Facile synthesis of polymerized ionic liquids with high thermal stability

Article abstract of DOI:10.1016/j.tetlet.2010.05.038

Twelve main-chain-type polymerized ionic liquids that have alkylimidazolium cation units were prepared using simple synthetic processes. The polymers were prepared using the self-polymerization of a single monomer; no polymerization initiators were required. The thermal stability and solvent miscibility of these polymers were studied. Results show that the combined anions greatly influence the solubility and thermal stability of the polymers. Among these polymers, poly-alkylimidazolium bis(trifluoromethylsulfonyl)imide polymers exhibited the highest thermal stability (>400 °C), which makes them candidates for many applications.

Full text of DOI:10.1016/j.tetlet.2010.05.038

Tetrahedron Letters 51 (2010) 3666–3669
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis of polymerized ionic liquids with high thermal stability
Yu-Nung Hsieh a, Chun-Hsiung Kuei a, Yu-Kai Chou b, Chia-Chyuan Liu b, Kuen-Lin Leu b, Tasi-Hsiu Yang c,
Mei-Ying Wang b, Wen-Yueh Ho b,
*
a Department of Chemistry, National Cheng Kung University, No. 1, Ta-Hsueh Road, Tainan 701, Taiwan
b Department of Cosmetic Science, Chia Nan University of Pharmacy and Science, 60 Erh-Jen Road, Sec.1 Pao-An, Jen-Te Hsiang, Tainan 717, Taiwan
c Department and Institute of Health and Nutrition, Chia Nan University of Pharmacy and Science, 60 Erh-Jen Road, Sec.1 Pao-An, Jen-Te Hsiang, Tainan 717, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Twelve main-chain-type polymerized ionic liquids that have alkylimidazolium cation units were pre-
pared using simple synthetic processes. The polymers were prepared using the self-polymerization of
a single monomer; no polymerization initiators were required. The thermal stability and solvent misci-
bility of these polymers were studied. Results show that the combined anions greatly influence the sol-
ubility and thermal stability of the polymers. Among these polymers, poly-alkylimidazolium
bis(trifluoromethylsulfonyl)imide polymers exhibited the highest thermal stability (>400 °C), which
makes them candidates for many applications.
Received 30 January 2010
Revised 4 May 2010
Accepted 11 May 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Ionic liquids (ILs) are salts with relatively low melting points
that are composed entirely of cations and anions. The term has
been generally restricted to salts whose melting point is at or be-
low 373 K. They also have negligible vapor pressure and relatively
high thermal stability when compared to those of conventional
molecular-type solvents. Their physical and chemical properties
are easily tunable by synthetic means. In the recent decades, they
have attracted a lot of research interests.1–5 More recently, ionic
liquids have been used in polymer science.6 Polymerized ionic liq-
uids (PILs) have some of the aforementioned advantages, and their
immobilized structure usually improves their mechanical strength,
which can eliminate possible liquid leakage. Therefore, PILs have
potential applications in many emerging technological fields. Many
ionic-liquid-type polymers have been produced via the radical
polymerization of vinyl-containing ionic liquid monomers, such
as vinylimidazolium-based ionic liquids. After the radical polymer-
ization reaction, the imidazolium moieties form a side-chain (or
comb)-type polymer. This type of polymer has been used in poly-
mer electrolytes,7 CO2 absorption media,8 stationary phases for
gas chromatography,9–11 and extraction fiber for solid-phase mic-
roextraction.12 Synthetic vinyl-containing ionic liquids are extre-
mely promising because their physical and chemical properties
can be readily tuned by chemical functionalization. Nevertheless,
the syntheses often require the addition of polymerization initiators
and the polymerization parameters need to be carefully selected.
The amount of initiators, the reaction time, and temperature greatly
affect polymerization. Several steps are required to remove the ini-
tiator and by-products after the polymerization, making the process
labor intensive and sometimes poorly reproducible. Recently, poly-
merization reactions that require no initiators have been developed.
Suzuki et al. demonstrated a new ionic polymer electrolyte for dye-
sensitized solar cells.13 They used diiodoalkane and alkylene-
bis(imidazole) as the monomers. The polymerization was carried
out via the alkylimidazolium salt formation reaction, which requires
no polymerization initiators. Yoshida et al. presented an oligomeric
gelator synthesized via a quaternization reaction.14 The reaction
was carried out using commercially available 4-aminopyridine
and 4-(chloromethyl)benzoyl chloride. An intermediate product
containing nucleophilic and electrophilic parts at both termini
was produced. The intermolecular quaternization reaction occurred
immediately after the condensation reaction, forming the ionic olig-
omeric gelator. The ionic gelator was successfully used as a
dispersant for single-walled carbon nanotubes. Both of the afore-
mentioned processes are polymerization-initiator-free and can be
applied for the in situ polymerization of main-chain-type co-poly-
mers. However, in the synthesis of main-chain-type polymers, stoi-
chiometric control is very sensitive to the molecular weight control.
For the AB-type co-polymer, the reaction quenches in the absence of
either monomer (A or B). The end groups of the polymer depend on
the excess monomer. Therefore, the reaction parameters must be
carefully selected.
In the present study, a process for the synthesis of PILs using a
self-polymerization reaction is presented. Only a single monomer
is used for self-polymerization. The proposed homopolymer-type
PILs have several desirable features: (i) easy preparation proce-
dure; (ii) tunable solubility; (iii) high thermal stability (ꢀ420 °C);
(iv) polymerization-initiator-free. The synthesis of the proposed
PILs was carried out in two or three steps: (1) synthesis of the
monomer; (2) self-polymerization; and (3) metathesis of the an-
ions (if different combinations of anions are required). The pro-
cesses are outlined in Scheme 1. Briefly, (1) 0.046 mol (3.17 g) of
* Corresponding author. Tel.: +886 6 2664911.
E-mail address: rickho@mail.chna.edu.tw (W.-Y. Ho).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.038

Y.-N. Hsieh et al. / Tetrahedron Letters 51 (2010) 3666–3669
3667
Scheme 1. Schematic illustrations of the synthesis procedures for the proposed PILs.
imidazole 1 was added into 10 mL of dry THF solution; 0.056 mol
(0.45 g) of lithium hydride was dissolved in a 100-mL round-bot-
tomed flask containing 10 mL of dry THF solution and a stir-bar
at 0 °C. The imidazole solution was slowly mixed with LiH solution
at 0 °C under nitrogen atmosphere. 0.027 mol (4.34 g) of 1-bromo-
3-chloropropane 2a were slowly added into the mixture for reac-
tion, which was carried out under nitrogen atmosphere for 24 h
at room temperature (25 1 °C). Then, 10 mL of distilled water
was added to terminate the reaction. The THF solvent was removed
using an aspirator, and then the product was extracted several
times using dichloromethane. After the removal of dichlorometh-
ane, the monomer 1-(3-chloropropyl)imidazole (ImC3Cl, 3a) was
obtained as a transparent oily liquid. The yield of the monomer
was around 94%. Monomers 3b–d were prepared using the same
procedure, but 2b–d were applied as the starting materials, respec-
tively. (2) Then, 0.02 mol (3 g) of 3a was added into a 50-mL round-
bottomed flask containing 2 mL of ethylene glycol solution, which
was then heated in an oil bath at 90 °C for 12 h. The solution was
cooled to room temperature after the reaction completed. Ten mil-
liliters of methanol were poured into the round-bottomed flask to
dissolve the product. The solution was added to 200 mL of acetone
to cause precipitation. After the removal of acetone, the precipitate
was washed using acetone three times, and then dried in a vacuum
to obtain the polymer product as a white powder (polypropylimi-
dazolium chloride, PImC3Cl, 4a). The yield of the polymers was
around 88%. Polymers 4b–d were prepared using the same proce-
dure, but monomers 3b–d were used for polymerization, respec-
tively. (3) One gram of 4a was dissolved in 50 mL of deionized
water in a 250-mL round-bottomed flask. Then, 100 mL of 0.35 M
potassium hexafluorophosphate was slowly added into stirred 4a
solution for metathesis. The reaction was carried out at room tem-
perature for 12 h. The precipitate (PImC3PF6, 5a) was first filtrated
and washed using deionized water three times, and then dried at
50 °C in an oven for three days. The yield of the product was
around 90%. Other PF6À-containing polymers (5b–d) were pre-
pared using the same procedure. For NTf2À-containing PILs (6a–
d), lithium bis-(trifluoromethylsulfonyl)imide was used as the
metathesis reagent. The ion-exchanged products were then sub-
jected to a silver nitrate test to make sure that there was no silver
chloride precipitate. All four monomers (3a–d) were characterized
using 1H NMR, 13C NMR, and HRMS-FAB. The 1H NMR spectra
(200 MHz) and 13C NMR spectra (50 MHz) were recorded in deu-
terated chloroform; high resolution mass spectra (HRMS) were
determined using a JEOL JMS-700 mass spectrometer in fast atom
bombardment (FAB) mode. The spectra of the four monomers are
listed in the notes.15–18 The four chloride-containing polymers
(4a–d) were characterized using 1H NMR; the spectra were re-
corded in deuterated water and are listed in the notes.19–22 The rel-
ative molecular weights of the PILs were measured using gel
permeation chromatography (GPC) (equipped with a Jasco-880PU
pump, Waters Styragel HR4E (Mw: 50–100,000) and a UV detector)
with DMF as the eluent. The flow rate was 0.60 mL minÀ1. The rel-
ative molecular weights were calculated using a series of poly-sty-
rene standards as references. Due to solvent compatibility (see
Table 1, 6a–d are totally miscible with DMF), only the four
NTf2À-containing polymers (6a–d) were tested. The relative
Table 1
Solvent compatibility test
Polymer
H2O
DMSO
MeOH
Acetone
EA
THF
DCM
Toluene
Hexane
DMF
4a
5a
6a
4b
5b
6b
4c
5c
6c
4d
5d
6d
+
+
+
+
+
+
+
+
+
+
+
+
+
+
À
À
À
À
À
À
À
À
À
À
À
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À
À
À
À
À
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+
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+
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+
+
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+: Miscible; : soluble on heating; À: immiscible; testing temperature: 25 1 °C

3668
Y.-N. Hsieh et al. / Tetrahedron Letters 51 (2010) 3666–3669
stability tests. The tests were carried out using a thermal gravimet-
ric analyzer (TGA-50, Shimadzu, Japan); the results are shown in
Fig. 1. The thermal stability of the 12 PILs can be divided into three
À
groups. Chloride-containing PILs, PF6À-containing PILs, and NTf2
-
containing PILs have significant weight loss at around 280––350 °C
(Fig. 1a), 320–400 °C (Fig. 1b), and 420–460 °C (Fig. 1c), respec-
tively. There seems to be no significant effect on the alkyl-chain
lengths. The differences between PILs containing the same anions
are minor. The results suggest that the thermal stability of the PILs
is mostly anion dependent. Of note, NTf2À-containing PILs exhib-
ited a very high thermal stability. Under nitrogen atmosphere, al-
most no weight loss was observed at temperatures below 400 °C.
The melting points of 5a–d and 6a–d were also measured; they
are listed in the notes.24 Chloride-containing PILs (4a–d) were
not tested because they are too hygroscopic. The results suggest
that melting point decreases with increasing alkyl-chain length
and larger anion size. This could be due to the asymmetry of the
ionic structure decreasing the lattice energy. Of note, 6d was suc-
cessfully applied to be a high-thermal-stable stationary phase for
gas chromatography.25 The column can be applied to separate very
low-volatile organic compounds (such as coronene, bp 525 °C), be-
cause the stationary phase can resist high temperature (maximum
programmable oven temperature ꢀ400 °C).
In conclusion, a facile procedure for the synthesis of the main-
chain-type ionic liquid homopolymers was reported. No polymer-
ization initiators are required; only a single monomer is used for
each polymerization. The solubility and thermal stability can be
easily tuned by adjusting the combination of anions and/or cations.
Results suggest that the anions have a major influence on the
polarity and thermal stability of the proposed PILs. NTf2À-contain-
ing PILs show very high thermal stability (ꢀ420 °C), which makes
them suitable for a variety of applications.
Acknowledgment
Fig. 1. Thermal gravimetric analysis of the 12 PILs. (a) ClÀ-containing PILs, 4a–d; (b)
PF6À-containing PILs, 5a–d; (c) NTf2À-containing PILs, 6a–d. Temperature program:
The authors acknowledge the financial support by the National
Science Council of Taiwan under Grant NSC-97-2221-E-041-017.
100–500 °C at a heating rate of 10 °C minÀ1; N2 flow: 20 mL minÀ1
.
References and notes
molecular weights of 6a–d are listed in note.23 They are in the
range of 50,000–90,000 g molÀ1
1. Welton, T. Chem. Rev. 1999, 99, 2071–2083.
2. Fraser, K.; Izgorodina, E. I.; Forsyth, M.; Scott, J. L.; MacFarlane, D. R. Chem.
Commun. 2007, 3817–3819.
.
Many reports on ILs and/or PILs have indicated that the solubil-
ity can be changed by changing the combination of cations and an-
ions, the latter of which seems to have a greater influence.9–11
Twelve prepared PILs (4, 5, 6 series) were subjected to solvent
compatibility tests. Table 1 summarizes the solvent compatibility
of the PILs in various solvents. The chloride-containing PILs (4 ser-
ies) are water soluble. However, when the anions were replaced by
PF6 or NTf2 (5 and 6 series, respectively), the PILs became more
hydrophobic. The alkyl-chain length also influences solvent com-
patibility. 6a and 6b are insoluble in methanol (at 25 1 °C) de-
spite containing hydrophobic anions. However, they became
soluble when the alkyl-chain length was increased (e.g., 6c and
6d). According to Table 1, none of the prepared PILs are soluble
in less polar organic solvents. Previous studies suggest that some
side-chain-type PILs can be dissolved in some less polar solvents
such as tetrahydrofuran (THF) and dichloromethane(DCM).11 This
could be attributed to the fact that for side-chain-type polymers,
the alky chains are exposed out of the polymer base, which leads
to better interaction with an organic solvent. In contrast, for
main-chain-type PILs, both hydrophobic alky-chains and ionic
moieties are in-line. Therefore, the contributions of the hydropho-
bic interactions from the alky-chains are suppressed.
3. Anderson, J. L.; Armstrong, D. W.; Wei, G. T. Anal. Chem. 2006, 78, 2893–2902.
4. Marcilla, R.; Blazquez, A.; Rodriguez, J.; Pomposo, J. A.; Mecereeyes, D. J. Polym.
Sci., Part A: Polym. Chem. 2004, 42, 208–212.
5. Koel, M. Crit. Rev. Anal. Chem. 2005, 35, 177–192.
6. Lu, J. M.; Yan, F.; Texter, J. Prog. Polym. Sci. 2009, 34, 431–448.
7. Matsumi, N.; Sugai, K.; Miyake, M.; Ohno, H. Macromolecules 2006, 39, 6924–
6927.
8. Tang, J.; Tang, H.; Sun, W.; Plancher, H.; Radosz, M.; Shen, Y. Chem. Commun.
2005, 3325–3327.
9. Anderson, J. L.; Armstrong, D. W. Anal. Chem. 2005, 77, 6453–6462.
10. Hsieh, Y. N.; Ho, W. Y.; Horng, R. S.; Huang, P. C.; Hsu, C. Y.; Huang, H. H.; Kuei,
C. H. Chromatographia 2007, 66, 607–611.
11. Hsieh, Y. N.; Horng, R. S.; Ho, W. Y.; Huang, P. C.; Hsu, C. Y.; Whang, T. J.; Kuei, C.
H. Chromatographia 2008, 67, 413–420.
12. Zhao, F.; Meng, Y. J.; Anderson, J. L. J. Chromatogr., A. 2008, 1208, 1–9.
13. Suzuki, K.; Yamaguchi, M.; Hotta, S.; Tanabe, N.; Yanagida, S. J. Photochem.
Photobiol., A 2004, 164, 81–85.
14. Yoshida, M.; Koumura, N.; Misawa, Y.; Tamaoki, N.; Matsumoto, H.; Kawanami,
H.; Kazaoui, S.; Minami, N. J. Am. Chem. Soc. 2007, 129, 11039–11041.
15. Spectral data of 1-(3-chloropropyl)imidazole (ImC3Cl, 3a): 1H NMR spectra
(200 MHz, CDCl3): d 7.46 (s, 1H), 7.03 (s, 1H), 6.89 (s, 1H), 4.12 (t, J = 6.4 Hz,
2H), 3.43 (t, J = 5.8 Hz, 2H), 2.22–2.10 (m, 2H); 13C NMR spectra (50 MHz,
CDCl3): d 137.5, 130.0, 119.0, 43.6, 41.2, 33.6; HRMS-FAB: m/z [M+H]+ calcd for
C6H10N2Cl: 145.0533; found: 145.0531.
À
À
16. Spectral data of 1-(4-chlorobutyl)imidazole (ImC4Cl, 3b): 1H NMR spectra
(200 MHz, CDCl3): d 7.44 (s, 1H), 7.04 (s, 1H), 6.89 (s, 1H), 3.96 (t, J = 6.7 Hz,
2H), 3.51 (t, J = 6.0 Hz, 2H), 2.00–1.87 (m, 2H), 1.80–1.67 (m, 2H); 13C NMR
spectra (50 MHz, CDCl3): d 137.2, 130.0, 118.9, 46.5, 44.3, 29.5, 28.6; HRMS-
FAB: m/z [M+H]+ calcd for C7H12N2Cl: 159.0689; found: 159.0689.
17. Spectral data of 1-(3-chloropentyl)imidazole (ImC5Cl, 3c): 1H NMR spectra
(200 MHz, CDCl3): d 7.43 (s, 1H), 7.02 (s, 1H), 6.88 (s, 1H), 3.92 (t, J = 7.0 Hz,
It is believed that the anions have a great influence on thermal
stability and solubility. The 12 PILs were subjected to thermal

Y.-N. Hsieh et al. / Tetrahedron Letters 51 (2010) 3666–3669
3669
2H), 3.49 (t, J = 6.6 Hz, 2H), 1.85–1.70 (m, 4H), 1.49–1.37 (m, 2H); 13C NMR
spectra (50 MHz, CDCl3): d 137.2, 130.0, 119.0, 47.0, 44.7, 32.1, 30.6, 24.1;
HRMS-FAB: m/z [M+H]+ calcd for C8H14N2Cl: 173.0846; found: 173.0848.
18. Spectral data of 1-(3-chlorohexyl)imidazole (ImC6Cl, 3d): 1H NMR spectra
(200 MHz, CDCl3): d 7.42 (s, 1H), 7.02 (s, 1H), 6.87 (s, 1H), 3.90 (t, J = 7.0 Hz,
2H), 3.48 (t, J = 6.6 Hz, 2H), 1.84–1.56 (m, 4H), 1.52–1.20 (m, 4H); 13C NMR
spectra (50 MHz, CDCl3): d 137.2, 130.0, 119.0, 47.0, 45.0, 32.4, 31.1, 26.5, 26.0;
HRMS-FAB: m/z [M+H]+ calcd for C9H16N2Cl: 187.1002; found: 187.1000.
19. Spectral data of PImC3Cl, 4a: 1H NMR spectra (200 MHz, D2O): d 9.02 (1H), 7.63
(2H), 4.40 (4H), 2.59 (2H).
21. Spectral data of PimC5Cl, 4c: 1H NMR spectra (200 MHz, D2O): d 8.87 (1H), 7.55
(2H), 4.25 (4H), 1.98 (4H), 1.40 (2H).
22. Spectral data of PimC6Cl, 4d: 1H NMR spectra (200 MHz, D2O): d 8.80 (1H), 7.50
(2H), 4.20 (4H), 1.88 (4H), 1.36 (4H).
23. Relative molecular weights obtained from GPC: PImC3 NTf2, 6a: 94,500; PImC4
NTf2, 6b: 49,800; PImC5 NTf2, 6c: 47,300; PImC6 NTf2, 6d: 51,300.
24. Melting point tests: PImC3 PF6, 5a: 266 °C; PImC4 PF6, 5b: 249 °C; PImC5 PF6, 5c:
138 °C; PImC6 PF6, 5d: 93 °C; PImC3 NTf2, 6a: 117 °C; PImC4 NTf2, 6b: 95 °C;
PImC5 NTf2, 6c: 79 °C; PImC6 NTf2, 6d: 65 °C.
25. Ho, W. Y.; Hsieh, Y. N.; Lin, W. C.; Kao, C. L.; Huang, P. C.; Yeh, C. F.; Pan, C. Y.;
Kuei, C. H. Anal. Methods 2010. doi:10.1039/b9ay00313d.
20. Spectral data of PimC4Cl, 4b: 1H NMR spectra (200 MHz, D2O): d 8.92 (1H), 7.59
(2H), 4.33 (4H), 2.00 (4H).