Well-Defined Polymeric Ionic Liquids with an Upper Critical Solution

Nov 19, 2012 - The polymer has a cloud point occurring near body temperature, at 32 °C, providing a versatile platform for preparing biomedical mater...
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Well-Defined Polymeric Ionic Liquids with an Upper Critical Solution Temperature in Water Hayato Yoshimitsu, Arihiro Kanazawa, Shokyoku Kanaoka, and Sadahito Aoshima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

ABSTRACT: An upper critical solution temperature (UCST)type phase separation in water was achieved using well-defined polymeric ionic liquids (ILs) with imidazolyl groups in their side chains, prepared based on living cationic polymerization using a cationogen/Et1.5AlCl1.5 initiating system with 1,4-dioxane as an added base. Aqueous solutions of the polymers with tetrafluoroborate as counteranions showed sharp and reversible UCST-type phase separation at 5−15 °C. The effect of polymer concentration, chain-end groups, and molecular weight on the phase separation temperature suggests that the phase separation resulted from interpolymer electrostatic interactions. Other polymeric ILs with SbF6− also showed a lower critical solution temperature-type phase separation in various organic solvents.

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their application in controlled drug loading and release.19−22 These studies have demonstrated that UCST-type phase separation in water requires strong but heat-dependent interpolymer interactions, such as those induced by hydrogen bonds. A recent trend is to design UCST-type polymers with pendants that can form multiple concerted hydrogen bonds. Aoki et al. synthesized poly(6-(acryloyloxymethyl)uracil) with hydrogen bond donors (N−H) and acceptors (CO) in single units that formed polymer complexes, resulting in a phase separation in cold water.23 Polymers with ureido groups that underwent a UCST-type transition under physiologically relevant conditions were synthesized by Maruyama et al.24 The UCST-type phase separation behavior of poly(N-acryloylglycinamide) and its random copolymers was studied by Onishi et al.25 and Agarwal et al.26,27 In addition, Lutz et al. reported the synthesis of poly(N-acryloylasparaginamide) by RAFT polymerization and its UCST-type phase separation behavior in water and physiological media.28 Very recently, Agarwal et al. presented a general approach for the synthesis of polymers that exhibit UCST-type phase separation in water.29 They demonstrated that UCST-type phase separation could be achieved using classical homo- and copolymer systems such as poly(methacrylamide) and poly(acrylamide-co-acrylonitrile). Despite this recent progress in the field, only a few UCST-type polymers prepared by living polymerization have been reported. We have synthesized various stimuli-responsive polymers such as thermo-,30 pH-,31 and photoresponsive32 poly(vinyl ether)s with controlled primary structures by base-assisting living

INTRODUCTION Thermoresponsive polymers1,2 are classified into two general types: those with a negative temperature dependence, which causes the collapse of polymer chains above a critical temperature (lower critical solution temperature, LCST), and those with a positive temperature dependence, which induces phase separation below an upper critical solution temperature (UCST). A number of hydrophilic polymers have been found to exhibit LCST-type phase separation in water. For example, poly(Nisopropylacrylamide) (PNIPAM),3 poly(vinyl ether)s,4 cellulose derivatives,5 poly(ethylene oxide) derivatives,6 poly(oxazoline)s,7 and poly(N-vinylcaprolactam)8 undergo LCST-type phase separation. The most extensively investigated thermoresponsive polymer is undoubtedly PNIPAM. The polymer has a cloud point occurring near body temperature, at 32 °C, providing a versatile platform for preparing biomedical materials, such as drug carriers,2 cell adhesive motifs,9 and microfluidics.10 In contrast, polymers showing UCST-type phase separation in water are relatively uncommon. One of the most investigated UCST-type polymers is poly(methacrylate) derivative, which features side-chain sulfobetaine groups that become insoluble in water at low temperatures due to electrostatic interactions among the sulfobetaine groups.11 Poly(sulfobetaine) can be successfully synthesized in a controlled manner via group transfer polymerization, atom transfer radical polymerization, and reversible addition−fragmentation chain transfer (RAFT) polymerization to facilitate its use in biomedical materials.12−17 Another well-known UCST-type polymer solution is a mixture of poly(acrylic acid) and poly(acrylamide) in water, in which hydrogen bonds between the carboxy groups and the amide groups in the side chains contribute to the collapse of the polymer chains.18 Interpenetrating polymer networks composed of poly(acrylic acid) and poly(acrylamide) have been studied for © XXXX American Chemical Society

Received: August 20, 2012 Revised: November 11, 2012

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cationic polymerization. Living polymerization facilitate research on the structure−property relationship because various stimuliresponsive polymers with precise structures are obtained by this technique. For example, the effect of the molecular weight and the chain-end structure on the LCST-type phase separation behavior of poly(vinyl ether)s in water was reported in our previous study.33 Furthermore, the self-assembly of block copolymers containing these stimulus-responsive segments leads to sensitive and reversible physical gelation.34−39 The thermoresponsivity employed in those systems exhibit LCSTtype phase separation behavior in water or UCST-type phase separation behavior in organic solvents. Polymers synthesized by living cationic polymerization and showing UCST-type phase separation behavior in water have not yet been achieved. Combining these opposite thermoresponsive segments, i.e., the UCST-type segment and LCST-type segment, the design of block copolymers that show unique self-assembly in response to temperature will be achieved. Those backgrounds motivated us to conduct further research on the UCST-type polymers. Ionic liquids (ILs) have been reported to exhibit unique interactions between cations and anions or ions and solutes, which are expected to contribute to interesting phase separation behaviors, including those of IL/water,40,41 IL/organic solvent,42 and IL/polymer43,44 systems. Additionally, polymeric ILs have attracted considerable attention as fascinating polymer materials with properties that can be easily tuned via simple anion exchange.45,46 These specific features motivated us to examine the phase separation behavior of polymeric ILs; in fact, we have recently demonstrated that some polymeric ILs exhibit LCSTtype phase separation behavior in chloroform.47 In this study, we report the synthesis and solubility characteristics of a series of polymeric ILs achieved by the living cationic polymerization of precursor polymers and successive chemical modifications. The structures of the polymers examined are shown in Chart 1. An interesting feature of the polymeric

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EXPERIMENTAL SECTION

Materials. 2-Chloroethyl vinyl ether (CEVE: TCI; >97.0%) and isobutyl vinyl ether (IBVE: TCI; >99.0%) were washed with 10% aqueous sodium hydroxide solution and then with water, dried overnight over potassium hydroxide (pellets), and distilled twice over calcium hydride before use. 1,4-Cyclohexanedimethanol divinyl ether (DVE) was washed and distilled under reduced pressure in a manner similar to that of CEVE and IBVE. 1,4-Dioxane (Wako; >99.5%) was distilled over calcium hydride and then over lithium aluminum hydride. Toluene (Wako; 99.5%) was dried by passage through solvent purification columns (Glass Contour) and then distilled over metallic sodium. 1-(Isobutoxy)ethyl acetate (IBEA) and 1-(methoxyethoxy)ethyl acetate (MOEA) were prepared from the addition reactions of IBVE or 2-methoxyethyl vinyl ether (Maruzen Petrochemical) and acetic acid as previously reported.48 1-(Cyclohexylmethoxy)ethyl acetate (CMEA) was synthesized from the addition reaction of cyclohexylmethyl vinyl ether (Maruzen Petrochemical) and acetic acid in a manner similar to that of IBEA and MOEA and then purified by column chromatography. The bifunctional initiator CHDA, an adduct of DVE and acetic acid, was obtained as a white solid from DVE and acetic acid in a manner similar to that of IBEA and then purified by recrystallization from hexane. Et1.5AlCl1.5 (Nippon Aluminum Alkyls; 1.0 M solution in toluene) was used as received. 1-Pentylimidazole was synthesized from imidazole and 1-bromopentane according to the literature.49 1-Methylimidazole (Aldrich; 99%), 1-ethylimidazole (TCI; >98%), 1,2-dimethylimidazole (Wako; >97%), 1-butylimidazole (Aldrich; 98%), sodium tetrafluoroborate (Aldrich; 98%), sodium hexafluoroantimonate (Aldrich; 90%), lithium bis(trifluoromethanesulfonyl)imide (Wako; >98%), and all solvents except for a polymerization solvent were used without further purification. Preparation of Precursor Polymers. Polymerization was carried out under a dry nitrogen atmosphere in a glass tube equipped with a three-way stopcock, dried using a heat gun (Ishizaki; PJ-206A; blow temperature ∼450 °C). The following is a typical example of the polymerization of CEVE. Toluene (3.09 mL), 1,4-dioxane (0.50 mL; 5.9 mmol) as an added base, CEVE (0.41 mL; 4.0 mmol), and a 40 mM IBEA solution in toluene (0.50 mL; 2 × 10−2 mmol) as a cationogen were added successively into a glass tube using a dry medical syringe. Polymerization was initiated by adding a 200 mM Et1.5AlCl1.5 solution (30 °C) in toluene (0.50 mL; 0.10 mmol) at 30 °C. The reaction was terminated with a methanol solution (30 °C) containing a small amount of aqueous ammonia (3.0 mL; 0.1%). The quenched reaction mixture was diluted with dichloromethane and washed with a dilute hydrochloric acid, an aqueous NaOH solution, and water to remove initiator residues. The organic layer was evaporated under reduced pressure to remove the remaining volatiles. The product was dried in vacuo for at least 6 h at room temperature. The degree of monomer conversion was determined by gravimetry. For end-functionalized poly(CEVE)s, MOEA or CMEA was used instead of IBEA. Random copolymers were prepared with a mixture of CEVE and IBVE in the same manner as the homopolymerization. In the block copolymerization, IBVE was polymerized by first being initiated with IBEA or CHDA as a cationogen (bifunctional initiator) at 0 °C, followed by the addition of CEVE after predetermined intervals. After the addition, the glass tube was immersed in a water bath (30 °C) to promote the polymerization of less active CEVE. For star-shaped polymers, CEVE was polymerized by first using IBEA at 30 °C; DVE was then added to the glass tube, where the temperature was kept at 30 °C until the reaction was terminated. Reaction of Precursor Polymers with Imidazoles. After poly(CEVE) (0.40 g) was dissolved in DMF (4.0 mL), an alkylimidazole (equivalent to 5 CEVE units) was added. The solution was stirred with a magnetic stir bar at 80 °C for 72 h. The reaction mixture was dialyzed against deionized water for at least 3 days to remove DMF and the unreacted imidazole. After dialysis, water was removed under reduced pressure to obtain a slightly yellow brittle polymer. The obtained polymer was vacuum-dried for at least 6 h. The degree of introduction of imidazolium salt moieties was determined by 1H NMR. Anion Exchange Reaction. Anion exchange was conducted according to the literature.46 The obtained polymeric ILs with Cl− as

Chart 1. Structures of Polymeric ILs Synthesized in This Study

ionic liquids is the design of a counteranion. In fact, control of interactions between side chains was achieved by altering the property of the anion, resulting in the difference in solubility. The obtained polymeric ILs with chloride (Cl−) or hexafluoroantimonate (SbF6−) conteranions underwent LCST-type phase separation in some organic solvents, while those with tetrafluoroborate (BF4−) showed UCST-type phase separation in water. The phase separation mechanism is discussed based on these comparisons. B

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Scheme 1. Synthesis of Polymeric ILs with Various Counteranions

Figure 1. Synthesis of poly(CEVE)s by living cationic polymerization: (A) time−conversion curve, (B) Mn (open circles) and Mw/Mn (filled circles) for the polymerization, and (C) GPC curves of the obtained polymers ([CEVE]0 = 0.80 M, [IBEA]0 = 4.0 mM, [Et1.5AlCl1.5]0 = 20 mM, [1,4-dioxane] = 1.2 M in toluene at 30 °C). an anion (0.60 g) were dissolved in water (3.0 mL) and then mixed with sodium salts or a lithium salt having various anions (equivalent to 3 imidazolium salt units). For water-insoluble polymers, the precipitate was repeatedly washed with water to remove excess salts. For the other polymers, the reaction mixture was purified by dialysis against deionized water for at least 3 days, followed by evaporation under reduced pressure. Each product was vacuum-dried for at least 6 h. An upfield shift in the signal assignable to the imidazolium ring proton confirmed the anion exchange. Characterization. The molecular weight distribution (MWD) of the precursor polymers was measured by gel permeation chromatography (GPC) in chloroform at 40 °C with three polystyrene gel columns (TSKgel G-4000HXL, G-3000HXL, G-2000HXL; 7.8 mm i.d., 300 mm length; flow rate = 1.0 mL/min or TSKgel MultiporeHXL-M × 3; 7.8 mm i.d., 300 nm length; flow rate = 1.0 mL/min) connected to a Tosoh DP8020 pump, a CO-8020 column oven, a UV-8020 ultraviolet detector, and an RI-8020 refractive-index detector. The number-average molecular weight (Mn) and polydispersity ratio [weight-average molecular weight/number-average molecular weight (Mw/Mn)] were calculated by chromatography using calibration curves with respect to 16 polystyrene standards (Tosoh; Mn = 291−1.09 × 106, Mw/Mn ≤ 1.1). The Mw of the star-shaped polymer was determined by GPC coupled with multiangle light scattering (GPC-MALS) in chloroform at 40 °C on a DAWN HELEOS (Wyatt Technology; Ga−As laser, λ = 690 nm). The average number of arms of the star-shaped polymer was calculated by the following equation: number of arms = (Mw of star-shaped polymer × weight fraction of linear segments)/(Mw of linear polymer). The refractive index increment (dn/dc) value was measured in chloroform on differential refractometer (Otsuka Electronics DRM3000; λ = 633 nm). 1H NMR spectra of the polymers with pendant imidazolium salts were recorded at 30 °C in DMSO-d6 using a JEOL JNM-ECA500 spectrometer (500 MHz). The phase separation temperatures of the polymer solutions were determined from the transmittance at 500 nm. The transmittance was recorded using a JASCO V-500 UV/vis spectrometer equipped with a Peltier-type ETC505 thermostatic cell holder.

was carried out because the imidazolium-salt-containing polymers were insoluble in common solvents used in cationic polymerization and because counteranions might cause undesirable side reactions. Poly(CEVE) was chosen as a precursor polymer because the 2-chloroethyl groups are easily converted to alkylimidazolyl groups, as reported in our previous study.47 In addition, CEVE can be copolymerized with VEs with various functional groups, which allows for the design of stimulusresponsive polymers with various architectures such as block, graft, gradient, and star-shaped polymers. The cationic polymerization of CEVE was conducted using Et1.5AlCl1.5 in conjunction with IBEA as a cationogen in the presence of 1,4-dioxane in toluene at 30 °C. The polymerization proceeded in a living fashion to produce poly(CEVE)s with narrow MWDs and Mn values that increased in proportion to the monomer conversion (Figure 1). Poly(CEVE)s with a hydrophilic or a hydrophobic end group were also synthesized via living cationic polymerization using MOEA or CMEA as a cationogen. The polymerization proceeded in a living fashion, as was the case for polymerization with IBEA. The end-functionalized polymers had molecular weights that were in proportion to the monomer-to-initiator ratio and relatively narrow MWDs (Figure 2). Various shapes of polymers containing poly(CEVE) segments, i.e., random copolymers, (di- and tri-) block copolymers, and star-shaped polymers, were also prepared under similar reaction conditions (Figure 3 and Table 1). The random copolymerization of IBVE and CEVE was carried out at 30 °C with different IBVE to CEVE ratios. The copolymerization was completed in one day, producing poly(IBVE-ran-CEVE)s with narrow MWDs. A series of diblock and triblock copolymers of IBVE and CEVE were synthesized by sequential living cationic copolymerization. IBVE was polymerized first at 0 °C using IBEA to produce diblock copolymers or CHDA as an initiator to produce triblock copolymers. The second monomer, CEVE, was successively fed into the polymerization mixture at the same temperature, and then the temperature was raised to 30 °C to efficiently polymerize CEVE. The second living polymerization continued to proceed smoothly and reached almost quantitative monomer

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RESULTS AND DISCUSSION Synthesis and Solubility of Polymeric ILs. The synthesis of polymeric ILs with various substituents and counteranions was conducted in three steps: (i) synthesis of a precursor polymer, (ii) reaction of the prepolymer with an alkylimidazole, and (iii) an anion exchange reaction (Scheme 1). This multistep process C

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Table 1. Synthesis of Precursor Polymers with Various Shapes by Living Cationic Polymerizationa entry 1 2 3 4 5 6 7 8 9 10

polymer IBVE60-ran-CEVE140 IBVE40-ran-CEVE160 IBVE20-ran-CEVE180 IBVE100-b-CEVE100 IBVE140-b-CEVE100 IBVE300-b-CEVE100 CEVE100-b-IBVE100-bCEVE100 CEVE150-b-IBVE100-bCEVE150 CEVE200-b-IBVE100-bCEVE200 star-shaped CEVE200

time (h)

conv (%)

Mn × 10−4 b

Mw/Mnb

22 22 22 22 22 22 21

92 91 92 193c 191c 174c 180c

1.80 1.81 1.74 2.04 2.22 4.42 2.44

1.11 1.09 1.10 1.07 1.06 1.07 1.10

21

187c

3.25

1.13

21

191c

4.06

1.15

8.25

188c

12.5

1.22

a

Polymerization conditions: for entry 1: [IBVE]0 = 0.24 M, [CEVE]0 = 0.56 M, [IBEA]0 = 4.0 mM at 30 °C; for entry 2: [IBVE]0 = 0.16 M, [CEVE]0 = 0.64 M, [IBEA]0 = 4.0 mM at 30 °C; for entry 3: [IBVE]0 = 0.08 M, [CEVE]0 = 0.72 M, [IBEA]0 = 4.0 mM at 30 °C; for entry 4: [IBVE]0 = 0.40 M, [IBEA]0 = 4.0 mM, IBVE/CEVE = 1/1 molar ratio at 0 or 30 °C; for entry 5: [IBVE]0 = 0.56 M, [IBEA]0 = 4.0 mM, IBVE/CEVE = 7/5 molar ratio at 0 or 30 °C; for entry 6: [IBVE]0 = 1.2 M, [IBEA]0 = 4.0 mM, IBVE/CEVE = 3/1 molar ratio at 0 or 30 °C; for entry 7: [IBVE]0 = 0.20 M, [CHDA]0 = 2.0 mM, IBVE/CEVE = 1/2 molar ratio at 0 or 30 °C; for entry 8: [IBVE]0 = 0.20 M, [CHDA]0 = 2.0 mM, IBVE/CEVE = 1/3 molar ratio at 0 or 30 °C; for entry 9: [IBVE]0 = 0.20 M, [CHDA]0 = 2.0 mM, IBVE/CEVE = 1/4 molar ratio at 0 or 30 °C; for entry 10: [CEVE]0 = 2.0 M, [IBEA]0 = 10 mM, CEVE/DVE = 20/1 molar ratio 30 °C; [Et1.5AlCl1.5]0 = 20 mM, [1,4-dioxane] = 1.2 M in toluene for all entries. bBy GPC (polystyrene calibration). cThe final conversion in sequential copolymerization is defined as 200%.

Figure 2. Synthesis of end-functionalized poly(CEVE)s by living cationic polymerization: (A) Mn plotted against the [monomer]/ [initiator] ratio (circles: MOEA as an initiator; squares: CMEA as an initiator), (B) GPC curve for poly(CEVE) initiated with MOEA, and (C) GPC curve for poly(CEVE) initiated with CMEA ([CEVE]0 = 0.08, 0.16, 0.40, or 0.80 M, [initiator]0 = 4.0 mM, [Et1.5AlCl1.5]0 = 20 mM, [1,4-dioxane] = 1.2 M in toluene at 30 °C).

conversion after an additional 20 h. Figures 3B,C show the GPC curves of the block copolymers. After the addition of CEVE, the MWD clearly shifted toward higher molecular weights, maintaining a very narrow shape without tailing or leading. These results demonstrated that the reactions proceeded smoothly to yield well-defined block copolymers without side reactions such as termination or chain transfer. Star-shaped polymers were prepared using DVE as a crosslinking agent according to the literature method,50 in which starshaped polymers with a narrow MWD was obtained. CEVE was polymerized first at 30 °C, and then DVE was fed into the polymerization mixture to bundle the linear poly(CEVE)s. After the addition of DVE, a higher molecular weight product was obtained at a relatively high yield. The Mn value of the polymer increased 9-fold to reach 12.5 × 104. The number of arms of the star-shaped polymer was calculated to be about 9 by the equation described in the Experimental Section. Although a small amount of linear poly(CEVE)s remained unreacted, the high molecular weight star-shaped polymer had a monomodal and narrow MWD, demonstrating the clear formation of star-shaped polymers without significant side reactions. To convert the Cl atoms to imidazolyl groups, reactions of poly(CEVE) with 5 equiv molar amounts of alkylimidazoles were conducted in DMF at 80 °C. After 72 h, the reactions yielded polymeric ILs with Cl− as an anion. Figure 4 shows the 1H NMR spectra of a precursor polymer and the obtained polymeric ILs. The peak intensity ratio of the 1H NMR confirmed a nearly quantitative reaction independent of the alkyl groups attached to the imidazolium cation. Anion exchange reactions were also

Figure 3. GPC curves for random copolymer, (di- and tri-) block copolymers, and star-shaped polymer: (A) poly(IBVE-ran-CEVE) ([IBVE]0 = 0.08 M, [CEVE]0 = 0.72 M, [IBEA]0 = 4.0 mM, [Et1.5AlCl1.5]0 = 20 mM, [1,4-dioxane] = 1.2 M in toluene at 30 °C), (B) poly(IBVE-b-CEVE) ([IBVE]0 = 0.40 M, [IBEA]0 = 4.0 mM, [Et1.5AlCl1.5]0 = 20 mM, [1,4-dioxane] = 1.2 M, IBVE/CEVE = 1/1 molar ratio in toluene at 0 or 30 °C), (C) poly(CEVE-b-IBVE-b-CEVE) ([IBVE]0 = 0.20 M, [CHDA]0 = 2.0 mM, [Et1.5AlCl1.5]0 = 20 mM, [1,4dioxane] = 1.2 M, IBVE/CEVE = 1/2 molar ratio in toluene at 0 or 30 °C), and (D) star-shaped poly(CEVE) ([CEVE]0 = 2.0 M, [IBEA]0 = 10 mM, [Et1.5AlCl1.5]0 = 20 mM, [1,4-dioxane] = 1.2 M, CEVE/DVE = 20/ 1 molar ratio in toluene at 30 °C).

D

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[BF4]) showed poor solubility compared to the Cl−-type polymer. The polymer was soluble in acetone and insoluble in water and exhibited UCST-type phase separation in methanol. Several other polymeric ILs also exhibited UCST-type phase separation in alcohols, as shown in Table 3. Figures 5A−C show the temperature dependence of the transmittance of polymeric IL solutions in alcohols. The cloud points varied depending on the structures of the imidazolium cations, counteranions, and alcohols. Another interesting feature of the obtained polymeric ILs was that the SbF6−-type polymers induced LCST-type phase separation in some organic solvents. LCST-type phase separation in organic solvents has rarely been reported.51−53 As shown in Figure 5D, a 1 wt % THF solution of poly([PeIm][SbF6]) was clear at 25 °C but became turbid at 30 °C as the temperature increased. The heterogeneous solution became transparent again as the solution cooled. Additionally, a 0.5 wt % acetone solution of poly([Me2Im][SbF6]) also showed LCSTtype phase separation at ∼25 °C. Both phase separation processes exhibited significant hysteresis when the solutions returned to the homogeneous state. UCST-Type Phase Separation of Polymeric ILs with BF4− in Water. The trends of UCST-type phase separation in alcohols of polymeric ILs suggest that the introduction of less hydrophobic substituents to the BF4−-type polymers may lead to UCST-type phase separation in water. Turbidity measurements in water were conducted for BF4−-type polymers with a methyl group, an ethyl group, and two methyl groups (2- and 3positions). The temperature-dependent transmittance changes of these polymeric ILs are shown in Figure 6. An aqueous solution of the BF4−-type polymer with a methyl group was transparent at room temperature but became turbid at 5 °C as the temperature decreased. Furthermore, as the temperature increased, a sensitive reverse phase transition occurred, and this transition was reversibly repeated many times. The polymers with an ethyl group and two methyl groups also showed sharp phase separations at 8 and 15 °C, respectively. A slight hysteresis was yet observed even when the measurement was conducted at slower heating or cooling rates (0.2 °C/min), as shown in Figure S2. This was probably because a higher temperature was required to break the strong intra- and interpolymer interactions in the aggregated state. To the best of our knowledge, this is the first example of a poly(vinyl ether) that exhibits UCST-type phase separation in water. The effect of concentration of polymeric ILs was investigated to obtain a further understanding of the phase separation behavior. A phase diagram of poly([MeIm][BF4]) is shown in Figure 7. The concentration of the aqueous solution had a strong

Figure 4. 1H NMR spectra of 1 wt % solutions of poly(CEVE) in CDCl3 and poly([MeIm][X])s in DMSO-d6 at 30 °C.

conducted by mixing the Cl−-type polymer and 3 equiv molar amount of NaBF4, NaSbF6, or LiNTf2 in water. The exchange reactions were nearly quantitative, as confirmed by the 1H NMR spectra, which showed upfield shifts in the signals assignable to the imidazolium ring protons relative to those of the Cl−-type polymer. The detailed data of the produced polymeric ILs are listed in Table 2. Table 2. Synthesis of Polymeric ILs by Chemical Modifications

a b

entry

R

Mn × 10−4 a

Mw/Mna

yield of poly(IL)s (mol %)b

1 2 3 4 5 6 7 8 9 10 11 12

Me Me Me Me Et Et Et Me2 Me2 Bu Pe Pe

1.53 1.70 2.00 2.46 0.30 1.53 7.47 1.70 2.28 1.70 1.00 1.38

1.07 1.06 1.08 1.09 1.19 1.07 1.22 1.06 1.15 1.06 1.09 1.09

90 98 93 100 97 96 100 100 90 82 98 98

By GPC for precursor polymers (polystyrene calibration). Determined from the conversion to imidazolyl groups by 1H NMR.

The solubilities of the obtained polymeric ILs were tested in various solvents ranging from highly polar DMSO to nonpolar toluene (Table 3). The Cl−-type polymers were soluble in polar solvents such as water and methanol, even when a relatively hydrophobic butyl group was attached to the imidazolium ring as previously reported by our group.47 However, poly([BuIm]-

Table 3. Solubility Characteristics of Polymeric ILs with Different Counteranions in Various Solventsa X− = Cl−

a

X− = BF4−

X− = SbF6−

X− = NTf2−

R=

Me

Me2

Bu

Pe

Me

Me2

Bu

Pe

Me

Me2

Bu

Pe

Me

Me2

Bu

Pe

DMSO water methanol ethanol acetone THF chloroform toluene

S S S S I I I I

S S S S I I I I

S S S S I I L I

S S S S I I S I

S U I I I I I I

S U I I I I I I

S I U I S I I I

S I U I S I I I

S S I I I I I I

S I I I L I I I

S I S I S I I I

S I S I S L I I

S I S I S I I I

S I U I S I I I

S I S U S S I I

S I S U S S I I

S: soluble; I: insoluble; U: UCST-type phase separation; L: LCST-type phase separation; 1 wt % solutions. E

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Figure 5. Turbidity measurements of polymeric ILs in organic solvents. (A) 0.5 wt % ([PeIm][BF4])150 methanol solution, (B) 1 wt % ([PeIm][NTf2])150 ethanol solution, (C) 1 wt % ([Me2Im][NTf2])200 methanol solution, (D) 1 wt % ([PeIm][SbF6])100 THF solution, and (E) 0.5 wt % ([Me2Im][SbF6])200 acetone solution, scan rate: 0.5 °C/min for (A) and 1 °C/min for (B−E); solid line: cooling; broken line: heating.

Figure 6. Turbidity measurements of 2 wt % aqueous solutions of polymeric ILs: (A) poly([MeIm][BF4]), (B) poly([EtIm][BF4]), and (C) poly([Me2Im][BF4]); scan rate: 1 °C/min; solid line: cooling; broken line: heating.

poly(2-methoxyethyl vinyl ether) in water, in which the cloud points are greatly affected by the molecular weight or the chainend structure.33 Upon studying the UCST-type phase separation of poly(sulfobetaines)s in water, Garner et al. proposed that ionic and hydrogen-bonding interactions of the zwitterionic groups induced phase separation at lower temperatures.11 They believed that the UCST-type phase behavior resulted from a shift from insolubility caused by intrapolymer associations to solubility induced by interpolymer interactions at low levels. However, the interpolymer interactions became so dominant that precipitation occurs at higher levels. Similarly, the BF4−-type polymers were most likely to aggregate by interpolymer interactions of the imidazolium salt side chains at lower temperatures. An increase in the temperature resulted in a cleavage of the interaction. The effective interaction among polymers may be an electrostatic interaction and a hydrogen bond. In addition, hydrogen bonds of the imidazolium cation with the ether group54,55 in the side chains or with the F atoms of BF4− would be strengthen by cooling, which possibly shortened the distance between the polymers and helped the association. The existence of hydrogen

effect on the cloud points. An increase in the concentration of poly([MeIm][BF4]) raised the cloud point to ∼25 °C. These results suggest that the phase separation is mainly due to interpolymer interactions rather than intrapolymer interactions. However, the contributions of the both interactions cannot be separated based on the phase diagram because the phase separation is a complex result of changes in entropy and enthalpy. The effects of molecular weight and chain-end structure were also investigated using a series of poly([EtIm][BF4]) initiated with MOEA or CMEA. MOEA has a hydrophilic 2-methoxyethyl group, and CMEA has a hydrophobic cyclohexylmethyl group. The effects on the cloud points are shown in Figure 8. Aqueous solutions of 3 wt % poly([EtIm][BF4]) with a hydrophilic end group had cloud points at 11−16 °C, whereas poly([EtIm][BF4]) with a hydrophobic end group had cloud points at 11−14 °C. These data demonstrate that the cloud point of this system is not dependent on the molecular weight or the chain-end structure, even in the low molecular weight region, where the existence of end groups is generally not negligible. This trend was also observed at different concentrations (6 or 10 wt %). The results are in sharp contrast to the LCST-type phase separation of F

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However, the effects of salts may not be simply explained because the added salts would also induce anion exchange reaction. The details of these effects are currently under investigation.

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CONCLUSION Poly(vinyl ether)s with pendant imidazolium salts were synthesized by base-assisting living cationic polymerization and subsequent chemical modifications. The design of the alkyl groups on the imidazolyl groups and the counteranions resulted in UCST-type phase separation in water. The UCST-type phase separation was most likely induced by interpolymer electrostatic interactions among the side-chain ionic groups. This mechanism is supported by the dependence of cloud points on the alkyl groups attached to the imidazolium cation, the concentration of the polymers, and the chain-end groups. The polymers with different structures also showed UCST- or LCST-type phase separation in organic solvents. Various shapes of precursor polymers, such as random copolymers, (di- and tri-) block copolymers, and star-shaped polymer, were also synthesized via living cationic polymerization. The self-assembly behavior of these copolymers containing segments that show a UCST transition in water will be the subject of a forthcoming article.

Figure 7. (A) Turbidity cooling curves of aqueous solutions of poly([MeIm][BF4]) with different concentrations (scan rate: 1 °C/ min) and (B) dependence of cloud points on the polymer concentration.

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ASSOCIATED CONTENT

S Supporting Information *

Effects of salts on the cloud points and turbidity measurements at slower scan rate. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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Figure 8. Mn dependence on cloud points of 3 wt % (black), 6 wt % (red), and 10 wt % (blue) aqueous solutions of poly([EtIm][BF4])s obtained with MOEA (circles) or CMEA (squares).

ACKNOWLEDGMENTS This research was supported in part by a Grant-in Aid for Scientific Research (No. 22107006) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206). H. Yoshimitsu thanks The Global COE Program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University.

bonds between the O atoms of PEO and the H atoms of the imidazolium cations and between F atoms and the H atoms of the imidazolium rings were expected based on the results of several experimental,56,57 theoretical,57 and computational58 works. These interactions would be weakened at higher temperatures to be compensated by hydrogen bonds with water molecules. However, the electrostatic interaction was likely to be the major interaction rather than the hydrogen-bond interaction because the aqueous solution of poly([Me2Im][BF4]) with no hydrogen atom at the 2-position of the imidazolium ring also underwent phase separation. The hydrogen at the 2-position is the most protic among the hydrogen atoms on the imidazolium ring, which suggests that the hydrogen-bonding interaction among the polymer chains was not crucial to the phase separation. The mechanism by which the relatively strong interpolymer electrostatic interaction induced phase separation is supported by the significant dependence of cloud points on polymer concentration and the small effect of chain-end groups on the cloud points. Moreover, the cloud points were greatly affected by salts (Figure S1), which strongly suggested that the phase separation in this system was mainly due to electrostatic interactions. Adding NaCl into the solution lowered the cloud points, while adding Na2CO3 raised it.

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