Synthesis of a new aromatic diacid containing pyridine ring and related polybenzimidazole

Article abstract of DOI:10.1016/j.cclet.2010.04.023

A new aromatic diacid monomer containing pyridine unit, 4-phenyl-2,6-bis[4-(4-carboxyphenoxy) phenyl] pyridine 3, was successfully synthesized in three steps, starting from 4-hydroxyacetophenone. The corresponding pyridine-containing polybenzimidazole was

Full text of DOI:10.1016/j.cclet.2010.04.023

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 976–978
www.elsevier.com/locate/cclet
Synthesis of a new aromatic diacid containing pyridine ring and
related polybenzimidazole
*
Tao Ma, Yan Feng Li , Shu Jiang Zhang, Feng Chun Yang,
Chen Liang Gong, Jiu Jiang Zhao
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry,
Institute of Biochemical Engineering & Environmental Technology, Lanzhou University, Lanzhou 730000, China
Received 16 December 2009
Abstract
A new aromatic diacid monomer containing pyridine unit, 4-phenyl-2,6-bis[4-(4-carboxyphenoxy) phenyl] pyridine 3, was
successfully synthesized in three steps, starting from 4-hydroxyacetophenone. The corresponding pyridine-containing polybenzi-
midazole was prepared via polycondensation of 3 with tetraaminodiphenyl ether (TADE) in poly(phosphoric acid) (PPA). The
resulting polybenzimidazole exhibits excellent solubility, good thermal stabilities and high thermal resistance.
# 2010 Yan Feng Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Polybenzimidazoles; Pyridine-containing diacid monomer; Synthesis; Thermal stabilities
Polybenzimidazoles (PBIs) are well known as an excellent high performance resin because of their outstanding
thermal stability, thermoplastic toughness, dispersibility, and chemical stability [1]. However, their applications have
been limited due to polybenzimidazoles are normally insoluble in common organic solvents and have extremely high
glass-transition temperatures or melting temperatures, which preclude melting processing. For the development of
polybenzimidazoles, synthesis of new monomers and related polybenzimidazoles with both good processability and
thermal stability would be a major research topic, and new monomers of aromatic diacid would play important rules in
synthesis of polybenzimidazoles [2,3]. Aromatic ether linkages in polymer main chains provide them significantly
lower energy of internal rotation. In general, such a structural modification leads to a lower glass-transition
temperature (Tg) and lower crystalline melting temperature as well as a significantly improvement in the solubility and
process characteristics without greatly sacrificing thermal stability. It is generally recognized that aryl-ether linkage
can impart better solubility and melt processing characteristics and improve toughness in comparison with those of
polymers without aryl-ether linkage.
The PBIs are generally synthesized by the step growth polymerization of tetramine with an aromatic diacid or
diacid derivate in poly(phosphoric acid) (PPA) as a reaction medium, activates both functional groups toward
condensation, and reacts with the water formed by the polycondensation to effectively dehydrate the system [4]. In
* Corresponding author.
E-mail address: liyf@lzu.edu.cn (Y.F. Li).
1001-8417/$ – see front matter # 2010 Yan Feng Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.04.023

T. Ma et al. / Chinese Chemical Letters 21 (2010) 976–978
977
Scheme 1. Synthesis of diacid monomer 3 (a) DMF, K2CO3, 90 8C, 8 h;(b) HAc, NH4Ac, reflux,24 h;(c) H3PO4, reflux, 2 h.
addition, the solvents of the step growth polymerization have been reported such as cresol, phenol, dimethyl sulfoxide,
dimethyl formamide, sulfolane and diphenylsulfone [5,6].
In this work, a new aromatic diacid monomer containing pyridine unit, 4-phenyl-2,6-bis[4-(4-carboxyphenoxy)-
phenyl] pyridine 3, was successfully synthesized by using 4-hydroxyacetophenone as starting material, as shown in
Scheme 1. Firstly, 0.05 mol of 4-hydroxyacetophenone, 0.051 mol of 4-fluorobenzonitrile and 0.1 mol of K2CO3 were
added into a flask with 100 mL of DMF.
The mixture was heated at 90 8C for 8 h with stirring under nitrogen. After cooling down, the mixture was poured
into a solution of ethanol and water (v/v = 1), the precipitation was filtrated. The crude product 1 was got and
recrystallized from ethanol. A mixture 1 (0.05 mol), benzaldehyde (0.025 mol), ammonium acetate (0.625 mol) and
40 mL of glacial acetic acid were placed into a 250-mL three-necked flask equipped with a mechanical stirred and a
reflux condenser. The mixture was refluxed with stirring for 24 h. Then the solid 4-phenyl-2,6-bis[4-(4-
nitrilephenoxy)phenyl]pyridine 2 was filtered off and washed thoroughly with glacial acetic acid and water,
respectively. Finally, in a 250-mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a reflux
condenser, 2 (0.01 mol) and 130 mL of 85% phosphoric acid were added and refluxed for 2 h. After cooling to room
temperature, the mixture was poured into 600 mL of cold, dilute aqueous potassium hydroxide. The solution was
stirred at room temperature for 6 h after modulating the pH value to 11, and then the solution was neutralized with 6 M
Table 1
Experimental data of compound.
Compounds
Yield (%)
m.p. (8C)
Molecule formula
Elemental analysis (wt.%)
C
H
N
1
88
81–84
C15H11NO2
(237.08)
Calcd.
75.94
4.67
5.90
Found
Calcd.
75.92
82.05
4.66
4.28
5.84
7.76
2
3
41
82
221–225
136–140
C37H23N3O2
(541.18)
Found
Calcd.
81.98
76.67
4.35
4.35
7.78
2.42
C37H25NO6
(579.17)
Found
76.74
4.32
2.38

978
T. Ma et al. / Chinese Chemical Letters 21 (2010) 976–978
Scheme 2. Synthesis of polybenzimidazole (a) PPA, 120 8C, 3 h; (b) 200 8C, 12 h.
hydrochloric acid to pH 3–4, followed by stirring for 6 h. The product was collected by filtration and recrystallized
from 50% of aqueous acetic acid using activated charcoal. Melting point of 3 is 136–140 8C. The experimental data of
diacid monomer 3 and its intermediates 1, 2 were listed in Table 1, respectively. The FABMS, m/z, shows M+: 579.4
(31%). FT-IR (KBr) peaks of 3 at 2500–3500 cmÀ1 and 1235 cmÀ1 belonged to CO(O)H and C–O–C groups,
respectively. 1H NMR (200 MHz, DMSO-d6): d 8.44 (d, 4H, J = 8.4, Ha), 8.22 (s, 2H, He), 8.05 (d, 4H, J = 6.2, Hd),
7.89 (d, 2H, J = 8.8, Hf), 7.57 (d, 2H, J = 7.2, Hg), 7.32 (d, 1H, J = 8.6, Hh), 7.20 (d, 8H, J = 8.6, Hb, Hc), and 13C NMR
(200 MHz, DMSO-d6): d 166.7, 160.6, 159.7, 156.3, 155.7, 137.6, 135.0, 131.7, 130.7, 129.1,127.4, 126.5, 125.6,
119.9, 118.7, 117.6, respectively. Elemental analysis for C37H25NO6: calcd. C 76.67, H 4.35, N 2.42, found C 76.74, H
4.32, N 2.38. The data agree with the structure of 3.
Related polybenzimidazole was synthesized by polycondensation of 3 and aromatic tetramine TADE via previously
method, as shown in Scheme 2 [7]. Firstly, a 250-mL three-necked flask equipped with a mechanical stirrer, a nitrogen
inlet, and an outlet CaCl2 drying tube was charged with PPA (40 g) and stirred with heating under constant flow of
nitrogen above 120 8C. The system was flushed with nitrogen for 10 min to remove the air then the diacid (1 mmol)
and tetramine (1 mmol) was added, the temperature was slowly raised to 200 8C for 12 h. The hot solution was poured
into water (150 mL) and solid polymer was filtrated and stirred in 5% aqueous sodium bicarbonate solution (5 h). The
polymer was collected by filtration and washed with water until neutral. The inherent viscosity of the resulting
polybenzimidazole in the NMP solution with a 0.5 g dLÀ1concentration at 30 8C was 0.251 g dLÀ1. FT-IR spectra of
the resulting polybenzimidazole revealed strong bands at 3378 cmÀ1 due to N–H group, 1597 cmÀ1 due to C N group
and 1082 cmÀ1 due to benzimidazole. The TGA curve in nitrogen exhibited two-stage degradation processes. The
value of 5% and 10% gravimetric loss in air (T5 and T10) of the resulting polybenzimidazole is 558 and 598 8C, which
showed the polybenzimidazole has good thermal stability. It also exhibited good solubility in common organic
solvents such as DMSO, NMP and m-cresol. According to the experimental results above, the new pyridine-containing
aromatic diacid monomer 3 also holds good polymerizability.
Acknowledgments
The authors are grateful for the research support from the Natural Science Foundation of Gansu Province (No.
096RJZA047) and the State Key Laboratory of Applied Organic Chemistry of People’s Republic of China.
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