A novel benzimidazole-containing phthalonitrile monomer with unique polymerization behavior

Article abstract of DOI:10.1002/pola.26331

A novel benzimidazole-containing phthalonitrile monomer (BIPN) was synthesized. The chemical structure of BIPN was confirmed by various spectroscopic techniques. Differential scanning calorimetry measurement revealed that the self-promoted polymerization reaction of the BIPN proceeds extremely sluggish and showed low polymerization exothermic effect. Subsequent rheological measurement displayed that the BIPN was able to keep a stable and low melt viscosity for 4 h at 300 °C, 2 h at 310 °C, and 50 min at 330 °C. The derived BIPN polymers showed excellent thermal properties revealed by thermogravimetric analysis, which were better than those of the corresponding polymer derived from phthalonitrile monomer without benzimidazole moiety. IR analysis confirmed the occurrence of the triazine ring within the polymer crosslinking sites.

Full text of DOI:10.1002/pola.26331

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A Novel Benzimidazole-Containing Phthalonitrile Monomer With
Unique Polymerization Behavior
Dimeng Wu, Yongchao Zhao, Ke Zeng, Gang Yang
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University,
Chengdu 610065, People’s Republic of China
Correspondence to: K. Zeng (E-mail: zk_ican@sina.com) or G. Yang (E-mail: yanggang65420@163.com)
Received 3 May 2012; accepted 23 July 2012; published online
DOI: 10.1002/pola.26331
ABSTRACT: A novel benzimidazole-containing phthalonitrile
monomer (BIPN) was synthesized. The chemical structure of BIPN
was confirmed by various spectroscopic techniques. Differential
scanning calorimetry measurement revealed that the self-pro-
moted polymerization reaction of the BIPN proceeds extremely
sluggish and showed low polymerization exothermic effect. Sub-
sequent rheological measurement displayed that the BIPN was
able to keep a stable and low melt viscosity for 4 h at 300 ꢀC, 2 h at
310 ꢀC, and 50 min at 330 ꢀC. The derived BIPN polymers showed
excellent thermal properties revealed by thermogravimetric analy-
sis, which were better than those of the corresponding polymer
derived from phthalonitrile monomer without benzimidazole moi-
ety. IR analysis confirmed the occurrence of the triazine ring within
C
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the polymer crosslinking sites.
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KEYWORDS: curing of polymers; high performance polymers;
high temperature materials; phthalonitrile resin
PEPI with the processing temperature kept below 300 ꢀC.5
This is required since strong chain–chain interactions and
the stiffness of the aromatic backbone are inherent to the
nature of aromatic heterocyclic polymers (polyimide, poly-
phenylquinoxaline, polybenzoxazole, etc.) and result in their
high melt viscosity.
INTRODUCTION Studies on high temperature/performance
resins with terminal reactive groups cured via an addition
reaction have received much attention in the past decades.
These resins cure via an addition reaction with no volatile
byproducts evolved during the processing stage. In addition,
the cured resins showed excellent thermal properties. Typi-
cal addition-type reactive groups such as cyanate, maleimide,
ethynyl, and benzocyclobutene have been fully character-
ized.1 Although, resins end-capped with these reactive
groups have witnessed application in hi-tech fields such as
optical material and microelectric industry,2 much effort was
concentrated on resins terminated with phenylethynyl (PE)
group. This is due to their wide processing window and
excellent combination of properties. Various spacers contain-
ing imide, phenylquinoxaline, and arylene ether moiety were
introduced between PE end-caps.3
On the other hand, phthalonitrile (PN) resins have been
studied over the past 30 years as a class of high tempera-
ture/performance polymers,6 having a variety of potential
applications for composite matrices,6,7 adhesives,8 electrical
conductors,9 and carbon precursors.10 The polymerization of
the neat resins is extremely sluggish and can be promoted by
various curing agents such as phenols,11 organic amines,12
strong organic acids,13 strong organic acid/amine salts,14 met-
als, and their salts.15 The PN resins exhibit an attractive com-
bination of properties, that is, outstanding thermal-oxidative
stability, high glass transition temperatures (Tg), and superior
flame resistance.6 In addition, a low melt complex viscosity
(0.01–1.0 Pa s) of the PN monomers or oligomers enables fac-
ile processing by the cost-effective, non-autoclavable process-
ing techniques such as RTM, resin infusion molding, and fila-
ment winding. During the past decades, systematic studies
have been conducted at Naval Research Laboratory (NRL) on
PN resins in terms of cure behavior, processabilities, and prop-
erties of the PN resins.10,16–20
As is well-known, low and stable melt viscosity is of utmost
importance for fabrication of thick composite components by
resin transfer molding (RTM) or resin infusion methods
(RIM). Hergenrother and coworkers have tried to design low
molecular weight oligomeric imide resins with terminal PE
groups (PEPI), enabling its processing by RTM (melt stability
3(a)
ꢀ
>2 h at 280 C).
Simone and Scola reported that even the
lowest reactive PEPI was unlikely to keep a stable and low
melt viscosity at 310 ꢀC over 1 h.4 Therefore, additional
improvements such as introduction of flexible or asymmetric
linkage are required to further reduce the viscosity of the
Additional Supporting Information may be found in the online version of this article.
C
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Oligomeric imide resins with terminal PN units were also
prepared and characterized at NRL.21 Unfortunately, the
same problem as that for PE end-capped imide-containing
resins cannot be avoided. Low molecular weight imide
spacers or chemical structure modifications were also
required when processing imide-containing PN resins by
RTM or RIM.
mol) under nitrogen atmosphere. The mixture was stirred in
ice water bath for 24 h. The reaction mixture was filtered
and the filtrate was collected. The product, N-(2-amino-
phenyl)-3,5-dihydroxybenzamide, was obtained by bubbling
with dry hydrogen chloride into the reaction system, collect-
ing the white precipitate by filtration, subsequently neutral-
izing the mixture of the white precipitate/THF with NaHCO3,
and evaporation of THF after filtration under reduced pres-
sure. The yield was 35% and the product was used in the
next synthetic step without further purification.
The previous investigation, in our laboratory, on amino or
hydroxy-containing PN model compounds (APN or HPN)
demonstrated that APN or HPN showed a self-promoted
thermal polymerization even in the absence of curing addi-
tives.22–24 The systematic investigation on the thermal poly-
merization behaviors of the HPN indicated that the polymer-
ization proceeds more readily with stronger acidity of the
hydroxy group.24 Therefore, curing of the PN derivatives con-
taining amino or hydroxy groups offers a new and alterna-
tive processing mode for PN resins, having the potential of
expanding their application range. However, the introduction
of amino or phenol groups to PN resulted in undesirable
thermal properties.
A
solution of N-(2-aminophenyl)-3,5-dihydroxybenzamide
(4.00 g, 0ꢀ.011 mmol) in acetic acid (51 mL) was refluxed at
120–130 C under nitrogen atmosphere for 16 h. After cool-
ing, the solution was maintained at room temperature over
night for crystallization. The resulting precipitate (compound
1) was collected by filtration and dried at 70 ꢀC under
reduced pressure for 24 h. The yield was 75%. IR (KBr,
cmꢁ1): 3195 (OAH stretching), 1608 (C¼¼N stretching). 1H
NMR (400 MHz, DMSO-d6): d 12.80 (s, 1 H, imidazole NH),
9.54 (s, 2 H, OH), 7.57 (s, 2 H, ArAH), 7.2 (d, 2 H, ArAH),
7.01 (s, 2 H, ArAH), 6.37 (s, 1 H, ArAH).
As an extension of this work, a benzimidazole ring with
acidic hydrogen proton was introduced into PN resin (benz-
imidazole-containing phthalonitrile, BIPN). This is because
benzimidazole ring having excellent thermal properties
would be more compatible with thermally stable polymers
relative to amino or hydroxy groups. We believe that the
BIPN may exhibit a similarly or more intensely self-promoted
polymerization reaction relative to the HPN, due to the
strong acidity of NAH proton on benzimidazole ring. Unex-
pectedly, an extremely mild self-promoted polymerization
was observed for BIPN. Inspired by this result, it would be
possible to emꢀploy higher initial processing temperatures (in
excess of 300 C) resulting in an initial lower melt viscosity,
and then the molecular weight range of the aromatic hetero-
cyclic spacers can be broadened and more flexible chemical
structures can be introduced into the spacers. In this work, a
type of novel high performance thermosetting resin with un-
precedented polymerization behavior (outstanding melt sta-
bility (>300 ꢀC) and low polymerization exothermic effect)
was found.
Synthesis of BIPN Monomer
To a 100-mL three-neck flask were added 4-nitrophthaloni-
trile (1.73 g, 10 mmol), compound 1 (1.13 g, 5 mmol), anhy-
drous potassium carbonate (2.41 g, 0.017 mol) and 30 mL of
DMSO. The mixture was stirred at 30 ꢀC under nitrogen for
24 h and then poured into a large amount of water. Then
the solution was acidified to pH ¼ 2 by adding concentrated
hydrochloric acid. The resulting precipitate was collected by
filtration, washed with water until the filtrate was neutral,
dried at 70 ꢀC under reduced pressure for 24 h. The yield
was 90%. IR (KBr, cmꢁ1): 3587 (NAH stretching), 2233
(CAN stretching), 1597 (CAN stretching). 1H NMR (400
MHz, DMSO-d6): d 13.00 (s, 1 H, imidazole NH), 8.19–8.21
(d, 2 H, ArAH), 8.01 (d, 2 H, ArAH), 7.87 (d, 2 H, ArAH),
7.66–7.73 (dd, 2 H, ArAH), 7.54–7.56 (d, 2 H, ArAH), 7.32
(d, 1 H, ArAH), 7.21–7.28 (m, 2 H, ArAH). ELEM. ANAL.: calcd
for C29H14N6O2 (H2O): C, 70.16; H, 3.25; N, 16.93. Found: C,
69.99; H, 3.31; N, 16.84.
Synthesis of BIPN Polymers
The BIPN polymers were produced by heat treatment in
thermogravimetric analysis (TGA) chamber under nitrogen
EXPERIMENTAL
ꢀ
Materials
with a floꢀw rate of 50 mL/min with conditions: 310 C forꢀ4
ꢀ
ꢀ
4-Nitrophthalonitrile was purchased from Aldrich Chemical
Co. (Aldrich). Dimethyl sulfoxide (DMSO), dicyclohexylcarbo-
diimide (DCC), potassium carbonate, and tetrahydrofuran
(THF) were purchased from Tianjin BoDi. 3,5-Dihydroxyben-
zoic acid, 4-hydroxybenzoic acid, and other chemicals were
purchased from Chengdu Kelong chemical reagent and used
as received.
h or 310 C for 8 h or 310 C for 8 h, 330 C for 2 h, 350 C
for 8 h or 310 ꢀC for 8 h, 330 ꢀC for 2 h, 350 ꢀC for 8 h,
ꢀ
375 C for 8 h (post-cured polymer).
Synthesis of Model Benzimidazole-Containing
Phthalonitrile Compound (MBIPN)
Synthesis of Compound 10
To a 100-mL three-neck flask were added 4-nitrophthaloni-
trile (1.74 g, 0.01 mol), 4-hydroxybenzoic acid (1.38 g, 0.01
mol), anhydrous potassium carbonate (3.00 g, 0.0ꢀ21 mol),
and 20 mL of DMSO. The mixture was stirred at 30 C under
nitrogen for 24 h and then poured into a large amount of
water. Then the solution was acidified to pH ¼ 2 by adding
concentrated hydrochloric acid. The resulting white
Synthesis of Benzimidazole-Containing
Phthalonitrile Monomer
Synthesis of Compound 1
To a solution of o-phenylenediamine (10.2 g, 0.094 mol) and
3,5-dihydroxybenzoic acid (13.86 g, 0.09 mol) in THF (132
mL) were added dicyclohexylcarbodiimide (19.47 g, 0.094
2
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H, ArAH). ELEM. ANAL.: Calcd for C21H12N4O(H2O): C, 71.18;
H, 3.98; N, 15.81. Found: C, 71.90; H, 4.21; N, 15.48.
Characterization
1H NMR (400 MHz) was measured on a Bruker Avance-400
NMR spectrometer with DMSO-d6 as the solvent and tetra-
methylsilane as the internal standard. IR spectra were
recorded with a Nicolet FTIR-380 Fourier transform infrared
spectrometer by KBr pellet. Elemental analyses (EA) were
performed on a Italy CARLO ERBA 1106 elemental analyzer.
TGA was carried out with a TA instrument Q500 thermogra-
ꢀ
vimetric analyzer at a heating rate of 10 C/min. Differential
scanning calorimetry (DSC) was performed on a TA instru-
ment Q200 differential scanning calorimeter under nitrogen
atmosphere at a heating rate of 10 ꢀC/min. Rheological
experiments were conducted on a TA instruments AR-2000
rheometer in conjunction with an environmental testing
chamber for temperature control. The measurements were
made using a 25 mm diameter parallel plates at low strain
values (2.5 ꢂ 10ꢁ4) and a frequency of 1 Hz. The sample
specimen disks, 25 mm in diameter, and 1.0–1.1 mm thick,
were prepared by press-molding the BIPN powder (around
0.5 g) at room temperature. The compacted sample disk was
subsequently loaded in the rheometer fixture. The dynamic
storage modulus as a function of temperature was measured
from 50 to 400 ꢀC at a heating rate of 5 ꢀC/min in air. The
dynamic storage modulus of BIPN was also monitored at
SCHEME 1 Synthesis and polymerization of BIPN monomer
and polymers.
precipitate was collected by suction filtration, and washed
with water until the filtrate was neutral, dried at 70 ꢀC
under reduced pressure for 12 h. Yield: 2.17 g, 82%. The
crude product was used in the next reaction without purifi-
cation. IR (KBr, cmꢁ1): 2230 (CAN stretching), 1675 (C¼¼O
stretching). 1H NMR (400 MHz, DMSO-d6): d 13.01 (s, 1 H,
COOH), 8.14–8.16 (d, 1 H, ArAH), 8.02–8.04 (d, 2 H, ArAH),
7.92–7.93 (d, 1 H, ArAH), 7.52–7.55 (dd, 1 H, ArAH), 7.25–
7.27 (s, 2 H, ArAH).
Synthesis of Compound 20
To a solution of o-phenylenediamine (1.14 g, 10.1 mmol) and
compound 10 (4.06 g, 0.01 mol) in THF (50 mL) were added
dicyclohexylcarbodiimide (2.16 g, 10.4 mmol) under nitrogen
atmosphere. The mixture was stirred in ice/water bath for
24 h and then filtered. The filtrate was poured into a large
amount of water. The resulting precipitate was collected by
filtration and dried at 55 ꢀC under reduced pressure for 12
h. The crude product was purified by purging dry HCl gas
into its THF solution, producing hydrochloric acid salt pre-
cipitate, which was collected by filtration and then neutral-
ized using 5 wt % aqueous solution of Na2CO3. Yield: 1.24 g,
35%. IR (KBr, cmꢁ1): 3458, 3371 (NAH symmetrical and
asymmetrical stretching), 2230 (CAN stretching), 1645
ꢀ
300, 310, and 330 C in air as a function of time, where the
temperatures were rapidly ramped to required values at a
heating rate of 50 ꢀC/min and equilibrated for 1 min,
respectively.
RESULTS AND DISCUSSION
Monomer Synthesis
The BIPN monomer was synthesized in three steps (Scheme 1).
Direct condensation between 3,5-dihydroxybenzoic acid and
1,2-phenylenediamine using dicyclohexylcarbodiimide (DCC) as
the dehydration catalysis and subsequent cyclodehydration in
acetic acid afforded compound 1. The BIPN was readily synthe-
sized by simple nucleophilic displacement between compound
1 and 4-nitrophthalonitrile in the yield of 90%. The chemical
1
(C¼¼O stretching). H NMR (400 MHz, DMSO-d6): d 9.87 (s, 1
H, amide NH), 8.11–8.17 (m, 3 H, ArAH), 7.89–7.90 (d, 1 H,
ArAH), 7.48–7.51 (dd, 1 H, ArAH), 7.31–7.33 (d, 2 H,
ArAH), 7.20–7.22 (d, 1 H, ArAH), 7.03–7.07 (t, 1 H, ArAH),
6.90 (d, 1 H, ArAH), 6.75 (s, 1 H, NH2).
1
structure of the BIPN was unambiguously confirmed by IR, H
NMR, and EA. The model benzimidazole-containing PN
(MBIPN) compound was synthesized according to Scheme 2. Its
chemical structure was also verified by IR, 1H NMR, and EA.
Synthesis of MBIPN
A solution of compound 20 (4.00 g, 0.011 mmol) in acetic
acid (51 mL) was refluxed for 6 h. After cooling to room
temperature, the reaction solution was poured into ice water.
The resulting precipitate was collected by filtration and puri-
fied by recrystallization from methanol/acetonitrile mixed
solvents. Yield: 1.84 g, 50%. IR (KBr, cmꢁ1): 3579 (NAH
stretching), 2230 (CAN stretching), 1595 (C¼¼N stretching).
1H NMR (400 MHz, DMSO-d6): d 12.99 (s, 1 H, imidazole
NH), 8.27–8.29 (d, 2 H, ArAH), 8.14–8.16 (d, 1 H, ArAH),
7.93–7.94 (d, 1 H, ArAH), 7.60 (s, 2 H, ArAH), 7.52–7.54
(dd, 1 H, ArAH), 7.36–7.38 (d, 2 H, ArAH), 7.20–7.22 (m, 2
SCHEME 2 Synthesis of MBIPN compound.
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Inspired by these results, it is interesting to investigate the
melt stability of the BIPN above 300 ꢀC. Modulus versus
time measurements at different temperatures of 300, 310,
ꢀ
and 330 C were conducted, as shown in Figure 2. The data
showed that a stable and low modulus can be kept for an
unexpectedly long time. The BIPN did not show a distinctive
increase in modulus within 4 h dwell at 300 ꢀC, 2 h at
310 ꢀC, and 50 min at 330 ꢀC. On the other hand, a rapid
increase in modulus for BIPN can be observedꢀafter 5 h
ꢀ
ꢀ
dwell at 300 C, 2.5 h at 310 C, and 1 h at 330 C. In con-
trast, model PN monomer, 1,3-bis(3,4-dicyanophenoxy)ben-
zene, did not show an increaꢀse in modulus even after 5.5 h
dwell at temperature of 310 C (Supporting Information Fig.
S2). The result unambiguously indicated the role of benzim-
idazole ring in promoting the polymerization rate of the PN
resins. This unique polymerization behavior is not only ad-
vantageous for BIPN to be processed into thick composite
sections by the cost-effective, non-autoclavable processing
techniques such as RTM or RIM, but also it can afford more
flexibility for the design of the aromatic heterocyclic spacers
between PN end-caps. To the best of our knowledge, the
polymerization behavior of the BIPN monomer was unprece-
dented among currently studied resin systems such as PE
resin, for which even the lowest reactive PEPI is unlikely to
FIGURE 1 DSC curves of BIPN under nitrogen atmosphere: (A)
First scan at a heating rate of 10 ꢀC/min; (B) second scan at a
heating rate of 10 ꢀC/min immediately after first scan; (C) Sec-
ond scan at a heating rate of 20 ꢀC/min immediately after iso-
thermal scan at 310 ꢀC for 8 h in the DSC.
Polymerization Behavior of BIPN
4
ꢀ
keep a stable and low melt viscosity at 310 C over 1 h.
The thermal behaviors of the BIPN were studied by DSC.
The DSC first and second scan curves are plotted in Figure
1(A, B). Two endothermic peaks at approximately 175 ꢀC
and 250 ꢀC are corresponding to loss of crystalline water
and melting transition of BIPN, respectively. The crystalline
water loss can be confirmed by its TGA result that a 3.44%
Thermal Properties of BIPN Polymers
The BIPN polymers were produced by heat treatment in TGA
chamber under nitrogen with a flow rate of 50 mL/min
(Scheme 1). Figure 3 shows the TGA curves of the BIPN
monomer and polymers. The thermal stability was found to
be a function of the curing cycle. As the cure time and tem-
perature was increased, an improvement in the thermal sta-
bility was observed. However, further heating of the BIPN to
ꢀ
weight loss between 80 and 180 C was found, which is con-
sistent well with the calculated value of 3.62%. The elemen-
tal analysis result of the BIPN also supported this inference.
An exothermic peak at approximately 185 ꢀC can be attributed
to the recrystallization of the BIPN after loss of the crystalline
ꢀ
350 C showed a less pronounced progression in the thermal
properties. Fortunately, the char yields or weight retention
ꢀ
ꢀ
water. A glass transition was observed at about 120 C in the
(800 C) of the BIPN post-cured polymer were still in excess
second scan curve, in which the crystallization and melting
peaks of the BIPN were similar to those in the first scan curve.
Neither curve showed any exothermic peaks at elevated tem-
peratures due to curing, implying that the polymerization is
progressing at a very slow rate. This nature is totally different
compared to those of other thermosetting resins. Following
ꢀ
the heat treatment (310 C for 8 h) in the DSC chamber, the
sample did not show a Tg, indicating that its crosslinking den-
sity had increased to a point where little molecular mobility
was present within the polymeric backbone [Figure 1(C)]. Af-
ter heat treatment in the DSC, the insolubility of the BIPN
polymers in solvents such as NMP, DMF, and concentrated
H2SO4 further indicated the occurrence of the crosslinking
reaction (Supporting Information Table S1).
Rheological measurements were conducted to further eluci-
date the polymerization behavior of the BIPN. The storage
modulus of the BIPN monomer was tested as a function of
ꢀ
temperature from 50 to 400 C (Supporting Information Fig.
S1). The monomer displayed a rapid decrease in modulus on
ꢀ
melting at 250 C, and did not show a distinctive increase in
FIGURE 2 Storage modulus vs. time curves for BIPN monomer
at 300, 310, and 330 ꢀC.
modulus within the temperature range of 250–400 ꢀC.
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FIGURE 4 Thermal and thermo-oxidative stability of BIPN
post-cured polymer (310 ꢀC for 8 h, 330 ꢀC for 2 h, 350 ꢀC for
8 h, 375 ꢀC for 8 h) under nitrogen and air atmosphere.
FIGURE 3 TGA curves of powdered BIPN (A) in nitrogen and
BIPN polymers cured with conditions: (B) 310 ꢀC for 4 h; (C)
310 ꢀC for 8 h; (D) 310 ꢀC for 8 h, 330 ꢀC for 2 h, 350 ꢀC for 8 h.
was promoted through protonated nitrile groups undergoing
nucleophilic attack by Lewis base.14 Our previous results on
hydroxy-containing PN model compounds can also be inter-
preted using this mechanism.24 However, in this article, we
believe new interpretation about the unique polymerization
behavior of the BIPN may be proposed. This will be our con-
cern in the future.
of 80% in N2 and 25% in air (Figure 4). The values were
higher than those of the 1,3-bis(3,4-dicyanophenoxy)ben-
zene-based PN polymer,25 suggesting that introduction of
benzimidazole ring is effective in maintaining the outstand-
ing thermal properties of PN resins.
The polymerization reaction was monitored by IR technique,
as shown in Figure 5. Nitrile group absorption peak was at
around 2227 cmꢁ1 for BIPN monomer, which obviously dimin-
ished in intensity after heat treatment. A less pronounced
change in absorption peak of nitrile group was observed for
the BIPN polymers and post-cured polymer. The tendency is
consistent with that of polymer thermal property improve-
ment. New absorption peaks at 1520 cmꢁ1 were observed,
indicating the formation of triazine ring during the polymer-
ization of the PN groups.26 In addition, the nitrile absorptions
of the polymers shifted to lower wavenumbers relative to that
of the BIPN monomer, similar to a previous report when cyano
groups are converted to triazine ring.26
CONCLUSIONS
A novel BIPN monomer was synthesized. Benzimidazole ring
is responsible for the self-promoted polymerization reaction
of the BIPN. The unique polymerization behavior (outstanding
melt stability (>300 ꢀC) and low polymerization exothermic
effect) of the BIPN is favorable to processing into complex
composite structures by RTM or RIM, and afforded more
Thermal Behavior of Model Compounds
Thermal behaviors of MBIPN compound and model hydroxy-
containing PN (m-HPN) were comparatively studied by DSC
and rheology analysis to further understand the unique poly-
merization behaviors of BIPN. m-HPN displayed an obviously
exothermic peak at about 230 ꢀC, which was not observed
for MBIPN on the DSC curve (Supporting Information Fig.
S3). The rheological measurement also indicated that the
cure reaction for the MBIPN was much less intense relative
to hydroxy-containing PN (Supporting Information Fig. S4).
One should keep in mind that MBIPN has much stronger
acidic hydrogen proton (NAH proton) relative to that of
m-HPN (AOH proton), determined by their different chemi-
cal shift on the 1H NMR spectra. The results seems to be
contradictory to the previously obtained principle that the
self-promoted polymerization reaction proceeds more
intensely and rapidly under stronger acidic conditions.24 It
has been proposed that the polymerization of PN monomer
FIGURE 5 IR spectra of BIPN (A) and BIPN polymers cured
under conditions: (B) 310 ꢀC for 4 h; (C) 310 ꢀC for 8 h; (D)
310 ꢀC for 8 h, 330 ꢀC for 2 h, 350 ꢀC for 8 h; (E) BIPN post-
cured polymer.
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6 Zeng, K.; Yang, G. Phthalonitrile Matrix Resins and Compo-
sites, Encyclopedia of Composites; Wiley: New York, 2012 (In
publication).
flexibility for designing aromatic heterocyclic spacers.
Improvement in the thermal properties of the BIPN polymers
can be attributed to addition-type polymerization reaction of
PN groups and resultant triazine formation. Studies on model
compounds (MBIPN and m-HPN) showed that the thermal po-
lymerization of the MBIPN was significantly less intense rela-
tive to that of hydroxy-containing PN, although MBIPN has
much stronger acidic hydrogen proton. These results imply a
significant breakthrough and a new opportunity for designing
high temperature/performance resins. To explain the origin of
the BIPN unique cure behavior, more in-depth studies on poly-
merization mechanism will be reported in a separate paper.
7 Sastri, S. B.; Armistead, J. P.; Keller, T. M. Polym. Compos.
1996, 17, 816–822.
8 Keller, T. M.; Roland, C. M. U. S. Patent. 5,242,755, 1993.
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ACKNOWLEDGMENTS
14 Burchill, P. J. J. Polym. Sci. Part A: Polym. Chem. 1994, 32,
Authors thank the China Postdoctoral Science Foundation (No.
20100481387) and the National Natural Science Foundation of
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