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Designed Monomers and Polymers 13 (2010) 207–220brill.nl/dmp

Synthesis and Characterization of New OpticallyActive Poly(amide-imide)s Based on

N,N′-(Pyromellitoyl)-bis-L-Amino Acidsand 1,3,4-Oxadiazole Moieties

Khalil Faghihi ∗ and Hassan Moghanian

Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Faculty of Science,Arak University, Arak 38156, Iran

AbstractNew optically active poly(amide-imide)s derived from chiral N,N′-(pyromellitoyl)-bis-L-amino acids (3a–f)and 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (BAO) (8) were synthesized by direct polycondensation. Chi-ral N,N′-(pyromellitoyl)-bis-L-amino acids were obtained by the reaction of pyromellitic dianhydride withtwo equimolar of L-alanine (2a), L-valine (2b), L-leucine (2c), L-isoleucine (2d), L-phenyl alanine (2e)and L-2-aminobutyric acid (2f) in acetic acid. The polycondensation reaction proceeded through the in situformation of an Vilsmeier adduct by dissolving tosyl chloride (TsCl) in a mixed solvent of pyridine andDMF. The resulting thermally stable poly(amide-imide)s were obtained in good to high yields and inherentviscosities ranging between 0.31 and 0.55 dl/g. The structures of the new polymers were confirmed by ele-mental analysis and spectral methods (FT-IR, 1H-NMR). Optical activity and thermal behavior investigatedby polarimetric measurements and thermogravimetric analysis, respectively.© Koninklijke Brill NV, Leiden, 2010

KeywordsPoly(amide-imide), 1,3,4-oxadiazole, polycondensation, optically active polymer, chiral amino acid

1. Introduction

Aromatic polymers such as polyimides and polyamides are thermally stable poly-mers which have received much interest over the past decades due to increasingdemands for high-performance polymers as replacement for ceramics or metals inthe microelectronic, aerospace and automotive industries [1–3]. They are difficult toprocess due to their insolubility in organic solvents and infusibility [4–9]. Consider-able effort has been made to improve their processing properties by structural mod-ifications. One such method is the synthesis of copolymers. Poly(amide-imide)s(PAIs) are a class of high-performance polymers, which show excellent mechani-

* To whom correspondence should be addressed. Tel.: (98-91) 8863-0427; Fax: (98-86) 1277-4031; e-mail:[email protected]

© Koninklijke Brill NV, Leiden, 2010 DOI:10.1163/138577210X12634696333514

208 K. Faghihi, H. Moghanian / Designed Monomers and Polymers 13 (2010) 207–220

cal and thermal properties and are also solvent resistant [4, 10]. There is a growinginterest in PAIs for a variety of applications, as they retain good mechanical proper-ties at high temperatures and are more processable than other aromatic thermostablepolymers, such as polyamides and polyimides [11].

Optically active compounds have attracted much attention because living sys-tems are chiral. Proteins and nucleic acids possess chiral characteristic structuresthat are related closely to their functions. Because of the chirality, living organismsusually show different biological responses to one or the other of a pair of enan-tiomers or optical isomers whether they are drugs, pesticides or wastes. Synthesisand characterization of optically active polymers has been a challenging theme inthe field of polymer synthesis in recent years for their important applications of op-tically active polymers as catalysts for asymmetric synthesis [12, 13] and as chiralstationary phases (CSP) for the direct optical resolution of enantiomers [14–17].Optically active polymers can be obtained by polymerization of optically activemonomers or by stereo-selective polymerization of racemic or prochiral monomersusing optically active catalysts.

Recently, we have synthesized a variety of optically active polymers by incor-poration of optically active segments in polymer’s backbone. In addition of opticalproperties of these polymers, the solubility of them was improved without signifi-cant loss of mechanical and thermal properties [18–22].

On the other hand, it was shown that aromatic polymers containing 1,3,4-oxadiazole rings in the main chain exhibit high thermal resistance in oxidativeatmosphere, good hydrolytic stability, low dielectric permittivity, high toughnessand other special properties which are determined by the electronic structure of thisparticular heterocycle [23–27]. The incorporation of oxadiazole and imide ringstogether with flexible groups into the polymer chain is expected to provide a combi-nation of high-performance properties and processability, particularly in thin filmsand coatings.

Here we present the research on the synthesis and characterization of new op-tically active PAIs containing 1,3,4-oxadiazole rings in the main chain, whichwere obtained by direct polycondensation reaction of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (7) with six chiral N,N′-(pyromellitoyl)-bis-L-amino acids (3a–f)using tosyl chloride (TsCl), pyridine (Py) and dimethylformamide (DMF) as con-densing agent.

2. Experimental

2.1. Materials

All chemicals were purchased from Fluka and Aldrich. Pyromellitic dianhydridewas purified by recrystallization from acetic anhydride and then dried in vacuo at125◦C for 12 h. Acetic anhydride was purified by distillation under reduced pres-sure.

K. Faghihi, H. Moghanian / Designed Monomers and Polymers 13 (2010) 207–220 209

2.2. Measurements

IR spectra were recorded on a Galaxy series FT-IR 5000 spectrophotometer. Bandintensities are assigned as weak (w), medium (m), strong (s) and band shapesas shoulder (sh), sharp (s) and broad (br). 1H-NMR and 13C-NMR spectra wererecorded on a Bruker 300 MHz instrument. Inherent viscosity was measured by astandard procedure using a Technico® viscometer. Specific rotations were measuredby an A-Kruss polarimeter. Thermogravimetric analysis (TGA) data for polymerswere recorded on a Mettler TA4000 System under N2 atmosphere at a rate of10◦C/min. Elemental analyses were performed using Vario EL equipment at ArakUniversity.

2.3. Monomer Synthesis

2.3.1. Synthesis of N,N′-(pyromellitoyl)-bis-L-amino Acid (3a–f)Pyromellitic dianhydride (1,2,4,5-benzenatetracarboxylic acid 1,2,4,5-dianhydride)1 (4.36 g, 20.00 mmol), 40.00 mmol L-amino acids 2a–f, 80 ml acetic acid anda stirring bar were placed into a 250-ml round-bottomed flask. The mixture wasstirred at room temperature overnight and refluxed for 4 h. The solvent was removedunder reduced pressure, and the residue was dissolved in 100 ml cold water, then thesolution was decanted and 5 ml concentrated HCl was added. A white precipitatewas formed, filtered off, and dried to give compounds N,N′-(pyromellitoyl)-bis-L-amino acid (3a–f).

Diacid (3a): 1H-NMR (300 MHz, DMSO-d6, δ, ppm): 13.5 (s, br, 2H), 8.39–8.40 (d, 1H, J = 9 Hz), 8.26 (s, 1H), 8.04–8.07 (d, 1H, J = 9 Hz), 4.91 (q, 1H),1.55 (d, 3H). FT-IR (KBr): 2500–3400 (s, br), 1728 (s, sh), 1722 (s, br), 1604 (w,sh), 1487 (w, sh), 1423 (s, sh), 1384 (s), 1290 (s), 1095 (m), 927 (m), 731 (s), 655(m), 532 (w) cm−1.

Diacid (3b): 1H-NMR (300 MHz, DMSO-d6, δ, ppm): 13.64 (s, br, 2H), 8.51–8.54 (d, 1H, J = 9 Hz), 8.25 (s, 1H), 8.02–8.05 (d, 1H, J = 9 Hz), 5.02 (d, 1H),2.32 (m, 1H), 1.06–1.08 (d, 3H), 0.83–0.85 (d, 3H). FT-IR (KBr): 2500–3400 (m,br), 1782 (m, sh), 1722 (s, br), 1487 (w), 1384 (s), 1290 (s), 1095 (w), 929 (m), 733(s), 609 (w), 532 (w) cm−1.

Diacid (3c): 1H-NMR (300 MHz, DMSO-d6, δ, ppm): 13.07 (s, br, 2H), 8.51–8.54 (d, 1H, J = 9 Hz), 8.41 (s, 1H), 8.02–8.05 (d, 1H, J = 9 Hz), 4.78–4.83 (dd,1H, J = 6, 3 Hz), 1.52 (m, 2H), 1.49 (m, 1H), 0.85–0.87 (d, 6H). FT-IR (KBr):2500–3400 (m, br), 1780 (m, sh), 1733 (s, br), 1485 (w), 1380 (s), 1295 (s), 1095(w), 920 (m), 743 (s), 609 (w), 533 (w) cm−1.

Diacid (3d): 1H-NMR (300 MHz, DMSO-d6, δ, ppm): 13.39 (s, br, 2H), 8.38–8.41 (d, 1H, J = 9 Hz), 8.27 (s, 1H), 8.02–8.05 (d, 1H, J = 9 Hz), 4.53–4.56 (d,1H), 2.36–2.38 (m, 1H), 1.46–1.52 (m, 2H), 1.00–1.06 (d, 3H), 0.77–0.82 (t, 3H).FT-IR (KBr): 2400–3500 (s, br), 1778 (s, sh), 1722 (s, br), 1379 (s), 1286 (s), 1093(m), 929 (w), 733 (m), 538 (w) cm−1.

Diacid (3e): 1H-NMR (300 MHz, DMSO-d6, δ, ppm): 13.57 (s, br, 2H), 8.33–8.53 (d, 1H, J = 6 Hz), 8.19 (s, 1H), 7.94–7.97 (d, 1H, J = 6 Hz), 7.15 (s, 5H),


5.12–5.17 (dd, 1H, J = 9, 3 Hz), 3.46–3.52 (dd, 1H, J = 9, 3 Hz), 3.29–3.35 (dd,1H, J = 9, 3 Hz). FT-IR (KBr): 2400–3500 (s, br), 1770 (s, sh), 1720 (s, br), 1383(s), 1278 (s), 1091 (m), 925 (w), 731 (m), 539 (w) cm−1.

Diacid (3f): 1H-NMR (300 MHz, DMSO-d6, δ, ppm): 13.45 (s, br, 2H), 8.37–8.40 (d, 1H, J = 9 Hz), 8.26 (s, 1H), 8.01–8.04 (d, 1H, J = 9 Hz), 4.66–4.71 (dd,1H, J = 6, 3 Hz), 2.01–2.19 (m, 2H), 0.82–0.87 (t, 3H). FT-IR (KBr): 2400–3500(s, br), 1780 (s, sh), 1720 (s, br), 1604 (m), 1487 (m), 1383 (s), 1284 (s, sh), 1082(s), 879 (s), 729 (s), 638 (m), 472 (w), 314 (w) cm−1.

2.3.2. Synthesis of N′-(4-nitrobenzoyl)-4-nitrobenzohydrazide (6)Into a 100-ml round-bottomed flask fitted with a magnetic stirrer was placed a so-lution of 4-nitrobenzoyl chloride (10 g, 53.9 mmol) and triethylamine (4 ml) in40 ml dry dimethylacetamide (DMAc). The reaction mixture was cooled in an icewater bath. To this solution, 6 ml hydrazine monohydrate was added drop-wise.The mixture was stirred in ice bath for 2 h and at room temperature overnight. Themixture was poured into 100 ml water. The precipitate was collected by filtrationand washed thoroughly with water and dried at 100◦C to yield 6 (6.5 g, 73%). Mp290–292◦C, FT-IR: 3221 (s, br), 3111 (w), 1619 (s), 1587 (s), 1529 (s), 1465 (s),1348 (s), 1261 (w), 1109 (w), 837 (m), 715 (m) cm−1.

2.3.3. Synthesis of 2,5-Bis(4-nitrophenyl)-1,3,4-oxadiazole (7)Into a 50-ml round-bottomed flask, 4 g (12.0 mmol) N′-(4-nitrobenzoyl)-4-nitrobenzohydrazide, 30 ml phosphoryl trichloride and a stirring bar were placed.The stirrer was started and the mixture was refluxed for 12 h. The reaction mix-ture was cooled to room temperature, poured into ice–water and precipitated. Theprecipitate was filtered, washed with water and then dried to afford 7 (3.5 g, 87%).Mp > 300◦C, FT-IR: 3109 (w), 1612 (m), 1583 (s), 1515 (s), 1469 (m), 1350 (s),1109 (w), 866 (m), 717 (m) cm−1.

2.3.4. Synthesis of 2,5-Bis(4-aminophenyl)-1,3,4-oxadiazole (BAO) (8)2,5-Bis(4-nitrophenyl)-1,3,4-oxadiazole (2 g, 6.4 mmol), 0.2 g 10% Pd–C and50 ml ethanol were introduced into a 100-ml round-bottomed flask to which 12 mlhydrazine monohydrate was added drop-wise over a period of 1 h at 85◦C. Afterthe complete addition, the reaction was continued at reflex temperature for another5 h. Then, the mixture was filtered to remove the Pd–C and the filtrate was pouredinto water. The product was filtered off, washed with water and dried to afford 8(1.2 g, 74%). Mp 260–261◦C, FT-IR: 3325 (m), 3215 (m), 1608 (s), 1493 (s), 1438(w), 1273 (w), 1178 (m), 1080 (w), 831 (w), 746 (w) cm−1. 1H-NMR (DMDO-d6):δ = 7.71 (d, J = 8.7, 4H), 6.69 (d, J = 8.7, 4H), 5.86 (s, 4H) ppm. 13C-NMR(DMDO-d6): δ = 168.6, 157.2, 133.0, 118.8, 115.4 ppm.

2.4. Polymer Synthesis

Polymer 8a, as an example of synthesis of PAIs, was prepared by the following pro-cedure: a pyridine (0.20 ml) solution of TsCl (0.18 g, 9.5 × 10−4 mol) after 30 minstirring at room temperature was treated with DMF (0.07 ml, 9.0 × 10−4 mol)


for additional 30 min. The reaction mixture was added drop-wise to a solutionof N,N′-(pyromellitoyl)-bis-L-alanine (3a) (0.137 g, 3.8 × 10−4 mol) in pyridine(0.20 ml). The mixture was maintained at room temperature for 30 min, and thento this mixture, a solution of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (7) (0.096 g,3.8 × 10−4 mol) in pyridine (0.30 ml) was added drop-wise and the whole solutionwas stirred at room temperature for 30 min and at 100◦C for 2 h. As the reactionproceeded, the solution became viscous, then was precipitated in 20 ml methanol,filtered off and dried in vacuum to yield 0.196 g (89%) of the polymer 9a.

PAIs 9a–f were analyzed using FT-IR spectroscopy with KBr pellets.9a: 3317 (m, br), 3067 (w), 2941 (w), 1774 (m), 1722 (s, br), 1606 (m), 1525

(m), 1496 (s), 1383 (sh), 1251 (m), 1157 (m), 1076 (w), 1016 (w), 939 (w), 846(m), 729 (m), 565 (w) cm−1.

9b: 3331 (m, br), 3099 (w), 2968 (w), 1776 (m), 1722 (s, br), 1604 (s), 1496 (s),1383 (sh), 1315 (m), 1251 (m), 1180 (m), 1076 (m), 1012 (w), 914 (w), 846 (m),727 (m), 563 (w) cm−1.

9c: 3352 (m, br), 2960 (m), 1776 (m), 1724 (s, br), 1606 (s), 1496 (s), 1411 (w),1356 (sh), 1317 (w), 1251 (w), 1182 (w), 1084 (w), 844 (w), 727 (w), 565 (w)cm−1.

9d: 3331 (m, br), 3107 (w), 2968 (w), 1776 (m), 1722 (s, br), 1606 (m), 1527(m), 1496 (s), 1383 (sh), 1253 (m), 1182 (m), 1078 (m), 1014 (w), 914 (w), 846(m), 729 (m), 565 (w) cm−1.

9e: 3369 (m, br), 3030 (w), 2928 (w), 1776 (m), 1724 (s, br), 1604 (m), 1525(m), 1497 (s), 1381 (sh), 1317 (m), 1249 (m), 1180 (m), 1105 (w), 914 (w), 842(m), 727 (m), 563 (w) cm−1.

9f: 3350 (m, br), 3068 (w), 2972 (w), 1776 (m), 1724 (s, br), 1604 (m), 1496 (s),1413 (w), 1353 (sh), 1248 (m), 1180 (m), 1068 (m), 960 (w), 842 (m), 725 (m), 565(w) cm−1.

3. Results and Discussion

3.1. Monomer Synthesis

As shown in Scheme 1, the asymmetric diimide-diacids 3a–f were synthesized bythe condensation reaction of pyromellitic dianhydride 1 with two equimolars of L-alanine (2a), L-valine (2b), L-leucine (2c), L-isoleucine (2d), L-phenyl alanine (2e)and L-2-aminobutyric acid (2f) in an acetic acid solution according to the reportedprocedure [19]. The yields and some physical properties of these compounds areshown in Table 1.

The chemical structure and purity of the optically active diimide-diacids 3a–fwere determined using elemental analysis, FT-IR and 1H-NMR spectroscopy. Asan example, the FT-IR spectrum of N,N′-(pyromellitoyl)-bis-L-2-aminobutyric acid(3f) showed a broad peak between 2500 and 3500 cm−1, which was assigned to theCOOH groups and two absorption bands at 1776 and 1726 cm−1 due to carbonyl


Scheme 1. Synthesis of asymmetric diimide-diacids 3a–f.

Table 1.Synthesis of chiral diimide-diacid derivatives 3a–f

Entry Amino-acid compound R Mp (◦C) Yield (%) [α]D25a

3a L-Alanine CH3 303–305 78 −6.53b L-Valine (CH3)2CH 276–278 85 −3.03c L-Leucine (CH3)2CHCH2 318–320 87 +0.23d L-Isoleucine (C2H5)(CH3)CH 279–282 88 −8.03e L-Phenylalanine PhCH2 305–307 90 +0.23f L-2-Aminobutyric acid CH3CH2 295–297 85 +12.1

a Measured at a concentration of 0.5 g/dl in EtOH at 25◦C.

of imide (asymmetrical and symmetrical C=O stretching vibration), and bands at1383, 1113 and 731 cm−1 (imide ring deformation) (Fig. 1).

The 1H-NMR spectrum of diimide-diacid 3f showed peaks between 0.85 and0.90 ppm as a triplet, which were assigned for two CH3(a), and peaks between2.10 and 2.17 ppm as a multiplet, which was assigned to the CH2(b) and between4.72 and 4.77 ppm as a doublet of doublet (J = 6 and 3 Hz), which was assignedto the CH(c) proton, which is a chiral center. The peak at 8.35 ppm was assignedto aromatic protons (e). Also a broad peak in 13.25 ppm was assigned to COOHgroups (Fig. 2).

Also, 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (8) was synthesized using athree-step reaction. At first N′-(4-nitrobenzoyl)-4-nitrobenzohydrazide (6) was pre-pared from condensation of two equivalents of 4-nitrobenzoyl chloride (4) withhydrazine monohydrate (5) in the presence of triethylamine in DMAc solution(Scheme 2).

Then N′-(4-nitrobenzoyl)-4-nitrobenzohydrazide (6) was cyclized to 2,5-bis(4-nitrophenyl)-1,3,4-oxadiazole (7) with phosphorus oxychloride as anhydrous re-


Figure 1. FT-IR spectrum of N,N′-(pyromellitoyl)-bis-L-2-aminobutyric acid 3f.

Figure 2. 1H-NMR spectrum of N,N′-(pyromellitoyl)-bis-L-2-aminobutyric acid 3f.


agent under reflux conditions. Finally, 7 was reduced using Pd/C to produce 8(Scheme 2).

The chemical structure and purity of 8 was determined by elemental analysis,FT-IR and 1H-NMR spectroscopic techniques. The 1H-NMR spectrum of 8 showeda peak at 5.86 ppm, which was assigned to the H(c) protons of the NH2 groups.Peaks at δ = 6.60 and δ = 7.72 were assigned to the H(b) and H(a) protons of thephenyl rings (Fig. 3).

Scheme 2. Synthesis of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (8).

Figure 3. 1H-NMR spectrum of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (8).


3.2. Polymer Synthesis

PAIs 9a–f were synthesized by direct polycondensation reaction of an equimo-lar mixture of diacids 3a–f with 8 using TsCl/Py/DMF as condensing agent(Scheme 3).

In this report of the polycondensation of aliphatic–aromatic diacids and 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole, a Vilsmeier adduct was prepared by dissolv-ing TsCl in a mixed solvent of pyridine and DMF. The polycondensation wascarried out according to reported procedure [28, 29].

The synthesis and some physical properties of these new optically active PAIs arelisted in Table 2. All of the polymers were obtained in good to high yields (78–89%)with moderate inherent viscosities (0.31–0.55 dl/g) and show optical rotation and,therefore, they are optically active.

Scheme 3. Synthesis of PAIs 9a–f.

Table 2.Synthesis and some physical properties of PAIs 9a–f

Imide-diacid Polymer Yield (%) ηinh (dl/g)a [α]D25a

3a 9a 89 0.55 86.23b 9b 84 0.47 80.93c 9c 81 0.31 110.23d 9d 78 0.40 117.53e 9e 85 0.44 97.73f 9f 87 0.49 105.4

a Measured at a concentration of 0.5 g/dl in DMSO at 25◦C.


3.3. Polymer Characterization

The elemental analysis values of the resulting polymers are in good agreement withthe calculated values for the proposed structures (Table 3).

The solubility of PAIs was tested quantitatively in various solvents. The sol-ubility of the PAIs is listed in Table 4. Most of the PAIs are soluble in organicpolar aprotic solvents such as DMF, N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), 1-methyl-2-pyrrolidone (NMP), and polar protic solvents suchas H2SO4 at room temperature, and are insoluble in solvents such as chloroform,methylene chloride, methanol, ethanol and water.

Table 3.Elemental analysis of PAIs 9a–f

Polymer Formula C (%) H (%) N (%)

9a C30H20N6O7 Calcd 62.50 3.50 14.58(576.52)n Found 61.25 3.77 14.37

9b C34H28N6O7 Calcd 64.55 4.46 13.28(632.62)n Found 63.10 4.69 13.49

9c C36H32N6O7 Calcd 65.45 4.88 12.72(660.68)n Found 64.50 4.70 12.93

9d C36H32N6O7 Calcd 65.45 4.88 12.72(660.68)n Found 64.38 4.94 12.54

9e C42H28N6O7 Calcd 69.23 3.87 11.53(728.71)n Found 68. 17 3.63 11.32

9f C32H24N6O7 Calcd 63.57 4.00 13.90(604.57)n Found 62.10 4.22 14.06

Table 4.Solubility of PAIs 9a–f

Solvent 9a 9 9c 9d 9e 9f

H2SO4 + + + + + +DMAc + + + + + +DMSO + + + + + +DMF + + + + + +NMP + + + + + +MeOH − − − − − −EtOH − − − − − −CHCl3 − − − − − −CH2Cl2 − − − − − −H2O − − − − − −

+, soluble at room temperature; −, insoluble at room temperature.


The structures of these polymers were confirmed as PAIs by means of FT-IR,1H-NMR spectroscopy and elemental analyses. The representative FT-IR spectrumof PAI 9a is shown in Fig. 4.

The FT-IR spectra of the polymer exhibited characteristic absorption bands at1774 and 1722 cm−1 for the imide ring (asymmetric and symmetric C=O stretch-ing vibration), 1383 cm−1 (C–N stretching vibration). The absorption band at3317 cm−1 corresponds to the N–H stretching. The others spectra show a similarpattern. The FT-IR spectra of the polymers show an absorption peak at 3050–3070 cm−1 characteristic of the aromatic C–H stretching vibrations. The bands at2960–2870 cm−1 are due to the aliphatic C–H stretching.

The 1H-NMR spectrum of PAI 9f (Fig. 5) shows peaks that confirm its chemicalstructure. It shows a peak for CH3 (Hf) that appears at 0.91 ppm. A peak for CH2(He) appears as broad peak at 2.20 ppm according to its coupling with CH3 (Hf) andCH (Hd). The proton of the chiral center (Hd) appeared at 4.90 ppm. The aromaticprotons related to phenyl groups of oxadiazole unit and pyromellitic ring appearedin the region of 8.03, 7.78 (Ha and Hb) and 8.38 ppm (Hg), respectively. The peakin the region of 10.34 ppm is assigned to N–H of amide groups in the main chain.The decaying peak related to carboxylic acid protons and appearing peaks related

Figure 4. FT-IR spectrum of PAI 9a.


Figure 5. 1H-NMR spectrum of PAI 9f.

to amide groups, oxadiazole moiety and pyromellitic ring protons in the polymerchain, confirmed the proposed structure of PAIs 9a–f.

3.4. Thermal Properties

The thermal stability of the polymers 9a and 9e was characterized by TGA con-ducted in nitrogen at a heating rate of 10◦C/min. In all cases, the thermal stabilitywas very good. The temperature at which the decomposition began was never under300◦C. The highest thermal stability was found for PAI 9e. This behavior could bea consequence of the phenyl group present in the side-chain of polymer. TypicalTGA curves of representative polymers are shown in Fig. 6.

The 5 and 10% weight loss temperatures together with char yield at 600◦C forPAIs 9a and 9e have been calculated from their thermograms. From these data it isclear that the resulting polymers are thermally stable. The thermo analyses data ofPAIs 9a and 9b are summarized in Table 5.

4. Conclusion

The present study involved the synthesis of several new optically active PAIs(9a–f) by the direct polycondensation reaction of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (8) with six asymmetric imide-diacids 3a–f and tosyl chloride(TsCl)/pyridine (Py)/dimethylformamide (DMF) as condensing agent. These PAIs


Figure 6. TGA curves of PAIs 9a and 9e.

Table 5.Thermal properties of PAIs 9a and 9e

Polymer T5 (◦C) T10 (◦C) Char yield (%)c

9a 325–330 370–375 56.39e 350–355 385–390 52.11

T5, T10, temperature at which 5 or 10% weight loss was recorded TGA at a heating rate of10◦C/min in N2; char yield, weight percentage of material left after TGA analysis at a maximumtemperature of 600◦C in N2.

were soluble in various organic solvents and had moderate to good thermal stability.Since the resulting polymers optically active and have good thermal stability, theyhave the potential to be used as a chiral stationary phase in chromatography for theseparation of racemic mixtures.

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