Curable aromatic polyimides containing enaminonitrile groups

Full text of DOI:10.1021/ma0115085

Volume 35, Number 12
J une 4, 2002
© Copyright 2002 by the American Chemical Society
Communications to the Editor
Sch em e 1
Cu r a ble Ar om a tic P olyim id es Con ta in in g
En a m in on itr ile Gr ou p s
Mi Kyu n g Kim a n d Sa n g You l Kim *
Center for Advanced Functional Polymers, Department of
Chemistry and School of Molecular Science (BK21), Korea
Advanced Institute of Science and Technology, 373-1,
Kusung-Dong, Yusung-Gu, Taejon, 305-701, Korea
Received August 21, 2001
Revised Manuscript Received April 1, 2002
Among thermally stable polymers, aromatic polyim-
ides find many applications in electronics and compos-
ites, because of their high glass transition temperature
(Tg), outstanding chemical stability, and mechanical
properties. However, their high Tg and insoluble nature
in most organic solvents make them difficult to fabri-
cate.1 Generally, the processing of polyimides has been
accomplished through the soluble poly(amic acid) pre-
cursor, which has some problems such as hydrolytic
instability of the poly(amic acid)s, corrosion of copper
interconnecting metal, and generation of water during
the cure process.2 To address these problems, several
approaches for soluble polyimide including introduction
of a flexible linkage or bulky substituents on the main
chain3 and use of noncoplanar or unsymmetrical mono-
mers4 have been developed. However, the approaches
for soluble polyimides through structural modification
are often accompanied by deterioration of some physical
properties.
Poly(enaminonitrile)s (PEANs) which were synthe-
sized by J . A. Moore show hydrolytic stability, excel-
lent thermal properties, and good solubility in organic
solvents, and can be cured thermally without evolu-
tion of volatile byproducts to the insoluble polymers.5
Bulky enaminonitrile groups not only enhance the
solubility of polymers but also increase the thermal
stability, Tg and chemical resistance of polymers through
curing without any volatile byproducts. Several aro-
matic polyamides containing enaminonitrile groups
were synthesized,6 but unsymmetrical diamines with
one enaminonitrile unit have not been reported. In this
communication, we describe our successful attempt to
obtain a soluble polyimide by using an unsymmetrical
diamine monomer containing a curable enaminonitrile
unit.
The diamine monomers containing one enaminonitrile
unit were prepared from (chloro-4-nitrophenylmeth-
ylidene)propanedinitrile (1) in two steps, as shown in
Scheme 1.
(Chloro-4-nitrophenylmethylidene)propanedinitrile (1)
was prepared according to the previously reported
procedure.7 The product 1 was reacted with 4-nitro-
aniline in a polar aprotic solvent in the presence of 1,4-
diazabicyclo[2.2.2]octane (DABCO) as an acid acceptor
to form the nitro compounds 2 and 3. Complete reduc-
tion of the dinitro compounds was found to be difficult
with a palladium catalyst, presumably because of the
complexation of palladium with the reactant or the
product.6a However, the nitro compounds were hydro-
genated successfully with stannous chloride/HCl to give
the corresponding diamine monomers 4 and 5 with 84
and 75% yields, respectively. The above result revealed
* To whom correspondence should be addressed. Telephone: 82-
42-869-2834. Fax: 82-42-869-2810. E-mail: kimsy@mail.kaist.ac.kr.
10.1021/ma0115085 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/01/2002

4554 Communications to the Editor
Macromolecules, Vol. 35, No. 12, 2002
Sch em e 2
Sch em e 3
the hydrolytic stability of the enaminonitrile group in
HCl solution. The chemical structure of the monomers
were confirmed by spectral analyses.8
The model reaction was conducted to investigate the
reactivity of the diamine monomers and to obtain a
model compound as a reference material for structural
analysis and curing reaction. Diamine monomer 4 was
reacted with monofunctional phthalic anhydride in
m-cresol in the presence of a catalytic amount of
isoquinoline at 190 °C. The water generated during the
imidization was removed by azeotropic distillation of
chlorobenzene and water. Diimide model compound 6
was obtained quantitatively (Scheme 2).
Ta ble 1. P r op er ties of P olyim id es
Td5b(°C)
d
[η]a
(dL/g)
in N2
(°C)
in air
(°C)
char yield at
800 °C (%)c
Texo
polymer
(°C)
The structure of model compound 6 was confirmed
with IR and 1H NMR analyses. FT IR spectrum of 6
shows absorption bands at 1782, 1718, 1375, and 719
cm-1 corresponding to the CdO imide asymmetric and
symmetric stretching, C-N imide stretching, and
CdO imide bending, respectively, and at 2206 cm-1
corresponding to the CN stretching. 1H NMR spectrum
of 6 shows a peak at 11.01 ppm corresponding to
enaminonitrile proton (-NH-CdC(CN)2). Also, thermal
behavior of the diimide model compound was examined
with DSC. DSC analysis showed two melting points at
358 and 442 °C, and irreversible exothermic transition
between 360 and 400 °C, due to the curing of the
enaminonitrile groups in the model compound 6. It
seems that upon initial melting, the compound under-
goes the exothermic curing and immediately crystal-
lizes, resulting in a second melting. The exothermic peak
was completely absent, and only the second melting
point was observed when the sample was cooled and
rescanned.
Polymerization of the diamine monomer 4 with
3,3′,4,4′-benzophenone tetracarboxylic dianhydride
(BTDA) or 4,4′-oxydiphthalic anhydride (ODPA) was
carried out via one-pot solution imidization method, but
precipitation occurred during the imidization in both
N-methylpyrrolidone (NMP) and m-cresol solvents. Po-
lymerization of the diamine monomer 4 with ODPA in
N,N-dimethylacetamide (DMAc) via chemical imidiza-
tion method was also attempted, but only a low molec-
ular weight polymer having inherent viscosity of 0.18
dL/g (DMAc, 30 °C) was obtained. However, polymeri-
zation of the diamine monomer 5 with commercially
available dianhydrides, pyromellitic dianhydride (PMDA),
3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA),
hexafluoroisopropylidene diphthalic anhydride (6FDA),
BTDA, and ODPA via the one-pot solution imidization
method in NMP proceeded homogeneously, and high
molecular weight polymers were obtained in quantita-
7
8
9
10
11
0.54
0.46
0.45
0.54
0.46
545
572
555
493
519
507
514
512
60
67
60
61
52
387
379
376
373
383
548 (567)e
524
a
b
Intrinsic viscosity in DMAc (0.5 g/dL) at 25 °C. Td5 (5%
weight loss temperature) was measured by TGA at a heating rate
of 10 °C/min. c Measured by TGA at a heating rate of 10 °C/min
in nitrogen. Measured by DSC at a heating rate of 10 °C/min in
nitrogen. Td5 (5% weight loss temperature) after curing at 450
d
e
°C was measured by TGA at a heating rate of 10 °C/min.
tive yield (Scheme 3).9 The polymers showed inherent
viscosities in the range 0.45-0.54 dL/g. The weight-
average molecular weight (Mw) of the polymer 11
measured by GPC was 51 000. The properties of the
polymers are summarized in Table 1. The formation of
1
polyimides was confirmed with IR and H NMR spec-
troscopic analyses.
All the polymers were soluble in polar aprotic solvents
such as NMP, DMAc, and N,N-dimethylformamide
(DMF) and also in hot m-cresol. Interestingly, polymer
11 was even soluble in acetone and THF. These solubil-
ity results indicate that the enaminonitrile units in the
polyimide chains effectively improve the solubility of
aromatic polyimides.
Thermal behavior of these polymers was followed by
DSC and TGA. DSC analysis showed a broad irrevers-
ible exothermic peak between 300 and 450 °C, depend-
ing on the dianhydride monomer employed, but the
exothermic peak was completely absent when the
samples were cooled and rescanned, indicating that the
exothermic peak was caused by the curing of the
polymers. After curing, the polymers were completely
insoluble in the solvents that dissolved the uncured
polymers.
The 5% weight loss temperatures of the polymers
were in the range of 524-572 °C in nitrogen and 493-

Macromolecules, Vol. 35, No. 12, 2002
Communications to the Editor 4555
F. W.; Sakaguchi, Y.; Shibata, M.; Hsu, S. L. C. High
Perform. Polym. 1997, 9, 251. (f) Scola, D. A. J . Polym. Sci.,
Polym. Chem. Ed. 1993, 31, 1997. (g) Kim, W. G.; Hay, A.
S. Macromolecules 1993, 26, 5275. (h) Mikroyannidis, J . A.
Macromolecules 1995, 28, 5177. (i) Spiliopoulos, I. K.;
Mikroyannidis, J . A. Macromolecules 1996, 29, 5313. (j)
Chung, I. S.; Kim, S. Y. Macromolecules 1998, 31, 5920.
(4) (a) Harris, F. W.; Hsu, S. L. C. High Perform. Polym. 1989,
1, 3. (b) Matsuura, T.; Hasuda, Y.; Nishi, S.; Yamada, N.
Macromolecules 1991, 24, 5001. (c) Wang, Z. Y.; Qi, Y.
Macromolecules 1995, 28, 4207. (d) Lin, S.-H.; Li, F.; Cheng,
S. Z. D.; Harris, F. W. Macromolecules 1998, 31, 2080. (e)
Zheng, H. B.; Wang, Z. Y. Macromolecules 2000, 33, 4310.
(f) Chung, I. S.; Kim, S. Y. Macromolecules 2000, 33, 3190.
(5) (a) Moore, J . A.; Robello, D. R. Macromolecules 1989, 21,
1084. (b) Moore, J . A.; Robello, D. R. Macromolecules 1986,
19, 2667. (c) J ones, G. Quinolines; Wiley: London, 1977.
(6) (a) Moore, J . A.; Kaur, S. Macromolecules, 1997, 30, 3427.
(b) Mikroyannidis, J . A. Eur. Polym. J . 1993, 29, 527. (c)
Mikroyannidis, J . A. Eur. Polym. J . 1991, 27, 859.
519 °C in air. The above TGA results reveal that the
polyimides with enaminonirile units have good thermal
stability in nitrogen comparable to that of conventional
polyimide like Kapton, but 5 wt % loss occurred at lower
temperatures in air than in nitrogen, indicating that
the polymers are somewhat susceptible to thermooxi-
dative degradation. It seems that thermooxidative
degradation of the polymer chains occurred before
complete curing.
In summary, the diamine monomers containing one
enaminonitrile unit were synthesized and polymerized
with various aromatic dianhydrides by using the one-
pot solution imidization method. While the polymeri-
zation of the diamine monomer 4 produced insoluble
polymers, the polyimides from the diamine monomer 5
were soluble in polar aprotic solvents and showed
excellent thermal stability. All the polymers exhibited
broad irreversible exothermic transitions between 350
and 450 °C, indicating curing of enaminonitrile groups.
The polymers became insoluble after curing.
(7) Chung, I. S.; Kim, S. Y. Polym. Bull. (Berlin) 1997, 38, 635.
(8) Spectral data of monomers 4 and 5. [(p-Aminophenylamino)-
4-aminophenylmethylidene]propanedinitrile (4): mp 292 °C
(DSC); FT IR (KBr, cm-1) 3437, 3350, 3250 (NH2 and NH),
2213, 2190 (CN), 1606 (aromatic CdC); 1H NMR (DMSO-
d6, 200 MHz) 9.96 (s, 1H, -NH-); 7.24 (s, 2H); 6.79 (s, 2H);
6.58, 6.54, 6.49, 6.45 (dd, 4H); 5.94 (s, 2H, -NH2); 5.16(s,
2H, -NH2); 13C NMR (DMSO-d6, 300 MHz) 167.93, 152.91,
147.19, 131.41, 127.27, 125.59, 119.03, 118.24, 116.04,
113.62, 112.74, 47.79. HRMS (m/e): calcd for C16H13N5,
275.1172; found, 275.1172. [(m-Aminophenylamino)-4-ami-
nophenylmethylidene]propanedinitrile (5): mp 238 °C (DSC);
FT IR (KBr, cm-1) 3473, 3364, 3232 (NH2 and NH), 2207
(CN), 1602 (aromatic CdC); 1H NMR (DMSO-d6, 200 MHz)
10.08 (s, 1H, -NH-); 7.29, 7.24 (d, 2H); 6.94, 6.90, 6.86 (t,
1H); 6.58, 6.54 (d, 2H); 6.34, 6.32 (m, 2H); 6.20, 6.17 (d, 1H);
6.04 (s, 2H, -NH2); 5.18 (s, 2H, -NH2); 13C NMR (DMSO-
Ack n ow led gm en t. This work was supported by
Center for Advanced Functional Polymers at Korea
Advanced Institute of Science and Technology (KAIST)
and by the Brain Korea 21 project.
Su p p or tin g In for m a tion Ava ila ble: Text giving syn-
thetic procedures and figures showing FTIR, 1H NMR, 13C
NMR, and HRMS spectra of monomers 4 and 5 and IR and
1H NMR spectra and DSC curves of model compound 6 and
polymer 9. This material is available free of charge via the
Internet at http://pubs.acs.org.
d
6, 300 MHz) 167.49, 153.32, 149.21, 139.67, 131.83, 129.08,
118.64, 117.45, 115.96, 112.84, 111.50, 111.18, 108.74, 49.82.
HRMS (m/e): calcd for C16H13N5, 275.1172; found, 275.1172.
(9) A typical polymerization procedure is as follows (polymer
9): A 50 mL three-neck round-bottomed flask equipped with
an argon inlet and mechanical stirrer was charged with
0.4434 g (1.6106 mmol) of diamine monomer 5 and 10 mL
of NMP. To this solution was added in one portion 0.5190 g
of BTDA (1.6106 mmol) at 0 °C. The solution was stirred at
room temperature for 8 h and then heated gradually to 190
°C for additional 12 h. The water generated by imidization
was distilled from the reaction mixture along with small
amount of chlorobenzene. The product was precipitated into
distilled water, filtered, washed with hot water and hot
MeOH repeatedly, and dried in vacuo at 80 °C for 24 h
(0.8815 g, 98%). FTIR (film, cm-1): 3250 (NH); 2218 (CN);
1781, 1717 (CdO of imide); 1658 (CdO of ketone); 1600,
1513 (aromatic); 1368 (C-N stretching); 725 (CdO of
bending).1H NMR (DMSO-d6, 200 MHz): 11.08 (s, 1H,
-NH-); 8.27-8.16 (m, 6H); 7.87, 7.83 (d, 2H); 7.68, 7.64
(d, 2H); 7.52-7.36 (br, 4H).
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MA0115085