Cationic polymerization of a novel oxetane-bearing ionic liquid structure and properties of the obtained poly(ionic liquid)

Article abstract of DOI:10.1002/pola.27342

An ionic liquid, 1-ethyl-3-(3-ethyl-3-oxetanylmethyl)imidazolium bis(trifluoromethanesulfonyl)imide (OXImTFSI), was synthesized, and its cationic polymerization was examined. The heating of a mixture of 1-ethylimidazole and 3-chloromethyl-3-ethyloxetane at 90 °C for 48 h yielded 1-ethyl-3-(3-ethyl-3-oxetanylmethyl)imidazolium chloride, which was transformed to a roomerature ionic liquid, OXImTFSI, by ion exchange with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). This ionic liquid was polymerized using boron trifluoride ethyl ether complex as a catalyst to give polyOXImTFSI. Five percent weight loss temperature (T _d5) of polyOXImTFSI evaluated by thermal gravimetric analysis was 409 °C, indicating the high thermal stability. Glass transition temperature (T _g) of the polymer evaluated by differential scanning calorimetry was -19 °C, indicating the high flexibility of the material. Ionic conductivity of polyOXImTFSI was determined to be 1.86 × 10^-8 S/cm at 23 °C, which was far lower than that of the OXImTFSI monomer (5.05 × 10^-4 S/cm).

Full text of DOI:10.1002/pola.27342

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Cationic Polymerization of a Novel Oxetane-Bearing Ionic Liquid
Structure and Properties of the Obtained Poly(ionic liquid)
Kozo Matsumoto,1,2 Tomoaki Chijiiwa,1 Takeshi Endo2
1Department of Biological and Environmental Chemistry, Kinki University, 11-6 Kayanomori, Iizuka,
Fukuoka Prefecture 820-8555, Japan
2Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka Prefecture 820-8555, Japan
Correspondence to: K. Matsumoto (E-mail: kmatsumoto@fuk.kindai.ac.jp)
Received 15 May 2014; accepted 23 July 2014; published online 6 August 2014
DOI: 10.1002/pola.27342
ABSTRACT: An ionic liquid, 1-ethyl-3-(3-ethyl-3-oxetanylmethyl)i-
midazolium bis(trifluoromethanesulfonyl)imide (OXImTFSI),
was synthesized, and its cationic polymerization was exam-
ined. The heating of a mixture of 1-ethylimidazole and 3-
chloromethyl-3-ethyloxetane at 90 ꢀC for 48 h yielded 1-ethyl-3-
(3-ethyl-3-oxetanylmethyl)imidazolium chloride, which was
transformed to a room-temperature ionic liquid, OXImTFSI, by
ion exchange with lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI). This ionic liquid was polymerized using boron trifluor-
ide ethyl ether complex as a catalyst to give polyOXImTFSI.
Five percent weight loss temperature (Td5) of polyOXImTFSI
evaluated by thermal gravimetric analysis was 409 ꢀC, indicat-
ing the high thermal stability. Glass transition temperature (Tg)
of the polymer evaluated by differential scanning calorimetry
was 219 ꢀC, indicating the high flexibility of the material. Ionic
conductivity of polyOXImTFSI was determined to be 1.86 3
1028 S/cm at 23 ꢀC, which was far lower than that of the
C
V
OXImTFSI monomer (5.05
3
1024 S/cm).
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2986–2990
KEYWORDS: cationic polymerization; ionomers; polyethers; ring-
opening polymerization
acidic conditions, although they are rather stable under neu-
tral or basic conditions.23 Oxetane-based polymers are soft
and flexible materials with high thermal and chemical stabil-
ity. Their applications to solid electrolyte usages have been
investigated.24,25
INTRODUCTION Ionic liquids, or room-temperature molten
salts, have recently been attracting the attention of many
researchers because of their unique properties such as
extremely low vapor pressure, nonflammability, high thermal
stability, specific solubility, and ionic conductivity.1–3 These
properties strongly depend on the combinations of cations
and anions. Polymerized ionic liquids, so-called poly(ionic
liquid)s, are a new class of polyelectrolytes.4–14 They may be
useful as functional materials applicable to polymer electro-
lytes, sensors, actuators, CO2 absorbents, and so forth. Sev-
eral poly(ionic liquid)s have already been synthesized and
characterized by some research groups. For example, linear
poly(meth)acrylates,5–8,12–14 polystyrenes,10,11 and poly(vinyl
ether)s15,16 having imidazolium pendant groups and net-
worked polymethacrylates,17–19 polystyrenes,20 or epoxides21
have been investigated so far. Recently, poly(ethylene oxide)s
having imidazolium pendant groups have been synthesized
and characterized.22 However, more materials should be
studied and developed to further understand the nature of
such poly(ionic liquid)s and to promote their applications to
novel functional materials.
In this study, we focused on the synthesis and polymeriza-
tion of a new polymerizable ionic liquid, an imidazolium salt
having a four-membered cyclic ether oxetanyl moiety on the
imidazolium cation, and elucidated their fundamental prop-
erties such as thermal stability, phase transition behavior,
and ionic conductivity in detail.
EXPERIMENTAL
Materials
3-Ethyl-3-oxetanemethanol, triphenylphosphine, tetrachloro-
methane, and 1-ethylimidazole were purchased from Tokyo
Chemical Industry (Tokyo, Japan) and used as delivered.
Bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) was
purchased from Wako Pure Chemical Industry (Osaka, Japan)
and dried at 120 ꢀC for 2 h in vacuo. Dichloromethane was
distilled over CaH2 before use. Boron trifluoride ethyl ether
complex was purchased from Tokyo Chemical Industry and
used after dilution to 0.5 mol/L solution with dry
Oxetanes are four-membered cyclic ether compounds, which
are well known as highly polymerizable compounds under
C
V
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dichloromethane. Acetone-d6 and dimethylsulfoxide-d6 were
purchased from Cambridge Isotope Laboratories (Cambridge,
MA).
45.98, 53.77, 77.31, 120.93 (q, JCF 5 309 Hz), 123.36, 124.54,
137.03.
Polymerization of 1-Ethyl-3-(3-ethyl-3-oxetanylmethyl)
imidazolium Bis(trifluoromethanesulfonyl)imide
Preparation of 3-Chloromethyl-3-ethyloxetane
In a 50-mL two-necked round-bottomed flask equipped with
a magnetic stirring bar, three-way stopcock, rubber septa,
and a rubber balloon filled with nitrogen, 1-ethyl-3-(3-ethyl-
3-oxetanylmethyl)imidazolium bis(trifluoromethanesulfonyl)i-
mide (OXImTFSI; 495 mg, 1.05 mmol) was charged, and the
flask was filled with nitrogen. Then, a dichloromethane solu-
tion of BF3 ethyl ether complex (0. 5 mol/L, 0.21 mL, 0.105
mmol) was added, and the mixture was stirred at room tem-
perature. The viscosity of the mixture gradually increased.
After stirring for 2 h, a methanol solution of triethylamine
(1 mol/L, 0.5 mL, 0.5 mmol) was added to terminate the
polymerization. The resulting mixture was diluted with a
small amount of acetone and was poured into an excess
amount of dichloromethane. Precipitated oil was washed
twice with dichloromethane, and the residue was heated at
80 ꢀC for 2 h with stirring in vacuo to give polyOXImTFSI
(350 mg, 0.707 mmol/monomer unit) in 67.3% yield.
In a 500-mL round-bottomed flask equipped with a magnetic
stirring bar, 3-ethyl-3-oxetanemethanol (13.7 mL, 120
mmol), triphenylphosphine (37.7 g, 144 mmol), and tetra-
chloromethane (110 mL) were mixed. The mixture was
stirred at 80 ꢀC for 2 h. After cooling the mixture to room
temperature, 220 mL of hexane was added to precipitate the
triphenylphosphine oxide. After removal of the precipitates
by filtration, the filtrate was concentrated under reduced
pressure, and the residual oil was distilled to give the title
compound (bp: 80–82 ꢀC/30 mmHg, 11.4 g, 84.6 mmol) in
71% yield.
Preparation of 1-Ethyl-3-(3-ethyl-3-oxetanylmethyl)
imidazolium Chloride
In a 50-mL round-bottomed flask equipped with a magnetic
stirring bar, 3-chloromethyl-3-ethyloxetane (773 mg, 5.00
mmol) and 1-ethylimidazole (481 mg, 5.00 mmol) were
ꢀ
mixed, and the mixture was heated at 90 C for 48 h. A por-
1
IR (ATR): 738, 790, 1050, 1090, 1130, 1180, 1330, 1350,
tion of the mixture was taken to check the conversion by H
1
2880, 2970, 3150 cm21. H NMR d (DMSO-d6): 0.75–0.9 (m,
NMR spectroscopy. The mixture was diluted with 2 mL of
dichloromethane, and the solution was poured into an excess
amount of diethyl ether to precipitate the ionic products.
The precipitated liquid was collected and purified by repreci-
3H), 1.20–1.50 (m, 2H), 1.46 (t, J 5 7.4 Hz, 3H), 3.00–3.45
(m, 4H), 4.05–4.30 (m, 4H), 7.45 (s, 1H), 7.87 (s, 1H), 8.92
(s, 1H). 13C NMR d (DMSO-d6): 7.30, 15.10, 22,90, 42.74,
44.44, 51.82, 70.70, 119.48 (q, JCF 5 319 Hz), 122.07, 123.89,
136.34.
ꢀ
pitation. The residual oil was dried at 70 C in vacuo for 3 h
to give the title compound (880 mg, 3.51 mmol) in 70.2%
yield as a brown liquid.
Measurements
1H (400 MHz) and 13C (100 MHz) NMR spectra were
recorded on a JEOL JME-ECS 400 NMR spectrometer in
CDCl3 or DMSO-d6. Chemical shifts were determined using
the residual protons as the internal standard. IR spectra
were recorded on a Thermo Scientific Nicolet iS10 spectrom-
eter equipped with a Smart iTR Sampling Accessory. Differ-
ential scanning calorimetry (DSC) was carried out with a
Seiko Instrument DSC-6200 using an aluminum pan under a
IR (ATR): 973, 1160, 1450, 1460, 1560, 2870, 2930, 2960,
3050 cm21 1H NMR d (DMSO-d6): 0.87 (t, J 5 7.5 Hz, 3H),
.
1.37 (t, J 5 7.4 Hz, 3H), 1.48 (q, J 5 7.5 Hz, 2H), 4.21 (q,
J 5 7.4 Hz, 2H), 4.24 (d, J 5 6.0 Hz, 2H), 4.43 (d, J 5 6.0 Hz,
2H), 4.45 (s, 2H), 7.89 (s, 1H), 7.94 (s, 1H), 9.71 (s, 1H). 13C
NMR d (DMSO-d6): 7.88, 15.17, 26.17, 42.91, 44.28, 52.25,
76.24, 122.17, 123.39, 136.69.
ꢀ
Preparation of 1-Ethyl-3-(3-ethyl-3-oxetanylmethyl)
imidazolium Bis(trifluoromethanesulfonyl)imide
In a 50-mL round-bottomed flask equipped with a magnetic
stirring bar, 1-ethyl-3-(3-ethyl-3-oxetanylmethyl)imidazolium
chloride (OXImCl; 693 mg, 2.77 mmol), distilled water
(3 mL), and LiTFSI (953 mg, 3.32 mmol) were mixed, and
the mixture was stirred at room temperature for 12 h. A vis-
cous liquid was precipitated, and the precipitate was col-
lected and washed four times with distilleꢀd water (3 mL).
The residual viscous liquid was dried at 70 C in vacuo for 3
h to give the title compound (1310 mg, 2.64 mmol) in
95.5% yield as a liquid.
20 mL/min N2 flow at a heating rate of 10 C/min. Thermal
gravimetric analysis (TGA) was performed with a Seiko
Instrument TG-DTA 6200 using an aluminum pan under a
ꢀ
50 mL/min N2 flow at a heating rate of 10 C/min. The ionic
conductivities of the ionic liquid monomer and polymers
were both measured by HIOKI 3532-80 chemical impedance
meter at 50 mV in a frequency range of 4 Hz to 100 KHz
using a two-electrode battery evaluation stainless coin cell
(2E-CELL-SUS; Eager Corporation) with a PTFE guide and
PTFE spacers (200 mm).
RESULTS AND DISCUSSION
IR (ATR): 738, 790, 977, 1050, 1130, 1180, 1330, 1340,
Synthesis of Ionic Liquid Monomer OXImTFSI
2890, 2970, 3150 cm21
.
1H NMR d (acetone-d6): 1.06 (t,
A new ionic liquid monomer OXImTFSI, which has a cationi-
cally polymerizable oxetanyl group on the cationic part,
was synthesized in three steps starting from a commer-
cially available 3-ethyl-3-oxetanemethanol (Scheme 1). First
of all, 3-ethyl-3-oxetanemethanol was transformed to
J 5 7.5 Hz, 3H), 1.63 (t, J 5 7.2 Hz, 3H), 1.76 (q, J 5 7.5 Hz,
2H), 4.45 (d, J 5 6.4 Hz, 2H), 4.48 (q, J 5 7.2 Hz, 2H), 4.59 (d,
J 5 6.4 Hz, 2H), 4.73 (s, 2H), 7.88 (s, 1H), 7.91 (s, 1H), 9.25
(s, 1H). 13C NMR d (acetone-d6): 8.09, 15.42, 27.17, 44.33,
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SCHEME 1 Synthetic route for OXImTFSI.
FIGURE 2 IR spectra of OXImTFSI and polyOXImTFSI.
3-chloromethyl-3-ethyloxetane by treatment with triphenyl-
phosphine in tetrachloromethane (Appel reaction), which
was further transformed to imidazolium chloride salt
(OXImCl) by heating at 90 ꢀC with 1-ethylimidazole. Then,
anion exchange of chloride to TFSI was carried out by mix-
ing OXImCl with LiTFSI in an aqueous solution. Liquid–liquid
phase separation occurred, and the lower liquid layer was
collected, washed four times with water, and dried in vacuo
OXImTFSI. Absorption of the oxetane ring CAOAC asymmet-
ric stretching was observed at 977 cm21. Absorption of the
sulfonamide group S@O stretching was observed at 1330
and 1340 cm21. Additionally, in the 13C NMR spectrum, CF3
carbon was observed at 111.64 ppm as a quartet peak with
ꢀ
at 70 C to give the desired ionic liquid monomer, OXImTFSI.
J
CF 5 309 Hz (see “Experimental” section). These data are
The absence of AgCl precipitates after addition of the
obtained OXImTFSI to an aqueous AgNO3 solution indicated
the absence of chloride contamination.
evidence of the formation of OXImTFSI.
The ionic conductivity was evaluated by impedance measure-
ments. Temperature dependence of the ionic conductivity of
OXImTFSI is plotted in Figure 3. The conductivity at 23 ꢀC
was found to be 5.05 3 1024 S/cm, which was slightly lower
than that (8.4 3 1023 S/cm) of a simple imidazolium TFSI
ionic liquid, 1-ethyl-3-methylimidazolium TFSI salt, reported
previously. This is probably because of the higher viscosity
Figure 1(a) shows the 1H NMR spectrum of the obtained
OXImTFSI. All the signals are assignable to OXImTFSI, which
are depicted in the figure. Particularly, four protons on the
oxetane ring were observed separately in two parts at 4.45
and 4.59 ppm. Figure 2(a) shows the IR spectrum of
FIGURE 1 1H NMR spectra of (a) OXImTFSI in acetone-d6 and (b) polyOXImTFSI in DMSO-d6.
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FIGURE 4 TGA curve of polyOXImTFSI (heating rate: 10 ꢀC/
min, under N2 flow).
FIGURE 3 Temperature dependence of ionic conductivity of
OXImTFSI and polyOXImTFSI.
Properties of PolyOXImTFSI
The obtained polyOXImTFSI was soluble in polar organic sol-
vent such as methanol, acetone, DMF, and DMSO, but insolu-
ble in nonpolar solvent such as toluene, chloroform,
dichloromethane, diethyl ether, and THF. It was also insolu-
ble in water. Thermal stability of polyOXImTFSI was exam-
ined by TGA. Figure 4 shows the TGA curve obtained under
a nitꢀrogen flow. Almost no weight loss was observed below
350 C, and 5 wt % decomposition temperature (Td5) of pol-
yOXImTFSI was 409 ꢀC. Previously, we reported that Td5 of
ImTFSI-containing methacrylate-based poly(ionic liquid) was
372 ꢀC. Comparing these values, it is evident that polyOX-
ImTFSI possesses markedly high thermal stability. It is prob-
ably because the polyoxetane main chains are thermally
more stable than the polymethacrylate main chains.
of OXImTFSI, which may be attributed to the bulky 3-ethyl-
3-oxetanyl substitutent on the imidazolium cation.
Polymerization of OXImTFSI
OXImTFSI was polymerized by using boron trifluoride ether
complex as a cationic polymerization catalyst (Scheme 2). By
precipitation of the polymeric products diluted in dichloro-
methane, a rubbery material was obtained in 67% yield.
Figure 1(b) shows the 1H NMR spectrum of the obtained
polymer. All the signals are assignable to polyOXImTFSI. Par-
ticularly, methylene protons in the polymer main chain were
observed at 3.00–3.40 ppm, whereas the corresponding oxe-
tane protons in the monomer observed at 4.45 and 4.59
ppm disappeared. Figure 2(b) shows the IR spectra of the
obtained polymer. Absorption of the oxetane ring CAOAC
asymmetric stretching observed at 977 cm21 disappeared,
whereas linear CAOAC asymmetric stretching in the poly-
mer main chain was observed at 1090 cm21. Absorption of
the sulfonamide group S@O stretching was also observed at
1330 and 1340 cm21. Additionally, in the 13C NMR spectrum,
CF3 carbon was observed at 119.48 ppm as a quartet peak
with JCF 5 319 Hz (see “Experimental” section). These data
are evidence of the formation of polyOXImTFSI. We have not
determined the molecular weight of the obtained polymer so
far. GPC measurements using DMF containing 10 mM LiBr as
an eluent were carried out, but no polymer peaks were
detected, which may be due to the favorable interaction
between the polymer and the polystyrene gels of the GPC
column.
The thermal phase transition behavior of polyOXImTFSI was
examined by DSC measurement. Figure 5 shows the DSC first
heating scan taken under a nitrogen flow after quenching
the polymer to 2100 ꢀC. Glass transition temperature (Tg)
was observed at 219 ꢀC, which is much lower if it is com-
ꢀ
pared with Tg (178 C) of methacrylate-based poly(ionic liq-
uid) having imidazolium TFSI salt pendant groups reported
previously. This low Tg value clearly indicates that
FIGURE 5 DSC heating scan of polyOXImTFSI (heating rate:
SCHEME 2 Polymerization of OXImTFSI.
10 ꢀC/min, under N2 flow).
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polyOBMSB possesses a highly soft and flexible nature,
which may be advantageous as an ion conducting material.
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Ionic conductivity of the polymer was measured, and the
ionic conductivity of the polyOBMSB was plotted against
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monomer. Although the ionic conductivity significantly
decreased (three orders of magnitude) after polymerization,
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A novel ionic liquid monomer having an oxetanyl group
(OXImTFSI) was synthesized and polymerized by cationic
ring-opening polymerization. The obtained poly(OXImTFSI)
showed high thermal stability, low glass transition tempera-
ture, and ionic conductivity. As the polyoxetane backbone
seems to be thermally, chemically, and electrochemically sta-
ble, this polymer is expected to be useful as a new functional
material, which may be applicable as novel polymer electro-
lytes in electrochemical devices. Further studies on the film
preparation, crosslinking, and mechanical and electrochemi-
cal properties of this polymer are currently in progress.
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