versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.54 n.4 Concepción dic. 2009
J. Chil. Chem. Soc., 54, N° 4 (2009), págs. 331-333
SYNTHESIS OF POLY (BIBENZIMIDAZOLE-P-PHENYL- BENZOBISOXAZOLE)
YANHUA LUa,b, JIANMIN CHENa*, HAIXIA CUIa, HUIDI ZHOUa
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences, Lanzhou 730000, China. e-mail: firstname.lastname@example.org
A diacid monomer of benzobisoxazole 2, 6-di(p-phenylene-carboxylic) acid (BBODPA) was synthesized in a simple procedure, and characterized by means of FTIR, 1H NMR and 13C NMR. The resulting BBODPA was polymerized with 3, 3'-diaminobenzidine tetrahydrochloride (DAB) to prepare poly (bibenzimidazole-p-phenyl-benzobisoxazole) (PBBIBBO) in polyphosphonc acid. The inherent viscosities values of PBBIBBO indicated that it was a kind of high molecular weight polymer. The structure of PBBIBBO was confirmed by 1H NMR, 13C NMR and FTIR. Thermal properties and dissolubility of PBBIBBO were also investigated. These results showed that PBBIBBO exhibited good thermal stability.
Key words: Copolymer, polybenzimidazole, polybenzoxazole, thermal stability.
Polybenzimidazole (PBI) and polybenzoxazole (PBO) have received considerable attention since 1960s, due to their chemical resistance, thermal stability and good mechanical properties at high temperature.12 Fibers,34 foams, 5,6 membrane 7,8 and structural materials 9,10 of PBI and PBO have been extensively investigated. Melt polycondensation2,11 and solution polycondensation12,16 have been frequently used to synthesize PBI and PBO. Attempts have been made to combine polybenzimidazole with polybenzoxazole, in order to obtain more thermal stable and tractable polymers. For example, copolymers containing both PBI and PBO segments have been prepared. 17,18 The authors claimed that phase separation reaction would not easily take place compared with the solution mixture of polybenzimidazole and polybenzoxazole.
In the present study, homopolymer of poly (bibenzimidazole-p-phenyl-benzobisoxazole) (PBBIBBO) was synthesized. Benzobisoxazole 2, 6-di(p-phenylene-carboxylic) acid (BBODPA) was firstly synthesized using 4, 6-Diaminoresorcinol Dichloride (DAR) and paraphthaloyl chloride (PPC), as described in Scheme 1. Then the resulting BBODPA polymerized with 3, 3'-diaminobenzidine tetrahydrochloride (DAB) in polyphosphoric acid (PPA) to prepare PBBIBBO (Scheme 2). The compound of PBBIBBO which combined both benzoxazole groups and benzimidazole groups on the polymer chain was expected to find a place in preparing fiber and membrane materials.
DAB and DAR were prepared by the procedures of Herward and Marvel.2 Benzidine, m-phenylene diamine, paraphthaloyl chloride, polyphosphoric acid, phosphorus pentoxide and acetic acid were obtained from commercial sources and used as received.
A Bruker AV-400 type superconducting magnetic resonance spectrometer (400 MHz, using H2S04-d2 as solvents) was performed to record the 1H-nuclear magnetic resonance spectra (1H NMR) and 13C-nuclear magnetic resonance spectra (13C NMR). Fourier transform infrared (FTIR) spectra were obtained with a Bruker IFS66v/S Fourier transformation spectrophotometer (KBr discs). Thermal gravimetric analysis (TGA) was performed using a STA449C device (NETZSCH, Germany). The samples for TGA were placed in 75 μl A1203 pans and heated from 25 °C to 1000 °C at a heating rate of 10 °C /min, under a constant N2 flow of 20 ml/min. Inherent viscosities (r|inh = In nra/C)of polymers were determined for solution of 0.5 g/dl in H2S04 (98%) at 30 °C using a Ubbelohde viscometer.
Synthetic procedure for the monomer of BBODPA
In a three necked 2000 ml round bottom flask equipped with a mechanical stirrer and nitrogen inlet was placed 500 ml of PPA, 54.46 g (0.40 mol) of PPC and 2.5 g of SnCL/2H20. In to this flask 49.82 g (0.20 mol) of DAR (C6H8N202-2HC1-2H20) was added gradually and dissolved to prevent the carry-over of the reactant by the emitting HC1 under 105 °C. Once that the addition was finished the temperature was raised to 200 °C and kept at this temperature for 7 h. The product was isolated by pouring the hot solution into water, washing with water, dipping in dilute KOH overnight and drying in a vacuum oven at 50 °C for 24 h. The yield of BBODPA was 95%.
Preparation of PBBIBBO
A 2000 ml three-necked flask equipped with a stirrer and nitrogen inlet was charged with 1000 ml of PPA (P2Os ≥85%) and 2 g of SnCL/2H20. The flask was then placed into a silicone oil bath preheated to 105 °C. Into this flask, 40.8035 g (0.1030 mol) of DAB (C12N4H144HC1-2H20) was added gradually and dissolved to prevent the carry-over of the reactant by the emitting HC1. To the resulting mixed solution 43.60 g (0.10 mol) of BBODPA (C22H12N206 ■ 2H20) were added. Under a thin stream of nitrogen, the solution was heated at 200 °C for 12 h. The polymeric target product was isolated by pouring the hot solution into water, washing with water by decantation, dipping in dilute KOH overnight, washing thoroughly with water and CH3OH, and drying in a vacuum oven at 50 °C, generating 46.42 g of PBBIBBO.
RESULTS AND DISCUSSION
We have attempted to use benzobisoxazole 2, 6-di carboxylic acid (BBODPA) (Figure 1) as the monomer to synthesize PBBIBBO. However, BBODPA decomposed under 140 °C. Leonard and Kaimierczak suggested that the 2-carboxyl acid in benzoxazole or benzimidazole molecular were thermally unstable. " These results agreed with our research. It seems that the carboxylic acid on the 2-benzoxazole or 2-benzimidazole was thermally unstable. The phenyl carboxylic acid like BBODPA was more thermally stable and selected as the monomer to synthesize the PBBIBBO.
The monomer of BBODPA was prepared by solution method, as depicted in Scheme 1. The resulting BBODPA is confirmed by FTIR and 1H NMR and 13C NMR. Figure 2 shows the FTIR spectrum of BBODPA. It is observed that a broad O-H stretch is presented at 2000-3700 cm1,20 carbonyl group at 1688 cm1 (C=0 acidic, stretching), asymmetric C=C at 1610 cm"1 (C=C, stretching) and CN of benzoxazole at 1268 cm"1 (C=N, stretching).
The 1H NMR spectrum of BBODPA was obtained in deuterated sulfuric acid. The aromatic proton was observed at 7.9-8.5 ppm. It should be noted that BBODPA was almost insoluble in DMSO and CHC13. H2S04-d2 was used as solvent to acquire the 1H NMR spectrum. The proton of carboxylic acid (COO-H) was not observed, which was attributed the proton-exchange between H2S04-d2 and carboxylic acid group of BBODPA.
[H NMR (H2S04-d2, 5 ppm): 7.95 (benzobisoxazole, 1H), 8.20 (benzene, 2H), 8.39 (benzene, 2H), 10.5 (H2S04, solvent).
13C NMR (H2S04-d2, 5 ppm): 111.0, 120.5, 125.3, 127.4, 128.4, 129.4, 145.3, 163.3, 170.4.
The obtained PBBIBBO was a high molecular weight polymer, as evidenced from its inherent viscosities (t] ) values of 4.2 dl/g for the polymer. Figure 2 show the FTIR spectrum of PBBIBBO. As can be seen, there is no strong O-H stretch at 2000-3700 cm"1 and carbonyl group at 1689 cm"1, indicating that complete closure of the oxazole rings in the polymerization has been achieved. The characteristic peaks of BBODPA and PBBIBBO in FTIR spectra are listed in Table 1.21
The :1H NMR spectrum of PBBIBBO was obtained in deuterated sulfuric acid. The proton of aromatic was observed at 7.6-8.5 ppm. The proton of benzimidazole was not observed, like that of BBODPA, which was attributed the proton-exchange between H2S04-d2 and benzimidazole moiety.
The characterization by 1H NMR, 13C NMR and FTIR confirmed that the resulting PBBIBBO have the expected chemical structures.
1H NMR (H2S04-d2; δ ppm): 7.63 (benzobisoxazole, 2H), 7.71 (benzimidazole, 1H), 8.09 (benzene, 4H), 8.27 (benzimidazole, 1H), 8.42 (benzimidazole, 1H), 10.5 (H2S04, solvent).
13C NMR (H2S04-d2; δ ppm): 110.8, 114.6, 115.7, 120.5, 122.9, 128.0, 130.6, 131.0, 134.5, 137.2, 139.4, 152.8, 162.7.
The weight vs. temperature curves for PBBIBBO and BBODPA are shown in Figure 3. BBODPA displayed three-step thermal degradation processes, namely, desorption of water, decomposition of carbonyl group and aromatic group. The initial 6-8 wt.% weight loss of BBODPA below 145 °C is mainly due to the volatilizing of water, which corresponds to about 2 crystal water molecules per unit of BBODPA (C22H12N2062H20). The degradation of carbonyl group of BBODPA begins at 200 °C and the decomposition of aromatic group begins at 410 °C.
It can be observed in Figure 3 that the initial decomposition temperature of PBBIBBO is about 590 °C. When temperature reached 770 °C, there is still about 50 wt.% of residual left. These values show that PBBIBBO has reasonably good thermal stability. It is also shown that PBBIBBO exhibit better thermal stability than its monomer of BBODPA and DAB.
Dissolubility of BBODPA and PBBIBBO was examined in different solvents and the results are summarized in Table 2. They are soluble in sulfuric acid. BBODPA is insoluble in other hot solvents except in hot DMF. PBBIBBO is soluble in hot DMF, DMAc and NMP.
The diacid monomer of BBODPA was synthesized from DAR and PPC, and was characterized in this work. BBODPA was then used as monomers for the synthesis of new PBBIBBO.
Investigation of dissolubility of PBBIBBO showed that it was insoluble in polar aprotic solvents but soluble in H2S04 at room temperature. Thermal stability of BBODPA and PBBIBBO were also studied with TGA. The result showed temperature of 5% weight loss of PBBIBBO stayed in the range of 590-600 °C, indicating good thermal stabilities of the polymer.
The authors are grateful to the National Natural Science Foundation of China (Grant No. 50575217), the program of "973" (Grant No. 20070B607601) and the Innovative Group Foundation from NSFC (Grant No. 50421502) for financial support.
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(Received: July 22, 2008 - Accepted: April 29, 2009).