Ion Conducting ROMP Monomers Based on (Oxa)norbornenes with

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Ion Conducting ROMP Monomers Based on (Oxa)norbornenes with Pendant Imidazolium Salts Connected via Oligo(oxyethylene) Units and with Oligo(ethyleneoxy) Terminal Moieties Terry L. Price, Jr.,† U Hyeok Choi,‡,§ Daniel V. Schoonover,† Murugan Arunachalam,† Renxuan Xie,§ Steven Lyle,† Ralph H. Colby,§ and Harry W. Gibson*,† †

Department of Chemistry and Macromolecules Innovations Institute, Virginia Tech, Blacksburg, Virginia 24061, United States Department of Polymer Engineering, Pukyong National University, Busan 48513, Korea § Department of Materials Science and Engineering, Penn State University, University Park, Pennsylvania 16802, United States Macromolecules Downloaded from pubs.acs.org by WEBSTER UNIV on 02/09/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: A matrix of 22 two-armed norbornene-based imidazolium TFSI monomers (8) was synthesized to determine the optimal structure in terms of single ion conductivity. For the chain tethering the imidazolium ring to the norbornene ring three or four oxyethylene units are optimal. A terminal group of two ethyleneoxy units was optimal. NMR studies indicated that both the tether oxyethylene units and the terminal ethyleneoxy units interact with the imidazolium cation via hydrogen bonding. 8r (X = 4, Y = 2) exhibited a conductivity of 9.57 × 10−5 S/cm at 25 °C and a Tg of −46 °C. Low Tg values do not correlate with higher conductivity as a result of the H-bonding interactions. Stability toward autopolymerization and reasonable conductivities provide an acceptable platform for ion conducting ROMP polymers. Four one-armed norbornene-based imidazolium TFSI monomers (15) were prepared with tetra(ethyleneoxy) linkers/spacers and variable terminal groups. All of these exhibited low Tgs (10−4 S/cm, the highest being 4.39 × 10−4 S/cm for 15c (Tg = −69 °C), the analogue of 8r, providing hope for outstanding polymers. Three oxanorbornene-based two-armed imidazolium TFSI monomers (18) were prepared with varied linkers and terminal groups. 18b possesses a room temperature conductivity of 1.2 × 10−4 S/cm, again augering well for polymers derived therefrom by ROMP.

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electrodes placed on both sides of the film. Under a direct current (dc) potential the “free” ions move toward the respective counter electrode. This electrical transport of mass leads to strain and thus deforms the polymer in the direction away from the ion movement. The actuator application thus depends on the induced strain, and as a result large, bulky mobile ions are more effective than smaller ones, even though the mobility or conductivity of the latter may be higher. For controllable movement in one direction, only one of the ions should be mobile, i.e., a single ion conductor. If both cations and anions are mobile, motion occurs in both directions sequentially. High ionic conductivity is most easily achieved in a lowviscosity liquid phase that allows free or minimally hindered movement of ions in an electric field.17 Consequently, using a liquid salt that would show high ionic mobility in its liquid phase was attractive. In addition to being liquids at low

INTRODUCTION Recently there has been considerable interest in ionically conductive polymers.1−4 In contrast to electronically conductive materials, ion conducting materials move charge by the transport of ions instead of electrons. These systems may be thought of as analogous to solutions of electrolytes.5 The focus of the present work was the synthesis of monomers and polymers that conduct by the movement of anionic counterions of polymer-bound imidazolium cations, as other workers have reported.6−12 Imidazolium cationic species were selected for this work due to their outstanding physical and ion transport properties. Many imidazolium salts fall into the category known as ionic liquids. Ionic liquids generally have melting temperatures below 100 °C; a subset, room temperature ionic liquids, have melting points below 25 °C.4 They possess large, delocalized ions that reduce Coulombic attractive forces, leading to lower melting points and less ion pairing than normal salts such as NaCl. The ultimate purpose of this work was to generate materials that can be used as electromechanical actuators.13−16 The ideal material is a thin film of an ionic polymer whose mobile ions are made to move using an electrical potential applied through © XXXX American Chemical Society

Received: October 26, 2018 Revised: January 14, 2019

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DOI: 10.1021/acs.macromol.8b02295 Macromolecules XXXX, XXX, XXX−XXX

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these systems is that the monomers very easily autopolymerize. Ideally for actuators the low-Tg, ion conducting component would be part of a block copolymer whose other component would provide the mechanical properties necessary to do real work; the modulus of the material is important. In this case, the ideal morphology would be a bicontinuous microphaseseparated block copolymerone phase for mechanical integrity and the other for ionic conduction. The autopolymerization issue basically prevents the preparation of such block copolymers from the (meth)acrylates. As an alternative ring-opening metathesis polymerization (ROMP) appeared to be a viable alternative for the production of well-defined ioncontaining polymers,8,11,42−46 and sequential addition of monomers can lead to the synthesis of the requisite block copolymers.46−48 The earlier work employing the imidazolium cation with a PF6− anion gave acceptable conductivity values, but switching to the bis(trifluoromethylsulfonyl)imide anion {(CF3SO2)2N− = TFSI−} improved conductivities dramatically;23,25 the thermal/chemical stability was also improved, and the larger, asymmetric TFSI anion is expected to induce more strain than the smaller, more symmetric PF6 anion. In addition, this original work provided a rubric for monomer design. Oxyethylene (OE) spacer units (X and Y, Figure 1) were employed to lower the Tg of the monomer and thus increase the conductivity.20 And Tg values were decreased and conductivities were increased by placing ethyleneoxy (EO) units on the terminus of the pendant moiety (Z, Figure 1).20,26 Our strategy was to use the well-developed chemistry of norbornene and oxanorbornene. By use of these basic monomer units, there are two different tactics: “one-armed” and “two-armed” derivatives. Our primary effort was devoted to “two armed” derivatives to maximize the ionic content, with the idea that this might afford the most conductive systems, but we also examined “one-armed” and oxanorbornene analogues in a selective manner. Here we report the results of these monomer studies, beginning with “two-armed” monomers. Our figure of merit here is the room temperature (RT) conductivity, σ25°C; elsewhere, we will discuss detailed analyses of the dielectric relaxation spectra to elucidate more basic parameters such as free carrier densities, mobilities, dielectric constants, and so forth.

temperatures, imidazolium salts are less toxic than some other organic cations such as paraquat.18,19 In previous work, we found that there is a general correspondence of the glass transition temperature of a monomer or polymer with its ionic conductivity;3,20−30 samples that have lower glass transition temperatures (Tg) tend to have higher room temperature ionic conductivities (σ25°C). Ethyleneoxy polymers often have very low glass transition temperatures and thus are liquids at room temperature.20,26,30−32 Ethyleneoxy moieties are believed to stabilize the imidazolium cation and encourage the separation and transport of the counterion away from it. Indeed, there are many examples of complexes of imidazolium cations with polyethers, both cyclic (crown ethers) and linear, e.g., poly(ethylene glycol)s.33−41 The ethyleneoxy moieties provide an electron-rich environment via the multiple lone pairs associated with the oxygen atoms, which stabilizes the cationic species and allows for ion pair dissociation, yielding more “free” or less tightly bound anions necessary for charge transport.29,30 Some of our previous work20,25,26,29 focused on the synthesis of (meth)acrylate esters that carried an imidazolium salt with differing tether chain lengths (Figure 1). A critical issue with

Figure 1. General structure of prior (meth)acrylate imidazolium TFSI polymers.

Scheme 1. Synthesis of Norbornene-Based Two-Armed Derivatives as Monomer Precursors

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RESULTS AND DISCUSSION Synthesis of Two-Armed Norbornene-Based Imidazolium TFSI Monomers. Taking into consideration the previous observations, synthetic schemes were developed to include oligo(ethylene glycol) units in the norbornene monomers (Schemes 1, 2, and 3). A uniform synthetic path

Scheme 3. Norbornene Two-Armed Imidazolium TFSI Monomer Synthesesa

Scheme 2. Synthesis of Substituted Imidazole Precursors

to ditosylates 3 was not chosen in Scheme 1 due to the differing availabilities and costs associated with oligo(ethylene glycol) precursors. Scheme 2 shows the syntheses of imidazole precursors. The exo-norbornene isomer was specifically selected over the endo isomer for its ability to more readily undergo ROMP;47−51 stereo purity of anhydride 1 was achieved through repeated recrystallizations and maintained throughout the synthetic pathways. Although previous results indicated longer ethyleneoxy chains would be advantageous,26 with the addition of every ethyleneoxy unit, the overall charge density of the molecule is diluted, so obviously there is a limit and the optimal length was not easily predicted. In an attempt to find the optimal lengths of the linkers and terminal groups a matrix of monomers 8 was synthesized (Scheme 3). X was varied from one to six oxyethylene (OE) units, while Y of the terminal group was varied from one to four ethyleneoxy (EO) units, with a butyl chain for comparison. Three routes were established for the syntheses of these norbornene monomers, and each has its own unique advantage for a specific subset of monomers. The first synthetic method (used for monomers 8a−8e) involved reaction of the norbornene precursor 1 with 2bromoethanol in an esterification reaction to provide dibromide 4 (Scheme 1). The imidazoles 6a−6e (Scheme 2) were then quaternized with 4 to provide monomers 7a−7e (A− = Br−, Scheme 3). The second synthetic method (for monomers 8f−8t) used the oligo(ethylene glycol) of the desired length X in an esterification reaction with anhydride 1 to produce the diols 2a−2c (Scheme 1). In this synthetic step the glycol was used both as reagent and solvent for the reaction; product isolation was performed via aqueous washings. These compounds were then tosylated to give compounds 3a−3c. Monomers 7f−7t (A− = Tos−) were produced by quaternization of the imidazoles 6a−6e with 3a−3c (Scheme 3). This method is straightforward and used purification steps that minimize the use of column chromatography; this method was advantageous for ethylene glycols that were inexpensive. The third synthetic method for monomers 8u and 8v was developed due to the costs associated with penta(ethylene glycol) and hexa(ethylene glycol). Their monotosylates were prepared (Scheme 1) by reaction with tosyl chloride at a stoichiometry of 1:1. Chromatography was used to isolate both

a

Abbreviations: NB = norbornene, OE = oxyethylene = OCH2CH2, Im = imidazolium, Bu = n-butyl, EO = ethyleneoxy = CH2CH2O, Me = CH3, TFSI = N(SO2CF3)2.

products, so that the ditosylate could be reconverted to the glycol and the reaction repeated. The monotosylates were then reacted with anhydride 1 via EDCI coupling to give the ditosylates 3d and 3e (purification by column chromatography, Scheme 1). The imidazole 6c was then quaternized with 3d and 3e to give monomers 7u and 7v (A− = Tos−), respectively (Scheme 3). Quaternizations to produce monomers 7a−7v were achieved by solventless reactions of either dibromide 4 or ditosylates 3a−3e with the desired imidazole 6 at 50 °C).59−62 Table 4 lists the glass transition temperatures of these oxanorbornene derivatives. The Tg values of 18a, 18b, and 18c are each within 2 °C of their norbornene analogues 8l, 8q, and 8r (Table 1) but >21 °C higher than the one-armed norbornenes 15b and 15c (Table 3). Table 4 also lists the RT conductivities of oxanorbornenes 18. The oxonorbornenes 18b and 18c display RT conductivity

Figure 14. 1D and 2D NOESY summary for monomer NB[C(O)OE4Im+EO2Me]2(TFSI−)2 (8r).

These NMR studies support the hypothesis that the oxyethylene tethers and the terminal ethyleneoxy segments both interact with the imidazolium cations. This has two consequences: (1) lowering of the binding energy of the TFSI anions, thus leading to an increase in the fraction of charge carriers, the free TFSI anions, and (2) an increase in the glass transition temperature relative to a situation without the Hbonding interactions (monomers with Y = 1 and those with a butyl tail, Table 1). As a final note, it should be mentioned that the two-armed norbornene imidazolium TFSI monomers developed here were shelf stable with no noticeable change in their 1H NMR spectra over the course of several years; i.e., they did not undergo autopolymerization. Synthesis and Properties of Two-Armed NorborneneBased Imidazolium Monomers with Other Counteranions. The advantages of the TFSI counterion over PF6 are due to it being a much larger, but flexible, ion with welldelocalized charge. This lowers the Tg, reduces ion pairing, and in turn results in higher conductivities.23,25 For this reason, we chose to evaluate the anions shown in Scheme 4: tetrakis[3,5bis(trifluoromethyl)phenyl]borate (BARF in 9r, X = 4, Y = 2), tetrakis(perfluorophenyl)borate (TPFB in 10r, X = 4, Y = 2), 1,4-bis-2-ethylhexylsulfosuccinate (AOT in 11a, X = 1, Y = nbutyl), and dodecyl sulfate (DDS in 12a, X = 1, Y = n-butyl). As detailed in Table S1, these resulted in RT conductivities 102−105 lower than the TFSI analogues! Synthesis and Properties of One-Armed NorborneneBased Imidazolium TFSI Monomers 15. One-armed norbornene-based imidazolium TFSI monomers were synthesized through a multistep process as depicted in Scheme 5. Methyl norbornenate was synthesized from methyl acrylate and freshly cracked cyclopentadiene. The majority of this Scheme 4. Bulky Anions Used in This Study

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Macromolecules Scheme 5. Synthesis of One-Armed Norbornene-Based Imidazolium TFSI Monomers 15

Table 3. Glass Transition Temperatures and Room Temperature Conductivities of One-Armed Norbornene Imidazolium TFSI Monomers 15 monomer

Y

NAMEa

15a 15b

0 1

NBC(O)OE4Im+Me TFSI− NBC(O)OE4Im+EOMe TFSI− NBC(O)OE4Im+ EO2MeTFSI− NBC(O)OE4Im+EO3Me TFSI−

15c 15d a

2 3

Tgb (°C)

αDC at 25 °C (S/cm)

−55 −65

2.80 × 10−4 2.07 × 10−4

−69

−4

−56

Table 4. Glass Transitions and Room Temperature Conductivities of Two-Armed Oxanorbornene Imidazolium TFSI Monomers 18 compd

X

Y

namea

18a

3

1

18b

4

1

18c

4

2

ONB[C(O) OE3Im+EOMe]2(TFSI−)2 ONB[C(O) OE4Im+EOMe]2(TFSI−)2 ONB[C(O) OE4Im+EO2Me]2(TFSI−)2

4.39 × 10

−4

2.79 × 10

Tgb (°C)

αDC at 25 °C (S/cm)

−47

−

−42

1.2 × 10−4

−44

7.9 × 10−5

a

Abbreviations for names: same as Scheme 3 with ONB = oxanorbornene. bDetermined by DSC at 10 °C/min during the second heating scan. The midpoint of the change in heat capacity was defined as the glass transition temperature.

b

Abbreviations for names: same as Scheme 3. Determined by DSC at 10 °C/min during the second heating scan. The midpoint of the change in heat capacity was defined as the glass transition temperature.

values comparable to their two-armed orbornene analogues 8q and 8r (Table 2) but lower than the one-armed norbornenes Scheme 6. Synthesis of Two-Armed Oxanorbornene-Based Imidazolium TFSI Monomers 18

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100 °C for 24 h. Each sample was then annealed in the Novocontrol at 120 °C in a heated stream of nitrogen for 1 h prior to measurements. However, since the oxanorbornenes are thermally labile and undergo retro-Diels−Alder reactions, these films were simply dried in vacuo at room temperature. The conductivity was measured using a sinusoidal voltage with amplitude 0.1 V and 10−1− 107 Hz frequency range for all experiments. Data were collected in isothermal frequency sweeps every 5 K from 120 to −100 °C. Materials. Reagents were purchased and used as received. Compounds 1,63 5a,64,65 5b,64 and 6c20 were prepared in accordance with literature procedures. General Procedure 1: Norbornene Esters 2. exo,exo-Bis(2-(2′hydroxyethoxy)ethyl) Bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate {NB[C(O)OE2OH]2, 2a}. exo-Norbornene anhydride (1) (19.63 g, 119.6 mmol) was added to a round-bottom flask containing di(ethylene glycol) (500 mL, 3.74 mol), and the mixture was stirred briefly before adding sulfuric acid (0.5 mL, 9 mmol). The reaction mixture was heated at 70 °C with stirring for 20 h. After cooling to room temperature, the reaction mixture was poured into water (1 L) and extracted with DCM (200 mL) 3×. The DCM extracts were combined and washed with 1 M HCl (100 mL × 3), saturated NaHCO3 (100 mL × 2), and water (100 mL × 3) and dried over sodium sulfate. Filtration and removal of solvent provided the desired product as a yellow tinted oil (32.98 g, 77%). 1H NMR (500 MHz, CDCl3): δ 6.22 (s, 2H), 4.24 (m, 4H), 3.77−3.65 (m, 8H), 3.59 (m, 4H), 3.18 (s, 2H), 3.13−3.08 (m, 2H), 2.68 (s, 2H), 2.12 (d, J = 9 Hz, 1H), 1.50 (d, J = 9 Hz, 1H). 13C NMR (126 MHz, CDCl3): δ 173.60, 137.94, 72.58, 68.82, 63.99, 61.57, 47.44, 45.55, 45.44 (9 signals expected and 9 signals found). HR MS: calcd for C17H27O8 [M + H]+: m/z 359.1700; found: m/z 359.1705 (error 1 ppm). General Procedure 2: Norbornene Ditosylates 3. exo,exo-Bis(2(2′-tosyloxyethoxy)ethyl) Bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate {NB[C(O)OE2OTs]2, 3a}. exo,exo-Norbornene diol 2 (5.05 g, 14.1 mmol), TEA (33 mL, 0.24 mol), and DCM (20 mL) were combined in a round-bottom flask and cooled to 0 °C. Tosyl chloride (41.97 g, 220.1 mmol) was dissolved in DCM (80 mL) and added dropwise via an addition funnel to the norbornene diol solution; once addition was complete, the reaction mixture was allowed to warm to room temperature and stirred for 2 days. The reaction mixture was poured into water (100 mL), and the organic layer was washed with water (15 mL × 4) and saturated NaCl (15 mL × 1) and dried over sodium sulfate. After filtration and removal of solvent the oil was washed with hexanes (100 mL × 2) and purified by flash column chromatography (silica eluting with DCM to acetonitrile) to yield a clear oil (3.97 g, 42%). 1H NMR (500 MHz, CDCl3): δ 7.81 (d, J = 8 Hz, 4H), 7.36 (d, J = 8 Hz, 4H), 6.22 (m, 2H), 4.23−4.15 (m, 6H), 4.11−4.04 (m, 2H), 3.72−3.67 (m, 4H), 3.62 (m, 4H), 3.10 (s, 2H), 2.65 (d, J = 2 Hz, 2H), 2.46 (s, 6H), 2.07 (d, J = 9 Hz, 1H), 1.49 (d, J = 9 Hz, 1H). 13C NMR (126 MHz, CDCl3): δ 173.43, 144.89, 137.95, 132.98, 129.86, 127.95, 69.15, 69.14, 68.58, 63.60, 47.23, 45.74, 45.33, 21.64 (14 peaks expected and 14 peaks found). HR MS: calcd for C31H39O12S2 [M + H]+: m/z 667.1877; found: m/z 667.1842 (error −5.3 ppm); calcd for C31H42O12S2N [M + NH4]+: m/z 684.2143; found: m/z 684.2124 (error 2.8 ppm); calcd for C31H39cO12S2Na [M + Na]+: m/z 689.1697; found: m/z 689.1681 (error 2.3 ppm). General Procedure 3: Monotosylation of Glycols. Penta(ethylene glycol) Monotosylate, HOE5OTs. Penta(ethylene glycol) (19.31 g, 81.04 mmol) and NaOH (3.21 g, 80.2 mmol) were dissolved in water (75 mL) and cooled to 0 °C with magnetic stirring. Tosyl chloride (15.51 g, 81.35 mmol) in THF (50 mL) was added dropwise to the reaction mixture via an addition funnel. After the tosyl chloride solution was completely added, the reaction mixture was stirred at room temperature for 24 h. THF was removed by rotary evaporation, and the aqueous layer was extracted with DCM (3 × 50 mL). The organic mixture was washed with 10% HCl (4 × 25 mL), water (3 × 25 mL), and saturated NaCl (1 × 25 mL). DCM was removed by rotary evaporation, and the resulting material was passed over silica; 100% DCM eluted unreacted TsCl; 100% EA eluted a ditosylated product (13.28 g), and last 100% acetonitrile eluted the desired product as a clear oil, 8.74 g (27%). 1H NMR (500 MHz, CDCl3): δ

15 (Table 3). Although there are only two points, unlike series 8 and 15, here the maximum conductivity is not observed for Y = 2, but rather for Y = 1.

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CONCLUSIONS A matrix of two-armed norbornene-based imidazolium TFSI monomers (8) was synthesized to determine the optimal structure in terms of single ion conductivity. Thirty-seven new compounds were synthesized, 22 of which were two-armed norbornene imidazolium TFSI monomers. For the chain tethering the imidazolium ring to the norbornene ring three or four oxyethylene units (X = 3 or 4) are optimal. A terminal group of two ethyleneoxy units (Y = 2) was optimal. NMR studies indicated that the terminal ethyleneoxy units interact with the imidazolium cation via hydrogen bonding; the longer tethers also interact with the imidazolium moieties. The optimal system 8r exhibited a conductivity of 9.57 × 10−5 S/ cm at 25 °C and a Tg of −46 °C. A decoupling of lower Tg values with higher conductivity values was observed as a result of the competition between the tether and terminal ethers for H-bonding with the imidazolium cations. The observed shelf stability and reasonably high conductivities of monomers 8a− 8v render them promising precursors for effective polymers for use in ion conducting devices. Four new imidazolium/anion combinations (9r, 10r, 11a, and 12a) were tested for RT conductivity with BARF, TPFB, AOT, and DDS anions, respectively. When compared to their TFSI counterpart, in terms of conductivity, the resulting materials were vastly inferior. Four new one-armed norbornene-based imidazolium TFSI monomers (15) were prepared with tetra(oxyethylene) linkers (X = 4) and variable ethyleneoxy terminal groups (Y). All of these exhibited room temperature conductivities >10−4 S/cm, the highest being 4.39 × 10−4 S/cm for 15c (Y = 2), the analogue of 8r. The results parallel those for the two-armed analogues and provide hope for outstanding polymers. Three new oxanorbornene-based two-armed imidazolium TFSI monomers (18) were prepared with varied linkers and terminal groups. 18b (X = 4, Y = 1) possesses a room temperature conductivity of 1.2 × 10−4 S/cm. However, retroDiels−Alder reactions are problematic with these monomers.

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EXPERIMENTAL SECTION

Measurements. 1H NMR spectra were obtained on JEOL ECLIPSE-500, BRUKER 500, and AGILENT NMR vnmrs400 spectrometers; 13C NMR spectra were collected on these instruments at 126, 126, and 101 MHz, respectively. 1H NMR splitting abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), sex (sextet), h (heptet); coupling constants, J, are in Hertz. HR-MS were obtained using an Agilent LC-ESI-TOF system. DSC data were obtained with a TA Instruments Q2000 differential scanning calorimeter under N2 at heating and cooling rates of 10 °C/ min; Tg values were recorded on second heating scans as the midpoint of the change in heat capacity. The ionic conductivity measurements were performed by dielectric relaxation spectroscopy. Samples were prepared by allowing them to flow to cover a 30 mm diameter freshly polished brass electrode at 100 °C in vacuo. To control the sample thickness at 100 μm, silica spacers were placed on top of the sample after it flowed to cover the electrode. Then a 15 mm diameter freshly polished brass electrode was placed on top to make a parallel plate capacitor cell which was squeezed to a gap of 100 μm in the instrument (with precise thickness checked after dielectric measurements were complete). The samples sandwiched between two electrodes were positioned in a Novocontrol GmbH Concept 40 broadband dielectric spectrometer, after being in a vacuum oven at N


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(t, J = 7 Hz, 6H). 13C NMR (126 MHz, DMSO-d6): δ 172.45, 137.71, 136.48, 122.76, 122.50, 119.44 (q, J = 323 Hz), 62.25, 48.62, 48.00, 46.46, 45.07, 44.70, 31.31, 18.68, 13.23 (14 signals expected and 14 signals found). HR MS: calcd for C29H40O8N5S2F6 [M − TFSI]+: m/z 764.2217; found: m/z 764.2215 (error −0.2 ppm); calcd for C27H40O4N4 [M − 2 TFSI]2+: m/z 242.1525; found: m/z 242.1524 (error −0.4 ppm). Tg (DSC) = −44 °C. Conductivity at 25 °C = 3.07 × 10−5 S/cm. exo,exo-Bis(2-bromoethyl) Bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate {NB[C(O)OEBr]2, 4}. Norbornene anhydride 1 (3.58 g, 21.8 mmol) was added to a round-bottom flask containing 2bromoethanol (37.8 g, 302 mmol), and the mixture was stirred briefly before adding sulfuric acid (0.1 mL, 2 mmol). The reaction mixture was heated at 70 °C with stirring for 20 h. After cooling to room temperature, the reaction mixture was poured into water and extracted with DCM three times. The DCM extracts were combined and washed with NaHCO3 (×4) and water (×3) and dried over sodium sulfate. Filtration and removal of solvent provided a material that was passed through a silica plug, eluted with DCM to provide the desired product as an oil (6.87 g, 80%). 1H NMR (500 MHz, CDCl3): δ 6.24 (m, 2H), 4.38 (m, 4H), 3.54−3.48 (m, 4H), 3.17− 3.13 (m, 2H), 2.68 (m, 2H), 2.07 (m, 1H), 1.52 (m, 1H). 13C NMR (126 MHz, CDCl3): δ 173.07, 137.94, 64.10, 47.13, 45.93, 45.27, 28.81 (7 signals expected and 7 signals found). HR MS: calcd for C11H13O4 fragment [M − C2H4Br2 + H]+: m/z 209.0814; found: m/z 209.0795 (error −9.1 ppm). exo,exo-NB[C(O)OE4Im+EO2Me]2(−OTs)2 (7r). Norbornene ditosylate 3c (1.89 g, 2.24 mmol) and imidazole 6c (0.708 g, 5.61 mmol) were combined in a round-bottom flask under nitrogen and heated at 70 °C with stirring for 3 days. After cooling to room temperature, the reaction mixture was taken up in water (50 mL) and extracted with DCM (200 mL) in a liquid−liquid apparatus for 2 days. The aqueous layer was collected, and water was removed by rotary evaporation. The resulting material was placed under a stream of nitrogen for 1 day and vacuum for 2 days (1.75 g, 80%, a viscous liquid). 1H NMR (500 MHz, D2O): δ 7.58 (d, J = 8 Hz, 4H), 7.48−7.43 (m, 4H), 7.26 (d, J = 8 Hz, 4H), 6.17 (t, J = 2 Hz, 2H), 4.33−4.28 (m, 8H), 4.25−4.17 (m, 2H), 4.06−3.99 (m, 2H), 3.80 (q, J = 5 Hz, 8H), 3.64 (t, J = 5 Hz, 4H), 3.60−3.52 (m, 20H), 3.49−3.45 (m, 4H), 3.24 (s, 6H), 3.03−2.98 (m, 2H), 2.66 (d, J = 2 Hz, 2H), 2.29 (s, 6H), 1.80 (d, J = 9 Hz, 1H), 1.37 (d, J = 9 Hz, 1H). 13C NMR (126 MHz, D2O): δ 175.87, 142.41, 139.42, 137.94, 129.40, 125.32, 122.62, 122.54, 70.88, 69.63, 69.51, 69.50, 69.47, 68.38, 68.31, 64.16, 58.01, 49.13, 49.11, 47.28, 45.47, 44.62, 20.44 (23 signals expected and 23 signals found). HR MS: calcd for C41H82N4O20 [M − 2 OTs]2+: m/z 420.2361; found: m/z 420.2350 (error 2.6 ppm). (1R,2S,4R)-Bicyclo[2.2.1]hept-5-ene-2-carboxylic Acid (13). Cyclopentadiene (66 g, 1.0 mol) was cracked from the dimer by heating under reflux and distilling the product below 45 °C. Methyl acrylate (86 g, 1.0 mol) was added to the cyclopentadiene. DCM (200 mL) was added. The resulting solution was heated under reflux for 8 h. DCM and unreacted starting materials were removed under vacuum. The product (145 g, 95% yield) was a light yellow oil. The mixture of endo and exo isomers of the methyl ester (50.01 g, 328.6 mmol) was heated in a solution of 250 mL of methanol and 20.12 g (372.6 mmol) of sodium methoxide under reflux to increase the exo isomer content by isomerization. The ester was then slowly hydrolyzed at low temperature by the slow addition of water (25 mL) over 8 h. The slow hydrolysis coupled with the isomerization inverted the isomer mixture from ∼80:20 endo:exo to ∼20:80 endo:exo. At this point the two isomers were separated by iodo-lactonization. The acid isomers (45.34 g, 328.6 mmol) were dissolved in 250 mL of an aqueous solution of sodium carbonate (36.26 g, 342.1 mmol) and titrated with 80 mL of an aqueous solution of iodine (20.0 g, 78.8 mmol) and potassium iodide (12.5 g, 75.3 mmol). The lactonized endo isomer was extracted with diethyl ether. The aqueous solution was acidified and the exo isomer was extracted with diethyl ether, 35.41 g (71%), mp 36.6−39.0 °C, lit. mp 32.5−35.0,66 43−44 °C.67 1H NMR (500 MHz, CDCl3): δ 6.13 (dd, J = 6, 3 Hz, 1H), 6.09 (dd, J = 6, 3 Hz,

7.80 (d, J = 8 Hz, 2H), 7.34 (d, J = 8 Hz, 2H), 4.21−4.11 (m, 2H), 3.78−3.52 (m, 18H), 2.45 (s, 3H). 13C NMR (126 MHz, CDCl3): δ 144.78, 133.01, 129.81, 127.98, 72.45, 70.75, 70.61, 70.60, 70.55, 70.53, 70.35, 69.24, 68.68, 61.75, 21.65 (15 signals expected and 15 signals found). HR MS: calcd for C17H32O8SN [M + NH4]+: m/z 410.1843; found: m/z 410.1836 (error 2 ppm); calcd for C17H29O8S [M + H]+: m/z 393.1583; found: m/z 393.1563 (error 5.1 ppm); calcd for C17H28O8SK [M + K]+: m/z 431.1142; found: m/z 431.1126 (error 3.7 ppm). General Procedure 4. Esterification of 5-Norbornene-exo-2,3dicarboxylic Anhydride. exo,exo-Bis(2-(2′-(2″-(2’’’-(2’’’’tosyloxyethoxy)ethoxy)ethoxy)ethoxy)ethyl) Bicyclo[2.2.1]hept-5ene-2,3-dicarboxylate {NB[C(O)OE5OTs]2, 3d}. 5-Norbornene-exo2,3-dicarboxylic anhydride (1) (0.88 g, 5.4 mmol) was combined in a round-bottom flask with penta(ethylene glycol) monotosylate (4.06 g, 10.3 mmol), EDCI (1.61 g, 10.4 mmol), DMAP (0.13 g, 1.1 mmol), and DCM (20 mL). The reaction mixture was stirred at room temperature for 3 days, after which 30 mL of DCM was added, and the mixture was poured into water. The organic layer was collected and washed with water (3 × 10 mL). Solvent was then removed by rotary evaporation. Column chromatography (silica) using 100% DCM provided the desired product, 4.16 g (83%), as a viscous oil containing slight amounts of EDCI and DMAP. The product was used without further purification. 1H NMR (400 MHz, CDCl3): δ 7.71−7.69 (m, 4H), 7.17−7.15 (m, 4H), 6.17 (s, 2H), 4.29−4.27 (m, 4 H), 3.62−3.55 (m, 48H; overlaps with EDCI protons), 3.06 (s, 2H), 2.61 (s, 2H), 2.33 (s, 6H), 2.04 (d, J = 9 Hz, 1H), 1.45 (s, 1H). 13 C NMR (101 MHz, CDCl3): δ 173.47, 144.79, 137.90, 132.88, 129.02, 126.08, 70.65, 70.50, 70.48, 70.36, 70.21, 70.13, 68.80, 67.03, 47.18, 45.70, 45.28, 21.36 (20 signals expected and 18 signals found, 2 signals in the ethyleneoxy region are suspected to be overlapping). EDCI and DMAP NMR peaks were omitted for clarity. HR MS: calcd for C43H66O18S2N [M + NH4]+: m/z 948.3716; found: m/z 948.3721 (error 0.5 ppm); calcd for C43H62O18S2Na [M + Na]+: m/z 953.3275; found: m/z 953.3205 (error 7.3 ppm). General Procedure 5: N-Alkylimidazoles 6. 1-n-Butylimidazole (ImBu, 6a). Imidazole (16.95 g, 249 mmol) and sodium hydroxide (9.40 g, 235 mmol) were added to a round-bottom flask with water (10 mL) and stirred until the imidazole had completely dissolved. 1Chlorobutane (21 mL, 0.26 mol) was diluted with THF (40 mL) and added to the reaction flask. Reflux was achieved and maintained for 12 h. THF was removed by rotary evaporation, and the crude material was poured into water (50 mL). After extraction with DCM (3 × 100 mL), the organic layers were combined and washed with 10% sodium hydroxide (20 mL × 2) and water (20 mL × 3) and dried over sodium sulfate. Filtration and removal of the solvent provided the desired product as a clear oil (21.27 g, 86%). 1H NMR (500 MHz, CDCl3): δ 7.45 (s, 1H), 7.05 (s, 1H), 6.90 (s, 1H), 3.93 (t, J = 7 Hz, 2H), 1.80−1.71 (m, 2H), 1.38−1.28 (m, 2H), 0.94 (t, J = 7 Hz, 3H). 13 C NMR (126 MHz, CDCl3): δ 137.09, 129.40, 118.78, 46.73, 33.10, 19.74, 13.51 (7 signals expected and 7 signals found). HR MS: calcd for C7H13N2 [M + H]+: m/z 125.1073; found: m/z 125.1073 (error 0 ppm). General Procedure 6: Norbornene Imidazolium TFSI Salts. exo,exo-NB[C(O)OEIm+Bu]2(TFSI−)2 (8a). exo,exo-Norbornene dibromide 4 (3.32 g, 8.39 mmol) and imidazole 6a (4.35 g, 35.0 mmol) were combined in a round-bottom flask under nitrogen and heated at 70 °C with stirring for 3 days. After cooling to room temperature, the reaction mixture was taken up in water (50 mL) and extracted with DCM (200 mL) in a liquid−liquid apparatus for 2 days. LiTFSI (5.85 g, 20.4 mmol) was dissolved in water (10 mL) and added to the aqueous solution. The aqueous mixture was stirred for 4 h and allowed to sit overnight for the salt to “oil out”. The aqueous layer was decanted and extracted twice with DCM (50 mL). The DCM extracts were combined with the remaining oil and washed with water (15 mL × 4). Solvent was removed by rotary evaporation, and the product was dried under high vacuum for 12 h (3.29 g, 37%). 1H NMR (500 MHz, DMSO-d6): δ 9.19 (s, 2H), 7.82 (s, 2H), 7.79 (s, 2H), 6.23 (s, 2H), 4.43 (m, 6H), 4.19 (m, 6H), 2.96−2.91 (m, 2H), 2.61 (m, 2H), 1.82−1.72 (m, 4H), 1.69 (d, J = 9 Hz, 1H), 1.34−1.18 (m, 5H), 0.90 O


Article

Macromolecules

OE4OTs]2, 17b}. A solution of tetra(ethylene glycol) monotosylate (8.71 g, 25 mmol), oxanorbornene dicarboxylic acid (16, 2.21 g, 12 mmol), a catalytic amount of DMAP (305 mg, 2.5 mmol), and EDCI (5.75 g, 30 mmol) in 100 mL of DCM was vigorously stirred for 3 days at room temperature. Then the reaction mixture was washed with water. The organic layer was concentrated and purified through flash column chromatography on a neutral alumina column using CHCl3 as eluent. The product was a mixture of retro-DA adduct and the title compound. The mixture was purified again by flash column chromatography with CHCl3−MeOH (95:5), which yielded 8.98 g (82%) of 17b as colorless viscous oil. 1H NMR (CDCl3, 500 MHz): δ 2.44 (s, 6H), 2.83 (s, 2H), 3.62−3.69 (m, 28H), 4.15 (t, 4H), 5.25 (s, 2H), 6.44 (s, 2H), 7.34 (d, J = 10 Hz, 4H), 7.79 (d, J = 10 Hz, 4H). 13 C NMR (CDCl3, 125 MHz): δ 21.6, 46.8, 64.2, 68.7, 68.9, 69.3, 70.4, 70.5, 70.6, 70.7, 80.6, 127.9, 129.8, 132.9, 136.6, 144.8, 171.4 (17 peaks observed, theory: 17). General Procedure 9. exo,exo-ONB[C(O)OE3Im+EOMe]2(TFSI−)2 (18a). The ditosylate 17a (1.89 g, 2.50 mmol) and 1-(2′methoxyethyl)imidazole (6b, 1 g, 6 mmol) were mixed and heated at 40−45 °C for 19 h. The resultant viscous liquid was dissolved in 25 mL of water and washed several times with DCM to remove the excess imidazole. To the aqueous solution was added a saturated LiTFSI solution; after stirring 6 h, the separated viscous liquid was decanted and washed freely with water and vacuum-dried to get the product as pale yellow oil (0.23 g, 7%). 1H NMR (CDCl3, 500 MHz): δ 2.90 (s, 2H), 3.37 (s, 6H), 3.63−3.70 (m, 12H), 3.84 (t, J = 5 Hz, 4H), 3.94 (m, 4H), 4.12−4.38 (m, 4H), 4.56 (t, J = 5 Hz, 8H), 5.24 (s, 2H), 6.53 (s, 2H), 7.75 (s, 2H), 7.79 (s, 2H), 9.04 (s, 2H). 13C NMR (acetone-d6, 125 MHz): δ 47.6, 50.5, 50.6, 59.0, 64.6, 69.4, 69.6, 70.9, 71.0, 71.1, 81.5, 121.0 (q, J = 320), 123.7, 123.8, 137.5, 137.7, 172.4 (17 peaks observed, theory: 17). HR MS: m/z 333.1734 (M − 2 TFSI)2+, calcd for (C32H50N4O11)2+ m/z 333.1733 (error 0.3 ppm); m/z 946.2639 (M − TFSI)+, calcd for C34H50F6N5O15S2 + m/ z 946.2644 (error 0.5 ppm). Tg = −46.5 °C. Conductivity at 25 °C not measured.

1H), 3.14 (s, 1H), 2.97 (s, 1H), 2.30 (ddd, J = 9, 4, 3 Hz, 1H), 2.01− 1.96 (m, 1H), 1.56 (d, J = 8 Hz, 1H), 1.42 (m, 2H). exo-(1R,2S,4R)-2-(2′-(2″-(2”’-(Tosyloxy)ethoxy)ethoxy)ethoxy)ethyl Bicyclo[2.2.1]hept-5-ene-2-carboxylate (NBC(O)OE4OTs, 14). exo-Norbornene acid 13 (10.01 g, 72.45 mmol) was dissolved in 50 mL of thionyl chloride with 2 drops of dimethylformamide. Magnesium turnings (2.04 g, 83.9 mmol) were added to the solution. The mixture was stirred for 4 h at room temperature. Excess thionyl chloride was removed under vacuum, and the resulting material was extracted with dry DCM and cooled in an ice bath. Tetra(ethylene glycol) monotosylate68 (12.08 g, 34.67 mmol) and dry pyridine (5.0 mL 62 mmol) were dissolved in dry DCM and added over 30 min to the acid chloride solution. The resulting mixture was stirred at room temperature for 8 h before the product was isolated by aqueous workup as a light brown liquid (13.97g, 86%). 1H NMR (400 MHz, CDCl3): δ 7.8 (m, 2H), 7.2 (m, 2H), 6.13 (dd, J = 6, 3 Hz, 1H), 6.09 (dd, J = 6, 3 Hz, 1H), 4.26−4.21 (m, 4H), 4.17−4.12 (m, 1H), 3.68 (m, 4H), 3.62 (m, 2H), 3.57 (d, J = 2 Hz, 2H), 3.15 (s, 1H), 2.96 (m, 2H), 2.44 (s, 3H), 2.32 (dd, J = 10, 4 Hz, 1H), 2.24 (dd, J = 10, 4 Hz, 1H), 1.99 (dt, J = 12, 4 Hz, 1H), 1.94−1.87 (m, 1H), 1.51 (d, J = 9 Hz, 1H), 1.42 (d, J = 10 Hz, 1H), 1.34 (d, J = 8 Hz, 1H). HR MS: m/ z 469.1841, (M + H)+, calcd for C23H33O8S m/z 469.1896 (error 12 ppm); m/z 486.2134, (M + NH4)+, calcd for C23H36O8NS m/z 486.2156 (error 4.5 ppm); m/z 491.1693 Da = (M + Na)+, calcd for C23H32NaO8S m/z 491.1716 (error 4.7 ppm). General Procedure 8: One-Armed Norbornene Imidazolium TFSI Salts, X = 4. exo-NBC(O)OE4Im+MeTFSI− (15a). Tosylate 14 (2.34 g, 5 mmol) was added to a round-bottom flask equipped with a stir bar. Imidazole 6f (1.36 g, 20 mmol) was added to the flask, and the contents were dissolved in 50 mL of nitromethane. The resulting solution was heated under reflux for 2 days. Nitromethane was removed under vacuum, and the residue was suspended in acetonitrile. The tosylate salt was isolated using silica gel column chromatography, eluting first with acetonitrile and then with a mixture of methanol, nitromethane, and 2 M aqueous ammonium chloride (7:2:1 v:v:v). The resulting solution was distilled under vacuum to remove as much solvent as possible, and the residue was dissolved in a minimum of water. Lithium TFSI (2.87 g, 10 mmol) was dissolved in water and added to the aqueous solution. The monomer salt was then extracted with DCM. The organic solution was washed three times with 10% HCl and dried over sodium sulfate, and DCM was removed under vacuum. The resulting product was a brown oil (3.07 g, 93%). 1 H NMR (400 MHz, DMSO-d6): δ 9.04 (s, 1H), 7.72 (m, 1H), 7.68 (m, 1H), 6.15 (m, 2H), 4.34 (t, J = 4, 2H), 4.15 (m, 2H), 3.86 (s, 3H), 3.76 (t, J = 4, 2H), 3.62 (t, J = 6, 2H), 3.45−3.57 (m, 8H), 2.96 (bs, 1H), 2.89 (bs, 1H), 2.17 (m, 1H), 1.81 (m, 1H) 1.40 (d, J = 8, 1H), 1.28 (m, 2H). 13C NMR (101 MHz, DMSO-d6): δ 175.63, 138.31, 137.20, 136.02, 123.76, 123.10, 119.89 (q, J = 323), 70.13, 70.08, 69.98, 69.93, 68.75, 68.55, 63.63, 49.17, 46.48, 46.29, 42.86, 41.46, 36.12 (21 signals expected and 21 signals found). HR MS: calcd for C20H31N2O5 (M − TFSI)+ m/z 379.2227; found m/z 379.2227, (error 0 ppm). DSC (5.0 °C/min): Tg = −55 °C. Conductivity at 25 °C = 2.80 × 10−4 S/cm. exo,exo-Bis(2-(2′-(2″-(tosyloxy)ethoxy)ethoxy)ethyl) 7Oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate {ONB[C(O)OE3OTs]2, 17a}. A solution of tri(ethylene glycol) monotosylate (7.63 g, 25 mmol), exo-oxanorbornene dicarboxylic acid (16, 2.21 g, 12 mmol), a catalytic amount of DMAP (305 mg, 2.5 mmol), and EDCI (5.75 g, 30 mmol) in 200 mL of DCM was vigorously stirred for 3 days at room temperature. The mixture was washed with water; the organic layers were concentrated and subjected to column chromatography on neutral alumina by elution with CHCl3−MeOH: 6.97 g (77%) of a viscous colorless oil. 1H NMR (CDCl3, 500 MHz): δ 2.45 (s, 6H), 2.84 (s, 2H), 3.59 (s, 8H), 3.65−3.70 (m, 8H), 4.15− 4.19 (m, 6H), 4.26−4.31 (m, 2H), 5.25 (s, 2H), 6.45 (s, 2H), 7.34 (d, J = 10 Hz, 4H), 7.80 (d, J = 10 Hz, 4H). 13C NMR (CDCl3, 125 MHz): δ 21.6, 46.9, 68.7, 69.0, 69.2, 70.4, 70.7, 80.7, 100.0, 127.96, 129.83, 133.0, 136.7, 144.8, 171.4 (15 peaks observed, theory: 15). exo,exo-Bis(2-(2′-(2″-(2”’-(tosyloxy)ethoxy)ethoxy)ethoxy)ethyl) 7-Oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate {ONB[C(O)-

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02295. Syntheses of various intermediates and monomers, highresolution mass spectra for intermediates and monomers; representative differential scanning calorimetric determinations of glass transition temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

U Hyeok Choi: 0000-0002-0048-9550 Harry W. Gibson: 0000-0001-9178-6691 Present Addresses

T.L.P.: Zestron Corp., 11285 Assett Loop, Manassas, VA 20109. D.V.S: Momentive Performance Materials, Frendly, WV 26146. M.A.: The Gandhigram Rural Institute-Deemed University, Gandhigram, India. S.L.: Department of Chemistry, University of California, Berkeley, CA 94720. Notes

The authors declare no competing financial interest. P


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liquids: biological effects in fish channel catfish ovary (CCO) cell line. Ecotoxicol. Environ. Saf. 2013, 92, 112−118. (19) Pereiro, A. B.; Araujo, J. M. M.; Martinho, S.; Alves, F.; Nunes, S.; Matias, A.; Duarte, C. M. M.; Rebelo, L. P. N.; Marrucho, I. M. Fluorinated ionic liquids: properties and applications. ACS Sustainable Chem. Eng. 2013, 1, 427−439. (20) Lee, M.; Choi, U H.; Colby, R. H.; Gibson, H. W. Structureproperty relationships: ion conduction in polymerizable ionic liquid acrylates and their polymers. Chem. Mater. 2010, 22, 5814−5822. (21) Lee, M.; Niu, Z.; Slebodnick, C.; Gibson, H. W. Structure and Properties of N,N-Alkylene Bis(N’-alkylimidazolium) Salts. J. Phys. Chem. B 2010, 114, 7312−7319. (22) Aitken, B. S.; Lee, M.; Hunley, M. T.; Gibson, H. W.; Wagener, K. B. Synthesis of precision ionic polyolefins derived from ionic liquids. Macromolecules 2010, 43, 1699−1701. (23) Lee, M.; Choi, U H.; Salas-de la Cruz, D.; Mittal, A.; Winey, K. I.; Colby, R. H.; Gibson, H. W. Imidazolium polyesters. Adv. Funct. Mater. 2011, 21, 708−717. (24) Aitken, B. S.; Buitrago, C. F.; Heffley, J. D.; Lee, M.; Gibson, H. W.; Winey, K. I.; Wagener, K. B. Precision ionomers: synthesis and thermal/mechanical characterization. Macromolecules 2012, 45, 681− 687. (25) Choi, U H.; Lee, M.; Wang, S.; Liu, W.; Winey, K. I.; Gibson, H. W.; Colby, R. H. Ionic conduction and dielectric response of poly(imidazolium acrylate) ionomers. Macromolecules 2012, 45, 3974−3985. (26) Choi, U H.; Mittal, A.; Price, T. L., Jr.; Gibson, H. W.; Runt, J.; Colby, R. H. Polymerized ionic liquids with enhanced static dielectric constant. Macromolecules 2013, 46, 1175−1186. (27) Kidd, B. E.; Lingwood, M. D.; Lee, M.; Gibson, H. W.; Madsen, L. A. Cation and anion transport in a dicationic imidazolium-based plastic crystal ion conductor. J. Phys. Chem. B 2014, 118, 2176−2185. (28) Lee, M.; Choi, U H.; Colby, R. H.; Gibson, H. W. Ion conduction in a semi-crystalline polyviologen and its polyether mixtures. Macromol. Chem. Phys. 2015, 216, 344−349. (29) Choi, U H.; Mittal, A.; Price, T. L., Jr.; Lee, M.; Gibson, H. W.; Runt, J.; Colby, R. H. Molecular volume effects on the dynamics of polymerized ionic liquids and their monomers. Electrochim. Acta 2015, 175, 55−61. (30) Choi, U H.; Mittal, A.; Price, T.; Colby, R. H.; Gibson, H. W. Imidazolium-based Ionic liquids as initiators in ring opening polymerization: ionic conduction and dielectric response of endfunctional polycaprolactones and their block copolymers. Macromol. Chem. Phys. 2016, 217, 1270−1281. (31) Jourdain, A.; Serghei, A.; Drockenmuller, E. Enhanced ionic conductivity of a 1,2,3-triazolium-based poly(siloxane ionic liquid) homopolymer. ACS Macro Lett. 2016, 5, 1283−1286. (32) Elmore, C. T.; Seidler, M. E.; Ford, H. O.; Merrill, L. C.; Upadhyay, S. P.; Schneider, W. E.; Schaefer, J. L. Ion transport in solvent-free, crosslinked, single-ion conducting polymer electrolytes for post-lithium ion batteries. Batteries 2018, 4, 28. (33) Gjikaj, M.; Brockner, W.; Namyslo, J.; Adam, A. Crown-ether enclosure generated by ionic liquid componentssynthesis, crystal structure and Raman spectra of compounds of imidazolium based salts and 18-crown-6. CrystEngComm 2008, 10, 103−110. (34) Noujeim, N.; Leclercq, L.; Schmitzer, A. R. N,N′-Disubstituted methylenediimidazolium salts: a versatile guest for various macrocycles. J. Org. Chem. 2008, 73, 3784−3790. (35) Ganesan, K.; Alias, Y. Synthesis and characterization of novel dimeric ionic liquids by conventional approaches. Int. J. Mol. Sci. 2008, 9, 1207−1213. (36) Luo, S.; Zhang, S.; Wang, Y.; Xia, A.; Zhang, G.; Du, X.; Xu, D. Complexes of ionic liquids with poly(ethylene glycol)s. J. Org. Chem. 2010, 75, 1888−1891. (37) Lee, M.; Niu, Z.; Schoonover, D.; Slebodnick, C.; Gibson, H. W. 1,2-Bis[N-(N’-alkylimidazolium)]ethane salts as new guests for crown ethers and cryptands. Tetrahedron 2010, 66, 7077−7082. (38) Suresh, M.; Mandal, A. K.; Kesharwani, M. K.; Adarsh, N. N.; Ganguly, B.; Kanaparthi, R. K.; Samanta, A.; Das, A. Folding and

ACKNOWLEDGMENTS We are very appreciative of financial support from the U.S. Army Research Office under Grant W911NF-07-1-0452 Ionic Liquids in Electroactive Devices (ILEAD) MURI. We are also grateful to Prof. James McGrath (VT, deceased) and Tim Long (VT) for use of their thermal analysis equipment and Prof. James Runt (PS) for use of his dielectric spectrometer.

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