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Polymer International
Polym Int 49:8±10 (2000)
Rapid Report
Preventing gelation in polyisoimide synthesis
Young Jun Kim* and Hae Pum Park
Department of Textile Engineering, Sung Kyun Kwan University, 300 Chunchun-Dong, Jangan-Ku, Suwon, Kyunggi-do 440-746, Korea
Abstract: The origin of the gel formation during the conversion of poly(amic acid)s to polyisoimides,
and methods for its prevention are described. The gelation behaviour was studied as a function of the
cyclodehydration temperature, the concentration and end-groups of 4,4'-oxydianiline/4,4'-oxydiphthalic anhydride poly(amic acid) systems, using 1,3-dicyclohexylcarbodiimide as a dehydrating
agent. The 1H NMR spectroscopic data showed that gel formation during polyisoimide synthesis
results from the amide formation reaction of terminal amine groups with acylated poly(amic acid)s.
Dilution of poly(amic acid) solutions and lowering cyclodehydration temperatures retarded gelation.
However, complete prevention of gel formation at relatively high concentration (15%) and
temperature (20 °C) was achieved only for phthalic anhydride end-capped polyamic systems.
# 2000 Society of Chemical Industry
Keywords: polyisoimide: gelation: polyimide: poly(amic acid)
Aromatic polyimides have found many applications as
high temperature insulators and dielectrics, adhesives
and matrices for high performance composites due to
their excellent electrical, thermal and high-temperature mechanical properties.1,2 Because most of them
show insolubility, infusibility, and thus poor processability, they are generally processed from the more
soluble poly(amic acid) precursor formed in a high
boiling solvent, such as N-methyl-2-pyrrolidone
(NMP). The poly(amic acid) is then converted to
the desired polyimide by thermal treatment. The
conversion of the poly(amic acid) to polyimide,
however, leads to the evolution of water, which in
turn causes the formation of voids or pin-holes in
laminates and in adhesive joints. As part of an effort to
eliminate these drawbacks, polyisoimides, which can
be converted to polyimides without evolution of water,
have been utilized as precursors for polyimides.3±6
Cyclodehydration of poly(amic acid)s to polyisoimides can be achieved at room temperature with
dehydrating agents such as 1,3-dicyclohexylcarbodiimide (DCC)7 and tri¯uoroacetic anhydride in combination with basic catalysts.8 However, the
fundamental chemistry of polyisoimide formation is
not fully understood, primarily due to the inherent
instability of the isoimide structure and very complicated features of isoimide chemistry. For example, the
formed isoimide groups are sensitive to hydrolysis and
also to other nucleophilic attacks.7,9 It is also reported
that the percentage conversion of poly(amic acid) to
polyisoimide is very dependent upon the reaction
conditions of the cyclodehydration step, including
temperature, nature of dehydrating agents and solvents.3 Consequently, the instability of the isoimide
structure and the complicated features of isoimide
chemistry obstructed quantitative synthesis and detailed studies of polyisoimides. Perhaps one of the
most frustrating features in polyisoimide synthesis is
gel formation during the cyclodehydration step. For
the successful synthesis of polyisoimides, gelation
should be suppressed or prevented completely. To
our knowledge, however, studies on the gelation
behaviour have not been reported in the literature
except by Landis et al 10 where attempts were made to
suppress gel formation by lowering the cyclodehydration temperature to 0±5 °C. Also in the literature,3,7
polyisoimide syntheses are generally performed in
dilute solutions (1±2%) probably for the purpose of
suppressing gel formation.
The objectives of this research were, therefore, to
elucidate the origin of gel formation and to ®nd
methods for its prevention.
High-purity 4,4'-oxydianiline (4,4'-ODA) and 4,4'oxydiphthalic anhydride (ODPA) were purchased
from Chriskev Co and vacuum dried before use.
Phthalic anhydride (PA) purchased from Aldrich was
puri®ed by sublimation. Solvent N-methylpyrrolidone
(NMP) was distilled from phosphorus pentoxide.
For the phthalic anhydride and amine terminated
poly(amic acid) syntheses, the number average molecular weights (Mn) of poly(amic acid)s were controlled to theoretical number average molecular
* Correspondence to: Young Jun Kim, Department of Textile Engineering, Sung Kyun Kwan University, 300 Chunchun-Dong, Jangan-Ku,
Suwon, Kyunggi-do 440-746, Korea
(Received 1 November 1999; accepted 9 November 1999)
# 2000 Society of Chemical Industry. Polym Int 0959±8103/2000/$17.50
Preventing gelation in polyisoimide synthesis
Mn a
Reaction temp ( °C)
Solids (%)
Gel time
30 000
ÿ10, 0, 20
5, 10, 15
15 h
18 min
5 min
7 h 50 min
4 h 40 min
1 h 3 min
17.5 min
22.2 min
4.5 min
No gelation
Theoretical number average molecular weight obtained via the Carothers equation.
A large amount of poly(amic acid) with intrinsic viscosity value of 1.47 dl gÿ1 was synthesized and
used for four different gelation reactions.
One batch for the poly(amic acid) synthesis and six different gelation reactions; Mn = 4400 g molÿ1 by
H NMR spectroscopy.
Intrinsic viscosity value of 0.75 dl gÿ1.
Table 1. Gelation behaviour during isoimidization
of 4,4'-ODA/ODPA poly(amic acid)s
weights using the Carothers equation. Three different
types of poly(amic acid)s (uncontrolled molecular
weight and end-groups, 5000 amine terminated, and
30 000 phthalic anhydride end-capped) were synthesized for gelation studies. A typical example of the
synthesis of a phthalic anhydride end-capped poly(amic acid) and its transformation to polyisoimide
with a number average molecular weight of
30 000 g molÿ1 is as follows: 4.400 g (21.974 mmol)
of 4,4'-ODA was dissolved in 100.8 g of dry NMP in a
reaction ¯ask. After the diamine was dissolved, 0.110 g
(0.745 mmol) of phthalic anhydride was added to the
stirring solution of the diamine and was allowed to
react for several minutes. Next, 6.701 g (21.612 mmol)
of ODPA was added to the mixture and the reaction
was allowed to proceed for an additional 12 h at room
temperature under nitrogen atmosphere. To this
solution 9.604 g (46.5 mmol) of N,N '-dicyclohexylcarbodiimide (DCC) was added with stirring and the
reaction mixture was stirred for 12 h at room temperature.
Intrinsic viscosity measurements were performed at
25 °C in anhydrous 0.1 M LiBr in NMP solution to
minimize polyelectrolyte effects.11 A Varian Unity
Figure 1. 1H NMR spectra of 4,4'-ODA/
ODPA poly(amic acid) systems: (A)
phthalic anhydride end-capped poly(amic
acid) of Mn = 30 000; (B) amine terminated
poly(amic acid) of Mn = 5000; (C)
Polyisoimide gel obtained from
cyclodehydration of sample (B) at 0 °C for
about 1 h.
Polym Int 49:8±10 (2000)
YJ Kim, HP Park
Figure 2. Crosslink formation reaction of a terminal amine group with a
carbonyl group of the acylated poly(amic acid).
Inova 500 spectrometer was used to obtain 1H NMR
spectra. Poly(amic acid) solutions were dissolved in
deuterated dimethylsulphoxide (DMSO-d6). In the
case of polyisoimide gel, the gel was diluted with a
large excess of NMP to swell it and DMSO-d6 was
The gelation behaviour during conversion of poly(amic acid)s to polyisoimides was investigated as a
function of end-groups, reaction temperature and
concentration using DCC as a dehydrating agent.
Table 1 shows the results of a gelation study of 4,4'ODA/ODPA polyamic acid systems with different
molecular weights and end-groups; as can be seen, the
nature of the end-groups and concentration of the
polyamic acid solutions have profound effects on
gelation behaviour. As expected, dilution of poly(amic
acid)s solutions and lowering the reaction temperatures retarded gelation, both for amine terminated
poly(amic acid) systems and for uncontrolled molecular weight and end-groups. However, gelation occurred for a low molecular weight (Mn = 5000) amineterminated system even at low concentration (5%) and
low temperature (0 °C), but incorporation of unreactive phthalic anhydride end-groups prevented gel
formation completely even at relatively high (15%)
concentration. These experimental observations
strongly suggest that amine functional groups have a
key role in the gelation mechanism.
Direct evidence of participation of amine functional
groups in gelation was obtained from 1H NMR
spectroscopy. Figure 1 shows 1H NMR spectra of
poly(amic acid)s and polyisoimide gel. As reported in
previous work12 where 1H±1H correlation spectroscopy was utilized for decoding the 1H NMR spectrum
of partially imidized 4,4'-ODA/ODPA poly(amic
acid), the up®eld-shifted peak at 6.60 ppm in spectrum
B is due to ortho protons with respect to amine groups
in the terminal aromatic moieties. In Fig 1(A), no
peaks are observed at 6.60 ppm, which con®rms that
poly(amic acid)s are successfully end-capped with
phthalic anhydride. Spectrum (C) was obtained from
the polyisoimide gel formed from amine terminated
poly(amic acid) of Mn = 5000 (10% solids). The
cyclodehydration was carried out at 0 °C for about
1 h. Even though the baseline of the spectrum of the
gel (Fig 1C) is very rough, the spectrum was used to
calculate the extent of reacted amine groups. By
comparing the ratios of the areas of the ortho protons to
those of the total aromatic protons, it was found that
about 45% of the amine groups reacted. The observation that a signi®cant amount of the amine groups
reacted was a quite natural result, because an amine
functional group is much more nucleophilic than a
nitrogen or oxygen atom of the ambident amide
nucleophile of the acylated amic acid (I in Fig 2).
Both the gelation behaviour shown in Table 1 and the
H NMR spectroscopic data suggest that the endgroups of the polyamic acid should be capped with
unreactive groups such as phthalic anhydride to
completely prevent gel formation in polyisoimide
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Applications. Ed by Ghosh MK and Mittal KL, Marcel
Dekker, New York. pp 759±814 (1996).
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Polym Int 49:8±10 (2000)
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