Synthesis and characterization of n-substituted polybenzimidazoles

Article abstract of DOI:10.14233/ajchem.2013.13881

Soluble N-substituted polybenzimidazoles have been prepared by aromatic nucleophilic displacement reaction of N-H sites in 1,3-bis(2- benzimidazolyl)benzene (mBBIB) with activated aromatic difluorides containing sulfonyl and carbonyl groups in sulfolane at 210 °C in the presence of anhydrous potassium carbonate. 1,3-Bis(2-benzimidazolyl)benzene was synthesized from isophthalic acid and 1,2- phenylenediamine in polyphosphoric acid. The structure of 1,3-bis(2-benzimidazolyl)benzene was confirmed by FT-IR, HRMS, 1H and 13C NMR. The characterizations of the resulting polymers were performed with FT-IR, 1H NMR, elemental analysis, GPC, DSC, TGA and solubility tests. DSC and TG measurements showed that polymers had high glass transition temperatures (Tg > 210 °C) and good thermal stability with high decomposition temperatures (TD > 430 °C). These novel polymers also showed good solubility.

Full text of DOI:10.14233/ajchem.2013.13881

Asian Journal of Chemistry; Vol. 25, No. 7 (2013), 4013-4016
http://dx.doi.org/10.14233/ajchem.2013.13881
Synthesis and Characterization of N-Substituted Polybenzimidazoles
1
2
1
1
1
1,*
YEWEI XU , JIE TANG , GUANJUN CHANG , XUAN LUO , FANGHUA ZHU and LIN ZHANG
1Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, P.R. China
2School of Chemistry and Chemical Engineering, Mianyang Normal University, Mianyang, P.R. China
*Corresponding author: Fax: +86 816 2493148; Tel: +86 816 2484234; E-mail: xywchem@sina.com
(Received: 10 April 2012;
Accepted: 21 January 2013)
AJC-12742
Soluble N-substituted polybenzimidazoles have been prepared by aromatic nucleophilic displacement reaction of N-H sites in 1,3-bis(2-
benzimidazolyl)benzene (mBBIB) with activated aromatic difluorides containing sulfonyl and carbonyl groups in sulfolane at 210 ºC in
the presence of anhydrous potassium carbonate. 1,3-Bis(2-benzimidazolyl)benzene was synthesized from isophthalic acid and 1,2-
phenylenediamine in polyphosphoric acid. The structure of 1,3-bis(2-benzimidazolyl)benzene was confirmed by FT-IR, HRMS, 1H and
13C NMR. The characterizations of the resulting polymers were performed with FT-IR, 1H NMR, elemental analysis, GPC, DSC, TGA
and solubility tests. DSC and TG measurements showed that polymers had high glass transition temperatures (Tg > 210 ºC) and good
thermal stability with high decomposition temperatures (TD > 430 ºC). These novel polymers also showed good solubility.
Key Words: N-Substituted, Polybenzimidazole, Thermal properties, Solubility.
thermal stability. In addition, pursuing to explore higher
molecular weight polybenzimidazoles with different structures,
sulfonyl and carbonyl groups in the polybenzimidazole
polymers would be increased solubility and flexibility12.
In this paper, the synthesis of 1,3-bis(2-benzimidazolyl)-
benzene (mBBIB) is reported along with the preparation of
N-substituted polybenzimidazoles from the N-H sites in
benzimidazole ring and bis(4-fluorophenyl) sulfone or 4,4'-
difluorobenzophenone. Because of the incorporation of
sulfonyl and carbonyl groups in the polybenzimidazole back-
bone and the reaction of the reactive benzimidazole N-H sites,
the resulting polymer exhibited good thermal resistance, high
glass transition temperature values and good solubility in
organic aprotic solvents.
INTRODUCTION
Due to outstanding physical properties, high performance
polymers were considered the most promising class of polymers
for special applications to automotive, aircraft and spacecraft
industries1. However, only a few types have been commercia-
lized because of their high production costs. Polybenzimidazoles
(PBIs) is one example. They are materials with high thermal
stability, excellent chemical resistance, retention of stiffness
and toughness2,3. However, the poor solubility of polybenzimi-
dazoles, a resulting of their highly rigid backones, is a major
barrier to their application. Thereby persistent efforts have been
made to improve the solubility by modifying the polymer struc-
tures of polybenzimidazoles while maintaining their desirable
properties. These modifications are accomplished in two ways,
either by synthetically modifying the monomers prior to poly-
merization or by the post-polymerization substitution of the
polymer at the reactive benzimidazole N-H sites4-8. However,
few papers were published on modifying the monomers prior
to polymerization at the reactive benzimidazole N-H sites9. In
air at 350 ºC, the concentration of N-H groups gradually and
continuously decreases over time. Hay et al.10,11, first reported
that the N-H group in benzimidazoles could undergo a C-N
coupling reaction with activated halides to produce linear
polymers with high molecular weight at appropriate tempe-
rature and these N-substituted homopolymers and copolymers
showed very high glass transition temperatures and pronounced
EXPERIMENTAL
All chemicals and reagents, except where otherwise speci-
fied, were purchased from Aldrich, Acros and TCI Chemical
Co. and used as received. All solvents were further purified
before use.
FT-IR spectra were recorded on a Nicolt 6700 FT-IR spectro-
meter for the measurement of infrared absorption spectra for
monomers and polymers. NMR spectra were recorded using
a Brucker NMR spectrometer in DMSO-d6. Mass spectra were
recorded onVGAutospec 3000 for ESI MS. Elemental analyses
were performed using a Vario EL III analyzer. The molecular
weights and molecular weight distributions were estimated

4014 Xu et al.
Asian J. Chem.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
by gel permeation chromatography (GPC) on a Wyatt DAWN
HELEOS using DMF as an eluent. Differential scanning calori-
metry (DSC) was recorded on a 910-TA instrument in a nitrogen
atmosphere at a heating rate of 10 ºC/min. Thermogravimetric
analysis (TGA) was conducted using a TA-SDT Q600 thermal
analysis in air or nitrogen atmosphere at a heating rate of 10
ºC/min.
mBBIB
Synthesis of 1,3-bis(2-benzimidazolyl)benzene (mBBIB):
A 25 mL three-neck round-bottomed flask equipped with a
magnetic stirrer, a water cooled condenser and an argon inlet
and outlet was charged with 1,2-phenylenediamine 0.454 g
(4.2 mmol), isophthalic acid 0.332 g (2 mmol) and 10 mL
polyphosphoric acid (PPA). The temperature was slowly raised
to 200 ºC and the reaction mixture was heated for 6 h. The
resulting solution was allowed to slowly cool to room tempe-
rature and subsequently poured into cold water. Following drop
15 % NaOH solution and neutralize to pH = 8. Then filtered
through celite, water washed and dried. The obtained crude
product was re-crystallized from ethanol and dried in vacuum
at 60 ºC. The yield is 94 %.
3500
3000
2500
2000
1500
1000
500
-1
)
Wavenumber (cm
Fig. 1. FTIR spectrum of mBBIB
of the isolated non-hydrogen bonded N-H groups of benzimidazole
ring. The characteristic absorptions for aromatic C-H groups
were detected at 3064 cm-1. Fig. 2 shows the signal assignments
Synthesis of N-substituted polybenzimidazoles: A 25
mL three-necked round-bottom flask was equipped with a
magnetic stirrer, a water cooled condenser and an argon inlet
and outlet.To this,we added 2 mmol of 1,3-bis(2-benzimidazolyl)-
benzene, 2 mmol of difluoride derivative and 4 mmol of
anhydrous potassium carbonate in 10 mL of sulfolane and 3
mL of chlorobenzene. The reaction mixture was heated to
azeotrope off the resulting water and the chlorobenzene. The
chlorobenzene was then removed and the reaction mixture was
brought up to 210 ºC for 10 h. The reaction mixture was cooled
down and the solution was poured into water to precipitate
out the polymer. Then the polymer was collected by filtration
and dried in a vacuum at 80 ºC for 24 h.
1
of the H NMR spectrum of mBBIB. The spectrum reveals
aromatic protons at 7.0-9.1 ppm and imidazole N-H protons
at 13 ppm. Furthermore, the m/e of the fragments in high
resolution mass measurements and the signal assignments of
the 13C NMR spectrum were in agreement with the predicted
structures of the monomer.
RESULTS AND DISCUSSION
Monomer synthesis:As shown in Scheme-I, the synthesis
of 1,3-bis(2-benzimidazolyl) benzene (mBBIB) is performed
via the reaction of isophthalic acid with 1,2-phenylenediamine
in polyphosphoric acid (PPA), as the condensation agent and
solvent. Polyphosphoric acid is a very convenient dehydration
agent for the preparation of benzimidazole from acid in a single
step.
Fig. 2. 1H NMR spectrum of mBBIB
Polymer synthesis: N-substituted polybenzimidazoles
were prepared from mBBIB with bis(4-fluorophenyl) sulfone
or 4,4'-difluorobenzophenone under the existence of anhydrous
potassium carbonate at 210 ºC. The detailed prepared process
is shown in Scheme-II and the yields of the polymerization
reactions were above 92 % (Table-1).
The structure of mBBIB was confirmed by FT-IR, HRMS,
1H and 13C NMR. Fig. 1 presents the FT-IR spectrum of
mBBIB. No residual carbonyl absorption was observed between
1780 and 1650 cm-1, suggesting the nearly complete closure
of the imidazole rings under the reaction condition and the
characteristic benzimidazole bands were clearly observed at
1630 cm-1 (C=N/C=C stretching), 1570 cm-1 (ring vibration
of conjugation between benzene and imidazole rings) and 1530
cm-1 (in-plane ring vibration of 2-substituted benzimidazole).
The bands at 3167 cm-1 were ascribed to the stretching vibration
The structures of N-substituted polybenzimidazoles were
1
confirmed by FT-IR, H NMR and elemental analysis. The
FT-IR spectra of the synthesized polymers are shown in Fig. 3.
The characteristic benzimidazole bands and characteristic
absorptions for aromatic C-H groups could also be observed.
The characteristic absorption bands for sulfonyl groups at
around 1070 cm-1 and carbonyl groups at around 1660 cm-1
appeared and the stretching vibration of the isolated non-
H
H2N
200 oC
PPA
Scheme-I: Synthesis of mBBIB
N
N
HOOC
COOH
N
N
H
H2N

Vol. 25, No. 7 (2013)
Synthesis and Characterization of N-Substituted Polybenzimidazoles 4015
H
N
N
F
R
F
N
N
H
R
K2CO3
210 oC
O
S
P1:
P2:
sulfolane
O
N
N
N
O
C
N
R
n
Scheme-II: Synthesis of N-substituted polybenzimidazoles
TABLE-1
CHARACTERIZATION OF N-SUBSTITUTED POLYBENZIMIDAZOLES
Polymer code
Mna
Mwa
26400
41900
Mw/Mna
T5 (ºC)b N2
T5 (ºC)c air
Tg (ºC)d
Yield (%)
P1
P2
12000
13500
2.2
439
468
211.3
230.2
92
94
3.1
519
543
aThe soluble fraction in DMF calibrated by GPC polystyrene standards. b5 % weight loss temperature under N2 with heating rate 10 ºC/min. c5 %
weight loss temperature under air with heating rate 10 ºC/min. dObtained by DSC under N2 with heating rate 10 ºC/min.
1.1
1.0
1660
0.9
0.8
0.7
0.6
0.5
P2
0.4
0.3
0.2
0.1
0.0
-0.1
1070
P1
Fig. 4. 1H NMR spectra of P1 and P2
110
100
90
3500
3000
2500
2000
1500
1000
500
-1
)
Wavenumber (cm
Fig. 3. FT-IR spectra of P1 and P2
80
hydrogen bonded N-H groups at 3175 cm-1 disappeared. Fig. 4
shows the 1H NMR spectroscopic data of P1 and P2. The spectra
show the disappearance of imidazole N-H protons at around
13 ppm in the case of unsubstituted mBBIB. The integration
of the aromatic protons (between 7.0 and 8.5 ppm) belonging
to the added phenylsulfonyl and benzophenone groups indicated
successful aromatic nucleophilic displacement reaction. Further-
more, the elemental analysis data were in agreement with the
predicted structures of P1 and P2.
70
P1 in N
2
P1 in air
60
50
P2 in N
2
P2 in air
40
30
0
200
400
600
o
800
1000
The molecular weights of the reprecipitated polymers
were measured by GPC (calibrated by polystyrene standards).
Mn and Mw/Mn of P1 were 12000 and 2.2, respectively. P2
was 13500 and 3.1.
Temperature ( C)
Fig. 5. TGA of P1 and P2 in nitrogen and in air
and 510 ºC in air, respectively (Table-1). The 50 % weight
losses take place at over 900 ºC in nitrogen. Char yield is an
important indicator. It is correlated to the ability to sustain
combustion. For these polymers, the char yields in nitrogen at
800 ºC were up to 60 %.
Thermal behaviours: The synthesized polymers showed
notable thermal stability and a high char yield in nitrogen at
500 ºC. Fig. 5 shows the primary therogravimetric curves of
P1 and P2 obtained by dynamic TGA. TG curves reveal that
the polymers are thermally stable at up to 440 ºC in nitrogen

4016 Xu et al.
Asian J. Chem.
TABLE-2
SOLUBILITY OF N-SUBSTITUTED POLYBENZIMIDAZOLESa
Polymer code
DMAc
++
DMF
++
NMP
++
DMSO
++
CH2Cl2
+-
CHCl3
+-
THF
--
H2SO4
++
P1
P2
++
++
++
+-
+-
+-
--
++
a++: Solid polymer was completely dissolved at room temperature; +-: Solid polymer was partly dissolved at 50 ºC; --: Solid polymer could be
swelled at 50 ºC.
A differential scanning calorimeter was used to determine
the glass transition temperatures (Tgs) of N-substituted
polybenzimidazoles. Obvious Tgs were observed upon the
second DSC heating traces. DSC thermograms of P1 and P2
are shown in Fig. 6. The thermograms of the P1 and P2 reveal
them to have almost the same differential scanning calorimetry
characteristics.A single glass transition temperature was obtained
and Tgs of P1 and P2 were 211.3 and 230.2 ºC, respectively
(Table-1). Tgs of the resulting polymers were found to lower
than that of traditional PBIs (Tg > 400 ºC)2. This decrease in
Tg could be correlated to the elimination of hydrogen bonding
after the N-substitution.
carbonyl groups in the polybenzimidazole backbone and the
disappearance of the N-H sites from the benzimidazole rings,
which hinders dense chain packing, thereby reducing the
interchain interactions and enhancing solubility.
Conclusion
A convenient synthetic route has designed to prepare 1,3-
bis(2-benzimidazolyl) benzene in high yield and high purity.
It has been used as condensation monomer, in combination
with bis(4-fluorophenyl) sulfone or 4,4'-difluorobenzophenone,
to yield novel soluble N-substituted polybenzimidazoles. The
reaction of the reactive benzimidazole N-H sites and the combi-
nation of sulfonyl and carbonyl groups along with the polymer
backbone remarkably endow the current polymers with some
special characteristics, such as good thermal resistance, high
glass transition temperature values and good solubility in
common organic solvents. The synthesized polymers may have
potential as one of the new solution-tolerant, high-temperature
polymeric material.
P1
P2
ACKNOWLEDGEMENTS
o
230.2
C
This work was financially supported by the Science and
Technology Development Foundation of China Academy of
Engineering Physics (Grant No. 2012B0302050) and the
Foundation of Double-Hundred Talents of China Academy of
Engineering Physics (Grant No. ZX8078).
o
211.3
C
0
50
100
150
200
o
250
300
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