Improved solubilization of pyromellitic dianhydride and 4,4′-oxydianiline in ionic liquid by the addition of zwitterion and their polycondensation

Article abstract of DOI:10.1016/j.tetlet.2007.01.012

Three different ionic liquids were prepared and examined as solvents for polyimide synthesis. The solubility of 4,4′-oxydianiline and pyromellitic dianhydride as starting materials in ionic liquids was first evaluated, and then their polycondensation was carried out. Although these starting materials were hardly soluble in 1-benzyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (3), addition of imidazolium type zwitterion, 1-(1-butyl-3-imidazolio)butane-4-sulfonate (ZI), certainly improved their solubility. When 3 containing 40 mol % ZI was used, nothing was phase separated from this mixed solution containing both starting materials after cooling down to room temperature. After preparing prepolymer in 3 containing 40 mol % of ZI at room temperature, polycondensation was carried out in the same solution at 100, 200, and then 300 °C for every 1 h to obtain polyimide. An inherent viscosity of the obtained polyimide (0.05 g in 10 ml concentrated sulfuric acid) was 1.3 dL g^-1, higher than that prepared in only 3 (0.9 dL g^-1). The higher average molecular weight of the polyimide was attributed to the improved solubility of the starting materials by the addition of ZI that enabled the preparation of the prepolymer, poly(amide acid), without heating before imidation.

Full text of DOI:10.1016/j.tetlet.2007.01.012

Tetrahedron Letters 48 (2007) 1553–1557
Improved solubilization of pyromellitic dianhydride
and 4,40-oxydianiline in ionic liquid by the addition
of zwitterion and their polycondensation
Masahiro Tamada, Takahiro Hayashi and Hiroyuki Ohno*
Department of Biotechnology, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
Received 1 November 2006; revised 27 December 2006; accepted 4 January 2007
Available online 7 January 2007
Abstract—Three different ionic liquids were prepared and examined as solvents for polyimide synthesis. The solubility of 4,40-oxy-
dianiline and pyromellitic dianhydride as starting materials in ionic liquids was first evaluated, and then their polycondensation was
carried out. Although these starting materials were hardly soluble in 1-benzyl-3-methylimidazolium bis(trifluoromethane sulfon-
yl)imide (3), addition of imidazolium type zwitterion, 1-(1-butyl-3-imidazolio)butane-4-sulfonate (ZI), certainly improved their sol-
ubility. When 3 containing 40 mol % ZI was used, nothing was phase separated from this mixed solution containing both starting
materials after cooling down to room temperature. After preparing prepolymer in 3 containing 40 mol % of ZI at room temperature,
polycondensation was carried out in the same solution at 100, 200, and then 300 ꢀC for every 1 h to obtain polyimide. An inherent
viscosity of the obtained polyimide (0.05 g in 10 ml concentrated sulfuric acid) was 1.3 dL gꢀ1, higher than that prepared in only 3
(0.9 dL gꢀ1). The higher average molecular weight of the polyimide was attributed to the improved solubility of the starting mate-
rials by the addition of ZI that enabled the preparation of the prepolymer, poly(amide acid), without heating before imidation.
ꢁ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Aromatic polyimides have excellent properties such as
thermal stability, chemical stability, mechanical
strength, and so on. Among these, polyimides prepared
from 4,40-oxydianiline (ODA) and pyromellitic dianhy-
Ionic liquids (ILs), composed of only ions, have a poten-
tial as liquid media with negligible vapor pressure, flame
resistance until decomposition temperature, high ionic
conductivity, electrochemical stability, and so on.1
Therefore, ILs have been studied as novel reaction med-
ia2 and ion conductive materials.3 Polymerization of
several monomers is one of the important subjects of
IL technology. From the viewpoint of the environmen-
tal care and safety, these polar and non-volatile ILs
should be required instead of volatile and toxic organic
solvents for the synthesis. In these studies, there are sev-
eral papers on the characterization of some ILs for poly-
merization.4 Most of these are based on free-radical
polymerization5 because of its widely applicable possi-
bility with small restriction. Against this, polycondensa-
tion, especially the synthesis of aromatic polyimides, is
quite difficult in ILs.
O
O
O
O
H2N
O
NH2
+
(a) ODA
O
O
(b) PMDA
H
O
N C
C OH
O
HO C
O
O
H
C N
O
n
PAA
O
O
N
Δ
O
N
-2n H2O
n
O
O
Keywords: Solubilization; Polycondensation; Polyimides; Ionic liquids;
Zwitterions.
PI
*
Corresponding author. Tel./fax: +81 42 388 7024; e-mail: ohnoh@
cc.tuat.ac.jp
Scheme 1. Synthesis of typical PI.
0040-4039/$ - see front matter ꢁ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.01.012

1554
M. Tamada et al. / Tetrahedron Letters 48 (2007) 1553–1557
dride (PMDA) (a and b, respectively, in Scheme 1) have
been most commonly studied and widely used.6 Hereaf-
ter, polyimide composed of ODA and PMDA is abbre-
viated as PI. Synthesis of PI is generally carried out by
two step reactions as also shown in Scheme 1. First,
both ODA and PMDA reacted generally in the amide
type polar solvents such as dimethylformamide to ob-
tain soluble poly(amide acid) (PAA) as a prepolymer.
Then, the solution of the prepared PAA was heated in
order both to proceed imidation and to remove solvents.
In these steps, especially the second step, the solvent was
vaporized to the environment and was also at risk of
ignition. ILs, which have negligibly small vapor pres-
sure, are expected as solvents for materials synthesis,
and some studies on the PI synthesis have already been
tried.7 In a previous study, polycondensation of various
starting materials including ODA and PMDA has been
carried out in 1,3-diisopropylimidazolium bromide.7b
The ILs containing halide anions are known as relatively
polar ILs and show better solubilizing ability of many
kinds of molecules. Also, biological polymers were
found to be soluble in some ILs containing halide
anions.8
liquid state at room temperature, (2) hydrophobic ILs,
and (3) thermally stable above 300 ꢀC.
2. Results and discussion
Figure 1 shows the structure and abbreviation of ILs
used in this study. We focus on the ILs containing imide
anion. ILs containing imide anion such as bis(trifluo-
romethane sulfonyl)imide (Tf2N) have advantages of
low melting point, low viscosity, hydrophobicity, ther-
mal stability, and so on.12 These characteristics are quite
favorable for the polycondensation media of PI. ILs
containing Tf2N (1, 2, and 3) were synthesized by apply-
ing the method as reported previously.12 Solubility of
the starting materials, ODA and PMDA, in 1, 2, and
3 was evaluated. These starting materials were insoluble
in ILs 1 and 2. On the other hand, 3 solubilized ODA
after heating (as shown in Table 1). However, dissolved
ODA in hot 3 was easily recrystallized after cooling the
solutions down to an ambient temperature. IL 3 also
solubilized PMDA but only slightly. At the initial stage,
ILs having Tf2N anions were concluded to be unsuitable
as a solvent for PI synthesis. It was requested to improve
the solibilizing ability of 3 by some polar additives.
However, these halide anion-containing ILs have a few
drawbacks as solvents for polycondensation reactions.
First, the melting point of halide-containing ILs is rela-
tively high. Almost all of these ILs containing halide an-
ions are solid at room temperature. It is quite important
to lower the reaction temperature for PAA synthesis,
because the molecular weight of PAA was reported to
be lower when the reaction was carried out at higher tem-
perature.9 Since the average molecular weight of PI
clearly reflects the molecular weight of the prepolymer
(PAA),10 the PAA should be prepared at a low tempera-
ture. However, halide-containing ILs were solid at room
temperature. These halide-containing ILs needed heating
when used as solvents. The ILs, having lower viscosity
and lower melting point than those ILs having halide an-
ions are quite favorable for the PAA syntheses. Second,
halide-containing ILs are hydrophilic and hygroscopic.
The reactivity of starting materials, especially anhydrous
dicarboxylic acid (e.g., PMDA), decreases considerably
in the presence of water.11 ILs containing halide anions
are generally hygroscopic, and thus, contaminated water
considerably lowers the reactivity. Therefore polycon-
densation in halide-containing ILs requires a special care
to avoid humidity from the reaction systems. Hydropho-
bic ILs are much more favorable for this point of view.
Even in the case of hydrophobic ILs, there are small
amounts of water. However, these water molecules are
not free but strongly hydrated. Accordingly, hydropho-
bicity is another important factor of ILs. Last, halide-
containing ILs generally decompose around 200 ꢀC.
The thermal imidation of prepolymers should be carried
out above 250 ꢀC, otherwise imidation was not com-
pletely proceeded. The decomposition temperature of
ILs containing halide anion was around 200 ꢀC, suggest-
ing insufficient imidation.
We have been studying zwitterionic type IL (ZI).13,14 ZIs
show unique characteristics because both cation and an-
ion of the IL were tethered.13 ZIs have similar character-
istics to general ILs except for a high melting point.
Although most ZIs are solid at room temperature, the
mixture of these solid ZIs with other solid salts or strong
acids is obtained as a liquid.13,14 The properties of solid
salts and acids are expected to be improved by adding
ZIs in many cases. In spite of the certain limitation of
the added salts and acids, ZIs are quite useful to liquid-
ize these salts and acids. These results as mentioned
above forced us to examine the properties of ILs after
mixing ZIs. The properties such as the solubility of start-
ing materials are expected to be improved by the ZIs.
We added ZIs to these ILs to improve the solubility of
both ODA and PMDA keeping their hydrophobicity
and thermal stability. The synthesis of ZI, 1-(1-butyl-
3-imidazolio)-butane-4-sulfonate (Fig. 1), will be men-
tioned in Section 4.3. The solubility of ODA and PMDA
in 3/ZI has also been evaluated. Table 1 shows the effect
of composition of 3 to ZI on the solubility of both ODA
and PMDA in the 3/ZI mixture. Solubility of the start-
R
=
1
2
3
N
N
R
O
O
N
F3C S
O
S CF3
O
ILs
N
N
SO3
ZI
In this study, we prepared ILs to satisfy the sufficient
solubility of both ODA and PMDA and the above three
requirements namely: (1) a low melting point, at least
Figure 1. Structure and abbreviations of ionic liquids and zwitterion
used in this study.

M. Tamada et al. / Tetrahedron Letters 48 (2007) 1553–1557
Table 1. Solubility (wt %) of ODA and PMDA in 3/ZI mixture
1555
mal properties of 1, 2, 3, and 3/ZI. A b value (hydrogen
bond basicity) of 1, 2, and 3 (0.22–0.24) was much lower
than that of DMF (0.69) in spite of higher a (hydrogen
bond acidity) and p* (polarizability) values. On the
other hand, b value of 3/ZI (0.57) was much higher than
that of 3 (0.22) in spite of the same a and p* values. The
b value of 3/ZI approached closely that of DMF (0.69).
BMICl is also compared and mentioned in Table 2 as a
typical IL, which is known to solubilize several valuable
materials, including biological polymers.8 These halide-
containing ILs are favorable for their high b value.
However, these ILs lack a few other properties as men-
tioned above. Addition of ZI was confirmed to elevate b
value, which is required for the solubilization of both
ODA and PMDA. The reason for the increased b value
by ZI addition will be reported elsewhere.
3: ZI/molar ratio
Temperature/ꢀC
20
50
80
7.5
7.0
7.0
5.5
5.0
(a) ODA
6:4
7:3
8:2
9:1
0
0
0
0.5
0
0.5
0.5
0.5
1.0
0.5
10:0
(b) PMDA
6:4
7:3
8:2
9:1
10:0
0
0
0.5
1.0
1.0
1.5
2
2.5
2.0
1.5
5.5
5.5
5.0
3.5
1.5
Thermal property of 3/ZI was evaluated as shown in
Table 2. Compound 3/ZI did not show melting point
but did a glass transition temperature at ꢀ48 ꢀC. The
decomposition temperature (10 wt % loss) was found
at 330 ꢀC. Compound 3/ZI was in a liquid-state at room
temperature and thermally stable above 250 ꢀC. The
temperature range of the liquid state of 3/ZI was much
wider than that of BMICl.
ing materials in 3 at 80 ꢀC was improved by adding ZI.
Even addition of 10 mol % ZI improved the solubility of
the starting materials, especially ODA, as seen in Table
1. Furthermore, in the mixture containing 20 mol % or
more amount of ZI, solubility of both ODA and PMDA
was significantly improved at 80 ꢀC. Especially, when
the mixture containing 40 mol % of ZI was used, no
component was recrystallized or phase separated from
this mixed solution by cooling down to room tempera-
ture. This improved solubility forced us to examine the
effect of ZI addition to other ILs (1 and 2, containing
40 mol % of ZI). However, the mixture of ZI with 1 or
2 showed a phase separation after a few days by keeping
them at room temperature. As described above, we
selected 3 containing 40 mol % ZI as a solvent for ODA
and PMDA. Hereafter, 3 containing 40 mol % ZI was
abbreviated as 3/ZI.
Since we finally got preferable solvents for both ODA
and PMDA, we solubilized and polymerized them in
both 3 and 3/ZI. Both systems gave brown powder after
polycondensation. Formation of imide bonds was con-
firmed by FT-IR measurement. For example, two peaks
at 1776 and 725 cmꢀ1 were characterized to the imide
groups according to the literature.6 The thermal proper-
ties of thus obtained PI were measured. DSC measure-
ment (from ꢀ130 to 200 ꢀC) and thermogravimetry
measurement (from 25 to 500 ꢀC) showed no peak and
no weight loss, respectively. These results are the same
as those for PI synthesized in DMF.
In order to discuss the effect of ZI on the solubility of
ODA and PMDA in 3, polarity of the ILs was evalu-
ated. Table 2 shows Kamlet–Taft parameters and ther-
Since PI was insoluble in organic solvents, average
molecular weight cannot be determined by ordinary
methods. Thus, the inherent viscosity of these PI was
measured in a concentrated sulfuric acid. The inherent
viscosity is an indicator of average molecular weight of
the polymers. Inherent viscosity is the ratio of the natu-
ral logarithm of the relative viscosity (the ratio of the
viscosity of the solution to the viscosity of the solvent)
to the mass concentration of the polymer. The PI pre-
pared in pure 3 and 3/ZI showed the inherent viscosity
of 0.9 and 1.3 dL gꢀ1, respectively. These values were
larger than 0.5 dL gꢀ1, which was previously reported
for the PI prepared in IL containing halide anion.7b
The average molecular weight of the PI prepared in
the present study was strongly suggested to be higher.
The effect of ZI was obvious, namely the solubility of
PMDA was considerably improved, and prepolymer,
PAA, was successfully prepared at room temperature.
Table 2. Kamlet–Taft parameters and thermal properties of ILs and 3/
ZI
Entries
Kamlet–Taft parameter
Thermal
properties/ꢀC
a
b
p
Tm
Td
1
0.71a
0.62b
0.57
0.23a
0.24b
0.22
0.98a
0.98b
1.03
ꢀ4d
453f,i
397j
381j
345j
2
ꢀ5e
3
ꢀ60h
ꢀ48h
73
ꢀ61
3/ZI
0.54
0.57
1.04
BMICl
DMF
0.41c
0
0.95c
0.69
1.17c
0.88
234g
153k
Tm: melting point, Td: decomposition temperature.
a Ref. 15.
b Ref. 16.
c Ref. 1 (p 97).
d Ref. 17.
e Ref. 12.
f Ref. 18.
g Ref. 19.
h Glass-transition temperature.
i Onset.
3. Conclusion
j 10 wt % loss.
k Boiling point.
A few ionic liquids were prepared and examined as sol-
vents for polyimide synthesis. The solubility of ODA

1556
M. Tamada et al. / Tetrahedron Letters 48 (2007) 1553–1557
and PMDA as starting materials in ionic liquids was
evaluated, and then the polycondensation of them was
carried out in the ionic liquids. Although ionic liquid,
1-benzyl-3-methylimidazolium bis(trifluoromethane sul-
fonyl)imide (3 in Fig. 1), was difficult to dissolve starting
materials, their solubility was significantly improved by
the addition of imidazolium type zwitterion, 1-(1-butyl-
3-imidazolio)butane-4-sulfonate (ZI), because of
increasing hydrogen-bond accepting ability. When poly-
condensation was carried out in 3 containing 40 mol %
of ZI, high average molecular weight of the polyimide
was obtained because PAA was prepared without
heating.
the solubility limit. The suspensions, which contain
insoluble fraction were heated stepwise on a hot plate
and the solubility was measured similarly. The highest
concentration thus observed at every temperature was
recorded as the solubility.
4.3. Synthesis of ZI (1-(1-butyl-3-imidazolio)butane-4-
sulfonate)
ZI was synthesized according to the previous reports.13
1-Butylimidazole (Aldrich) (6.2 g, 0.05 mol) was dis-
solved in acetonitrile (100 ml), and then 1,4-butanesul-
tone (TCI) (6.8 g, 0.05 mol) was added to the solution.
The solution was stirred under dry nitrogen at 80 ꢀC
for 7 days, and then acetonitrile was evaporated. The
obtained yellow powder was washed with acetone. It
was further purified by recrystallization from acetoni-
trile to give 10.5 g (82%) white solid. 1H NMR
(DMSO-d6, 500 MHz): d = 0.90 (t, J = 7 Hz, 3H), 1.26
(m, J = 7 Hz, 2H), 1.54 (m, J = 8 Hz, 2H), 1.77 (m,
J = 7 Hz, 2H), 1.89 (m, J = 7 Hz, 2H), 2.45 (t, J =
8 Hz, 2H), 4.18 (m, J = 8 Hz, 4H), 7.81 (d, J = 2 Hz,
2H), 9.24 (s, 1H).
4. Experimental
4.1. Synthesis of ILs
1-Methylimidazole (Wako, distilled, 1.6 · 10ꢀ2 mol) was
dissolved in acetonitrile (100 ml), and then excess of cor-
responding alkyl bromide (TCI) was added to the solu-
tion. The solution was stirred under dry nitrogen gas at
room temperature for 3 days. Then acetonitrile was
evaporated. The obtained transparent viscous liquid
was washed with ethyl acetate twice and dried at 45 ꢀC
under vacuum for 1 day. This prepared 1-alkyl-3-methyl-
imidazolium bromide (9.1 · 10ꢀ3 mol) was mixed with
equimolar lithium bis(trifluoromethane sulfonyl)imide
(LiTf2N, 3 M) in H2O (200 ml). After decantation, the
obtained transparent liquid was washed with H2O and
dried at 45 ꢀC under vacuum for 7 days. No bromide
ion was detected by the silver (I) nitrate titration test.
4.4. Synthesis of PI
Synthesis of PI was carried out by the following proce-
dure. ODA (TCI) (0.048 g, 2.4 · 10ꢀ4 mol) was added
to 3 or 3/ZI mixture (2.0 g) and then the solution was
heated to 70 ꢀC to dissolve it completely. After cooling
down the solution to room temperature, PMDA (Al-
drich) (0.052 g, 2.4 · 10ꢀ4 mol) was added to the solu-
tion. The mixture was stirred for 1 h at room
temperature, and then heated at 100, 200, and 300 ꢀC
for every 1 h. The prepared PI was washed with metha-
nol and the precipitate was separated by filtration. The
solid fraction was then dried under vacuum at 45 ꢀC
for 1 day. Yield: 73 mg, 73% (in 3) and 75 mg, 75% (in
3/ZI).
4.1.1.
1 (1-Ethyl-3-methylimidazolium Tf2N).
Methylimidazole was reacted with ethyl bromide (TCI);
yield 76%; 1H NMR (DMSO-d6, 500 MHz): d = 1.42 (t,
J = 8 Hz, 3H), 3.86 (s, 3H), 4.21 (m, J = 7 Hz, 2H), 7.76
(s, 1H), 7.82 (s, 1H), 9.16 (s, 1H).
4.1.2. 2 (1-Butyl-3-methylimidazolium Tf2N). The same
procedure as for the case of 1 was used with n-butyl bro-
mide (TCI) instead of ethyl bromide; yield 84%; 1H
NMR (DMSO-d6, 500 MHz): d = 0.90 (t, J = 8 Hz,
3H), 1.27 (m, J = 8 Hz, 2H), 1.77 (m, J = 6 Hz, 2H),
3.86 (s, 3H), 4.17 (m, J = 7 Hz, 2H), 7.72 (s, 1H), 7.79
(s, 1H), 9.17 (s, 1H).
4.5. Evaluation of inherent viscosity in PI
The inherent viscosity was determined by the following
measurement. PI (0.05 g) was dissolved in concentrated
sulfuric acid (10 ml) and the solution viscosity was mea-
sured with Ubbelohde-type viscometer (KRK, No. U-
3195) at 25.0 ꢀC.
4.1.3.
3 (1-Benzyl-3-methylimidazolium Tf2N). The
same procedure as above was applied with benzyl bro-
mide (TCI); yield 81%; 1H NMR (DMSO-d6,
500 MHz): d = 3.87 (s, 3H), 5.42 (s, 2H), 7.43 (m,
J = 5 Hz, 5H), 7.72 (s, 1H), 7.78 (s, 1H), 9.20 (s, 1H).
Acknowledgments
The present study was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan
(#17205020 and #17073005). The present study was car-
ried out under the 21st Century COE program, Future
Nano Materials. LiTf2N was a gift from Sumitomo 3M.
4.2. Solubility of ODA and PMDA in IL or IL/ZI
mixtures
ODA or PMDA (10 mg) was added in 2.0 g of IL or IL/
ZI mixture, and then the mixtures were stirred for 1 h.
After that, solubility of them was evaluated by the visual
observation. When the mixed solution turned clear, fur-
ther PMDA or ODA (10 mg) was added again to the
transparent solutions and repeated the addition until
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