhydride (1.97 g, 12.0 mmol) and 2-(2-(2-(2-ethoxy)ethoxy) ethoxy) ethanol (5.0 g, 28.0 mmol) were dissolved in CH2Cl2 (80 ml), DMAP (0.70 g, 5.6 mmol) was added to the solution at 0 °C and the mixture was stirred for 48 h at room temperature. EDC HCl (2.68 g, 14.0 mmol) was added to the reaction mixture and stirred for further 48 h. The mixture was subsequently washed with 1 M HCl, saturated NaHCO3 aq., and water. The organic layer was separated and dried over anhydrous MgSO4 and concentrated. The residue was purified by silica gel chromatography (3:1 ethyl acetate/petroleum ether) yield 2.88 g of colorless liquid (48%). 1H NMR (CDCl3): d (ppm) = 6.24 (s, 2H, CH=CH), 4.06–4.22 (m, 4H, 2 COOCH2CH2), 3.49–3.65 (m, 24H, 6 CH2OCH2), 3.31(s, 2H, 2 CH), 3.16(s, 2H, bridge position), 1.29–1.46 (m, 2H, CH2), 1.18–1.21 (m, 6H,
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2 CH2CH3). 13C NMR (CDCl3): d (ppm) = 173.81, 136.33, 78.22, 72.12, 71.99, 71.89, 71.23, 70.50, 68.05, 68.05, 64.88, 50.02, 49.45, 47.79, 16.57. Elem. Anal. Calcd. for C25H42O10 (502.57): C, 59.74; H, 8.42. Found: C, 59.21; H, 8.56. 2.5. ROMP using 1 as CTA A typical procedure (Table 1, entry 1) is as follows. In a nitrogen-filled Schlenk tube, a solution of second generation Grubbs catalyst (1.2 mg, 1.4 lmol) in 0.2 ml of CHCl3 was added to a solution of monomer NBEDE 2 (702.8 mg, 1.4 mmol) and CTA 1 (108.0 mg, 0.28 mmol) in 1.8 ml of [bmim][BF4] to give an initial monomer concentration of 0.70 mol/l. The ratio of [2]0/[1]0 was 1000/200. The reaction mixture was stirred at 60 °C for 12 h and the polymer was precipitated by adding dropwise into water. The product was dried under vacuum to give the polymer (PNBEDE) as a soft solid. Yield: 90%. 1H NMR (DMSO-d6): d (ppm) = 5.64–5.16 (m, 2H, CH=CH), 4.61–4.50 (m, 4H, 2 CH2CH=CH), 4.21–3.86 (m, 4H, COOCH2CH2), 3.69– 3.38 (m, 24H, 6 CH2OCH2), 3.30–2.68 (m, 4H, 4 CH), 2.07–1.50 (m, CH2 + OCO(CH3)2), 1.15–1.01 (m, 6H, 2 CH2CH3). 2.6. Polymerization procedure for preparing triblock copolymers
Table 2 ROMP of monomer at different conditions in ionic liquida Entry
Time (h)
Temperature (°C)
Yieldb (%)
Mn,thc
Mn,GPCd
Mw/Mn
1 2 3 4 5 6 7
0.2 0.5 1.0 6.0 12.0 1.0 1.0
60 60 60 60 60 40 20
39 73 85 90 92 81 64
2400 4100 4700 4900 5000 4500 3600
4800 5300 6300 5600 5800 6700 7100
1.25 1.30 1.29 1.29 1.38 1.46 1.55
a IL = 1.8 ml, CHCl3 = 0.2 ml (dissolving for catalyst), [Cat] = 7.0 10 4 mol/l, [2]0/[1]0/[Cat]0 = 1000:100:1. b Isolated yields after purification. c Mn,th = (% Yield) ([2]/[1]) M(2) + M(1). d Determined by gel permeation chromatography in THF relative to monodispersed polystyrene standards.
Table 3 ATRP of DMAEMA using PNBEDE (Mn,NMR = 4400) as macroinitiator in ionic liquida Entry
M/I
Yieldb (%)
Mn,thc
Mn,NMRd
md
Mn,GPCe
Mw/Mn
1 2 3
70 210 420
70 75 78
12100 29200 55900
11600 33100 61500
23 91 181
9500 22000 48600
1.34 1.44 1.28
a
IL = 2.0 ml, T = 60 °C, t = 7 h. Isolated yields after purification. c Mn,th = (% yield) ([M]/[I]) M(DMAEMA) + Mn(PNBEDE). d Mn,NMR = (m M(DMAEMA)) 2 + Mn(PNBEDE) and m = (3SP/Sg) n were determined by 1H NMR spectroscopy, where M(DMAEMA) = 157 and Mn(PNBEDE) = 4400 were the molecular weight of monomer DMAEMA and macroinitiator PNBEDE, respectively. e Determined by gel permeation chromatography in THF relative to monodispersed polystyrene standards. b
A typical procedure (Table 3, entry 1) is as follows. A mixture of ROMP produced macroinitiator PNBEDE (96.0 mg, 0.022 mmol), DMAEMA (240.0 mg, 1.53 mmol), CuBr (6.3 mg, 0.044 mmol), PMDTA (23 ll, 0.11 mmol), and [bmim][BF4] (2.0 ml) was degassed by three freezepump-thaw cycles, and then heated at 60 °C for 7 h. After cooling to room temperature, the polymer was precipitated from the reaction mixture, filtered off and dissolved in THF, passed through a short silica column to remove the copper residues, and then precipitated from a large amount of petroleum ether, the product was dried under vacuum to give the triblock copolymer (PDMAEMA–PNBEDE–PDMAEMA) as a white solid. Yield: 70%. 1H NMR (CDCl3): d (ppm) = 5.71–5.25 (m, 2H, CH=CH), 4.58 (m, 4.36–3.94 (m, COOCH24H, 2 CH2CH=CH), CH2 + OCH2CH2N), 3.71–3.46 (m, 6 CH2OCH2), 3.39–2.69 (m, 4 CH), 2.56 (m, CH2NCH3), 2.27 (m, N(CH3)2), 2.08– 1.73 (m, CH2 + OCO (CH3)2 + CH2CCH3), 1.23–1.16 (t, 2 CH2CH3), 0.81–1.14 (d, CH2CCOCH3).
2.7. Sample preparation for DLS The copolymers were dissolved in THF at room temperature to make a polymer solution at 0.2 wt%. This copolymer solution (0.6 ml) was then added to 20 ml of selective solvent (Water) under vigorous stirring at a rate of 1 drop every 5 s to give a content of 50 mg/l followed by vigorous stirring for 30 min and standing overnight before the DLS measurements. All solvents and micelle solutions were filtered through filters (0.45 lm) to remove dust particles and the glassware was cleaned thoroughly with reagent-grade acetone.
Table 1 Effect of the molar ratio of monomer to chain transfer agent on ROMP in ionic liquida Entry
[2]/[1]
Yieldb (%)
Mn,thc
Mn,NMRd
nd
Mn,GPCe
Mw/Mn
1 2 3 4
1000/200 1000/100 1000/50 1000/20
90 92 86 89
2700 5000 10400 25500
4400 8400 12500 28000
8 16 24 55
4500 5800 9600 23000
1.51 1.38 1.43 1.45
a b c d e
IL = 1.8 ml, CHCl3 = 0.2 ml (dissolving for catalyst), [Cat] = 7.0 10 4 mol/l, [2]0/[Cat]0 = 1000, T = 60 °C, t = 12 h. Isolated yields after purification. Mn,th = (% yield) ([2]/[1]) M(2) + M(1), where M(2) = 502 and M(1) = 386 are the molar masses of monomer 2 and CTA 1, respectively. Mn,NMR = (Se/Sh) M(2) + M(1) and n = Se/Sh were determined by 1H NMR spectroscopy assuming Fn = 2.0. Determined by gel permeation chromatography in THF relative to monodispersed polystyrene standards.
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2.8. Sample preparation for AFM The morphologies of the micelles were observed by AFM. Ultrathin films of PDMAEMA91–PNBEDE8–PDMAEMA91 (Table 3, entry 2) were prepared by spin-coating a solution of known concentration on freshly cleaved mica surfaces, which were then air-dried at room temperature. The sample was examined at least twice under the same conditions, and the images were found to be very reproducible. 2.9. Sample preparation for TEM Samples for TEM measurement were prepared by dropping colloidal solution onto the copper grids coated with carbon, followed by air-drying. Additionally, the samples were not stained before measurement. 3. Results and discussion 3.1. Synthesis of monomer and CTA The CTA 1 was obtained by coupling the 2-butene-1,4diol with a-bromoisobutyryl bromide, and the synthetic procedure for this CTA is shown in Scheme 1. We chose to synthesize the bromo-terminated CTA 1 mainly due to proven effectiveness of this compound and its derivatives
as ATRP agents for the polymerization of styrene, methyl methacrylate, and a variety of other monomers [28]. It is well-known that monomer solubility can be increased through the introduction of either long side chains or bulky groups. We therefore synthesized monomer 2 containing 2-(2-(2-(2-ethoxy) ethoxy) ethoxy) ethyl ester groups, which possess good solubility in [bmim][BF4] and other organic solvents at room temperature. 2 was synthesized by the reaction of 5-norbornene-endo,endo-2,3-dicarboxylic anhydride with 1 equiv of 2-(2-(2-(2-ethoxy) ethoxy) ethoxy) ethanol, followed by the condensation of the formed half ester with another 1 equiv of 2-(2-(2-(2ethoxy) ethoxy) ethoxy) ethanol, EDC HCl was employed as a condensation agent, the coupling chemistry based on EDC HCl/DMAP-mediated esterification provided the target compounds in good yield under mild reaction conditions. The structure of the monomer was confirmed by 1 H, 13C NMR spectroscopy, and elemental analysis. We show that the strained olefin monomer 2 can be polymerized with ROMP and the ROMP products were used subsequentially as initiators for ATRP. 3.2. Synthesis and characterization of macroinitiator via ROMP ROMP of 2 with CTA 1 was carried out under an inert atmosphere at 60 °C for 12 h in 1.8 ml of [bmim][BF4] to
Scheme 1. Synthesis of triblock copolymer via ROMP and ATRP.
M. Xie et al. / Reactive & Functional Polymers 68 (2008) 1601–1608
give an initial monomer concentration of 0.70 mol/l. This is depicted in Scheme 1. Second generation Grubbs catalyst was used as an initiator because of its high functional group tolerance and thermal stability [17]. The polymerizations were performed by addition of a small amount of CHCl3 solution of catalyst to the monomer/CTA homogeneous solution of [bmim][BF4], and an increase in solution viscosity was observed visually, suggesting the formation of polymer. While a decrease of the viscosity was also observed over several hours, which is a consequence of chain transfer events that shorten the polymer chains [15]. One of the advantages of using ROMP with functionalized CTA for the preparation of telechelic polymers is the ability to control the polymer molecular weights through the ratio of monomer to CTA ([M]/[CTA]) [29]. A series of a-bromoisobutyryl-functionalized telechelic PNBEDEs were synthesized with [2]/[1]/[Cat] ratios ranging from 1000:200:1 to 1000:20:1. As shown in Table 1, all the polymerizations were proceeded with good conversion of monomer to produce the telechelic polymers (PNBEDEs). The molecular weights of PNBEDEs increased as decreasing of the amount of CTA 1, and thus suggested that 1 can really play a role of CTA to tune the molecular weight of the telechelic polymer, which would be used as the difunctional macroinitiator for the subsequent ATRP. The obtained polymer was characterized via 1H NMR spectroscopy (Fig. 1A). The alkene proton signals of the norbornene ring appeared approximately at 6.24 ppm. Upon ring-opening and subsequent polymerization, these proton signals shifted upfield to approximately 5.64– 5.16 ppm (Ha), this indicated the polymerization had occurred. The resonances of methylene protons adjacent to the ester bond (CH=CHCH2OCO) shift from 4.82 ppm in the CTA to 4.56 ppm (Hh) in the polymer. These character-
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istic proton resonances indicate that the CTA have been incorporated into the polymer chain. Furthermore, other types of end-groups, specifically terminal phenyl and ruthenium carbene species, were not observed in their expected regions of the 1H NMR spectrum (7.2–7.4 ppm for phenyl and 19.5–20 ppm for ruthenium carbenes). Hence, the result of terminal-group analysis by using 1H NMR spectroscopy has demonstrated that the polymer obtained under the condition of CTA/catalyst ratio of 200 was really a difunctionalized telechelic macroinitiator with a high degree of bromine end functionalization (Fn 2), which was employed according to that of previous studies where a CTA/catalyst ratio of 200 was necessary for the preparation of a telechelic polymer [30]. The molecular weights (Mn,NMR) of the polymers were estimated from 1H NMR spectra. By comparing the integral intensity of the methylene protons (He, 4 protons for each NBEDE unit) [Se/4H] from the monomer units at 4.10 ppm to the integral intensity of the methylene protons (Hh, 4 protons) [Sh/4H] from the telechelic end-groups at 4.56 ppm, the ratio of Se/Sh = n, which is the average degree of polymerization of the monomer 2, was used to determine the numberaverage molecular weight of the polymers, Mn,NMR = (Se/Sh) M(2) + M(1), and the values were shown in Table 1, which were comparatively in accordance with those of molecular weights (Mn,GPC) from GPC and the theoretical ones (Mn,th). The polydispersity index (PDI) of polymers was relatively low ranging from 1.38 to 1.51, and the representative GPC curve for PNBEDE is shown in Fig. 2A. The polymerization behaviors of monomer 2 with CTA 1 were investigated in detail in [bmim][BF4] at different reaction conditions and the results were presented in Table 2. For polymerization of 2 in the presence of CTA, it was found that the conversion of monomer was increased with
Fig. 1. 1H NMR spectra of PNBEDE (A, Mn,NMR = 4400) in DMSO-d6, and triblock copolymer PDMAEMA–PNBEDE–PDMAEMA (B, Mn,NMR = 33,100) in CDCl3.
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3.3. Synthesis, characterization, and self-assembly of amphiphilic triblock copolymer
Fig. 2. GPC curves for PNBEDE (A, Mn,NMR = 4400, PDI = 1.51), and PDMAEMA-PNBEDE-PDMAEMA (B, Mn,NMR = 11600, PDI = 1.34).
the elongation of reaction time, and they were 39% and 92% for 0.2 h and 12 h at 60 °C, respectively. It was seen from Table 2 that all the polymers had much or less higher molecular weights from GPC than those of calculating values, and the PDI values of polymers were slightly lower (1.25–1.38). Additionally, the polymerization efficiency was found to be strongly dependent on the solubility of the resulting polymer [26]. With progress of the polymerization, the reaction mixture became very viscous in several minutes but no precipitate formed and still kept a pseudo-homogeneous system, means the monomers and the catalytic living centers contacted fully and had a good compatibility with each other, which facilitates the polymerizations of 2 and allows for the preparation of telechelic polymers with lower PDI and better control over the polymerization in [bmim][BF4] than those in ordinary solvents such as CH2Cl2 [26]. Therefore, [bmim][BF4] could be considered as a suitable solvent in terms of the control over the polymerization and the molecular weight distribution. The polymerizations of 2 were also investigated at different temperature and the results were shown in Table 2. It was found that the conversions of monomer were increased with the temperature elevation from 20 to 60 °C in [bmim][BF4] (Table 2, entries 3, 6, and 7), and the molecular weights from GPC have a slight increasing and the PDI has a variation from 1.29 to 1.55.
For synthesis of triblock copolymers with PDMAEMA blocks, the ATRP of DMAEMA was carried out by using PNBEDE (Mn = 4400, PDI = 1.51) as macroinitiator and CuBr/PMDTA as catalyst/ligand at 60 °C for 7 h in [bmim][BF4]. The homogeneous reaction mixture was obtained by dissolving macroinitiator, CuBr, PMDTA, and DMAEMA in [bmim][BF4]. With progress of the reaction, the viscosity of reaction mixture increased, and the resulting polymers have not precipitated but dispersed well in the mixture over time. After cooling down and standing for a few minutes, the copolymers were aggregated to be a tacky soft solid, filtered off and dissolved in THF, and then passed through a short silica column to remove the copper residue. The block copolymers were precipitated from a large amount of petroleum ether. The copolymers with various lengths of PDMAEMA segment were prepared by varying the ratio of the macroinitiator to DMAEMA (Table 3). For confirming the formation of the copolymers, their 1 H NMR spectra were measured and the representative spectrum was shown in Fig. 1B. The formation of the copolymers were verified through the characteristic resonances of OCH2CH2N and N(CH3)2 protons from the block of PDMAEMA at 2.63–2.52 (Hp) and 2.35–2.24 ppm (Hq), respectively, and resonances of methyl protons of C(CH3) from the same block at 1.12–0.80 ppm (Hk). The integration area ratios of these characteristic resonances of 2.00:5.96:2.92 were well agreed with the ratios of corresponding protons of 2:6:3. The molecular weights of the copolymers were calculated by comparing the integral intensity of the methylene protons of PDMAEMA at 2.56 ppm (Hp, 2 protons for each DMAEMA unit) [SP/2H] to the integral intensity of the methyl protons from PNBEDE at 1.23–1.16 ppm (Hg, 6 protons for each NBEDE unit) [Sg/6H], the average degree of polymerization of the monomer DMAEMA can be written as follows: m = (SP/2H)/ (Sg/6H) n = (3SP/Sg) n, which was used to determine the number-average molecular weight of the polymers, Mn,NMR = (m M(DMAEMA)) 2 + Mn (PNBEDE) = ((3SP/Sg) n M(DMAEMA)) 2 + Mn(PNBEDE). The GPC curve in Fig. 2B showed the molecular weight and a monomodal molecular weight distribution of the triblock copolymer prepared from ROMP macroinitiator. After the polymerization, the molecular weight of the copolymer is shifted to high
Fig. 3. TEM microphotograph of spherical aggregates from amphiphilic triblock copolymer PDMAEMA91–PNBEDE8–PDMAEMA91 (Mn,NMR = 33,100).
M. Xie et al. / Reactive & Functional Polymers 68 (2008) 1601–1608
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Fig. 4. Micelles observed by AFM for PDMAEMA91–PNBDDE8–PDMAEMA91 (Mn,NMR = 33,100).
molecular weight position, and no peak attributed to the starting macroinitiator was observed, which means the ROMP macroinitiator used has been completely converted to the ABA triblock copolymer. The molecular weight of the triblock copolymer has a multiple accretion as the enlarged feed ratio of monomer to initiator, and the values of Mn obtained by GPC analysis were much lower than those of calculated Mn,th and those estimated by NMR (Mn,NMR) as shown in Table 3, this is likely due to the adsorption of PDMAEMA blocks onto the GPC column, which would result in an increase in retention time and lead to lower detected molecular weights [31,32]. The triblock copolymers in three cases (Table 3) have reasonable low molecular weight distributions with a PDI of less than 1.44. Further evidence for the formation of block copolymer was obtained by TEM analysis. As seen from Fig. 3, triblock copolymer of PDMAEMA91-PNBEDE8-PDMAEMA91 (Mn,NMR = 33100, PDI = 1.44 from entry 2 in Table 3) exhibited a microphase-separated structure, characteristic of copolymers having polar and nonpolar blocks. The polar block (PDMAEMA) is totally soluble in H2O. However, the nonpolar block (PNBEDE) is insoluble but precipitated in H2O. The solubility discrepancy of the PDMAEMA and PNBEDE blocks in PDMAEMA91–PNBEDE8–PDMAEMA91 triblock copolymer provides an opportunity of self-aggregation to form micelles in the solvent (H2O). The projection of the micelles is circular, and the micelles must be spherical. The micelles seem to have a light core and a dark shell. The overall diameter, including the core and shell, of the micelles is about 20 nm. The morphology of copolymer PDMAEMA91–PNBEDE8– PDMAEMA91 in THF/H2O was examined in preliminary investigations by AFM. Although AFM can only probe the topography of adsorbed species on a substrate surface, it is a facile technique available to produce high-resolution 3D topographic images without the need to stain the specimen. Fig. 4 shows AFM height image of the micelles. In the
AFM image, one can see many separate and densely deposited particles as spherical morphology on the mica surface, the spherical particles appear to be comparatively uniform in size, i.e., they are essentially monodisperse with an average diameter of 100 nm, while these nanoparticles exhibited collapsed shapes [33] on the mica surface with average height of 2.5 nm. There are seldom large particles in the image, perhaps caused by global micelles overlapping from THF/H2O evaporation and does not suggest agglomeration in mixed solvents. DLS measurements were used to estimate the hydrodynamic diameters of the micelles in aqueous solution. The hydrodynamic diameter of the micelles was 223 ± 0.16 nm with a narrow distribution and the polydispersity index is 0.209. The hydrodynamic diameter of the micelles was larger than the electron microscopy diameter [34] because DLS and TEM measure the diameters of the micelles in the solvent-swollen and dry states, respectively.
4. Conclusion In this work, we have demonstrated synthesis of a new amphiphilic ABA triblock copolymers based on the combination of ROMP and ATRP in [bmim][BF4]. ROMP with CTA in [bmim][BF4] exhibited a different performance by changing the reaction conditions, and there has been a good control over polymerization process and relatively low polydispersity index of polymer in [bmim][BF4]. The prepared PDMAEMA–PNBEDE–PDMAEMA triblock copolymer can self-assemble spontaneously in H2O to form polymeric micelles, which was confirmed by a DLS technique. The AFM and TEM measurements indicated that the spherical micelles formed from the triblock copolymer are composed of a light core of PNBEDE blocks, and PDMAEMA blocks stretching out into the H2O to form the densely packed dark shell.
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Acknowledgement We thank the National Natural Science Foundation of China (No. 20474017) for the financial support of this work. References [1] P. Zheng, X. Jiang, X. Zhang, W. Zhang, L. Shi, Langmuir 22 (2006) 9393. [2] B. Jeong, Y.H. Bae, D.S. Lee, S.W. Kim, Nature 388 (1997) 860. [3] Y. Kakizawa, K. Kataoka, Adv. Drug Deliv. Rev. 54 (2002) 203. [4] P. Ritz, P. Làtalovà, M. Janata, L. Toman, J. Krˇízˇ, J. Genzer, P. Vlcˇek, React. Funct. Polym. 67 (2007) 1027. [5] G. Burillo, E. Bucio, E. Arenas, G.P. Lopez, Macromol. Mater. Eng. 292 (2007) 214. [6] F. Marchal, A. Roudot, N. Pantoustier, P. Perrin, J. Daillant, P. Guenoun, J. Phys. Chem. B 111 (2007) 13151. [7] Z. Zhang, G. Liu, S. Bell, Macromolecules 33 (2000) 7877. [8] A.P. Narrainen, S. Pascual, D.M. Haddleton, J. Polym. Sci. Part A: Polym. Chem. 40 (2002) 439. [9] L. Mespouille, M. Vachaudez, F. Suriano, P. Gerbaux, W.V. Camp, O. Coulembier, P. Degée, R. Flammang, F.D. Prez, P. Dubois, React. Funct. Polym. 68 (2008) 990. [10] X. Zhang, K. Matyjaszewski, Macromolecules 32 (1999) 1763. [11] H. Katayama, Y. Fukuse, Y. Nobuto, K. Akamatsu, F. Ozawa, Macromolecules 36 (2003) 7020. [12] Y. Miura, Y. Sakai, I. Taniguchi, Polymer 44 (2003) 603. [13] M.K. Mahanthappa, F.S. Bates, M.A. Hillmyer, Macromolecules 38 (2005) 7890. [14] C.W. Bielawski, T. Morita, R.H. Grubbs, Macromolecules 33 (2000) 678.
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Amphiphilic ABA triblock copolymers via combination of ROMP and ATRP in ionic liquid: Synthesis, characterization, and self-assembly Meiran Xie *, Yi Kong, Huijing Han, Jiaxin Shi, Liang Ding, Chunmei Song, Yiqun Zhang Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 15 June 2008 Received in revised form 17 September 2008 Accepted 18 September 2008 Available online 25 September 2008
Keywords: Ring-opening metathesis polymerization (ROMP) Atom transfer radical polymerization (ATRP) Block copolymers Ionic liquid Micelles
a b s t r a c t A new amphiphilic ABA triblock copolymer was synthesized by ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP) in ionic liquid (IL). The bromo-terminated macroinitiator was prepared via ROMP of norbornene derivative, which is compatible with ionic liquid [bmim][BF4], in conjunction with a chain transfer agent (CTA) in [bmim][BF4]. ROMP with CTA in [bmim][BF4] exhibited a different performance by changing the reaction conditions, and importantly there has been a good control over polymerization process and relatively low polydispersity index of polymer in [bmim][BF4]. Based on the telechelic macroinitiator, the amphiphilic ABA triblock copolymer was synthesized by ATRP in [bmim][BF4]. The prepared PDMAEMA–PNBEDE– PDMAEMA triblock copolymer can self-assemble spontaneously in H2O to form polymeric micelles, which was characterized by DLS, AFM, and TEM. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, amphiphilic block copolymers have attracted much attention due to their applications as stabilizers in emulsions or dispersions and drug delivery carriers, etc. [1–4]. The amphiphilic block copolymers containing hydrophilic poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA) block are of particular scientific interest, since they are sensitive to temperature and pH. The PDMAEMA block exhibits a low critical solution temperature (LCST) in the range of 38–40 °C and pH sensitivity characterized by a critical point at pH = 5.4 [5]. Atom transfer radical polymerization (ATRP) is commonly used to produce amphiphilic diblock or triblock copolymers with PDMAEMA as a hydrophilic block, such as PS-b-PDMAEMA [6], PSMA-b-PDMAEMA [7], PtBMA-b-PDMAEMA [8], PCLb-PDMAEMA [9], and PDMAEMA-b-PMMA-b-PDMAEMA [10]. However, as in any single polymerization mechanism,
* Corresponding author. Tel.: +86 21 62233493; fax: +86 21 62232414. E-mail address: [email protected] (M. Xie). 1381-5148/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2008.09.001
the range of suitable monomers and therefore resultant polymer blocks is somewhat limited. Many studies have been published on the synthesis of block copolymers through different polymerization mechanisms [11–16]. For example, ring-opening metathesis polymerization (ROMP), which is a well-known polymerization technique for the synthesis of highly functionalized block copolymers because the development of a well-defined ruthenium catalyst with improved control features and functional group tolerance [17], has been paired with anionic polymerization [11], nitroxide-mediated radical polymerization (NMP) [12], reversible addition–fragmentation chain transfer polymerization (RAFT) [13], and ATRP [14–16]. Hillmyer and coworkers synthesized ABA triblock copolymers of mechanistically incompatible monomer pairs such as 1,5-cyclooctadiene with styrene and acrylate monomers by a tandem ROMP and RAFT [13]. Grubbs and coworkers prepared difunctional macroinitiators by Ru-catalyzed ROMP of 1,5-cyclooctadiene in the presence of chain transfer agent (CTA) containing latent ATRP initiating sites [14]. Until now, however, few PDMAEMA-containing block copolymers have been synthesized by ROMP and ATRP.
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Ionic liquids (ILs) have received substantial interest as prospective environmentally friendly reaction media for a wide range of organic reactions and polymer synthesis [18–20]. The merits in the application of ILs as solvents in polymerization processes have been demonstrated. For example, in ATRP, the use of ILs as solvents facilitates separation of the polymer from residual catalyst and reduces the extent of side reactions [21–23], whereas scarce reports have been published for ROMP processes on the use of ILs as the reaction medium [24–26]. Our strategy for preparing amphiphilic ABA triblock copolymers is based on the combination of ROMP and ATRP in IL medium. The work here involves ROMP of norbornenyl derivative with the pendant of polyethylene glycol monoethyl ether in the presence of 1,2-bis(bromoisobutyryloxy)- 2-butene (1) as CTA to produce the bromo-ended telechelic polymers via the ruthenium-based catalyst in IL, namely 1-butyl-3-methylimidazoliumtetrafluoroborate ([bmim][BF4]) at different reaction conditions, and the synthesis of amphiphilic ABA triblock copolymers containing PDMAEMA block by ATRP using the bromoended telechelic ROMP polymers as macroinitiators in [bmim][BF4]. We have chosen the low-viscous [bmim][BF4], which is miscible with the norbornenyl derivative monomer, offering the possibility to control over ROMP reaction, but is immiscible with the ROMP product, facilitating the product recovery. To our knowledge, this is the first report on the synthesis of amphiphilic ABA triblock copolymers by ROMP and ATRP in [bmim][BF4]. In addition, the micellar characteristics of the amphiphilic block copolymers were investigated by using dynamic light scattering (DLS), atom force microscopy (AFM), and transmission electron microscopy (TEM) measurements. 2. Experimental 2.1. Materials 5-Norbornene-endo,endo-2,3-dicarboxylicanhydride, pentamethyldiethylenetriamine (PMDTA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDC HCl), triethylamine, a-bromoisobutyryl bromide, 2(dimethylamino)ethylmethacrylate (DMAEMA), cis-1,4butenediol, 2-(2-(2-(2-ethoxy)ethoxy)ethoxy)-ethanol, CuBr, benzylidene [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(tricyclohexylphosphine) ruthenium [(SIMes)(PCy3)Ru(CHPh)Cl2], and 4-dimethylaminopyridine (DMAP) were purchased from Aldrich, Fluka, or Alfa Aesar and used without further purification unless otherwise noted. Solvents were distilled over drying agents under nitrogen prior to use: chloroform from calcium hydride, tetrahydrofuran from sodium. 1,2-Bis(bromoisobutyryloxy)-2-butene (1) was prepared using a modified literature procedure [27]. 2.2. Characterization 1
H, 13C NMR spectra were recorded using tetramethylsilane (TMS) as an internal standard on a BRUKER AVANCE-500 spectrometer. Relative molecular weights
and molecular weight distributions were measured by gel permeation chromatography (GPC) system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR3, HR4, and HR5; 7.8 300 mm). GPC measurements were carried out at 35 °C using THF as the eluent with a flow rate of 1.0 ml/min. The system was calibrated with polystyrene standards. The size distribution was determined with a Malvern Zetasizer, A Zetasizer Nano ZS light scattering apparatus (Malvern Instruments, U.K.) with a He–Ne laser (633 nm, 4 mW) was used for DLS measurements. The Nano ZS instrument incorporates noninvasive backscatter (NIBS) optics with a detection angle of 173°. The z-average diameter and the polydispersity of the sample were automatically provided by the instrument using cumulant analysis. AFM observations were performed on SPM AJ-III atomic force microscope at a measure rate of 1.0005 Hz in the tapping mode, and the AFM images were obtained at room temperature in air. TEM was performed on a JEM-2100 microscope operating at an acceleration voltage of 200 kV. Elemental analysis was carried out with an Elementar vario EL 4578. 2.3. Synthesis of 1,2-bis (bromoisobutyryloxy)-2-butene (1) To a solution of a-bromoisobutyryl bromide (5.5 mL, 44.0 mmol) in CH2Cl2 (25 mL), which was cooled to 0 °C, was added dropwise a mixture of cis-1,4-butenediol (1.64 mL, 20.0 mmol), triethylamine (6.7 mL, 60.0 mmol), and CH2Cl2 (25 mL) under a nitrogen atmosphere over 20 min. The reaction was allowed to warm to room temperature and stirred for an additional 24 h. During this time a white precipitate formed. The precipitate was filtered off and the filtrate was subsequently washed with 1 M HCl, saturated NaHCO3 aq., and water. The organic layers were dried over Na2SO4, and concentrated to yield 6.72 g of yellow liquid (87%). 1H NMR (CDCl3): d (ppm) = 5.83 (m, 2H, CH=CH), 4.82 (d, 4H, 2 CH2CH=CH), 1.95 (m, 12H, 4 CH3). 13C NMR (CDCl3): d (ppm) = 171.34, 127.92, 61.46, 55.49, 30.76. 2.4. Synthesis of 5-norbornene-endo,endo-2,3-dicarboxylic acid bis-2-(2-(2-ethoxy ethoxy) ethoxy) ethyl ester (NBEDE, 2) 5-Norbornene-endo,endo-2,3-dicarboxylic anhydride (1.97 g, 12.0 mmol) and 2-(2-(2-(2-ethoxy)ethoxy) ethoxy) ethanol (5.0 g, 28.0 mmol) were dissolved in CH2Cl2 (80 ml), DMAP (0.70 g, 5.6 mmol) was added to the solution at 0 °C and the mixture was stirred for 48 h at room temperature. EDC HCl (2.68 g, 14.0 mmol) was added to the reaction mixture and stirred for further 48 h. The mixture was subsequently washed with 1 M HCl, saturated NaHCO3 aq., and water. The organic layer was separated and dried over anhydrous MgSO4 and concentrated. The residue was purified by silica gel chromatography (3:1 ethyl acetate/petroleum ether) yield 2.88 g of colorless liquid (48%). 1H NMR (CDCl3): d (ppm) = 6.24 (s, 2H, CH=CH), 4.06–4.22 (m, 4H, 2 COOCH2CH2), 3.49–3.65 (m, 24H, 6 CH2OCH2), 3.31(s, 2H, 2 CH), 3.16(s, 2H, bridge position), 1.29–1.46 (m, 2H, CH2), 1.18–1.21 (m, 6H,
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2 CH2CH3). 13C NMR (CDCl3): d (ppm) = 173.81, 136.33, 78.22, 72.12, 71.99, 71.89, 71.23, 70.50, 68.05, 68.05, 64.88, 50.02, 49.45, 47.79, 16.57. Elem. Anal. Calcd. for C25H42O10 (502.57): C, 59.74; H, 8.42. Found: C, 59.21; H, 8.56. 2.5. ROMP using 1 as CTA A typical procedure (Table 1, entry 1) is as follows. In a nitrogen-filled Schlenk tube, a solution of second generation Grubbs catalyst (1.2 mg, 1.4 lmol) in 0.2 ml of CHCl3 was added to a solution of monomer NBEDE 2 (702.8 mg, 1.4 mmol) and CTA 1 (108.0 mg, 0.28 mmol) in 1.8 ml of [bmim][BF4] to give an initial monomer concentration of 0.70 mol/l. The ratio of [2]0/[1]0 was 1000/200. The reaction mixture was stirred at 60 °C for 12 h and the polymer was precipitated by adding dropwise into water. The product was dried under vacuum to give the polymer (PNBEDE) as a soft solid. Yield: 90%. 1H NMR (DMSO-d6): d (ppm) = 5.64–5.16 (m, 2H, CH=CH), 4.61–4.50 (m, 4H, 2 CH2CH=CH), 4.21–3.86 (m, 4H, COOCH2CH2), 3.69– 3.38 (m, 24H, 6 CH2OCH2), 3.30–2.68 (m, 4H, 4 CH), 2.07–1.50 (m, CH2 + OCO(CH3)2), 1.15–1.01 (m, 6H, 2 CH2CH3). 2.6. Polymerization procedure for preparing triblock copolymers
Table 2 ROMP of monomer at different conditions in ionic liquida Entry
Time (h)
Temperature (°C)
Yieldb (%)
Mn,thc
Mn,GPCd
Mw/Mn
1 2 3 4 5 6 7
0.2 0.5 1.0 6.0 12.0 1.0 1.0
60 60 60 60 60 40 20
39 73 85 90 92 81 64
2400 4100 4700 4900 5000 4500 3600
4800 5300 6300 5600 5800 6700 7100
1.25 1.30 1.29 1.29 1.38 1.46 1.55
a IL = 1.8 ml, CHCl3 = 0.2 ml (dissolving for catalyst), [Cat] = 7.0 10 4 mol/l, [2]0/[1]0/[Cat]0 = 1000:100:1. b Isolated yields after purification. c Mn,th = (% Yield) ([2]/[1]) M(2) + M(1). d Determined by gel permeation chromatography in THF relative to monodispersed polystyrene standards.
Table 3 ATRP of DMAEMA using PNBEDE (Mn,NMR = 4400) as macroinitiator in ionic liquida Entry
M/I
Yieldb (%)
Mn,thc
Mn,NMRd
md
Mn,GPCe
Mw/Mn
1 2 3
70 210 420
70 75 78
12100 29200 55900
11600 33100 61500
23 91 181
9500 22000 48600
1.34 1.44 1.28
a
IL = 2.0 ml, T = 60 °C, t = 7 h. Isolated yields after purification. c Mn,th = (% yield) ([M]/[I]) M(DMAEMA) + Mn(PNBEDE). d Mn,NMR = (m M(DMAEMA)) 2 + Mn(PNBEDE) and m = (3SP/Sg) n were determined by 1H NMR spectroscopy, where M(DMAEMA) = 157 and Mn(PNBEDE) = 4400 were the molecular weight of monomer DMAEMA and macroinitiator PNBEDE, respectively. e Determined by gel permeation chromatography in THF relative to monodispersed polystyrene standards. b
A typical procedure (Table 3, entry 1) is as follows. A mixture of ROMP produced macroinitiator PNBEDE (96.0 mg, 0.022 mmol), DMAEMA (240.0 mg, 1.53 mmol), CuBr (6.3 mg, 0.044 mmol), PMDTA (23 ll, 0.11 mmol), and [bmim][BF4] (2.0 ml) was degassed by three freezepump-thaw cycles, and then heated at 60 °C for 7 h. After cooling to room temperature, the polymer was precipitated from the reaction mixture, filtered off and dissolved in THF, passed through a short silica column to remove the copper residues, and then precipitated from a large amount of petroleum ether, the product was dried under vacuum to give the triblock copolymer (PDMAEMA–PNBEDE–PDMAEMA) as a white solid. Yield: 70%. 1H NMR (CDCl3): d (ppm) = 5.71–5.25 (m, 2H, CH=CH), 4.58 (m, 4.36–3.94 (m, COOCH24H, 2 CH2CH=CH), CH2 + OCH2CH2N), 3.71–3.46 (m, 6 CH2OCH2), 3.39–2.69 (m, 4 CH), 2.56 (m, CH2NCH3), 2.27 (m, N(CH3)2), 2.08– 1.73 (m, CH2 + OCO (CH3)2 + CH2CCH3), 1.23–1.16 (t, 2 CH2CH3), 0.81–1.14 (d, CH2CCOCH3).
2.7. Sample preparation for DLS The copolymers were dissolved in THF at room temperature to make a polymer solution at 0.2 wt%. This copolymer solution (0.6 ml) was then added to 20 ml of selective solvent (Water) under vigorous stirring at a rate of 1 drop every 5 s to give a content of 50 mg/l followed by vigorous stirring for 30 min and standing overnight before the DLS measurements. All solvents and micelle solutions were filtered through filters (0.45 lm) to remove dust particles and the glassware was cleaned thoroughly with reagent-grade acetone.
Table 1 Effect of the molar ratio of monomer to chain transfer agent on ROMP in ionic liquida Entry
[2]/[1]
Yieldb (%)
Mn,thc
Mn,NMRd
nd
Mn,GPCe
Mw/Mn
1 2 3 4
1000/200 1000/100 1000/50 1000/20
90 92 86 89
2700 5000 10400 25500
4400 8400 12500 28000
8 16 24 55
4500 5800 9600 23000
1.51 1.38 1.43 1.45
a b c d e
IL = 1.8 ml, CHCl3 = 0.2 ml (dissolving for catalyst), [Cat] = 7.0 10 4 mol/l, [2]0/[Cat]0 = 1000, T = 60 °C, t = 12 h. Isolated yields after purification. Mn,th = (% yield) ([2]/[1]) M(2) + M(1), where M(2) = 502 and M(1) = 386 are the molar masses of monomer 2 and CTA 1, respectively. Mn,NMR = (Se/Sh) M(2) + M(1) and n = Se/Sh were determined by 1H NMR spectroscopy assuming Fn = 2.0. Determined by gel permeation chromatography in THF relative to monodispersed polystyrene standards.
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2.8. Sample preparation for AFM The morphologies of the micelles were observed by AFM. Ultrathin films of PDMAEMA91–PNBEDE8–PDMAEMA91 (Table 3, entry 2) were prepared by spin-coating a solution of known concentration on freshly cleaved mica surfaces, which were then air-dried at room temperature. The sample was examined at least twice under the same conditions, and the images were found to be very reproducible. 2.9. Sample preparation for TEM Samples for TEM measurement were prepared by dropping colloidal solution onto the copper grids coated with carbon, followed by air-drying. Additionally, the samples were not stained before measurement. 3. Results and discussion 3.1. Synthesis of monomer and CTA The CTA 1 was obtained by coupling the 2-butene-1,4diol with a-bromoisobutyryl bromide, and the synthetic procedure for this CTA is shown in Scheme 1. We chose to synthesize the bromo-terminated CTA 1 mainly due to proven effectiveness of this compound and its derivatives
as ATRP agents for the polymerization of styrene, methyl methacrylate, and a variety of other monomers [28]. It is well-known that monomer solubility can be increased through the introduction of either long side chains or bulky groups. We therefore synthesized monomer 2 containing 2-(2-(2-(2-ethoxy) ethoxy) ethoxy) ethyl ester groups, which possess good solubility in [bmim][BF4] and other organic solvents at room temperature. 2 was synthesized by the reaction of 5-norbornene-endo,endo-2,3-dicarboxylic anhydride with 1 equiv of 2-(2-(2-(2-ethoxy) ethoxy) ethoxy) ethanol, followed by the condensation of the formed half ester with another 1 equiv of 2-(2-(2-(2ethoxy) ethoxy) ethoxy) ethanol, EDC HCl was employed as a condensation agent, the coupling chemistry based on EDC HCl/DMAP-mediated esterification provided the target compounds in good yield under mild reaction conditions. The structure of the monomer was confirmed by 1 H, 13C NMR spectroscopy, and elemental analysis. We show that the strained olefin monomer 2 can be polymerized with ROMP and the ROMP products were used subsequentially as initiators for ATRP. 3.2. Synthesis and characterization of macroinitiator via ROMP ROMP of 2 with CTA 1 was carried out under an inert atmosphere at 60 °C for 12 h in 1.8 ml of [bmim][BF4] to
Scheme 1. Synthesis of triblock copolymer via ROMP and ATRP.
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give an initial monomer concentration of 0.70 mol/l. This is depicted in Scheme 1. Second generation Grubbs catalyst was used as an initiator because of its high functional group tolerance and thermal stability [17]. The polymerizations were performed by addition of a small amount of CHCl3 solution of catalyst to the monomer/CTA homogeneous solution of [bmim][BF4], and an increase in solution viscosity was observed visually, suggesting the formation of polymer. While a decrease of the viscosity was also observed over several hours, which is a consequence of chain transfer events that shorten the polymer chains [15]. One of the advantages of using ROMP with functionalized CTA for the preparation of telechelic polymers is the ability to control the polymer molecular weights through the ratio of monomer to CTA ([M]/[CTA]) [29]. A series of a-bromoisobutyryl-functionalized telechelic PNBEDEs were synthesized with [2]/[1]/[Cat] ratios ranging from 1000:200:1 to 1000:20:1. As shown in Table 1, all the polymerizations were proceeded with good conversion of monomer to produce the telechelic polymers (PNBEDEs). The molecular weights of PNBEDEs increased as decreasing of the amount of CTA 1, and thus suggested that 1 can really play a role of CTA to tune the molecular weight of the telechelic polymer, which would be used as the difunctional macroinitiator for the subsequent ATRP. The obtained polymer was characterized via 1H NMR spectroscopy (Fig. 1A). The alkene proton signals of the norbornene ring appeared approximately at 6.24 ppm. Upon ring-opening and subsequent polymerization, these proton signals shifted upfield to approximately 5.64– 5.16 ppm (Ha), this indicated the polymerization had occurred. The resonances of methylene protons adjacent to the ester bond (CH=CHCH2OCO) shift from 4.82 ppm in the CTA to 4.56 ppm (Hh) in the polymer. These character-
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istic proton resonances indicate that the CTA have been incorporated into the polymer chain. Furthermore, other types of end-groups, specifically terminal phenyl and ruthenium carbene species, were not observed in their expected regions of the 1H NMR spectrum (7.2–7.4 ppm for phenyl and 19.5–20 ppm for ruthenium carbenes). Hence, the result of terminal-group analysis by using 1H NMR spectroscopy has demonstrated that the polymer obtained under the condition of CTA/catalyst ratio of 200 was really a difunctionalized telechelic macroinitiator with a high degree of bromine end functionalization (Fn 2), which was employed according to that of previous studies where a CTA/catalyst ratio of 200 was necessary for the preparation of a telechelic polymer [30]. The molecular weights (Mn,NMR) of the polymers were estimated from 1H NMR spectra. By comparing the integral intensity of the methylene protons (He, 4 protons for each NBEDE unit) [Se/4H] from the monomer units at 4.10 ppm to the integral intensity of the methylene protons (Hh, 4 protons) [Sh/4H] from the telechelic end-groups at 4.56 ppm, the ratio of Se/Sh = n, which is the average degree of polymerization of the monomer 2, was used to determine the numberaverage molecular weight of the polymers, Mn,NMR = (Se/Sh) M(2) + M(1), and the values were shown in Table 1, which were comparatively in accordance with those of molecular weights (Mn,GPC) from GPC and the theoretical ones (Mn,th). The polydispersity index (PDI) of polymers was relatively low ranging from 1.38 to 1.51, and the representative GPC curve for PNBEDE is shown in Fig. 2A. The polymerization behaviors of monomer 2 with CTA 1 were investigated in detail in [bmim][BF4] at different reaction conditions and the results were presented in Table 2. For polymerization of 2 in the presence of CTA, it was found that the conversion of monomer was increased with
Fig. 1. 1H NMR spectra of PNBEDE (A, Mn,NMR = 4400) in DMSO-d6, and triblock copolymer PDMAEMA–PNBEDE–PDMAEMA (B, Mn,NMR = 33,100) in CDCl3.
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3.3. Synthesis, characterization, and self-assembly of amphiphilic triblock copolymer
Fig. 2. GPC curves for PNBEDE (A, Mn,NMR = 4400, PDI = 1.51), and PDMAEMA-PNBEDE-PDMAEMA (B, Mn,NMR = 11600, PDI = 1.34).
the elongation of reaction time, and they were 39% and 92% for 0.2 h and 12 h at 60 °C, respectively. It was seen from Table 2 that all the polymers had much or less higher molecular weights from GPC than those of calculating values, and the PDI values of polymers were slightly lower (1.25–1.38). Additionally, the polymerization efficiency was found to be strongly dependent on the solubility of the resulting polymer [26]. With progress of the polymerization, the reaction mixture became very viscous in several minutes but no precipitate formed and still kept a pseudo-homogeneous system, means the monomers and the catalytic living centers contacted fully and had a good compatibility with each other, which facilitates the polymerizations of 2 and allows for the preparation of telechelic polymers with lower PDI and better control over the polymerization in [bmim][BF4] than those in ordinary solvents such as CH2Cl2 [26]. Therefore, [bmim][BF4] could be considered as a suitable solvent in terms of the control over the polymerization and the molecular weight distribution. The polymerizations of 2 were also investigated at different temperature and the results were shown in Table 2. It was found that the conversions of monomer were increased with the temperature elevation from 20 to 60 °C in [bmim][BF4] (Table 2, entries 3, 6, and 7), and the molecular weights from GPC have a slight increasing and the PDI has a variation from 1.29 to 1.55.
For synthesis of triblock copolymers with PDMAEMA blocks, the ATRP of DMAEMA was carried out by using PNBEDE (Mn = 4400, PDI = 1.51) as macroinitiator and CuBr/PMDTA as catalyst/ligand at 60 °C for 7 h in [bmim][BF4]. The homogeneous reaction mixture was obtained by dissolving macroinitiator, CuBr, PMDTA, and DMAEMA in [bmim][BF4]. With progress of the reaction, the viscosity of reaction mixture increased, and the resulting polymers have not precipitated but dispersed well in the mixture over time. After cooling down and standing for a few minutes, the copolymers were aggregated to be a tacky soft solid, filtered off and dissolved in THF, and then passed through a short silica column to remove the copper residue. The block copolymers were precipitated from a large amount of petroleum ether. The copolymers with various lengths of PDMAEMA segment were prepared by varying the ratio of the macroinitiator to DMAEMA (Table 3). For confirming the formation of the copolymers, their 1 H NMR spectra were measured and the representative spectrum was shown in Fig. 1B. The formation of the copolymers were verified through the characteristic resonances of OCH2CH2N and N(CH3)2 protons from the block of PDMAEMA at 2.63–2.52 (Hp) and 2.35–2.24 ppm (Hq), respectively, and resonances of methyl protons of C(CH3) from the same block at 1.12–0.80 ppm (Hk). The integration area ratios of these characteristic resonances of 2.00:5.96:2.92 were well agreed with the ratios of corresponding protons of 2:6:3. The molecular weights of the copolymers were calculated by comparing the integral intensity of the methylene protons of PDMAEMA at 2.56 ppm (Hp, 2 protons for each DMAEMA unit) [SP/2H] to the integral intensity of the methyl protons from PNBEDE at 1.23–1.16 ppm (Hg, 6 protons for each NBEDE unit) [Sg/6H], the average degree of polymerization of the monomer DMAEMA can be written as follows: m = (SP/2H)/ (Sg/6H) n = (3SP/Sg) n, which was used to determine the number-average molecular weight of the polymers, Mn,NMR = (m M(DMAEMA)) 2 + Mn (PNBEDE) = ((3SP/Sg) n M(DMAEMA)) 2 + Mn(PNBEDE). The GPC curve in Fig. 2B showed the molecular weight and a monomodal molecular weight distribution of the triblock copolymer prepared from ROMP macroinitiator. After the polymerization, the molecular weight of the copolymer is shifted to high
Fig. 3. TEM microphotograph of spherical aggregates from amphiphilic triblock copolymer PDMAEMA91–PNBEDE8–PDMAEMA91 (Mn,NMR = 33,100).
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Fig. 4. Micelles observed by AFM for PDMAEMA91–PNBDDE8–PDMAEMA91 (Mn,NMR = 33,100).
molecular weight position, and no peak attributed to the starting macroinitiator was observed, which means the ROMP macroinitiator used has been completely converted to the ABA triblock copolymer. The molecular weight of the triblock copolymer has a multiple accretion as the enlarged feed ratio of monomer to initiator, and the values of Mn obtained by GPC analysis were much lower than those of calculated Mn,th and those estimated by NMR (Mn,NMR) as shown in Table 3, this is likely due to the adsorption of PDMAEMA blocks onto the GPC column, which would result in an increase in retention time and lead to lower detected molecular weights [31,32]. The triblock copolymers in three cases (Table 3) have reasonable low molecular weight distributions with a PDI of less than 1.44. Further evidence for the formation of block copolymer was obtained by TEM analysis. As seen from Fig. 3, triblock copolymer of PDMAEMA91-PNBEDE8-PDMAEMA91 (Mn,NMR = 33100, PDI = 1.44 from entry 2 in Table 3) exhibited a microphase-separated structure, characteristic of copolymers having polar and nonpolar blocks. The polar block (PDMAEMA) is totally soluble in H2O. However, the nonpolar block (PNBEDE) is insoluble but precipitated in H2O. The solubility discrepancy of the PDMAEMA and PNBEDE blocks in PDMAEMA91–PNBEDE8–PDMAEMA91 triblock copolymer provides an opportunity of self-aggregation to form micelles in the solvent (H2O). The projection of the micelles is circular, and the micelles must be spherical. The micelles seem to have a light core and a dark shell. The overall diameter, including the core and shell, of the micelles is about 20 nm. The morphology of copolymer PDMAEMA91–PNBEDE8– PDMAEMA91 in THF/H2O was examined in preliminary investigations by AFM. Although AFM can only probe the topography of adsorbed species on a substrate surface, it is a facile technique available to produce high-resolution 3D topographic images without the need to stain the specimen. Fig. 4 shows AFM height image of the micelles. In the
AFM image, one can see many separate and densely deposited particles as spherical morphology on the mica surface, the spherical particles appear to be comparatively uniform in size, i.e., they are essentially monodisperse with an average diameter of 100 nm, while these nanoparticles exhibited collapsed shapes [33] on the mica surface with average height of 2.5 nm. There are seldom large particles in the image, perhaps caused by global micelles overlapping from THF/H2O evaporation and does not suggest agglomeration in mixed solvents. DLS measurements were used to estimate the hydrodynamic diameters of the micelles in aqueous solution. The hydrodynamic diameter of the micelles was 223 ± 0.16 nm with a narrow distribution and the polydispersity index is 0.209. The hydrodynamic diameter of the micelles was larger than the electron microscopy diameter [34] because DLS and TEM measure the diameters of the micelles in the solvent-swollen and dry states, respectively.
4. Conclusion In this work, we have demonstrated synthesis of a new amphiphilic ABA triblock copolymers based on the combination of ROMP and ATRP in [bmim][BF4]. ROMP with CTA in [bmim][BF4] exhibited a different performance by changing the reaction conditions, and there has been a good control over polymerization process and relatively low polydispersity index of polymer in [bmim][BF4]. The prepared PDMAEMA–PNBEDE–PDMAEMA triblock copolymer can self-assemble spontaneously in H2O to form polymeric micelles, which was confirmed by a DLS technique. The AFM and TEM measurements indicated that the spherical micelles formed from the triblock copolymer are composed of a light core of PNBEDE blocks, and PDMAEMA blocks stretching out into the H2O to form the densely packed dark shell.
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