Polyamide-imides bearing furan moieties. 1. Solution polycondensation of

aromatic dianhydrydes with 2-furoic acid dihydrazides

Souhir Abida, Rachid El Gharbia, Alessandro Gandinib,*

aLaboratoire de Chimie Appliquee HPCG, Faculte des Sciences de Sfax, 3038 Sfax, TunisiabEcole Francaise de Papeterie et des Industries Graphiques (INPG), BP 65, 38402 Saint Martin d’Heres, France

Received 18 March 2004; received in revised form 23 June 2004; accepted 24 June 2004

Abstract

Novel polyamide-imides containing flexible segments were synthesized from aromatic dianhydrides and various furan dihydrazides by a

two-steps procedure that included ring opening polyaddition to give polyhydrazide-acids, followed by thermal or chemical cyclodehydration.

The polyhydrazide-acids had inherent viscosities ranging from 0.045 to 0.11 l/g. The polyamide-imides showed good solubility in aprotic

polar solvents and had inherent viscosities up to 0.070 l/g, glass transition temperatures close to 250 8C and good thermal stability up to

350 8C.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Furan polymers; Polyamide-imides; Dianhydrides

1. Introduction

Aromatic polyimides are thermally stable polymers,

which exhibit excellent mechanical and electrical properties

[1–3]. However, these polymers are difficult to process

because of their high softening temperatures and poor

solubility in organic solvents. In order to improve the

solubility, several approaches have been proposed, based

on the incorporation of flexible segments bearing ester,

amide, ether or urethane moieties in the polymer

backbone, without sacrificing the heat resistance [4].

Thus, several copolymers have been developed, such as

polyamide-imides, which are known to have high

thermal stability and good solubility in polar amide-

type solvents [5]. The solubility of polyimides can also

be improved by using heterocyclic diamines such as

thionine, proflavine [6,7] and acridine yellow [8]. In a

short report, Sivaraj and Nanjan [9] described the

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.06.065

* Corresponding author. Address: Insituto de Quımica de Sao Carlos,

Universidad de Sao Paulo, Av. Trabalhador Sao Carlense, 400, 13566-590

Sao Carlos-SP, Brazil. Tel.: C55-16-273-9938; fax: C55-16-273-9952.

E-mail address: gandini@iqsc.usp.br (A. Gandini).

synthesis of processable polyamide-imides based on

3,3 0,4,4 0-benzophenone-tetracarboxylic dianhydride and

aliphatic, alicyclic or pyrrolic dihydrazides.

Given the extensive experience of our laboratories in the

field of furan-based polymers [10–13], we decided to study

the synthesis of a novel class of polyamide-imides

incorporating 2,5-substituted furan moieties (1378) in their

backbone, which would be more flexible than their fully

1,4-aromatic counterparts (1808), by breaking the chain

linearity and reduce the polymer packing density, as in the

case of 1,3-aromatic monomer units [1,2]. Thus, the

presence of both furan and amide groups in the final

macromolecules was thought to lead to improved solubility

and lower softening temperatures, while maintaining good

thermal and mechanical properties. To the best of our

knowledge, no reports on furan polyimides have appeared in

the literature. In the present paper, we describe the synthesis

of furan polyamide-imides by solution polycondensation of

furan dihydrazides with 3,3 0,4,4 0-benzophenone-tetracar-

boxylic dianhydride or 1,2,4,5-benzene-tetracarboxylic

anhydride. The choice of these two dianhydrides was

based on their well-documented role in aromatic polyamide-

imides.

Polymer 45 (2004) 6469–6478

www.elsevier.com/locate/polymer

S. Abid et al. / Polymer 45 (2004) 6469–64786470

2. Experimental

2.1. Materials

The 2-furoic acid dihydrazides 1a–1f (Scheme 2) were

prepared as described previously [13] by treating with

hydrazine hydrate (Aldrich) the corresponding diesters

obtained in turn by the condensation of ethyl 2-furoate

with the corresponding aldehyde or ketone, as reported in

detail elsewhere [14]. 1,2,4,5-Benzene-tetracarboxylic

anhydride 2 and 3,3 0,4,4 0-benzophenone-tetracarboxylic

dianhydride 3 (both Aldrich) were purified by refluxing

for 2 h in acetic anhydride and subsequently crystallising

them from the same medium. 2-furoic acid hydrazide FH a

high purity Aldrich product, was employed as received.

N,N-dimethylacetamide (DMA), N-methyl pyrrolidinone

(NMP), dimethyl sulfoxide (DMSO), pyridine (Py) were

treated with potassium hydroxide pellets for 24 h and

distilled under reduced pressure over phosphorus pentoxide.

All the other solvents were used as received.

2.2. Synthesis of model compounds CM1–CM4

To a stirred solution of 10 mmol of FH in 25 ml of DMA,

were added gradually 5 mmol of dianhydride. The mixture

was stirred at room temperature for 6 h in a nitogen

atmosphere. Diacids CM1 and CM2 were isolated by

precipitation in water and filtration. Imidisation was carried

out by thermal cyclodehydration by heating the diacid at

150 8C under vacuum for 12 h. Chemical cyclodehydration

was also carried out by heating the diacid at reflux with

thionyl chloride for 1 h. Thereafter, the precipitate CM3 and

CM4 were collected by filtration, washed thoroughly with

water and acetone and finally vacuum dried at 100 8C.

2.3. Polymerisations

The following procedure was applied to all polymeris-

ations. 5 mmol of dihydrazide were added to 25 ml of DMA

and heated under stirring to 60 8C in a nitrogen atmosphere.

To the ensuing clear solution, cooled to 25 8C, 5.05 mmol of

dianhydride were slowly added in small portions while

stirring. The reaction mixture became viscous as the solid

dissolved. The magnetic stirring was continued for 8 h. The

pale yellow-orange polyhydrazide-acid PHA thus obtained

was recovered by precipitation into an excess of water,

washed with hot acetone and methylene chloride and finally

vacuum dried at 60 8C to constant weight. The term ‘yield’

will be used in this work to express the amount of material

obtained following these operations.

2.4. Imidisations

Both thermal and chemical cyclodehydrations were

carried out in order to convert each PHA into the

corresponding polyamide-imide PAI, namely: (i) the PHA

was heated in a drying chamber at 150 8C under high

vacuum for 12 h; (ii) 2 g of PHA were dissolved in 10 ml of

SOCl2 under nitrogen; the reaction mixture was then stirred

for 1 h at 80 8C before being poured into an excess of water

in order to induce the precipitation of the ensuing PAI,

which was then washed with acetone and vacuum dried to

constant weight. It was noted that during the imidisation

processes the colour of the polymers turned dark brown.

2.5. Measurements

Structures were examined by FTIR (KBr pellets),

300 MHz 1H and 13C NMR (in DMSO-d6) spectroscopy.

Inherent viscosities were measured in DMSO (1.5 g/l) at

25 8C. DSC and TGA tracings were obtained under nitrogen

with a heating rate of 10 deg/min.

3. Results and discussion

3.1. Model compounds

In order to gain a useful insight into the feasibility of the

corresponding polycondensations and optimization of the

imidisation reactions, four model compounds CM1–CM4

were prepared. The syntheses involved the reaction between

FH and aromatic dianhydrides 2 or 3, followed by thermal

or chemical cyclodehydration, as shown in Scheme 1. The

synthesis of CM1 and CM2 was carried out in different

solvents, viz. DMA, NMP, DMSO and Py at various

temperatures. The best yields (CM1: 98%, CM2: 95%) were

obtained after 6 h in DMA at room temperature. Given the

results of this survey, it was decided to carry out the

polycondensations of dihydrazides with dianhydrides under

identical conditions.

CM3 and CM4 were obtained by imidisation of CM1 and

CM2, respectively using thermal and chemical cyclodehy-

dration. The complete thermal imidisation was attained after

12 h, whereas the chemical route only needed 1 h. It was

established that the cyclodehydration reaction involving NH

and COOH groups gave selectively imide functions, without

formation of any detectable 1,3,4-oxadiazole rings by

cyclodehydration of the OaC–NH–NH–CaO moieties

[13].

The FTIR spectra of CM3 and CM4 showed the presence

of the characteristic peaks of imide and amide functions

(Table 1) and the absence of the original peaks arising from

the COOH and OaC–NH–NH–CaO groups in the

corresponding CM1 and CM2 precursors (Table 2). The1H and 13C NMR spectra were also entirely consistent with

the structures of these model compounds (Tables 3 and 4).

In particular, the imidisation was confirmed, on the one

hand, by the downfield shift of the resonance of both

aromatic and furan protons and, on the other hand, by the

presence of only one peak related to the NH amide functions

Scheme 1. Synthesis of the four model compounds CM1–CM4 using

hydrazide FH and dianhydrides 2 or 3.

S. Abid et al. / Polymer 45 (2004) 6469–6478 6471

in the region of 10–12 ppm, instead of the typical two peaks

of the hydrazide NH protons.

3.2. Polymers

A series of furan-aromatic polyamide-imides was

synthesised in two steps, i.e. the ring opening polyaddition

and the subsequent thermal or chemical cyclodehydration

(Scheme 2). In the first step, the polymerization of 2-furoic

acid dihydrazides 1a–1f (structures given in Scheme 2 and

Table 5) with aromatic dianhydrides 2 and 3 was carried out

in a polar solvent (DMA) at 20 8C giving polyhydrazide-

acids PHA1–12 (see Table 5 for specific structures). The

mode of monomer addition played an important role for

Table 1

FTIR data (cmK1) related to CM1 and CM2

CM1 CM2

Hydrazide group

(NH; CaO)

3358–3229; 1694–

1652

3361–3227; 1681–

1643

Acid group (OH; Ca

O)

3458; 1735 3458; 1720

Furan aCH 3110 3110

Furan ring breathing 1027 1024

2-Substituted furan

ring

958; 888; 770 960; 883; 780

successful polymerization, as emphasised previously

[15,16]. In fact, the polyhydrazide-acids were obtained by

multi-step addition of 2 or 3 to the dihydrazide solution, as

described in the experimental part, since no polymer was

obtained by the reverse addition or by initially mixing the

entire amounts of both reactants. These polymers were

obtained in almost quantitative yields and possessed

inherent viscosities ranging from 0.045 to 0.106 l/g, which

suggested the formation of relatively high molecular

weights. The results of all these syntheses are summarised

in Table 5.

In the second step, the cyclodehydration of the

polyhydrazide-acids to give the polyamide-imides PAI1–12was carried out by two alternative procedures, namely (i)

thermal imidisation at temperatures up to 250 8C under

vacuum or (ii) chemical imidisation in thionyl chloride.

PAI1–12 had very good inherent viscosities ranging from

0.070 to 0.155 l/g (Table 5).

3.3. Structural characterisation

The major FTIR features of the polyhydrazide-acids

PHA1–12 included peaks related to the acid groups and the

OCNHNHCO functions and those arising from the furan

heterocycles (Table 6). The imidisation of these polymers

was confirmed by the presence of absorptions related to

imide rings and the absence of acid and hydrazide peaks in

the spectra of the ensuing PAI1–12 (Table 7). Fig. 1 shows a

typical spectrum of such a polyamide-imide.

The 1H NMR spectra (Table 8) of these new polyamide-

imides also agreed with the expected structures. Two

polymers were also characterised by 13C NMR and the

corresponding spectroscopic data (Table 9) were similar to

those reported in Table 4 for the corresponding model

compounds.

3.4. Properties

The solubility of polyamide-imides PAI1–12 was tested

qualitatively in various solvents. The solubility of most of

these polymers in highly polar solvents such as DMA, NMP,

DMF and DMSO may be attributed to the incorporation of

2,5-bifuranic units into the polymer backbones, as antici-

pated in the introduction. More specifically, the results

reported in Table 10 suggested that the solubility was

somewhat influenced by the structure of the actual furan

units incorporated into the polymer chains, whereas the

nature of the aromatic units did not influence this property.

PAI2 and PAI8 showed limited solubility and dissolved only

in highly polar hot solvents. This behaviour may have

resulted from the introduction of a phenyl group in the

bridge between the furan heterocycles, which could have

increased chain stiffness. On the other hand, it can be noted

that polyamide-imides prepared from symmetrical dihydra-

zide, namely 2,2 0bis[2-(5-furoic acid hydrazyl)] propane,

were less soluble compared with polyamide-imides

Scheme 2. Synthesis of polyamide-imides PAI1–12: Table 5 gives the corresponding structures, based on the use of the six different furan hydrazides 1a–1f and

the two aromatic dianhydrides 2 and 3.

Table 2

FTIR data (cmK1) related to CM3 and CM4

CM3 CM4

Imide I (asym. and sym. imide CaO) 1800–1758 1805–1745

Imide II (C–N) 1394 1385

Imide III and IV (imide ring deformation) 1111; 703 1118; 718

NH 3319–3235 3388–3263

Amide I (CaO) 1682 1687

Amide II and III (d N–H and n C–N coupling) 1511; 1305 1510; 1361

aCH furanic 3111 3166

Furanic ring breathing 1017 1018

2-Substituted furan ring 946; 840; 770 941; 873; 763

Table 31H NMR data related to model compounds (DMSO-d6/TMS)

Compound d (ppm)

H3Fu H4Fu H5Fu NH Ar

CM1 7.33 6.67 7.91 10.50–10.60 8.02

CM2 7.15 6.35 7.95 10.40–10.65 8.05

CM3 7.43 6.77 8.04 11.52 8.52

CM4 7.44 6.84 8.10 11.50 8.47

Table 413C NMR data related to CM1 and CM3 (DMSO/TMS)

d (ppm)

CM1 CM3

Hydrazide-acid or amide-imide groups 157–163; 165 160–167

Furan carbons 112–115; 144–146 116–120; 148–151

Aromatic carbons 119; 135–138 123–139

S. Abid et al. / Polymer 45 (2004) 6469–64786472

Fig. 1. FTIR spectrum of polyamide-imide PAI1.

Table 5

Yields and inherent viscosities of PHA1–12 and PAI1–12

PHA/PAI R1/R2 Ar Yield (%)a hinh (l gK1)a hinh (l g

K1)b

PHA1/PAI1 CH3/CH3 84 0.088 0.145

PHA2/PAI2 CH3/C6H5 92 0.106 0.155

PHA3/PAI3 CH3/C2H5 70 0.055 0.082

PHA4/PAI4 CH3/H 76 0.062 0.112

PHA5/PAI5 CH3/C5H11 66 0.045 0.072

PHA6/PAI6 CH3/CF3 86 0.075 0.105

PHA7/PAI7 CH3/CH3

C

O 90 0.088 0.137

PHA8/PAI8 CH3/C6H5

C

O 84 0.083 0.150

PHA9/PAI9 CH3/C2H5

C

O 65 0.065 0.088

PHA10/PAI10 CH3/H

C

O 80 0.058 0.106

PHA11/PAI11 CH3/C5H11

C

O 68 0.048 0.070

PHA12/PAI12 CH3/CF3

C

O 92 0.054 0.120

a Related to PHA.b Related to PAI.

S. Abid et al. / Polymer 45 (2004) 6469–6478 6473

Fig. 2. DSC tracing of polyamide-imide PAI2.

Table 6

FTIR data related to PHA1–12

Polyhydrazide-

acid

n (cmK1)

Hydrazide

groups

Acid functions Furan-related

peaks

PHA1 3322; 1678–

1652

3448; 1728 3134; 1014;

976; 808; 752

PHA2 3278; 1680–

1655

3484; 1733 3152; 1027;

944; 808; 766

PHA3 3278; 1691–

1635

3433; 1733 3138; 1027;

944; 754

PHA4 3273; 1689–

1635

3458; 1729 3136; 1027;

980; 804; 752

PHA5 3316; 1698–

1632

3436; 1719 3111; 1013;

975; 807; 766

PHA6 3239; 1694–

1630

3458; 1720 3125; 1013;

972; 815; 780

PHA7 3250; 1680–

1646

3454; 1722 3125; 1024;

970; 804; 752

PHA8 3277; 1694–

1626

3426; 1720 3108; 1023;

958; 813; 766

PHA9 3227; 1698–

1655

3395; 1722 3125; 1024;

947; 801; 754

PHA10 3239; 1680–

1643

3446; 1720 3111; 1013;

973; 806; 780

PHA11 3227; 1689–

1643

3472; 1715 3111; 1018;

942; 802; 768

PHA12 3265; 1681–

1643

3472; 1733 3124; 1024;

960; 806; 741

S. Abid et al. / Polymer 45 (2004) 6469–64786474

containing two different groups at the bifuran junction. This

may have arisen from the fact that PAI1 and PAI7 possessed

a higher structural regularity, which would facilitate a closer

chain packing than in the other polymers. It can be

concluded that, on the whole, the solubility of these novel

polyamide-imides was greatly increased when compared

with that of homologous polymers bearing linear stiff

sequences of 1,4-aromatic moieties [1,2].

The thermal behaviour of all the PAIs and some of the

PHAs was examined by thermogravimetric analysis (TGA)

and differential scanning calorimetry (DSC). Table 11 gives

the results related to the PAIs and Fig. 2 shows a typical

DSC tracing. These results suggested that the specific

structure of the two series of monomers did not intervene in

terms of major changes in thermal behaviour, since all PAIs

showed glass transitions in the vicinity of 250 8C and a

thermal stability in the range of 360–550 8C.

The TGA traces of PAIs were much less complex than

those of the corresponding PHAs. The typical curve in Fig. 3

shows that PHA1 exhibited a small weight loss between

room temperature and 50 8C, corresponding to residual

solvent evaporation, followed by the first significant break

in the range 140–225 8C corresponding to the conversion of

PHA1 to PAI1, viz. thermal imidisation. A final step in the

curve took place at 360–480 8C and corresponded to the

thermal decomposition of the PAI1 structure formed in situ.

Fig. 3. TGA tracing of polyhydrazide-acid PHA1.

Fig. 4. TGA tracing of polyamide-imide PAI1.

S. Abid et al. / Polymer 45 (2004) 6469–6478 6475

Fig. 5. TGA tracing of polyhydrazide-acid PHA8.

Fig. 6. TGA tracing of polyamide-imide PAI8.

S. Abid et al. / Polymer 45 (2004) 6469–64786476

Table 7

FTIR data related to PAI1–10

Polyamide-imide n (cmK1)

Imide functions Amide functions Furan-related peaks

PAI1 1797–1746 1398; 1111; 715 3253; 1655 1514; 1295 3110; 1012 973; 805; 767

PAI2 1802–1748 1385; 1117; 701 3228; 1671 1572; 1296 3104; 1023 968; 806; 771

PAI3 1799–1743 1399; 1112; 710 3218; 1657 1542; 1299 3111; 1022 958; 802; 757

PAI4 1799–1741 1402; 1111; 711 3238; 1647 1527; 1290 3100; 1021 972; 805; 752

PAI6 1800–1744 1371; 1114; 711 3228; 1671 1528; 1294 3142; 1028 971; 806; 757

PAI7 1805–1744 1388; 1113; 703 3291; 1644 1530; 1295 3111; 1021 961; 805; 735

PAI8 1793–1746 1406; 1121; 703 3270; 1681 1518; 1290 3114; 1017 977; 808; 743

PAI10 1805–1741 1408; 1111; 710 3250; 1628 1528; 1293 3138; 1022 963; 805

Table 81H NMR data related to polyamide-imides PAI1–12 (DMSO-d6/TMS)

Polymer d (ppm)

R1 R2 H3Fu H4Fu NH Ar

PAI1 1.77 1.77 6.52 7.38 11.43 8.50

PAI2 2.15 7.40 6.37 7.34 11.50 8.51

PAI3 1.71 0.81; 2.22 6.56 7.41 11.35 8.53

PAI4 1.65 4.50 6.50 7.35 11.40 8.40

PAI5 1.75 0.85; 1.20;1.33; 1.

68; 2.20

6.50 7.30 11.30 8.38

PAI6 2.11 – 6.52 7.33 11.42 8.48

PAI7 1.74 1.74 6.52 7.36 11.46 8.2

PAI8 2.17 7.44 6.43 7.36 11.40 7.80; 8.27

PAI9 1.66 0.82; 2.20 6.50 7.35 11.30 7.80; 8.30

PAI10 1.70 4.47 6.57 7.33 11.45 7.85; 8.39

PAI11 1.72 0.91; 1.18;1.32; 1.

72; 2.16

6.41 7.30 11.51 7.82; 8.45

PAI12 2.15 – 6.50 7.35 11.49 7.77; 8.43

S. Abid et al. / Polymer 45 (2004) 6469–6478 6477

The tracing of Fig. 4, showing the thermal degradation of

the synthesised PAI1, confirmed the latter interpretation.

The thermogravimetric behaviour of PHA8 and PAI8 was

more complex than that of the other homologous polymers.

With the former, after the solvent loss up to 90 8C, the break

in the curve (Fig. 5), corresponding to the conversion of

PHA8 to PAI8, was followed by two breaks which occurred

in the ranges 340–420 8C and 530–610 8C, respectively.

However, the TGA trace of the PAI8 prepared by the

thermal imidisation of PHA8, showed a single decompo-

sition step at 520–655 8C (Fig. 6).

It was also found that PAI2 and PAI8, which contained a

phenyl group in the bifuran bridge, were more stable than

Table 913C NMR data related to PHA1 and PAI1 (DMSO/TMS)

d (ppm)

PHA1 PAI1

Hydrazide-acid or

amide-imide groups

163; 166; 169 159; 164

Furan carbons 107; 117; 143; 157 107; 118; 143; 156

Aromatic carbons 124; 130; 135 124; 129

those bearing aliphatic groups. Thus, the decomposition of

PAI1 occurred in the range 360–500 8C, with a weight loss

of more than 50%, whereas PAI8 remained stable up to

about 500 8C and 65% of its original weight remained at

700 8C without any further weight loss up to 900 8C.

On the whole, these new materials maintained the same

good thermal stability as their fully aromatic counterparts

[1,2].

4. Conclusion

A series of new polyimides incorporating both furan

moieties and amide functions in their backbone were

successfully prepared. The synthesis involved solution

polycondensation of furanic dihydrazides with aromatic

dianhydrides followed by thermal or chemical cyclodehy-

dration. These polymers exhibited a regular structure, high

inherent viscosities and good thermal stability and most of

them were soluble in aprotic polar solvents. The replace-

ment of the aromatic group from the dihydrazide precursor

by differently substituted difuran bridges appears to have

provided an interesting alternative to previous attempts to

Table 10

Solubility of polyamide-imides PAI1–12

Polyamide-imide Solvent

DMA NMP DMSO DMF m-cresol Py CHCl3

PAI1 CC Ch Ch CCG

K K

PAI2 Ch Ch Ch Ch s K KPAI3 CC CC CC CC Ch s K

PAI4 CC CC CC CCG

s K

PAI5 CC CC CC CC ChG

K

PAI6 CC CC CC CC ChG

K

PAI7 CCG

Ch CCG

K K

PAI8 ChG

Ch Ch s K K

PAI9 CC CC Ch CCG

s K

PAI10 CC CC CC CC Ch s K

PAI11 CC CC CC CC ChG

K

PAI12 CC CC CC CC sG

K

CC: soluble at room temperature; Ch: soluble after heating; G: partly soluble; s: swelling; K: insoluble.

Table 11

Thermal properties (8C) of PAIs

Polyamide-imide Tg Td10a

PAI1 250 402

PAI2 262 540

PAI3 245 358

PAI4 242 361

PAI7 261 410

PAI8 267 553

PAI9 255 377

PAI10 251 391

a Temperature at which 10% weight loss was recorded in the TGA trace.

S. Abid et al. / Polymer 45 (2004) 6469–64786478

handle polyimide-amides more easily in terms of melt- or

solution-processes. Work is in progress to study these

processing techniques and to characterise the mechanical

properties of films of these novel materials.

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