Preparation and characterization of new soluble benzimidazole-imide copolymers

Article abstract of DOI:10.1039/b205150h

The present necessity to use heat-resistant materials in electronics justifies the scientific interest in different heterocyclic polymers. This paper is especially concerned with the preparation of novel heat-resistant polyimides having bisbenzimidazole moieties in the main chain and their applications as dielectric films. A soluble copolyimide was prepared by a two-step synthesis from aromatic dianhydrides and aromatic diamines. The bisbenzimidazole diamine was prepared by reduction of the corresponding dinitro compound. The diamine was reacted with various aromatic dianhydrides to prepare a series of alternating benzimidazole-imide copolymers via the poly(amic acid) precursors and thermal or chemical imidization. Monomers and polymers were characterized by conventional methods and their physical properties such as solution viscosity, solubility properties, thermal stability and thermal behaviour were studied. All copolymers were obtained in high yields having inherent viscosities η_inh that ranged from 0.60 to 0.98 dL g^-1. Thin films of the copolymer were tough and flexible, having tensile strengths as high as 100 MPa. Glass transition temperatures were observed between 275 and 328°C. Thermogravimetric analyses indicated that the thermal degradation of poly(benzimidazole-imide) occurs around 530°C, which is ca. 80°C higher than polyimide, confirming that the introduction of the bisbenzimidazole component improved the thermal stability of polyimide.

Full text of DOI:10.1039/b205150h

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Preparation and characterization of new soluble
benzimidazole–imide copolymers
a
M. Berrada,* F. Carriere, Y. Abboud, A. Abourriche, A. Benamara, N. Lajrhed,
b
c
c
c
c
c
M. Kabbaj and M. Berrada
c
a
Biosyntech Canada Inc, 475 Armand-Frappier, Laval, Quebec, Canada H7V 4B3.
E-mail: berrada@biosyntech.com
b
c
Laboratoire de Chimie Macromol e´ culaire, UMR 7610, Universit e´ Pierre et Marie Curie,
Tour 44, BP.185, 4 Place Jussieu, 75252 Paris Cedex 05, France
Universit e´ Hassan II, Facult e´ des Sciences Ben M’sik sidi Othmane, Casablanca, Morocco
Received 28th May 2002, Accepted 25th September 2002
First published as an Advance Article on the web 16th October 2002
The present necessity to use heat-resistant materials in electronics justifies the scientific interest in different
heterocyclic polymers. This paper is especially concerned with the preparation of novel heat-resistant
polyimides having bisbenzimidazole moieties in the main chain and their applications as dielectric films.
A soluble copolyimide was prepared by a two-step synthesis from aromatic dianhydrides and aromatic
diamines. The bisbenzimidazole diamine was prepared by reduction of the corresponding dinitro compound.
The diamine was reacted with various aromatic dianhydrides to prepare a series of alternating
benzimidazole–imide copolymers via the poly(amic acid) precursors and thermal or chemical imidization.
Monomers and polymers were characterized by conventional methods and their physical properties such as
solution viscosity, solubility properties, thermal stability and thermal behaviour were studied. All copolymers
2
1
were obtained in high yields having inherent viscosities ginh that ranged from 0.60 to 0.98 dL g . Thin films of
the copolymer were tough and flexible, having tensile strengths as high as 100 MPa. Glass transition
temperatures were observed between 275 and 328 uC. Thermogravimetric analyses indicated that the thermal
degradation of poly(benzimidazole–imide) occurs around 530 uC, which is ca. 80 uC higher than polyimide,
confirming that the introduction of the bisbenzimidazole component improved the thermal stability of
polyimide.
weight poly(amic acid) is generated at low or ambient
temperatures in the first stage, and is subsequently converted
via bulk, chemical or solution processes to fully cyclized
1
. Introduction
Polybenzimidazoles and polyimides are classes of high-
temperature/high-performance polymers already commercia-
7
,8
polyimide during the second stage.
1
lized. They are heterocyclic polymers that are attractive
because of their outstanding mechanical and dielectric pro-
Polybenzimidazoles (PBIs) are a class of high-performance
polymers that have received considerable attention in recent
years because of their outstanding ability to withstand extreme
conditions without an extensive loss of properties. They have,
for example, been widely used in the aerospace industry, where
thermal stability is a primary requirement. However, whereas
in inert atmospheres the total weight loss of polybenzimida-
zoles is negligible up to 600 uC, a much more pronounced effect
has been observed for the same temperature range in oxidizing
environments. PBIs are generally prepared via melt polymeri-
zation of aromatic bis(o-diamine)s with aromatic dicarboxylic
2
perties at high temperatures. They have been widely used for
high temperature aerospace applications (Fig. 1).
Polyimides (PIs) are well known for their outstanding
mechanical properties and thermal stability. Several poly-
imides have been developed that are suitable as adhesives,
coatings and matrix resins, and have found a wide range of
applications as high-performance materials in the aerospace
and electronics industries. However, further development of
polyimides is required to meet the increasing demands on
high-performance materials. In advanced polymer composites,
PIs are just one of a number of resin families currently in use.
There is a well-established need to produce polyimide resins
that are processable and cost-effective, and can withstand
3
–5
9
acid derivatives. However, the reactions must be carried out at
very high temperatures. Solution polymerizations were per-
1
formed in poly(phosphoric acid), sulfolane and diphenyl
0
1
sulfone. Other synthetic routes used to prepare PBIs include
1
6
the alkoxide-catalyzed reaction of aromatic bis(o-diamine)s
temperatures of 316–371 uC for extended periods of time.
Polyimides are commonly synthesized from dianhydrides and
diamines in a two-stage process: a soluble, high-molecular
1
2
with dinitriles and the reaction of the bis(bisulfite adduct)s of
1
aromatic dialdehydes with aromatic bis(o-diamine)s. PBIs
3
1
4
have only limited solubility and are intractable and infusible
and consequently difficult to process. Several approaches
currently are being pursued to improve the processability of
PBI polymers. Putting units in the polymer that provide more
flexibility can overcome this problem. For example, O, CH
1
2
O,
SO , and CMe units have been used for this purpose. Other
5
2
2
approaches to modify the properties of PBIs are to make PBI
1
6
copolymers, to make mixed polymers such as PBI-poly-
1
7
18
imides or PBI-polyamides, or to blend them with other
1
polymers. As co-monomers for PBI synthesis, the aromatic
9
Fig. 1 Aromatic polybenzimidazoles and polyimides.
DOI: 10.1039/b205150h
J. Mater. Chem., 2002, 12, 3551–3559
This journal is # The Royal Society of Chemistry 2002
3551

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bis(o-diamines) are also expensive and difficult to purify. There-
fore, there is incentive to develop new alternative synthetic
routes to these materials.
The present work is especially focused on the development of
processable and fully cyclized poly(benzimidazole–imide)s
copolymers with very high glass transition temperatures to
meet many of these demands (Fig. 2).
2.2. Purification of solvents
Dimethyl sulfoxide (DMSO) was distilled over sodium chloride
and calcium hydride using a column packed with glass helices.
1
-Methyl-2-pyrrolidinone (NMP) was purified by vacuum
distillation over phosphorus pentoxide. N,N-dimethylaceta-
mide (DMAc) was distilled over calcium hydride. Toluene was
washed twice with sulfuric acid, water, 5% aqueous bicarbo-
nate then with water. It was dried over calcium sulfate, then
phosphorus pentoxide and distilled over sodium. The other
reagents and solvents were used after appropriate purification
in the normal manner.
To date, the preparation of copolymers based on aromatic
2
0–25
benzimidazoles and imides is not well known.
This
deficiency is partly due to the experimental difficulties
encountered in the synthesis of benzimidazoles. In the past,
we have shown that incorporation of the benzhydrol
tetracarboxylic dianhydride (BHTDA) along with the utiliza-
tion of molecular weight control and solution cyclization
2
.3. Analytical equipment and techniques
2
6
methods result in processable, thermally stable polyimides.
Using these techniques, we have incorporated novel diamine
monomers based on bisbenzimidazole in polyimides.
Melting points were measured in capillaries on a B u¨ chi
apparatus (Model BUCHI 535). Infrared spectra were recorded
on a Bruker IFS 45 infrared Fourier transform spectrometer.
The FT-IR samples were prepared by casting films of the
materials on KBr plates. The FT-IR spectrometer acquired
The present paper describes some recent investigations on
the synthesis of poly(benzimidazole–imide)s copolymers that
are performed to enhance the thermal properties of conven-
tional polyimides. The route to these copolymers involved the
synthesis of 2,2’-alkylene- or arylene-bis(5-nitrobenzimida-
2
1 1
4 complete IR spectra with a spectral resolution of 4 cm . H
1
3
(200 MHz) and C (50.3 MHz) NMR spectra were obtained
with a Bruker ACE 200 spectrometer. Chemical shifts were
given in parts per million from tetramethylsilane at 0 ppm. The
structures of compounds were confirmed by elemental analysis
2
7
zole)s, which are then reduced into the corresponding
bis(5-amino) compounds. The latter are condensed with
2
8,29
(
Analytical Department of ICSN–CNRS 91198 Gif sur Yvette,
aromatic tetracarboxylic anhydrides
to yield benzimida-
France). Viscosity was measured in NMP at 30 uC using a
Canon Ubbelohde type viscometer. Differential scanning
calorimetry (DSC) analysis was performed with a Mettler
zole–imide copolymers. An unsubstituted 2,2’-(1,2-phenylene)-
bisbenzimidazole (o-PBI), is obtained by condensation of 1,
2
-phenylene diamine with phthalic anhydride. The nitration
2
1
1
2E at a heating rate of 10 uC min . The apparatus was
of o-PBI followed by the reduction of the dinitro compound
leads to the 2,2’-(1,2-phenylene)-bis(5-aminobenzimidazole),
21
calibrated with Indium and Zinc standards, at 10 uC min
.
The lag between the sample and pan holder temperatures was
also taken into account. Glass transition temperatures (T ’s)
(
o-P5ABI). When condensed either with aromatic dianhydrides
g
or with phthalic anhydride, the o-P5ABI gives respectively
alternating copolymers or a model compound. The thermal
properties of these copolymers were determined by DSC and
TGA.
were read at the middle of the change in the heat capacity and
were taken from the second heating scan after quick cooling.
Thermogravimetric analysis (TGA) was performed under air
2
1
.
atmosphere on a DuPont 9900 at a heating rate of 10 uC min
The temperature and the weight scales were calibrated using
high-purity standards (nickel and iron) at the specific heating
rate with a calibration parameter of their respective Curie
points. The high-resolution mass spectra were recorded on a
MAT 311 Varian Spectrometer with C.P.G. couplage (Analy-
tical Department of the University of Rennes 1, France).
Dielectrical measurements were carried out in the temperature
range 30–350 uC and at frequencies 1–200 kHz with an
Analyzer 2970 TA Instrument. Tensile properties were deter-
mined from stress–strain curves obtained by an Orientec
Tensilon with a load cell of 10 kg. A gauge of 3 cm and a strain
2
. Experimental methods
2
.1. Materials
1
,2-Phenylene diamine was recrystallized from absolute
ethanol. Phthalic anhydride was purified by sublimation and
polyphosphoric acid (Aldrich) was used as received. The
aromatic dianhydrides, 4,4’-oxydiphthalic anhydride (ODPA),
4
,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)
and benzophenone tetracarboxylic dianhydride (BTDA), were
obtained from Merck and used without further purification.
Benzhydrol tetracarboxylic dianhydride (BHTDA) was pre-
pared as reported by Berrada et al. and purified by Soxhlet
extraction with diethyl ether and dried in a vacuum to give the
2
1
rate of 2 cm min were used for this study. Measurements
were performed at room temperature with film specimens (0.4 cm
wide, 6 cm long, and ca. 0.05 mm thick).
3
0
1
2.4. Synthesis of monomers
product in 75% yield, mp 183 uC; H NMR (dimethylsulfoxide-
d ) d 8.1–8.4 ppm (m, Ar H, 6H), 6.81 ppm (d, OH, 1H),
6
2.4.1. Total synthesis of 2,2’-(1,2-phenylene)-bis(5-aminoben-
zimidazole), 5a (Scheme 1). 1- 2,2’-(1,2-Phenylene)-bisbenzimi-
dazole, 3a (o-PBI). 1,2-Phenylene diamine, 1, (10.8 g, 0.1 mol)
was added under N2 to poly(phosphoric acid) (PPA, 90 g).
2
Phthalic anhydride, 2, (7.4 g, 0.05 mol) was added under N to
the well-stirred mixture, then the solution was heated to 180 uC
1
.23 ppm (d, –CH–, 1H); C NMR (DMSO-d ) d 163.17,
3
6
1
6
62.97, 153.21, 134.01, 131.93, 130.43, 125.79, 122.81, 72.66;
2
1
21
IR (KBr) n 3517 cm (s, OH), 1858 and 1784 cm (s, CO).
Anal. Calcd. for C17
H, 2.68.
8
H O
7
: C, 62.96; H, 2.46. Found: C, 62.75;
for 3 h. The solution was cooled to 100 uC and poured into ice–
water (60 mL). The precipitate in suspension in water was
neutralized with solid sodium carbonate (2 g), filtered, and then
washed several times with water. The crude product was
refluxed in absolute ethanol (500 mL), the insoluble fraction
was filtered, dried and sublimed to give pure white crystals.
1
Yield 85%, mp 114–116 uC; H NMR (DMSO-d
H, ArH), 8.02 (m, 2H, ArH), 7.62 (m, 4H, ArH), 7.50 (m, 4H,
6
) d 8.13 (m,
2
1
3
ArH); C NMR (DMSO-d
14.72; C8, 133.85; C9, 132.69; C10, 123.89; C11, 126.92; C12,
133.87.
6
) C2, 146.96; C4, 101.53; C5,
1
Fig. 2 Novel class of high-temperature/high-performance polymers.
552 J. Mater. Chem., 2002, 12, 3551–3559
3

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Scheme 1 Synthesis of 2,2’-(1,2-phenylene)-bis(5-aminobenzimidazole).
2
-2,2’-(1,2-Phenylene)-bis(5-nitrobenzimidazole), 4a (o-P5NBI).
yellow crystals.
Yield 78%; mp w 300 uC; H NMR (DMSO-d
(m, 2H, ArH), 7.94 (m, 2H, ArH), 7.73 (d, 2H, ArH), 7.63 (s,
1
This compound was obtained by nitration of o-PBI, 3a.
Thus, 4 g (0.013 mol) of 3a were dissolved into concentrated
6
, TFA) d 8.14
1
3
H
2
SO
bath to maintain the temperature below 50 uC. A mixture of
fuming HNO (1.385 mL) and concentrated H SO (1.184
4
(11 mL), and the solution was cooled in an ice–water
6
2H, ArH), 7.37 (d, 2H, ArH); C NMR (DMSO-d , TFA) C2,
149.99; C4, 108.26; C5, 134.97; C6, 119.29; C7, 115.57; C8,
130.02; C9, 133.11; C10, 125.55; C11, 131.39; C12, 131.88; IR
3
2
4
2
1
mL) was added dropwise in ca. 10 min. Then the solution
was stirred for 1 h and poured into an ice–water mixture.
The raw product was filtered, washed with water, neutralized
with sodium carbonate until pH 9, and washed with water to
give raw 4a. The solid was then refluxed twice in ethanol and
filtered. The recrystallization from tetrahydrofuran (THF)
(KBr) d 1626 (CN) and 3250 cm (NH). Anal. Calcd. for
: C, 70.57; H, 4.74; N, 24.69. Found: C, 70.47; H,
4.70; N, 24.65. High-resolution MS m/z: found 340.1441
(Calcd. for C20 , 340.14364).
20 16 6
C H N
16 6
H N
2.4.2. Synthesis of other bisbenzimidazole diamines, 5c–h
(Scheme 2). The synthesis and characterization of dinitro
gave pale yellow crystals.
1
31
Yield 60%; mp w 300 uC; H NMR (DMSO-d
.44 (s, 2H, ArH), 8.18 (d, 2H, ArH), 8.07 (m, 2H, ArH), 7.89
6
, TFA) d
compounds, 4b–h (see Fig. 3), were previously described.
Other diamines were prepared by reduction of the correspond-
ing dinitro compound using the above procedure.
8
(
1
3
m, 2H, ArH), 7.72 (d, 2H, ArH); C NMR (DMSO-d
C2, 150.79; C4, 111.38; C5, 135.65; C6, 122.14; C7, 115.45; C8,
46.46; C9, 131.76; C10, 122.56; C11, 133.43; C12, 134.63; IR
6
, TFA)
1
2,2’-(4,4’-Oxybisphenylene)-bis(5-aminobenzimidazole), 5c
(Ox5ABI). The recrystallization from ethanol gave a white-
2
1
(
(
KBr) n 1626 (CN); 1512 (NO); 1342 (NO) and 3250 cm
NH). Anal. Calcd. for C20 : C, 60.00; H, 3.02; N,
0.99. Found: C, 59.82; H, 2.92; N, 20.85 (Calcd. for
C H N O 400.031, Found 400.030).
1
H
12
N
6
O
4
orange solid. mp w 300 uC, H NMR (DMSO-d ) d 8.25 (d,
6
2
4H, ArH), 7.52 (d, 2H, ArH), 7.35 (d, 4H, ArH), 6.86 (s, 2H,
1
ArH), 6.72 (d, 2H, ArH); C NMR (DMSO-d ) C2, 149.35;
3
2
0
12
6
4
6
3
-2,2’-(1,2-Phenylene)-bis(5-aminobenzimidazole), 5a (o-P5ABI).
C4, 97.08; C5, 145.70; C6, 112.64; C7, 117.93; C8, 135.02; C9,
139.30; C10, 127.25; C11, 128.61; C12, 119.97; C13, 157.87; IR
Compound 4a (1 g, 0.025 mol) was dissolved into glacial acetic
acid (100 mL). Tin chloride (7 g, 0.0037 mol) was dissolved into
concentrated hydrochloric acid (8 mL); both solutions were
heated to boiling, then the acetic acid solution was added
dropwise to the tin chloride solution in ca. 30 min. The mixture
was cooled to 5 uC and a bright orange precipitate was filtered,
washed with water and dried for 3 h at 50 uC under vacuum.
The crude product was then refluxed in ethanol containing
active charcoal. The recrystallization from ethanol gave pale
2
(KBr) n 1636 (CN) and 3250 cm (NH). Anal. Calcd. for
1
C H N O: C, 63.41; H, 3.27; N, 17.07. Found: C, 63.38; H,
6
2
6
20
3.20; N, 17.05.
2,2’-(4,4’-Hexafluoroisopropylidene)-bis(5-aminobenzimida-
zole), 5e (H5ABI). The recrystallization of the crude product
1
from ethanol gave a yellow solid. mp w 300 uC; H NMR
(DMSO-d ) d 8.17 (d, 4H, ArH), 7.49 (d, 4H, ArH), 7.32 (d,
6
Scheme 2 Synthesis of bis(aminobenzimidazolyl) compounds.
J. Mater. Chem., 2002, 12, 3551–3559
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Fig. 3 Dinitro compounds and their corresponding diamines used for the synthesis of the copolymers.
1
H, ArH), 6.77 (s, 2H, ArH), 6.60 (d, 2H, ArH); C NMR
6
DMSO-d ) C2, 153.42; C4, 97.46; C5, 148.60; C6, 98.31; C7,
3
2
(
atmosphere. The reaction mixture was stirred at room
temperature, and then toluene (5 mL) was added as an
azeotropic agent. The solution was heated to 140 uC for 3 h to
dehydrate it then to 190 uC and kept at this temperature for 4 h.
The solution was cooled to room temperature, and the model
compound 6 was precipitated into methanol (50 mL). It was
washed with distilled water then with methanol to remove
occluded reagents such as phthalic anhydride. The crude
product was recrystallized from acetone, and dried under high
vacuum for 16 h at 150 uC to yield the pure model compound
1
1
1
13.28; C8, 131.97; C9, 145.27; C10, 129.03; C11, 130.90; C12,
26.55; C13, 133.51; C14, 53.19; C15, 144.15, 142.13, 121.82,
18,11; IR (KBr) n 1627 (CN) and 3250 cm (NH). Anal.
: C, 55.60; H, 2.57; N, 13.41. Found: C,
5.47; H, 2.43; N, 13.40.
2
1
20 6 6
Calcd. for C29H N F
5
2
recrystallization of the crude product in ethanol gave a white
,2’-Ethylene-bis(5-aminobenzimidazole), 5f (E5ABI). The
1
solid. mp 194–196 uC; H NMR (DMSO-d ) d 7.32 (d, 2H,
6 as a yellow powder.
6
1
ArH), 6.85 (s, 2H, ArH), 6.62 (d, 2H, ArH), 3.45 (s, 4H, -CH
1
2
-);
C NMR (DMSO-d ) C2, 152.17; C4, 97.63; C5, 144.33; C6,
Yield 87%; mp w 300 uC; H NMR (DMSO-d
ArH), 13.49 (s, 2H, NH imidazole); C NMR (DMSO-d ) C2,
6
) d 7–8.10 (m,
3
13
6
6
1
11.59; C7, 116.79; C8, 133.58; C9, 138.68; C10, 27.64; IR
153.51; C4, 116.33; C5, 140.17; C6, 122.81; C7, 115.77; C8,
139.60; C9, 130.59; C10, 126.91; C11, 132.28; C12, 130.87; C13,
168.33; C14, 132.48; C15, 135.40, C16, 124.15; IR (KBr) n 1765,
2
1
(
KBr) n 1615 (CN) and 3250 cm (NH). Anal. Calcd. for
: C, 54.55; H, 3.43; N, 23.85. Found: C, 54.47; H,
.36; N, 23.82.
16 16 6
C H N
3
2
1
21
1717 (s, CO imide), 1625 cm (s, CN) and 1380 cm (s, C–N
imide). Anal. Calcd. for C36 : C, 71.99; H, 3.36; N,
13.99. Found: C, 71.42; H, 3.57; N, 13.71.
20 6 4
H N O
2
recrystallization from ethanol gave a white solid. mp 224–226
,2’-Butylene-bis(5-aminobenzimidazole), 5g (B5ABI). The
1
uC; H NMR (DMSO-d
6
) d 11.82 (2H, imidazole N–H), 7.31
d, 2H, ArH), 6.77 (s, 2H, ArH), 6.59 (d, 2H, ArH), 4.94 (4H,
2.6. Synthesis of alternating poly(benzimidazole–imide)s,
8
(
(PIBI) (Scheme 4)
1
3
amine NH
NMR (DMSO-d
1
2
Calcd. for C18H N
20 6
5
2
), 2.92 (s, 4H, –CH
) C2, 153.85; C4, 97.90; C5, 144.25; C6,
11.97; C7, 116.87; C8, 133.61; C9, 138.70; C10, 28.54; C11,
2
–), 1.96 (s, 4H, –CH
2
–);
C
The synthesis of the PIBI 8, that is prepared from BHTDA/
o-P5ABI 8a (Fig. 4) is given as a typical example.
BHTDA (0.48 g, 0.00147 mol) was added to a solution of
o-P5ABI 5a (0.50 g, 0.00147 mol) in NMP (70 wt%) in a
250 mL four necked flask equipped with a stirrer, Dean Stark,
condenser, nitrogen inlet, and thermometer. The reaction
6
2
1
7.95; IR (KBr) n 1620 (CN) and 3250 cm (NH). Anal.
: C, 56.84; H, 4.24; N, 22.09. Found: C,
5.4; H, 4.2; N, 21.8.
2,2’-Octamethylene-bis(5-aminobenzimidazole), 5h (O5ABI).
Therecrystallizationfromethanolgave awhitesolid. mpw300 uC;
1
H NMR (DMSO-d ) d 7.08 (d, 2H, ArH), 6.57 (s, 2H, ArH), 6.38
6
(
4
1
d, 2H, ArH), 4.94 (4H, amine NH ), 2.67 (t, 4H, –CH
2
2
–), 1.66 (q,
13
H, –CH –), 1.30 (s, 8H, –CH –); C NMR (DMSO-d ) C2,
2
2
6
52.49; C4, 96.59; C5, 143.70; C6, 110.34; C7, 115.56; C8, 132.41;
C9, 137.89; C10, 28.59; C11, 27.59; IR (KBr) n 1626 (CN) and
2
1
3250 cm (NH). Anal. Calcd. for C22
N, 19.25. Found: C, 59.83; H, 5.42; N, 19.15.
28 6
H N : C, 60.54; H, 5.54;
2
(
.5. Synthesis of model benzimidazole–imide compound, 6
Scheme 3)
An amine solution was prepared by dissolving 2,2’-(1,2-
phenylene)-bis(5-aminobenzimidazole) 5a (0.50 g, 0.00147 mol)
in NMP (25 mL) in a 250 mL four necked flask equipped with a
stirrer, Dean Stark trap, condenser, nitrogen inlet, and
thermometer. Phthalic anhydride (0.435 g, 0.00294 mol) was
gradually added to the amine solution under a nitrogen
Scheme 3 Synthesis of model benzimidazole–imide compound.
3
554
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Scheme 4 Synthesis of poly(benzimidazole–imide)s.
All of the procedures were carried out under dry nitrogen gas
or argon or in a vacuum to avoid ambient moisture because of
the hydrolytic instability of the anhydride groups.
3. Results and discussion
3.1. Synthesis and characterization of arylene- and alkylene-
bis(5-aminobenzimidazole)s
Fig. 4 Copolymer based on BHTDA–o-P5ABI or 7b/5a.
These diamines (R or Ar-5ABI) are obtained via the reduction
of dinitro precursors (listed in Table 1). The reduction of nitro
compounds to amines is a very useful route to a large range of
mixture was stirred overnight at room temperature. Toluene
5 mL) was added as an azeotropic agent. The solution was
3
2,33
(
reagents already developed.
heated to 140 uC for 3 h then to 190 uC and kept 4 h at this
temperature for imidization. The solution was cooled to room
temperature and added drop wise to methanol (250 mL) to
precipitate the copolymer PIBI 8a. The isolated polymer was a
beige powder, it was dried under vacuum for 16 h at 150 uC.
From 2,2’-(1,2-phenylene)-bisbenzimidazole, 3a (o-PBI), to
2,2’-(1,2-phenylene)-bis(5-aminobenzimidazole), 5a (o-P5ABI),
a complete synthesis was carried out from 1,2-phenylene
diamine, 1, and phthalic anhydride, 2, as indicated in Scheme 1.
The dinitro compound, 4a (o-P5NBI), was prepared by
direct nitration of the unsubstituted o-PBI, 3a. It is significant
to note that the nitration of o-PBI leads to virtually
quantitative substitution in position 5. It seems that the
strong conjugation due to o- or p-phenylene rings bridging the
benzimidazole cycles, prevents the nitration on them, whereas a
2
1
Yield 95%; IR (KBr) n 1774, 1715 (s, CO imide), 1625 cm
2
s, CN); inherent viscosity: 0.85 dL g in NMP (c ~ 1 g dL
at 30 uC. The molecular weight obtained by GPC (10 mg mL
1
21
(
)
2
1
4
THF solution, polystyrene standards) is M
4
n
~ 5.7 6 10 ,
Mw ~ 8.6 6 10 .
Table 1 Synthesis and properties of dinitro compounds 4 and the corresponding diamines 5
a
Reactions conditions
Compound
R
T/uC
Time/h
Solvent
Yield (%)
Melting/uC
4
5
4
5
4
5
4
5
4
5
4
5
4
5
4
5
a
a
a
b
b
c
c
d
d
e
e
f
1,2-C
1,2-C
1,4-C
1,4-C
1,4-C
1,4-C
1,4-C
1,4-C
1,4-C
1,4-C
6
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
H
4
4
4
4
4
4
4
4
4
4
41
1
0.5
6
0.5
6
Conc. H SO
Acetic acid
PPA
Acetic acid
PPA
2
4
60
78
69
56
80
90
53
40
70
85
60
83
85
88
90
91
w300
118
220
118
220
w300
w300
w300
210
–O–C
–O–C
–CHCH–C
–CHCH–C
–C(CF
–C(CF
6
H
H
4
6
4
w300
6
6
H
H
4
100
220
100
100
100
48
6
4 M HCl
PPA
w300
4
w300
3
)
)
2
2
–C
–C
6
H
4
4
238–240
w300
3
6
H
(CH
(CH
(CH
(CH
(CH
(CH
2
)
2
)
2
)
2
)
2
)
2
)
2
48
48
48
4 M HCl
4 M HCl
4 M HCl
286–288
194–196
264–266
224–226
220–222
w300
f
2
4
4
8
8
g
g
h
h
Same reaction conditions from 5a to 5h.
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3.2. Synthesis and characterization of model compound 6
m-phenylene ring is nitrated to a significant extent. In addition,
some dinitro compounds described herein have been previously
3
4,35
The one-step synthesis of benzimidazole–imide, 6, that is
also bisimide compound, is used as a reaction model for
the preparation of poly(benzimidazole-imide)s. The model is
prepared by reacting phthalic anhydride compound with a
bis(5-aminobenzimidazole) derivative in NMP solution under
a dry nitrogen atmosphere to avoid the hydrolysis of anhy-
dride groups. After complete reaction, the amic acid is imidized
by heat.
Anhydrides are very reactive with aromatic diamines even
at room temperature. We prepared the bisimide model 6 to
verify if the anhydrides could or not react with the amine of
the imidazole ring, which would lead to crosslinked polymer.
The characterization of the resulting model compound shows
clearly that only primary amines can react with the aromatic
anhydrides to produce the model compound as shown in
Scheme 3.
reported in literature,
4g) or greater synthetic convenience (4f). The raw materials,
,2-diamino-4-nitrobenzene and the dicarboxylic acids are
but we obtained an improved yield
3
1
(
1
either commercially available or easy to prepare and they are
readily converted to stable alkylene- or arylene-bis(5-nitro-
benzimidazole)s as indicated in Scheme 2.
A number of new bis(5-nitrobenzimidazoles) and the corres-
ponding bis(5-aminobenzimidazoles) have been prepared and
are shown in Fig. 3:
The stannous chloride reduction is known to be acid
catalyzed. Thus, during almost a century, all reductions with
stannous chloride have been carried out in the presence of
3
6
water. Recently, Satoh et al. proposed the system sodium
borohydride–stannous chloride for the selective reduction of
aromatic nitro compounds in ethanol. In our search for a mild,
2 2
selective and general method, we chose SnCl .2 H O in glacial
acetic acid to reduce a wide variety of substituted nitro com-
pounds in almost quantitative yields.
The reduction of alkylene-bis(5-nitrobenzimidazole)s could
be also achieved using hydrazine method (see Scheme 2). The
dinitro compound was dissolved in ethanol and the solution
was heated under reflux with 10% palladium–activated carbon
The characterization of the model compound was achieved
by NMR and IR spectroscopy, (see Experimental section). The
infrared spectrum shows the presence of CO imide bands at
2
1
1
1
765 and 1717 cm , CC and CN stretching vibrations at
2
625 cm
1
21
and C–N imide at 1380 cm . This model is
soluble in usual NMR solvents, which allows NMR spectra to
be recorded (see Fig. 5), unlike the copolymers which are
insoluble in conventional NMR solvents.
(
Pd/C). Hydrazine hydrate (100%) was added dropwise to the
above solution. Then, the mixture was refluxed until the
solution became colourless. The mixture was cooled, filtered
and the insoluble material washed with ethanol. The ethanol
solvent was then removed under vacuum. The resulting residue
was recrystallized from ethanol to give pure alkylene-bis(5-
aminobenzimidazole)s.
3.3. Synthesis and characterization of poly(benzimidazole–
imide) 8
New soluble poly(benzimidazole–imide) copolymers 8 were
prepared by polycondensation of these dianhydrides 7 with
1
3
Fig. 5 C NMR spectra of model compound 6. Using DMSO-d
6
as solvent.
3
556
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aromatic diamines 5 in solution in NMP under dry conditions
to avoid the hydrolysis of the dianhydride groups, which affects
strongly the molecular weight, followed by thermal imidization
method as shown in Scheme 4.
N-methyl-2-pyrrolidinone, N,N-dimethylacetamide, N,N-
dimethylformamide and dimethylsulfoxide. The good solubi-
lity was possibly governed by the structural modification
through the incorporation of flexible moieties as
hexafluoroisopropylidene and arylene ether groups into the
poly(benzimidazole–imide) structure. The polymerization of
benzimidazole diamines with aromatic dianhydrides gave
Alternating poly(benzimidazole–imide) copolymers are
obtained by condensation of dianhydrides and aromatic
diamines carrying benzimidazole groups as shown in Scheme 4.
This technique eliminates a number of detrimental side
reactions and has proved to be useful if a rigorous dianhy-
dride:diamine stoichiometry was maintained. The viscosity
increases gradually by the polycondensation reaction, and
viscous solutions suitable for thick coatings were obtained. The
characterization of the resulting polymers was realized with the
help of bis(benzimidazole–imide) model. The amide absorption
2
1
moderate-to-high inherent viscosities of 0.60–0.98 dL g
The molecular weight of the soluble copolymers were measured
.
2
1
by GPC (10 mg mL tetrahydrofuran solution as an eluent)
after calibration with the standard polystyrenes, except for
polymer 7d/5b, which was insoluble in tetrahydrofuran. These
n
polymers had number-average molecular weight (M ) in the
4
4
range from 3.7 6 10 to 11.8 6 10 . All of the polyimide films
could be obtained by casting from their NMP solutions and
also showed a transparent and flexible nature.
2
1
peak at 1650 cm
served as a criterion for the degree of
imidization, and complete cyclization to imide was achieved
under the above-described conditions.
The fully cured alternating benzimidazole–imide copolymers
reported here are designated by taking the symbols for dia-
mines and combined them with the symbols of the dianhydrides
3
.4.1. Electrical properties. Dielectric measurements were
carried out in the temperature range 30–350 uC and at
frequencies of 1–200 kHz. The values of dielectric constants
e are particularly constant over the corresponding range of
temperature. The dielectric constant e of Polymer 7b/5h was
evaluated as 4.8. This value is higher than those of com-
mercially available polyimides (e v 3.5), which is probably due
to the lower rigidity of the chain and the greater polarity of
benzimidazole group.
(
see Fig. 6): ODPA 7a, for diphenyloxy tetracarboxylic
dianhydride; BHTDA 7b, benzhydrol tetracarboxylic dianhy-
dride; 6FDA 7c, hexafluoro tetracarboxylic dianhydride;
BTDA 7d, benzophenone tetracarboxylic dianhydride.
Thus, the 2,2’-(1,2-phenylene)-bis(5-aminobenzimidazole)
moiety can be represented either by o-P5ABI or by its
number 5a. Therefore, the polymer 8a can be designed as
BHTDA/o-P5ABI or 7b/5a.
2
1
The IR amide I absorption at 1650 cm served as a crite-
rion for the degree of imidization, and a complete cyclization
to imide was achieved as described above. However, it was
easier to dissolve the bis(5-aminobenzimidazole)s bridged by
o-phenylene rings than the similar diamines bridged for
example by p-phenylene rings. In the former case, the diamines
are readily soluble in NMP to give 70% solution whereas a
diamine of the latter case is soluble only to an extent of 8%
of the solid. In this case, high polymer solutions can be
obtained since the poly(amic acid) formed is rather soluble and
the insoluble diamine enters into solution as the polymer is
formed. This is an inconvenient aspect of processing when
using the poly(amic acid) as a precursor.
3
.4.2. Mechanical properties. Poly (benzimidazole – imide)
was dissolved in NMP to give a 5% solution. Then, polymer
solution was cast on a glass plate, and solvent (NMP) was
evaporated under vacuum at 50 uC for 16 h. The cast films were
then thermally treated at 100 uC and 200 uC for 1 h each. The
polymer films showed excellent thermal resistance and were
transparent and colourless, desirable characteristics for prac-
tical applications in the field of polymer engineering.
The tensile properties of the copolymer films prepared by
thermal treatment are summarized in Table 2. The films had
tensile strength of 77–95 MPa, elongation at break of 7.2–
1
0.6%, and initial modulus of 2.0–2.4 GPa. Furthermore, the
films were strong and tough. The extensive delocalization of p
electrons is well known to be responsible for the array of
remarkable characteristics that polybenzimidazoles tend to
exhibit. These properties include non-linear optical behavior,
electronic conductivity, and exceptional mechanical properties
such as tensile strength and resistance to harsh environments.
3
.4. Properties of poly(benzimidazole–imide)s
Polyimides and polybenzimidazoles are known to be hygro-
scopic. The poly(benzimidazole–imide)s take up water from
the air. Therefore, the thermostable products were kept in dry
boxes. Most of the water can be released again by means of
an additional drying treatment under vacuum. However, for
polyimides this process is not totally reversible. Some of the
water can be consumed irreversibly by hydrolysis reactions of
the imide groups. Water uptake has an effect on the mechanical
properties and can cause stress relaxation.
3.4.3. Thermal stability. Thermal stability has been deter-
mined for poly(benzimidazole–imide)s by weight loss measure-
ments in an air atmosphere. The results of thermogravimetric
analysis (TGA) given in Fig. 7 show that poly(benzimidazole–
imide)s as a class possess outstanding thermal stability. The
polymers exhibited a one-step pattern of decomposition with
no significant weight loss below 420 uC. The degradation
Poly(benzimidazole–imide)s showed excellent to medium
solubility in a variety of polar aprotic solvents such as
temperature T
range 420–600 uC. Characterizations of 7b/5a, 7b/5c, 7c/5e, 7c/
g and 7d/5a are given as typical examples for thermogravi-
d
of the polymers is situated in the temperature
5
metric behaviour. Thermogravimetric analyses indicated that
the thermal degradation of poly(benzimidazole–imide) occurs
Table 2 Mechanical properties of poly(benzimidazole–imide)s, 8
Polymer
code
Tensile strength/
MPa
Elongation at
break (%)
Initial modulus/
GPa
7
7
7
7
7
b/5a
b/5c
c/5e
c/5g
d/5a
86
92
95
81
77
7.2
8.7
10.6
10.6
9
2.1
2.1
2.4
2.0
2.1
Fig. 6 Dianhydrides used for the copolymer synthesis.
J. Mater. Chem., 2002, 12, 3551–3559
3557

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be employed to modify the polymer properties, either by
lowering the interchain interactions or by reducing the stiffness
of the polymer chain. Depending on the type and amount of
structural modifications, the transition temperatures can be
lowered and the solubilities improved, resulting in processable
materials.
4. Conclusions
Polyimides can be used for extended periods of time but have
only acceptable thermal performance when compared to
polybenzimidazoles that possess excellent thermal performance
but can be used only for short time. A novel class of copolymer
based on alternating aromatic benzimidazole and aromatic
imide moieties was prepared, which enhanced the properties of
conventional polyimides and improved the processability of
polybenzimidazoles. While the synthesized poly(benzimida-
zole–imide)s do not necessarily have high temperature stability
when compared with the conventional polybenzimidazoles,
they do have much more desirable handling characteristics
which would allow them to be used in different applications.
A new family of polyimides with desirable properties for
manufacturing has been prepared by condensation of dianhy-
drides and aromatic diamines carrying benzimidazole groups,
and alternating poly(benzimidazole–imide) copolymers were
obtained. Highly processable thermostable copolymers having
high-molecular weight and containing o-phenylene ring brid-
ging bis(5-aminobenzimidazole) have been prepared by con-
densation of these diamines with aromatic dianhydrides. The
arylene- or alkylene-bis(5-aminobenzimidazole)s were pre-
pared by reduction of the corresponding dinitro compound.
These diamines were polycondensed in solution in NMP with
various dianhydrides to yield poly(amic acid) precursors that
gave rise to alternating imide–benzimidazole copolymers via
thermal cyclodehydration.
Fig. 7 Thermograms of poly(benzimidazole–imide)s, 8.
at ca. 530 uC, except for 7c/5g, which is ca. 80 uC higher than
the conventional polyimide, confirming that the introduction
of bisbenzimidazole component improved the thermal stability
of polyimide.
The poly(benzimidazole–imide) 7c/5g with an aliphatic chain
as part of the main chain are not as thermally stable as the fully
aromatic polymer. The degradation temperature of the com-
1
37
mercially aromatic poly(benzimidazole) named Celazole
1
for
0% weight loss was 465 uC in air and 530 uC in nitrogen
respectively. The degradation trends in 7b/5a, 7b/5c, and 7d/5a
are similar but the residues left behind after decomposition are
different. The TGA curves for 7c/5e and 7c/5g also exhibit
similar profiles with high char yields, which is probably due to
the nature of the dianhydride and the possible degradation
mechanism that the copolymers might undergo. Some poly-
mers listed in Table 3 have about 5% of weight loss at about
20 uC. At 600 uC, the residue is about 26% for copolymer 7b/
a; 73% for 7b/5c; 90% for 7d/5a; 54% for 7c/5g; and 63% for
c/5e. The residue left at 800 uC is about 42% for 7c/5e, and it is
4
5
7
found to be 20% for 7d/5a. These values are remarkably high
and they vary according to the structure of the copolymers.
However, decomposition under air left negligible residues for
both polymers 7b/5c and 7b/5a.
Acknowledgements
The authors would like to acknowledge the assistance of Mr.
Pierre Guenot and Mrs. Monique Contassot for their work
related to spectroscopic characterization. They thank Professor
Jean-Pierre Vairon for helpful discussions.
The glass transition temperatures, T , have values ranging
g
from 275–328 uC. No melting endotherm peak was observed
from DSC curves. Thus DSC measurements also revealed the
amorphous nature of the poly(benzimidazole–imide)s. When
the copolymers are heated up to 400 uC then cooled to 25 uC
and reheated, the DSC curves for both heating are very similar.
These results confirm the TGA values, which show that the
degradation begins only above 500 uC, except for 7c/5g. These
References
1
A. Buckley; D. E. Stuet and G. A. Serad, in Encyclopedia of
Polymer Science and Technology, ed. H. F. Mark, N. M. Bikales,
C. G. Overberger and G. Menges, John Wiley & Sons Inc., New
York, 2nd edn., 1988, vol. 11, p. 571.
copolymers had glass transition temperatures (T
g
) between
2
75 and 328 uC, which were higher than the T value of the
g
commercialized polyimide Ultem1000 (215 uC). As can be
expected, the benzophenone containing copolymer 7d/5a exhi-
2
E. J. Powers; G. A. Serad, in High Performance Polymers: Their
Origin and Development, ed. R. B. Seymour and
G. S. Kirshenbaum, Elsevier, New York, 1986; pp. 355–373;
P. E. Cassidy, Thermally Stable Polymers, Marcel Dekker, New
York, 1980; pp. 163–173.
bited the highest T
Polymer 7c/5e having the most flexible backbone structure gave
relatively higher T value. This may be attributed to its higher
g
due to its higher intermolecular force.
g
3
4
Polyimides: Materials, Chemistry and Characterization, ed.
C. Feger, M. M. Khojasteh and J. E. McGrath, Elsevier,
Amsterdam, 1989.
Polyimides, ed. D. Wilson, P. Hergenrother and H. Stenzenberger,
Chapman and Hall, London, 1990.
molecular weight. Introducing flexible units, bulky substitu-
ents, non-coplanar biphenylene moieties, and kinked units, can
Table 3 Thermal properties of polymers, 8, prepared by condensation
of bis(5-diaminobenzimidazole)s, 5, with dianhydrides, 7, in NMP
solution
5
6
7
D. A. Scola and J. H. Vontell, Chemtech, February 1989, p.112.
A. B. Thayer, C&E News, 23 July 1990.
F. Kataoka, F. Shoji, I. Takemoto, I. Obara, M. Kojima,
H. Yokono and T. Isogai, in Polyimide, Synthesis, Characteriza-
tion and Applications, ed. K. L. Mittal, Plenum, New York, 1984,
vol. 2, p. 933.
a
b
c
d
Polymer code
Yield (%)
T
g
/uC
T
d
/uC
Residue (%)
7
7
7
7
7
a
b/5a
b/5c
c/5e
c/5g
d/5a
95
85
94
90
92
288
307
275
296
328
530
560
530
420
580
26
73
63
54
90
8
9
W.-H Shen; A.-J Yo and B.-M. Gong, in Polyimide, Synthesis,
Characterization and Applications, ed. K. L. Mittal, Plenum, New
York, 1984, vol. 2, p. 947.
H. Vogel and C. S. Marvel, J. Polym. Sci., 1961, 50, 511.
10 Y. Iwakura, K. Uno and Y. Imai, J. Polym. Sci., Part A, 1964, 2,
2605.
11 F. L. Hedberg and C. S. Marvel, J. Polym. Sci., Polym. Chem.
Edn., 1974, 12, 1823.
For example, polymer 7b/5a could be also called BHTDA/o-P5ABI.
b
21
c
DSC, 10 uC min
. Starting of degradation in air, TGA, 10 uC
21
d
21
.
min . Percentage of residue at 600 uC, TGA, 10 uC min
3
558 J. Mater. Chem., 2002, 12, 3551–3559

View Article Online
12
D. I. Packham, J. D. Davies and H. M. Paisley, Polymer, 1969,
0(12), 923.
25 N. A. Adrova, V. N. Bagel, A. M. Dubonova, A. Ya Kvitko and
M. M. Koton, Vysokomol. Soedin, 1973, B-15, 509.
1
13
14
15
J. Higgins and C. S. Marvel, J. Polym. Sci., Part A-1, 1970, 8, 171.
I. Imai, K. Uno and Y. Iwakura, Makromol. Chem., 1965, 83, 179.
K. J. Scariah, V. N. Krishnumarthy, K. V. C. Rao and
M. Srinivasan, J. Polym. Sci., Polym. Chem. Ed., 1987, 25,
26 M. Berrada, H. Sekiguchi, F. Carriere, B. Coutin, P. Monjol and
R. Mercier, Chem. Mater., 1996, 8(5), 1029–1034.
27 M. Berrada, M. Vaultier, Z. Anbaoui, N. Lajhred, M. Berrada and
N. Knouzi, Ann. Chim., Sci. Mat., 1998, 23, 351–354.
2
675; A. A. Izyneev, I. Varga, Sh. P. Mazurevskaya, I. S. Novak,
28 M. Berrada, F. Carri e` re, P. Monjol, H. Sekiguchi and R. Mercier,
Chem. Mater., 1996, 8, 1022.
29 M. Berrada and H. Sekiguchi, presented at the 2nd European
Technical Symposium on Polyimides and High-Temperature
Polymers, Montpellier, France, 1991, June 4–7.
V. P. Mazurevski, D. M. Mognonov, A. A. Angotkina and
L. D. Radnaeva, Acta Polym., 1988, 39(8), 429.
V. V. Korshak, G. L. Berestneva, Z. Pelzbauer, K. V. Nikol’skii
and I. I. Dubovic, Acta Polym., 1986, 37(5), 281.
1
6
7
1
L. D. Radnaeva, D. M. Mognonov, A. L. Batotsyrenova,
A. A. Izyneev, B. V. Erzh and I. Varga, Acta Polym., 1988,
30 M. Berrada, PhD Thesis, Pierre et Marie Curie University, Paris,
1
M. Berrada, Z. Anbaoui, N. Lajrhed, M. Berrada, N. Knouzi,
992.
3
1
39(8), 437.
M. Vaultier, H. Sekiguchi and F. Carri e` re, Chem. Mater., 1997, 9,
1
1
8
9
G. Mandric, G. A. Neamtu, S. Peccincu and G. Racaru, Rev.
Roum. Chim., 1986, 31(1), 25.
D. S. Lee and G. Quin, Polym. J., 1989, 21, 751; L. Leung,
D. J. Williams, F. E. Karasz and W. MacKnight, J. Polym. Bull.
(Berlin), 1986, 16(5), 457.
J. Preston, J. Polym. Sci., Part A-1, 1969, 7(10), 3027.
J. Preston and J. W. Carson, Jr, Polym. Preprints, 1992, 33(1), 390.
J. Preston and J. W. Carson, Jr, Polymer, 1993, 34(4), 830.
F. Bower, F. Burgman and T. Ruffing, J. Polym. Sci., Part A-1,
989.
3
3
3
2
3
4
J. George and S. Chandrasekaram, Synth. Commun., 1983, 495.
W. K. Xing and Y. Ogata, J. Org. Chem, 1982, 47, 3577.
T. A. Fairley, R. R. Tikewell, I. Doukor, N. A. Naiman,
K. A. Ohemeg, R. J. Lombardy, J. A. Bentley and M. J. Corey,
J. Med. Chem., 1933, 36, 1476.
1
20
21
22
23
3
3
3
5
6
7
H. Ertepinar, Y. G o¨ k and M. K u¨ c¸ u¨ kislamoglu, Eur. Polym. J.,
1
995, 31, 603.
T. Satoh, N. Mitsuo, M. Nishiki, Y. Inoue and Y. Doi, Chem.
Pharm. Bull., 1981, 29, 1443.1
1968, 6, 215–233.
N. P. Kuznetsov, M. I. Bessonov, T. M. Kiseleva and
M. M. Koton, Vysokomol. Soedin, 1972, A-14, 2034.
24
Charlotte, and NC Celazole PBI Technical Bulletins, Hoeschst
Celanese Specialty Operations.
J. Mater. Chem., 2002, 12, 3551–3559
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