Synthesis of Poly(ionic liquid)s by Atom Transfer Radical

Sep 17, 2014 - Martina M. Cecchini , Jan Steinkoenig , Samantha Reale , Leonie Barner ... Hongkun He , Heesung Chung , Elliot Roth , David Luebke , Da...
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Synthesis of Poly(ionic liquid)s by Atom Transfer Radical Polymerization with ppm of Cu Catalyst Hongkun He,†,‡ David Luebke,‡ Hunaid Nulwala,*,†,‡ and Krzysztof Matyjaszewski*,† †

Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States ABSTRACT: Well-defined poly(ionic liquid)s (PILs) were synthesized by activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). The ionic liquid monomer 1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (VBBI+Tf2N−) was directly polymerized with tris(2pyridylmethyl)amine (TPMA) ligand and ppm level of copper catalyst. The addition of reducing agent tin(II) 2-ethylhexanoate (SnII(EH)2) to continuously regenerate the catalyst in the ARGET ATRP enabled the synthesis of PILs with a significantly lower concentration of catalysts than required for a normal ATRP. PILs with well-controlled molecular weight and relatively low dispersity (Mw/Mn < 1.3) were obtained. ATRP chain extension of the resulting PILs with polystyrene revealed that the chain-end functionality of the PILs was significantly improved by slow feeding of the reducing agent during the polymerization. Simple and effective methods were developed to remove the residual halide ions from the ionic liquid monomer by ion exchange with LiTf2N or precipitation with AgTf2N, which allowed ARGET ATRP of the purified ionic liquid monomer to occur with as low as 20 ppm copper catalyst. Additionally, ARGET ATRP was used for the preparation of PIL block copolymers, including AB diblock and ABA triblock with PIL segment(s) as the middle or side blocks, using ionic liquid monomer or mono/difunctional PIL macroinitiators.

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INTRODUCTION Polymerized ionic liquids or poly(ionic liquid)s (PILs), a subclass of polyelectrolytes with ionic moieties as the repeating unit, have attracted much attention because of their combination of properties from both ionic moieties and polymers.1,2 The ionic moieties in PILs typical contain bulky cations (i.e., alkylammonium, alkylphosphonium, N,N′-dialkylimidazolium, N-alkylpyridinium, 1,2,3-triazolium, etc.) and noncoordinating anions (i.e., bis(trifluoromethylsulfonyl)imide (Tf2N−), BF4−, PF6−, etc.).3−6 PILs exhibit unique properties compared to common neutral polymers, such as high ion conductivity, chemical and thermal stability, and tunable solubility.7 PILs have wide applications in many realms of chemical and material science, such as precursors for micro/ mesoporous nitrogen-doped carbons,8,9 solvent and reaction media,10 stabilizer in emulsion polymerization,11 colloidal template for meso- and macroporous silica,12 functional surfaces,13 ionic liquid gel polymer electrolyte for electrochemical capacitors,14,15 solid state organic electrochromic devices,16 and CO2 separation.6,17,18 PILs with various structures have been synthesized by direct polymerization of ionic liquid monomers or through postmodification of precursor polymers, i.e., polymerization of neutral monomers followed by subsequent incorporation of ionic liquid groups.19−22 The majority of the PILs that have been reported previously were synthesized by conventional free radical polymerization.2,23,24 However, reversible-deactivation © XXXX American Chemical Society

radical polymerization (RDRP) procedures can provide better control over the polymerization, resulting in PILs with wellcontrolled molecular weight (MW) and narrow molecular weight distribution (MWD). For instance, atom transfer radical polymerization (ATRP),13,19,25−28 reversible addition−fragmentation chain-transfer (RAFT) polymerization,29,30 and organotellurium-mediated living radical polymerization (TERP)31 have been applied to the polymerization of styrenic, methacrylate, or N-vinylimidazolium based ionic liquid monomers containing BF4−, PF6−, or Tf2N− counterions. In addition, PIL block copolymers were also synthesized by ATRP,32 RAFT polymerization,29,30,33−35 nitroxide-mediated polymerization (NMP),36 ring-opening metathesis polymerization (ROMP),37 and cobalt-mediated radical polymerization,38,39 either through a direct polymerization from macroinitiators by ionic liquid monomers29,30,32−34,37−39 or by selective post-treatment of one block of the precursory neutral copolymers.35,36 In the current study, we present a simple synthetic approach for the preparation of PIL homopolymers and block copolymers via activator regenerated by electron transfer (ARGET) ATRP of ionic liquid monomers. ARGET ATRP allows an ATRP process to be conducted with significantly Received: July 18, 2014 Revised: September 3, 2014

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raphy (GPC). The GPC system used a Waters 515 HPLC pump and a Waters 2414 refractive index detector using PSS columns (Styrogel 102, 103, and 105 Å) with tetrahydrofuran (THF) or THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent at a flow rate of 1 mL/min at 35 °C. Synthesis of PILs by ARGET ATRP. TPMA, BuCN, an ATRP initiator (EBiB or DMDBH), and VBBI+Tf2N− were added to a Schlenk flask. The reaction conditions are provided in Table 1. The flask was then degassed by three freeze−pump−thaw cycles. The flask was allowed to warm up to room temperature and placed in an oil bath thermostated at 90 °C. Deoxygenated SnII(EH)2 in BuCN was added to the Schlenk flask to start the polymerization, and an initial sample (t = 0) was collected by syringe. Samples were taken periodically to measure conversion via 1H NMR and molecular weights via GPC. The polymer was precipitated in methanol/water (4/1, v/v), purified by dialysis (MWCO = 3.5 kDa) against THF, and dried under vacuum at room temperature. Synthesis of PIL-b-PS and PS-b-PIL-b-PS by ATRP. Styrene, BuCN, the macroinitiator (PIL-Br or Br-PIL-Br), CuBr2, and PMDETA were added to a Schlenk flask. The flask was then degassed by three freeze−pump−thaw cycles. While the contents were frozen in liquid nitrogen, the flask was backfilled with nitrogen and CuBr was added. The flask was then degassed and backfilled with nitrogen thrice. The flask was allowed to warm up to room temperature, and an initial sample (t = 0) was collected by syringe. The flask was then placed in an oil bath thermostated at 90 °C. Samples were taken periodically to measure conversion via 1H NMR and molecular weights via GPC. The polymer was precipitated in methanol/water (4/1, v/v), purified by dialysis (MWCO = 3.5 kDa) against THF, and dried under vacuum at room temperature. Synthesis of PIL-b-PEO-b-PIL by ARGET ATRP. TPMA, BuCN, Br-PEO-Br, and VBBI+Tf2N− were added to a Schlenk flask. The flask was then degassed by three freeze−pump−thaw cycles. The flask was allowed to warm up to room temperature and placed in an oil bath thermostated at 90 °C. Deoxygenated SnII(EH)2 in BuCN was added to the Schlenk flask to start the polymerization, and an initial sample (t = 0) was collected by syringe. Samples were taken periodically to measure conversion via 1H NMR and molecular weights via GPC. The polymer was precipitated in methanol/water (4/1, v/v), purified by dialysis (MWCO = 3.5 kDa) against THF, and dried under vacuum at room temperature. Synthesis and Purification of Ionic Liquid Monomer VBBI+Tf2N− Using LiTf2N. LiTf2N (1:1 molar ratio) was added to the aqueous solution of VBBI+Cl−, and an oily liquid was instantly visible at the bottom of the flask. The reaction was allowed to stir for 24 h at room temperature. The aqueous layer was removed, and a solution of LiTf2N (0.5:1 molar ratio) in water was added to the organic layer. The mixture was allowed to stir for 24 h at room temperature. Then, the oil was taken up in ethyl acetate (EtOAc) and washed with deionized water. The organic phase was dried over anhydrous MgSO4 and filtered, and the solvent was removed via rotary evaporation. Excess solvent was removed under vacuum to give a viscous liquid. 1H NMR (300 MHz, DMSO-d6): δ (ppm) = 9.3 (1H, s, N-CH-N), 7.8 (2H, d, N-CHCH-N), 7.5 (2H, d, Ph), 7.4 (2H, d, Ph), 6.8 (1H, m, CH2CH), 5.9 (1H, d, CH2CH), 5.4 (2H, s, PhCH2-N), 5.3 (1H, d, CH2CH), 4.2 (2H, t, N-CH2-CH2-CH2-CH3), 1.8 (2H, m, N-CH2-CH2-CH2-CH3), 1.3 (2H, m, N-CH2-CH2-CH2CH3), 0.9 (3H, t, N-CH2-CH2-CH2-CH3). Synthesis and Purification of Ionic Liquid Monomer VBBI+Tf2N− Using AgTf2N. AgTf2N was synthesized according to a previously published procedure.48 A solution of AgTf2N (3.00 g, 7.7 mmol) in water (5 mL) was added dropwise to a solution of VBBI+Cl− (1.97 g, 7.0 mmol) in deionized water (5 mL), and the mixture was stirred for 1 h. The precipitate was collected and washed with water. The solid was washed with EtOAc (10 mL) thrice, and the EtOAc was separated from the solid by centrifugation, dried over anhydrous Na2SO4, and filtered, and the solvent was removed via rotary evaporation. Excess solvent was removed under vacuum to give a viscous liquid. 1H NMR (300 MHz, DMSO-d6): δ (ppm) = 9.3 (1H, s, N-CH-N), 7.8 (2H, d, N-CHCH-N), 7.5 (2H, d, Ph), 7.4 (2H, d,

lower concentrations of a very active copper catalyst in the presence of appropriate reducing agents (e.g., tin(II) 2ethylhexanoate (SnII(EH)2), ascorbic acid).40−43 The presence of ppm level of copper catalyst (10−4 mol % vs monomer) in the ARGET ATRP system has minimal influence on the ionic groups, which substantially reduces the chances of the incorporation of the halide ions present on the copper complex into PILs by ion exchange. Compared to normal ATRP conditions, which employ a high concentration of copper catalyst, ARGET ATRP with a very low concentration of copper could simplify the postpolymerization purification process and prevent the impact of the residual ionic impurities on the structures and properties of the resulting PILs. Another advantage of using low level of copper catalyst in ATRP is that catalyst-induced side reactions,44 including CuI-induced catalytic radical termination,45 can be significantly suppressed. In addition, we developed facile methods to remove the residual halide ions in the pristine ionic liquid monomer by ion exchange or precipitation, which allowed ARGET ATRP of purified ionic liquid monomer to be conducted with as low as 20 ppm copper catalyst. ARGET ATRP under optimized conditions with slow addition of the reducing agent resulted in the synthesis of well-defined PILs with a high degree of chainend functionality. Furthermore, PIL block copolymers were synthesized by using ionic liquid monomer or PIL macroinitiator via ARGET ATRP, resulting in the formation of AB or ABA block copolymers. Scheme 1. Mechanism for ARGET ATRP

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EXPERIMENTAL SECTION

Materials. 1-Butylimidazole (98%), lithium bis(trifluoromethanesulfonyl)imide (LiTf2N, ≥99%), butyronitrile (BuCN, 99%), ethyl 2-bromoisobutyrate (EBiB, 98%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), silver nitrate (≥99%), dimethyl 2,6-dibromoheptanedioate (DMDBH, 97%), poly(ethylene glycol) (average Mn = 4600), and tin(II) 2ethylhexanoate (SnII(EH)2, 95%) were purchased from Aldrich. 4Vinylbenzyl chloride (90%) and styrene (St, ≥99%) were purchased from Aldrich and purified by passing over a column of basic alumina to remove the inhibitor. CuBr was purchased from Aldrich in the highest available purity. NaOH (99.2%) was purchased from Fisher Scientific. All solvents and chemicals are of reagent quality and were used as received unless special explanation. Tris(2-pyridylmethyl)amine (TPMA),46 difunctional poly(ethylene oxide)-based macroinitiator (Br-PEO-Br, Mn = 5300),47 1-(4-vinylbenzyl)-3-butylimidazolium chloride (VBBI+Cl−), and 1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (VBBI+Tf2N−)27 were synthesized according to previously published procedures. Instrumentation. 1H nuclear magnetic resonance (NMR) measurements were performed on a Bruker Avance 300 or 500 MHz spectrometer. Molecular weight and molecular weight distribution (Mw/Mn) were determined by gel permeation chromatogB

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Table 1. ARGET ATRP of VBBI+Tf2N− a entry

M/I/CuBr2/TPMA/Sn(EH)2

Cub (ppm)

time (h)

conv (%)

Mn,theoc × 10−3

Mn,GPCd × 10−3

Mw/Mnd

e

100/1/0.040/0.4/0.2 100/1/0.035/0.4/0.2 100/1/0.030/0.4/0.2 100/1/0.040/0.4/0.2 300/1/0.120/0.6/0.3 100/1/0.005/0.4/0.2 100/1/0.005/0.4/0.2 100/1/0.002/0.4/0.2 100/1/0.005/0.4/0.2 400/1/0.160/1.6/0.2

400 350 300 400 400 50 50 20 50 400

28.5 22 44 35 11.6 10 9 8 8 4

57.4 37.3 33.7 36.4 57.7 60.7 48.5 45.9 49.2 54.4

30.1 19.7 17.8 19.2 90.5 31.8 25.5 24.1 25.9 57.1

32.6 25.9 22.5 22.5 81.0 39.5 31.0 25.6 29.0 60.3

1.21 1.19 1.20 1.19 1.27 1.26 1.14 1.25 1.16 1.27

1 2e 3e 4f 5g 6h 7i 8j 9k 10l

a All polymerizations were conducted with VBBI+Tf2N− as the monomer and EBiB as the initiator (except entry 10), BuCN/VBBI+Tf2N− = 1/1 (w/ w), at 90 °C. bCalculated by the initial molar ratio of CuBr2 to the monomer. cMn,theo = ([M]0/[I]0) × conversion × Mmonomer. dGPC were conducted with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using PIL standards. eThe monomer was synthesized from VBBI+Cl− by ion exchange with LiTf2N (1:1 molar ratio). Sn(EH)2 was added at 0 h. fThe monomer was synthesized from VBBI+Cl− by ion exchange with LiTf2N (1:1 molar ratio). Sn(EH)2 was added separately: 1/4 at 0 h, 1/4 at 2 h, 1/4 at 9 h, and 1/4 at 12 h. gThe monomer was synthesized from VBBI+Cl− by ion exchange with LiTf2N (1:1 molar ratio). Sn(EH)2 was added separately: 1/3 at 0 h, 1/3 at 2 h, and 1/3 at 4.5 h. hThe monomer was synthesized from VBBI+Cl− by ion exchange with LiTf2N (1.0 equiv and then 1.0 equiv). Sn(EH)2 was added separately: 1/2 at 0 h and 1/2 at 4 h. i,jThe monomer was synthesized from VBBI+Cl− by ion exchange with LiTf2N (1.0 equiv and then 0.5 equiv). i Sn(EH)2 was added separately: 1/2 at 0 h and 1/2 at 3 h. jSn(EH)2 was added separately: 1/2 at 0 h and 1/2 at 2 h. kThe monomer was synthesized from VBBI+Cl− by reaction with AgTf2N. Sn(EH)2 was added separately: 1/2 at 0 h and 1/2 at 3 h. lDimethyl 2,6-dibromoheptanedioate (DMDBH) was used as the initiator. Sn(EH)2 was added separately: 1/2 at 0 h and 1/2 at 1 h.

Figure 1. (a) GPC traces of poly(4-vinylbenzyl chloride) in THF calibrated using linear polystyrene standards. (b) GPC traces of polyVBBI+Tf2N−RAFT standards in THF (containing 10 mM LiTf2N and 10 mM 1-butylimidazole). (c) GPC calibration curve using 14 PVBBITf2N−RAFT standards in THF (containing 10 mM LiTf2N and 10 mM 1-butylimidazole) by a third-order polynomial fit of the data points.

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Ph), 6.8 (1H, m, CH2CH), 5.9 (1H, d, CH2CH), 5.4 (2H, s, PhCH2-N), 5.3 (1H, d, CH2CH), 4.2 (2H, t, N-CH2-CH2-CH2-CH3), 1.8 (2H, m, N-CH2-CH2-CH2-CH3), 1.3 (2H, m, N-CH2-CH2-CH2CH3), 0.9 (3H, t, N-CH2-CH2-CH2-CH3). GPC Measurements. The precise GPC measurement of PILs is important for the evaluation of their MWs and MWDs and study of the structure−property relationship. However, direct GPC characterization of PILs by using pure THF as the eluent does not display normal GPC traces due to the strong interaction between ionic groups of PILs and the stationary phases of GPC columns. Therefore, GPC measurements of polyVBBI+Tf2N− were performed with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent, and the MWs and MWDs of polyVBBI+Tf2N− were obtained by using self-made PILs as the calibration standards.27 The GPC calibration standards (denoted as polyVBBI+Tf2N−RAFT) were obtained by using poly(4-vinylbenzyl chloride) synthesized by RAFT polymerization, which were quaternized with 1-butylimidazole and ion-exchanged with LiTf2N. We have made a GPC calibration curve with MW range of 4160−63 700 in a previous report.27 To measure the polyVBBI+Tf2N− with higher MWs, more self-made GPC calibrations with high MWs were synthesized and characterized (Figure 1a,b). A GPC calibration curve using 14 PVBBI-Tf2N−RAFT standards by a third-order polynomial fit of the data points with MW range of 4160−331 000 (Figure 1c).

RESULTS AND DISCUSSION Determination of the Minimal Cu Catalyst Concentration for ARGET ATRP of Ionic Liquid Monomer. Scheme 2. Synthesis of PILs by ARGET ATRP of VBBI+Tf2N−

ARGET ATRP of neutral monomers (e.g., styrene, acrylate, methacrylate) enable the synthesis of well-defined polymers using very low concentration of the copper catalyst (1−100 ppm) in the presence of a suitable reducing agent and ligand capable of forming an active catalyst complex (e.g., tris(2pyridylmethyl)amine (TPMA) and tris[2-(dimethylamino)ethyl]amine (Me6TREN)).40,49 The amount of copper complex C

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Figure 2. Kinetic plots of ln([M]0/[M]) vs time (a, d, g, k), plots of Mn and Mw/Mn vs conversion (b, e, h, l), and GPC traces (with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using PIL standards) (c, f, i, m) for ARGET ATRP of VBBI+Tf2N−. Conditions are shown in Table 1: (a−c) entry 1; (d−f) entry 2; (g−i) entry 3; (k−m) entry 5.

Scheme 3. Synthesis of Poly(VBBI+Tf2N−)-b-polySt by ATRP Using PIL-Br Macroinitiators

can be reduced to ppm levels due to the fact that the ATRP polymerization rate (Rp) depends on the ratio of [CuI]/[X− CuII] and not on the absolute concentration of the copper complexes (eq 1). In addition, the presence of a sufficient amount of X−CuII is required for a well-controlled ATRP because molecular weight distribution (= Mw/Mn) is affected by the rate constants of propagation (kp) and deactivation (kdeact), monomer conversion (p), and the concentration of dormant species (PnX) and deactivator (X−CuII) (eq 2). Therefore, continuous regeneration of the CuI activator by controlled addition of reducing agents is necessary to compensate for any loss of CuI by termination reactions.50,51 Previous studies on the influence of salt/counterion on normal ATRP showed that the presence of noncoordinating anions such as Tf2N− has no obvious influence on the progress of an ATRP, whereas strongly coordinating anions such as Cl− suppress the active D

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Figure 3. GPC traces (with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using polystyrene standards) for ATRP of styrene with PIL-Br macroinitiators. The PIL-Br macroinitiators were synthesized by ARGET ATRP of VBBI+Tf2N− with copper concentrations of 400 (a), 350 (b), and 300 ppm (c). Conditions: [St]0/[PIL-Br]0/[CuBr]0/[CuBr2]0/[PMDETA]0 = 1000/1/1.8/0.2/2, BuCN/ VBBI+Tf2N− = 1/1 (w/w), 90 °C.

from incomplete conversion of the ion-exchange process would be expected to affect the copper catalyst in ARGET ATRP. In addition, it was found that normal ATRP could occur in the presence of sufficient amount of copper catalyst with respect to the residual Cl−.28 Thus, it was anticipated that the minimal concentration of copper catalyst needed for the ARGET ATRP of ionic liquid monomers containing trace amounts of Cl− would be higher than that required for polymerization of a pure neutral monomer. ⎛ [P X][Cu I/L][M] ⎞ ⎟ R p = k p[M][P*n ] = k pKATRP⎜ n II ⎝ [X−Cu /L] ⎠

(1)

⎛ ⎞⎛ 2 ⎞ k p[Pn X] Mw 1 ⎟⎜ − 1⎟ =1+ +⎜ II Mn DPn ⎠ ⎝ kdeact[X−Cu /L] ⎠⎝ p

(2)

A series of control experiments were conducted to determine the minimal concentration of copper catalyst needed for the ARGET ATRP of ionic liquid monomer. As shown in Scheme 2, the reaction was conducted with 1-(4-vinylbenzyl)-3butylimidazolium bis(trifluoromethylsulfonyl)imide (VBBI+Tf2N−) as the ionic liquid monomer, ethyl 2bromoisobutyrate (EBiB) as the initiator, TPMA as the ligand, Sn(EH)2 as the reducing agent, 1-butylimidazole (BuCN) as the solvent, and with targeted degree of polymerization (DP) of 100. No polymerization was observed when 100 or 200 ppm of CuIIBr was added to the reaction, while polymerizations occurred when the copper concentration was increased to 300, 350, and 400 ppm (entry 1−3 of Table 1). The polymerization rates decreased when the concentration of CuIIBr concentration was reduced from 400 to 350 ppm and then to 300 ppm, as shown in the kinetic plot in Figure 2a,d,g. All these polymerizations showed well-controlled MWs and narrow MWDs (Mw/Mn ∼ 1.1−1.3) (Figure 2b,c,e,f,h,i). Another ARGET ATRP of VBBI+Tf2N− was conducted with higher targeted DP of 300 under similar reaction conditions in the presence of 400 ppm of CuIIBr, producing PIL with Mn = 810 000 and Mw/Mn = 1.27 (Figure 2k−m and entry 5 of Table 1). Optimizing Reaction Conditions To Increase Retention of CEF. Chain-end functionality (CEF) refers to the fraction of extendable chains, and retention of CEF is important for the synthesis of well-defined block copolymers, segmented copolymers, and telechelic polymers used in postpolymerization reactions.55 To check the CEF of the PIL-Br obtained from ARGET ATRP of VBBI+Tf2N−, they were used as macroinitiators for the synthesis of PIL-b-PS (Scheme 3). All of the three PIL-Br macroinitiators could initiate the ATRP of St to

Figure 4. GPC traces (with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using PIL standards) for (a) ARGET ATRP of VBBI+Tf2N− with 400 ppm copper catalyst and (b) ATRP of styrene with PIL-Br macroinitiator synthesized in (a). Conditions for (a) are shown in entry 4 of Table 1; conditions for (b): [St]0/[PIL-Br]0/[CuBr]0/[CuBr2]0/[PMDETA]0 = 1000/1/1.8/ 0.2/2, BuCN/St = 1/1 (w/w), 90 °C.

Scheme 4. Synthesis of VBBI+Tf2N− from VBBI+Cl−

Cu+/L catalyst, where L stands for ligand.28,52−54 Therefore, trace amounts of Cl− in the ionic liquid monomer resulting E

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Figure 5. Kinetic plots of ln([M]0/[M]) vs time (a, d, g, j), plots of Mn and Mw/Mn vs conversion (b, e, h, k), and GPC traces (with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using PIL standards) (c, f, i, l) for ARGET ATRP of VBBI+Tf2N−. Conditions are shown in Table 1: (a−c) entry 6; (d−f) entry 7; (g−i) entry 8; (j−l) entry 9.

Table 1), but the reducing agent was added intermittently during the polymerization, instead of a single addition at the beginning of the polymerization (Figure 4a and entry 4 of Table 1); i.e., 1/4 of the total amount of Sn(EH)2 (0.05 equiv to the initiator) was added at 0, 2, 9, and 12 h. The resulting PIL-Br (Mn = 22 500, PDI = 1.19) was used as the macroinitiator for the ATRP of St. The GPC traces (Figure 4b) showed nearly complete peak shift from macroinitiator, and no shoulder peak was observed in the resulting PIL-b-PS. These results highlight the importance of slow feeding, or intermittent addition, of reducing agent in ARGET ATRP of the ionic liquid monomer to achieve a high degree of CEF of the resulting PILs, which was required to obtain well-defined PIL block copolymers by chain extension.

form block copolymers; however, large shoulders were noticed in all the GPC traces (Figure 3), which suggested a large portion of dead chains were present in the PIL-Br macroinitiators. The CEF was estimated to be 50%, 48%, and 23% for the PIL-Br synthesized with Cu catalyst concentrations of 400, 350, and 300 ppm, respectively. The significant loss of CEF indicated that the ARGET ATRP conditions for ionic liquid monomer need to be optimized. Previous studies have demonstrated that slow feeding of reducing agent into an ongoing ATRP reaction mixture, when very active ligand was used, could minimize termination and allow the reaction to reach high monomer conversion.56,57 The ARGET ATRP of VBBI+Tf2N− using 400 ppm copper catalyst was conducted again under the same conditions (entry 1 of F

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Scheme 5. (a) Synthesis of Difunctional PIL Macroinitiator (Br-PIL-Br) by ARGET ATRP; (b) Synthesis of PS-b-PIL-b-PS by ATRP; (c) Synthesis of PIL-b-PEO-b-PIL by ARGET ATRP

Figure 6. Kinetic plot of ln([M]0/[M]) vs time (a), plot of Mn and Mw/Mn vs conversion (b), and GPC traces (with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using PIL standards) (c) for ARGET ATRP of VBBI+Tf2N− with 400 ppm copper catalyst using the difunctional initiator, dimethyl 2,6-dibromoheptanedioate (DMDBH). Conditions are shown in entry 10 of Table 1.

Reduction in the Copper Catalyst Concentration for ARGET ATRP of Ionic Liquid Monomer. The minimal amount of copper catalyst that was needed for ARGET ATRP in this study (i.e., 300 pm for the VBBI+Tf2N−) was much higher than the minimal copper concentration for ARGET ATRP of neutral monomers. This could be attributed to the residual Cl− in the ionic liquid monomer originating from an incomplete ion exchange reaction from its Cl−-containing precursor (Scheme 4). Indeed, it was previously demonstrated that the copper catalyst in ATRP is affected by strongly coordinating anions (e.g., Cl−) but not by noncoordinating anions (e.g., Tf2N−).28 Thus, the residual halogen anion (i.e., Cl−) in the ionic liquid monomer should be removed in order to allow the concentration of the copper catalyst to be reduced from 300 to 400 ppm to below 50 ppm. The amount of residual Cl− could be reduced by increasing the efficiency of the ionexchange reaction. The initial VBBI+Tf2N− monomer was

prepared by one-step ion-exchange with LiTf2N (1.0 equiv) followed by extracting with ethyl acetate and washing with water. This monomer was further purified by a second ionexchange step with LiTf2N (1.0 equiv) in water, extracted with ethyl acetate, and washed with water. ARGET ATRP of the purified VBBI+Tf2N− monomer occurred with only 50 ppm of CuBr2, producing PILs with narrow MWD (Figure 5a−c and entry 6 of Table 1). This purification procedure was further improved by using less LiTf2N (0.5 equiv) in the second ion-exchange step to save the reagent, and the water washing before the second ionexchange step was eliminated to simplify the procedure. The purified VBBI+Tf2N− monomer showed well-controlled polymerization by ARGET ATRP with 50 ppm of CuBr2 (Figure 5d−f and entry 7 of Table 1) and even 20 ppm of CuBr2 (Figure 5g−i and entry 8 of Table 1). GPC measurements showed that the Mw/Mn (∼1.2−1.3) of the PILs synthesized G

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macroinitiators can be based on PILs or other telechelic polymers, resulting in ABA block copolymers with PILs in the middle block or in the two side blocks, respectively (Scheme 5). A difunctional PIL macroinitiators (Br-PIL-Br) was synthesized by ARGET ATRP of ionic liquid monomer using a difunctional initiator, dimethyl 2,6-dibromoheptanedioate (DMDBH) (Figure 6). The Br-PIL-Br was chain-extended by two polystyrene side blocks via ATRP of styrene, producing PSb-PIL-b-PS (Figure 7a). Alternatively, a difunctional poly(ethylene oxide) (PEO) macroinitiator (Br-PEO-Br) was chain-extended by two PIL side blocks via ARGET ATRP in the presence of 100 ppm copper catalyst, producing PIL-bPEO-b-PIL (Figure 7b).

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CONCLUSIONS In summary, ARGET ATRP was successfully utilized to synthesize PILs with well-controlled MWs and narrow MWDs (1.1−1.3) in the presence of ppm level of copper catalyst. A high degree of CEF of the PILs synthesized by ARGET ATRP was achieved by periodic addition of the reducing agent. The minimal copper concentration for the ARGET ATRP could be reduced to less than 50 ppm by decreasing the concentration of residual chloride ions in the ionic liquid monomer through the LiTf2N ion-exchange method or AgTf2N precipitation method. Both AB and ABA type PIL block copolymers were synthesized using ionic liquid monomers/neutral macroinitiators or PIL macroinitiators/ neutral monomers through ARGET ATRP. The significantly reduced concentration of copper catalyst in the ARGET ATRP system minimized the influence of the ionic impurities on the resulting PILs, which is important for the preparation of highpurity PILs and their block copolymer with various structures for a wide range of applications. This approach could be easily extended to synthesize other PILs by ARGET ATRP with various IL monomers, such as acrylate/methacrylate-based IL monomers or IL monomers with different cations or anions. Further research toward this direction is currently in progress.

Figure 7. GPC traces for the synthesis of PIL block copolymers (with THF containing 10 mM LiTf2N and 10 mM 1-butylimidazole as the eluent and calibrated using polystyrene standards). (a) ATRP of St with Br-PIL-Br prepared by ARGET ATRP using 400 ppm copper catalyst as the macroinitiator. Conditions: [St]0/[Br-PIL-Br]0/ [CuBr]0/[CuBr2]0/[PMDETA]0 = 2000/1/1.35/0.15/1.5, BuCN/ VBBI+Tf2N− = 2/1 (w/w), 90 °C. (b) ARGET ATRP of VBBI+Tf2N− with 100 ppm copper catalyst. Conditions: [VBBI+Tf2N−]0/[Br-PEOBr] 0 /[CuBr 2 ] 0 /[TPMA] 0 /[Sn(EH) 2 ] 0 = 100/1/0.01/0.4/0.03, BuCN/VBBI+Tf2N− = 1/1 (w/w), 90 °C. Sn(EH)2 was added separately: 1/3 at 0 h, 1/3 at 1 h, and 1/3 at 2.5 h.

with 20 ppm of CuBr2 was slightly larger than the Mw/Mn (∼1.1−1.2) of the PILs with similar DP synthesized with 50 ppm of CuBr2, which could be due to that lower concentration of deactivating species (i.e., X−CuII) resulted in broader MWD according to eq 2. Another method to produce halide-free ionic liquid monomer involves precipitation of the halide by formation of a silver salt. The precursory monomer containing Cl− was mixed with an aqueous solution of AgTf2N, resulting in VBBI+Tf2N− monomer and precipitation of AgCl (Scheme 4). The obtained VBBI+Tf2N− monomer was used in ARGET ATRP with 50 ppm of CuBr2 and resulted in a well-controlled polymerization (Figure 5j−l and entry 9 of Table 1). Although the AgCl precipitation method is more efficient than the ionexchange method for the removal of Cl−, the relatively high cost of silver salts makes this method inferior to the ionexchange method in practice, especially for the large-scale synthesis of ionic liquid monomers. It should be noted that ionic liquid monomers can be synthesized by other methods without using precursory monomers containing halide ions, such as using N-methyl bis[(trifluoromethyl)sulfonyl]imide, to form ionic groups;58 however, this method also suffers from use of more expensive reagents. Synthesis of ABA Type PIL Block Copolymers by ARGET ATRP. Difunctional macroinitiators with an ATRP initiating moieties on both chain ends were prepared for the synthesis of ABA type PIL block copolymers. The difunctional

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.N.). *E-mail [email protected] (K.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS NSF support (CHE-1039870 and DMR-0969301) and DoE support (ER-45998) are acknowledged. This technical effort was also performed in support of U.S. Department of Energy’s National Energy Technology Laboratory’s ongoing research on CO2 capture under contract DE-FE0004000.

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dx.doi.org/10.1021/ma501487u | Macromolecules XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ma501487u | Macromolecules XXXX, XXX, XXX−XXX