A novel benzimidazole moiety-containing benzoxazine: Synthesis, polymerization, and thermal properties

Article abstract of DOI:10.1002/pola.25873

A novel benzoxazine-containing benzimidazole moiety (P-PABZ) was synthesized from 2-(4-aminophenyl)-1H-benzimidazole-5-amine and characterized. With the aid of differential scanning calorimetry and in situ Fourier transform infrared, we found the thermal

Full text of DOI:10.1002/pola.25873

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A Novel Benzimidazole Moiety-Containing Benzoxazine: Synthesis,
Polymerization, and Thermal Properties
Po Yang,1,2 Yi Gu1
1State Key Laboratory of Polymeric Materials Engineering, College of Polymer Science and Engineering, Sichuan University,
Chengdu 610065, China
2National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, China
Correspondence to: Y. Gu (E-mail: guyi@scu.edu.cn)
Received 23 September 2011; accepted 29 November 2011; published online 29 December 2011
DOI: 10.1002/pola.25873
ABSTRACT: A novel benzoxazine-containing benzimidazole moi-
ety (P-PABZ) was synthesized from 2-(4-aminophenyl)-1H-benz-
imidazole-5-amine and characterized. With the aid of differential
scanning calorimetry and in situ Fourier transform infrared, we
found the thermal polymerization of P-PABZ in bulk started
around 140 ꢀC and its favored polymerization pathway. Com-
pared to the benzoxazine derived from 4,40-diamine diphenyl
methane (P-MDA), P-PABZ exhibited lower processing tempera-
ture, and the corresponding polymers had higher glass transition
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temperature and enhanced thermal stability. 2011 Wiley Period-
icals, Inc. J Polym Sci Part A: Polym Chem 50: 1261–1271, 2012
KEYWORDS: benzimidazole; benzoxazine; catalysts; high per-
formance polymers; polybenzoxazine; thermal polymerization
INTRODUCTION As shown in Scheme 1, polybenzoxazines
are the polymers with ACH2ANRACH2A (Mannich-type)
structure via the thermal polymerization of correspondent
benzoxazine monomers synthesized from formaldehyde, phe-
nol or phenol derivatives, and primary amines.1–7 As an al-
ternative to traditional phenolic resins, polybenzoxazines
have been received considerable interest from scientific and
industrial community. The major advantages of polybenzoxa-
zines are good thermal stability, good flame retardance,
excellent resistance to chemicals and UV light, low-dielectric
properties, low-surface energy, low-water absorption, near-
zero volumetric shrinkage, and no volatile release during
polymerization.8–17
plasticizers and migrate to the surface to produce fragile
materials. For this reason, an approach without using addi-
tives is desirable. The benzoxazine contains both special
groups and oxazine rings will act as catalysts and comono-
mer, and will be part of network, which should not interfere
with the properties. Hence, synthesis of the benzoxazines-
containing catalytic groups or special molecular structures is
a considerable approach. As for this approach, a series of
benzoxazines containing special groups such as carboxylic
acid, side-chain liquid crystal, hydroxyl, diacetylene, and ure-
thane have been synthesized; however, they will release vola-
tile during polymerization or interfere with the polymers’
properties, affording polybenzoxazines of poor processability
and thermal properties.21–28
Although polybenzoxazines are synthesized very early, they
have not been applied widely for a long time because of the
high-polymerization temperature.1 The research for decreas-
ing the polymerization temperature of benzoxazines is there-
fore receiving a great deal of attention. The target is an
approach that can decrease the polymerization temperature
and will not have any major adverse effects on their proper-
ties. Two approaches have been considered for decreasing
the polymerization temperature: (1) by adding catalysts like
imidazoles or acidic compounds and (2) by synthesizing
novel benzoxazines containing catalytic groups or special
molecular structures.17–28 As for the first approach, imida-
zoles or acids act as catalysts or initiators, but they are not
incorporated into the network, generating residues.17–20
These free residues can decrease the thermal properties as
In this study, we attempt to decrease the polymerization
temperature of benzoxazines through synthesis of a benzox-
azine-containing special structure. The special structure in
this work is benzimidazole. Benzimidazole structure has an
active NAH structure and does not release volatile at ele-
vated temperature; moreover, it can restrict the chain mo-
bility as a rigid structure to give the polymers many excel-
lent properties such as good mechanical properties and
thermal properties.29,30 Therefore, if a benzoxazine contains
both benzimidazole moiety and oxazine rings, it will act as
catalysts and comonomer simultaneously and will be part
of network, which will not interfere with or enhance the
thermal properties. The aim of this work is synthesis of
such benzoxazine (Scheme 5; P-PABZ and isoP-PABZ) and
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SCHEME 1 Synthesis of benzoxazines and polybenzoxazines.
determination of its thermal polymerization behavior and
thermal properties. Efforts to decrease the polymerization
temperature, we also explore the possibility of using this
benzoxazine to blend with another benzoxazine derived
from 4,40-diamine diphenyl methane. According to the
results, we find that P-PABZ has low-polymerization tem-
perature and can effectively decrease the processing tem-
perature of other benzoxazines, while the thermal proper-
ties of their polymers, such as Tgs and thermal stability, are
not low.
Figure 1(c). The signals at 4.24, 5.67, and 5.90 ppm were
assigned to ArANHACH2A, which confirmed the reduction
of imine linkage. Because of the isomer of intermediate-2,
two signals of benzimidazole unit were observed at 11.86
and 11.97 ppm, and they disappeared after exchanging by
deuterium oxide [Fig. 1(d)].
In this study, trial has been made to find the suitable sol-
vents for the synthesis of P-PABZ using intermediate-2 and
paraformaldehyde. As shown in Table 1, when toluene, chlo-
roform, and 1,4-dioxane were used as the reaction solvents,
no reaction occurred, because intermediate-2 was insoluble
in these solvents, even though nonpolar solvents, such as
chloroform, toluene and xylene, were regarded as appropri-
ate solvents to synthesize benzoxazines generally.34,35 Gela-
tion occurred when N,N-dimethylformamide (DMF) and N,N-
dimethylacetamide (DMAC) were used. To obtain P-PABZ, so,
DMF was added to chloroform as a cosolvent due to the
poor solubility of intermediate-2 in nonpolar solvent. The
RESULTS AND DISCUSSION
Because of the poor solubility of PABZ and the formation of
gelation between PABZ and paraformaldehyde, P-PABZ was
not synthesized by traditional method. Hence, we used a
three-step synthetic method to synthesize P-PABZ, according
to the procedure presented in Scheme 2.31–33 In the first
step, intermediate-1 was synthesized by ortho-hydroxyben-
zaldehyde and PABZ. Figure 1(b) showed the 1H NMR spec-
tra of intermediate-1. The signal at 9.08 ppm for ACH¼¼NA
was observed, confirming the formation of imine linkage.
Because of intermediate-1’s asymmetry structure, the
resonances of phenolic OH were observed at 12.97 and
13.10 ppm. Additionally, the signal of benzimidazole unit
appeared. After that, the imine linkage of intermediate-1 was
1
chemical structure of P-PABZ was confirmed by FTIR and H
NMR. The absorption band appeared at 945 cmꢁ1 proved
the formation of the oxazine ring. The absorption for the
benzimidazole unit appeared at 3408 and 1662 cmꢁ1 1H
.
NMR was used to further confirm the structure of P-PABZ
[Fig. 1(e)]. The characteristic oxazine resonances typically
showed downfield singlet for AOACH2ANA and upfield sin-
glet for ArACH2ANA. In this study, because of
the asymmetry structure, the AOACH2ANA resonance
split into 5.45 and 5.52 ppm, whereas the ArACH2ANA
ꢀ
reduced by NaBH4 below 10 C by ice bath, while the benz-
imidazole structure should be preserved. The 1H NMR spec-
tra of intermediate-2 and the assignment were shown in
SCHEME 2 Synthesis of P-PABZ.
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FIGURE 1 1H NMR spectra (DMSO-d6) of PABZ (a), intermediate-1 (b), intermediate-2 (c) and it exchanged by deuterium oxide
(D2O) (d), P-PABZ (e), and it exchanged by D2O (f).
resonance split into 4.66 and 4.74 ppm. Two signals of benz-
imidazole moiety were observed at 12.45 and 12.51 ppm
and disappeared after exchanging by deuterium oxide
[Fig. 1(f)], suggesting that benzimidazole unit was preserved
in P-PABZ.
The thermal polymerization behaviors of P-MDA and P-PABZ
are investigated by differential scanning calorimetry (DSC)
(Fig. 2) at a heating rate of 5 ꢀC/min. As can be seen from
Figure 2, the onset of polymerization for P-MDA did not
appear until 200 ꢀC, and the exothermic peak centered at
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TABLE 1 The Effect of Reaction Conditions on Synthesis of P-PABZ
Runa
Solvent
Solubilityb
Reaction temperature (ꢀC)
Reaction time (h)
Result
1
2
3
4
5
a
DMF
þ
þ
ꢁ
6
6
85
85
85
55
85
5
5
5
5
5
Gelation
DMAC
Gelation
Toluene
Chloroform
1,4-Dioxane
No reaction
No reaction
No reaction
Intermediate-2 and paraformaldehyde are in a mole ratio of 1:2.
b
Solubility of intermediate-2 in the solvent, þ, soluble; 6, partial soluble; ꢁ, insoluble.
222 ꢀꢀC. However, the exotherm of P-PABZ started at around
140 C and maximized at 181 ꢀC with broad polymerization
exotherm peak. The results showed that the thermal poly-
merizationꢀ of P-PABZ occurred at a temperature that was
nearly 60 C lower than that of P-MDA, indicating that benz-
imidazole moiety could decrease the thermal polymerization
temperature. Moreover, the sharp melting peak of P-PABZ
indicated its high purity.
obtained from FTIR was similar to that from DSC. Both the
curves showed that the polymerization of P-MDA started at
around 200 ꢀC and ended at 250 ꢀC. However, the conver-
sion curves of P-PABZ obtained from FTIR were quite differ-
ent from that from DSC. The FTIR results showed that P-
PABZ reached the conversion of 50% at nearly 170 ꢀC, but,
to reach the same conversion, the curves obtained from DSC
appeared at ꢃ220 ꢀC. The results probably suggested that
different polymerization behaviors took place for P-PABZ and
P-MDA, although no additional initiators or catalysts were
added. This is probably caused by the benzimidazole moiety
of P-PABZ, which acted as the internal catalyst and been part
of the benzoxazine molecule.
To further study the polymerization behavior, the relation-
ship of conversion versus temperature obtained from DSC is
used and can be expressed as36,37
HðTÞ
Conversion ¼
ꢂ 100%
HR
During the thermal polymerization of benzoxazines, two
main structures, Mannich-type structure and N,O-acetal-type
structure (Mannich-type structure and N,O-acetal-type struc-
ture in Scheme 3), will be gained.17,39–41 For the two struc-
tures, phenolic ArAOH exists in Mannich-type structure but
does not exist in N,O-acetal-type structure, and Mannich-type
structure is the thermodynamically more stable structure.
Without adding any initiators or catalysts, a benzoxazine
monomer relies on the thermal activation of the oxazine ring
to produce Mannich-type structure.39 In this case, Mannich-
type structure immediately follows after the oxazine ring
opening, the fraction of oxazine ring absorbance band
where H(T) is the enthalpy of reaction up to temperature T,
and HR is the total enthalpy of polymerization.
Benzoxazine monomers undergo oxazine ring-opening poly-
merization to give the corresponding polymers, so that the
conversion of oxazine ring consumed can be approximately
used as an indicator of the extent of polymerization.18 In
this work, in situ FTIR is applied to follow the oxazine ring-
opening reaction. As in situ FTIR study, polymerization of the
ꢀ
benzoxazines from 100 to 300 C is continuously monitored
using a heated transmission cell placed in the spectrometer.
The absorbance peak located between ꢃ875 and 1000 cmꢁ1
assigning to oxazine ring consumes sustainingly during ther-
mal polymerization (Fig. 3). Because this absorbance band
does not have any significant overlapping peaks, the conver-
sion can be calculated from the change in area of oxazine
ring absorbance band.38 The relationship of conversion ver-
sus temperature obtained from FTIR is expressed as
ꢀ
ꢁ
AðTÞ=A0ðTÞ
AðRÞ=A0ðRÞ
Conversion ¼ 1 ꢁ
ꢂ 100%
where A(T) and A(R) are the integrated areas of oxazine ring
absorbance band at given temperature (T) and 100 ꢀC,
respectively, and A0 (T) and A0 (R) are the integrated areas of
benzene band absorbance around 750 cmꢁ1 at given temper-
ꢀ
ature (T) and 100 C, respectively (Fig. 3).
The conversion versus temperature curves of P-MDA and
P-PABZ were shown in Figures 4 and 5, respectively. As can
be seen from Figure 4, the conversion curve of P-MDA
FIGURE 2 DSC plots of P-PABZ and P-MDA.
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FIGURE 3 In situ FTIR spectra of P-MDA and P-PABZ.
cmꢁ1 when the polymerization started (around 200 C), indi-
cating the existence of Mannich-type structure. The intensity
increases with the rise of conversion, but it increases little
ꢀ
consumed can approximately signify the polymerization pro-
cess.18,38 Hence, the conversion curves obtained from FTIR
are closer to that from DSC. However, by adding initiators or
catalysts, the thermal polymerizations become that corre-
sponding polybenzoxazines via formation of intermediary
N,O-acetal-type structure first and then rearrangement into
Mannich-type structures at higher temperature.17,19,40,41
There is a temperature lag between the oxazine ring opening
and polymerization; therefore, the conversion curves
obtained from FTIR that is dependent on the oxazine ring
opening is different from that from DSC. The polymerization
behaviors suggest that the main polymerization of P-MDA is
that consumes oxazine ring to produce Mannich-type struc-
ture, while P-PABZ is the formation of intermediary N,O-ace-
tal-type structure first and then rearrangement into Man-
nich-type structures.
ꢀ
above the end temperature (about 250 C). The results prob-
ably suggested that the main polymerization pathway of
P-MDA was that consumed oxazine ring to produce Mannich-
type structure directly as Scheme 3 Path A shown. For
P-PABZ, no obvious bands appeared near 3500 cmꢁ1 until at
175 ꢀC indicated little ArAOH groups existed; however,
about 60% of oxazine ring has been consumed according to
the conversion curve obtained from FTIR. This probably sug-
gested that N,O-acetal-type structure was favored at lower
temperature. As temperature rises, the bands of ArAOH
groups appeared, and the intensity increased constantly,
indicating that produce Mannich-type structure at higher
temperature. Hence, the favored polymerization pathway of
P-PABZ is probably that formed N,O-acetal-type structure
first (Path B: Step a in Scheme 3) and then transformed into
Mannich-type structure at elevated temperature (Path B:
Step b in Scheme 3). This pathway is the same as the ben-
zoxazines with adding initiators or catalysts, but no
This is also supported by the changes of intensity for pheno-
lic ArAOH on the FTIR spectra, because ArAOH just exists
in Mannich-type structure. In the case of P-MDA [Fig. 6(b)],
ArAOH-related absorbance peak appeared at near 3500
FIGURE 4 The conversion versus temperature curves of
FIGURE 5 The conversion versus temperature curves of
P-MDA.
P-PABZ.
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1227 cmꢁ1, and the 1,2-disubsituted benzene absorptions at
1484 cmꢁ1 disappeared after curing at 200 ꢀC, indicating
complete ring-opening reaction of the oxazine ring. However,
the absorption band at 1260 and 1478 cmꢁ1 revealed the
formation of phenolic ArAOH and 1,2,3-trisubstituted ben-
zene. Additionally, the absorption appeared at 1185 cmꢁ1
,
which corresponded to the CANAC (Mannich-type) struc-
ture, was observed in P(P-PABZ), demonstrating P(P-PABZ)
formed Mannich-type polymer. The benzimidazole unit
absorption at 1662 shifted to 1657 cmꢁ1 after polymeriza-
tion, illustrating the benzimidazole moiety incorporated into
the polymeric network. According to the FTIR analysis, the
structure of P(P-PABZ) was proposed and shown in Scheme
4. The benzimidazole moiety was still preserved into the net-
work after thermal polymerization.
In this work, we also explored the possibility of decreasing
the thermal polymerization temperature of other benzoxa-
zines by blending with P-PABZ. Imidazoles or acides have
been used to decrease the polymerization temperature of
benzoxazines.17–20 In this case, oxazine ring generated the
corresponding N,O-acetal-type structure as an intermediate.
Then this intermediate generated the thermodynamically
more stable Mannich-type structure through rearrangement
upon heating.17 For example, 3 wt % of imidazole was added
as a catalyst to P-MDA produced a decrease of 60 ꢀC in the
onset of polymerization temperature as Figure 8 (EMI)
shown, whereas it was not incorporated into the network.
Thus, some benzoxazines were attempted to use as additives
to decrease other monomers’ polymerization tempera-
ture.21,38 It was shown previously that the onset tempera-
tures observed to date for the polymerization of P-PABZ
were much lower than P-MDA. Significantly, P-PABZ contains
both benzimidazole moiety and oxazine ring, so that it will
be the catalyst and comonomer and be incorporated into the
SCHEME 3 Thermal polymerization pathways of P-MDA and
P-PABZ.
additional initiators or catalysts are added into P-PABZ.17
Hence, P-PABZ may have self-catalyzed characteristics
ascribed to the benzimidazole moiety. The oxygen atoms of
oxazine rings were protonated by the active NAH structure
of benzimidazole moiety to form iminium ions, and then
these iminium ions were preferred to combine with the cor-
responding phenoxide into N,O-acetal-type structure at lower
temperature.18,40
To the better understanding of the polymeric structure, the
region between 2000 and 600 cmꢁ1 in FTIR spectra of
P-PABZ after different polymerization temperatures is
recorded and shown in Figure 7. The oxazine ring absorp-
tions at 945 cmꢁ1, the ArAOAC absorptions at 1056 and
FIGURE 6 FTIR spectra of P-PABZ (a) and P-MDA (b) at different temperature in the 3800–2500 cmꢁ1 region.
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FIGURE 7 FTIR spectra of P-PABZ between 2000 cmꢁ1 to 600 cmꢁ1 at room temperature (P-PABZ), cured after 160 ꢀC (a), cured af-
ter 180 ꢀC (b), and cured after 200 ꢀC (c).
network structure. This will not give any major adverse
effects on the properties.
approximately as EMI. Hence, P-PABZ can effectively serve as
a catalyst for P-MDA by lowering the temperature for poly-
merization significantly.
In an effort to exploit the possibility of decreasing the ther-
mal polymerization temperature of benzoxazines by blending
with P-PABZ, several mixtures of P-MDA and 0 (P-MDA), 10
(B10), 30 (B30), and 50 (B50) wt % of P-PABZ were pre-
pared and studied by DSC. The results are collected in Figure
8. As can been seen from Figure 8, the onset of polymeriza-
tion was shifted almost 45 ꢀC lower, and the exotherm with
the peak centered at 220 ꢀC became broad, when 10%
P-PABZ (B10) was added. The ꢀ onset of polymerization
temperature decreased nearly 55 C when the percentage of
P-PABZ reached at 30%. As the percentage continued to
increase to 50%, the polymerization temperatures became
The glass transition temperature is an important property of
thermosetting polymer. In this work, dynamic mechanical an-
alyzer (DMA) is applied to probe the polymers. The results
are shown in Figure 9 and Table 2.
As can be seen, the measured values of P(P-PABZ) were much
higher than that of P(P-MDA), because the rigid benzimidazole
moiety restricted the segmental movement effectively. For the
blend with P-MDA and imidazole, a decrease of about 15 ꢀC in
Tgs was detected, probably indicating that imidazole hinders
the thermal properties of polybenzoxazines, because it was
SCHEME 4 Possible structure of P(P-PABZ).
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Finally, the thermal stability of these polymers is analyzed
by thermogravimetric analysis (TGA). The TGA thermograms
and the values obtained from TGA are shown in Figure 10
and Table 2, respectively. As can be seen, both the 10%
weight loss temperature and the residue of P(P-PABZ)
exceed those of P(P-MDA), indicating P(P-PABZ) has better
thermal stability than P(P-MDA). Simultaneously, little
decrease in 10% weight loss temperature and a slight
increase in residue of the polymers by adding P-PABZ, illumi-
nating incorporation of P-PABZ could improve polybenzoxa-
zines’ thermal stability. However, the results obtained from
TGA also showed that small amounts of imidazole produced
ꢀ
a decrease in 10% weight loss temperature (20 C) and resi-
due, suggesting that imidazole may interfere with the ther-
mal stability of polybenzoxazines.
FIGURE 8 DSC plots of the mixtures of P-MDA and its blends.
EXPERIMENTAL
not incorporated into the network. For the blends with
P-MDA and P-PABZ, scarcely increased in Tgs with 10 and
30%, but, an increase of about 10 ꢀC was detected when 50%
P-PABZ was used. This is because P-PABZ can incorporate
directly into the network, and the rigid benzimidazole units
help to reduce the segmental mobility.
Materials
2-(4-Aminophenyl)-1H-benzimidazole-5-amine (PABZ) was
obtained from ChangZhou Sunlight Medical Raw Material. Co.
(China). Paraformaldehyde was supplied by Fydsa (Spain).
Orth-hydroxybenzaldehyde, sodium borohydride (NaBH4),
imidazole, DMF, DMAC, toluene, 1,4-dioxine, chloroform,
FIGURE 9 Storage modulus (a), loss modulus (b), and tan d (c) of P(P-MDA), P(P-PABZ), and polymers from their blends.
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TABLE 2 DMA and TGA data for P(P-MDA), P(P-PABZ), and
polymers from their blends
Polymer
Tg(ꢀC)a
Tg(ꢀC)b
Tg(ꢀC)c
T
10%(ꢀC)d
R(%)e
P(P-MDA)
PB10
192
190
194
203
231
178
210
210
206
221
251
194
223
228
224
365
368
362
364
386
346
39
42
42
41
50
37
PB30
f
PB50
–
P(P-PABZ)
P-EMI
a
276
209
Glass transition temperature obtained from the storage modulus of
DMA.
b
c
d
e
f
Peak of loss modulus, measured by DMA.
Peak of tan d, measured by DMA.
Temperature of 10% weight loss in nitrogen measured by TGA.
Residue at 800 ꢀC in nitrogen measured by TGA.
The data cannot be obtained from DMA.
FIGURE 10 TGA thermograms of P(P-MDA), P(P-PABZ), and
polymers from their blends.
Synthesis of Intermediate-2
butanone, acetone, and ethanol were purchased from
ChengDu Kelong Chemical Reagents Corp. (China) and used
without further purification. Benzoxazine derived from 4,40-
diamine diphenyl methane (P-MDA) was prepared by our
laboratory. The chemical structures of imidazole and P-MDA
were shown in Scheme 5.
Intermediate-1 (36.75 g and 0.085 mol) and ethanol (150
mL) were introduced into a 250-mL three-necked flask with
a nitrogen inlet and a magnetic stirrer. The mixture was
cooled by ice bath, followed by adding sodium borohydride
(NaBH4) (9.36 g and 0.255 mol) portionwise with stirring
for 12 h below 10 ꢀC by ice bath. Finally, the mixture was
poured into deionized water (500 mL), and the precipitate
ꢀ
was filtered and dried in a vacuum oven at 110 C for 2 h to
Synthesis of Intermediate-1
obtain 34.1 g (92% yields) of tawny powder intermediate-2
Orth-hydroxybenzaldehyde (24.4 g and 0.2 mol), PABZ (22.4
g and 0.1 mol), and DMF (120 mL) were introduced into a
250-mL three-necked flask under nitrogen atmosphere. The
ꢀ
with a melting point of 208 C (DSC).
1H NMR (DMSO-d6, ppm): d ¼ 4.24 (4H, ANHACH2AArA),
5.67 and 5.90 (1H, ArANHAC), 6.42 (1H, ArANHAC), 9.57
(2H, ArAOH), 11.86 and 11.97 (1H, NAH of benzimidazole),
ꢀ
mixture was stirred at 50 C for 4 h. After the mixture cool-
ing to room temperature, the precipitate was filtered and
dried in a vacuum oven to give brown powder intermediate-
1 (42.8 g and 99% yields) with a melting point of 243 ꢀC
(DSC).
1
and 6.61–7.81 (15H, Aromatic H). H NMR (D2O þ DMSO-d6,
ppm): d ¼ 4.22 (4H, ANACH2AArA), and 6.61–7.81 (15H,
Aromatic H). 13C NMR (DMSO-d6, ppm): d ¼ 41.7 and 43.0
(NHACH2AAr), 112–156 (Ar). FTIR (KBr, cmꢁ1): 1240 (CAN
stretch), 1636 (vibration of benzimidazole), 3424 (NAH of
benzimidazole stretch), and 3200–3500 (OH stretch).
1H NMR (DMSO-d6, ppm): d ¼ 6.96–8.30 (15H, Aromatic H),
9.08 (2H, ArACH¼¼N), 12.97 and 13.10 (2H, ArAOH), and
13.38 (1H, NAH of benzimidazole). 13C NMR (DMSO-d6,
ppm): d ¼ 117–152 (Ar), 160.7 and 160.9 (CH¼¼N), 162.3,
and 164.3 (ACACH¼¼N). FTIR (KBr, cmꢁ1): 1616 (CH¼¼N),
3419 (NAH of benzimidazole), and 3200–3500 (OH).
C27H24N4O2: Calcd. C 74.31%, H 5.50%, N 12.84%, and O
7.35%. Found C 74.43%, H 5.37%, N 12.94%, and O 7.26%.
Synthesis of P-PABZ
C
27H20N4O2: Calcd. C 75.06 %, H 4.66%, N 12.97%, and O
Intermediate-2 (21.8 g and 0.05 mol), paraformaldehyde
(3.3 g and 0.11 mol), chloroform (100 mL), and DMF
7.31%. Found C 75.12%, H 4.72%, N 12.74%, and O 7.42%.
SCHEME 5 Chemical structures of imidazole and the benzoxazines described in the present contribution.
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(10 mL) were introduced into a 250-mL three-necked flask.
The mixture was stirred at reflux temperature for 4 h. At
last, solvents were removed using a rotary evaporator. After
recrystallization in butanone/ethanol and drying in a vac-
uum oven, 14.3 g (63% yields) of white powder (P-PABZ)
reference. Elemental analysis was conducted with Elementar
vario EL III analyzer. In situ Fourier transform infrared (in
situ FTIR): Nicolet 560 IR spectrometer, which was equipped
with a deuterated triglycine sulfate detector. The spectral re-
solution was 4 cmꢁ1, and the number of scans of each spec-
trum was 40. The sample was protected by dried high-purity
nitrogen gas during measurement. The IR spectra were col-
lected from 100 to 300 ꢀC; meanwhile, the temperature
increased at a constant rate of 5 ꢀC/min. DSC scans were
obtained with a TA Instruments Q20 under nitrogen atmos-
ꢀ
with a melting pꢀeak of 116 C (DSC) and a curing peak tem-
perature of 187 C were obtained.
1H NMR (DMSO-d6, ppm):
d
¼
4.66 and 4.74 (4H,
ANHACH2AArA), 5.45 and 5.52 (4H, ANACH2AOA), 12.45
and 12.51 (1H, NAH of benzimidazole), and 6.61–7.95 (15H,
Aromatic H). 1H NMR (D2OþDMSO-d6, ppm): d ¼ 4.63 and
4.70 (4H, ANHACH2AArA), 5.41 and 5.48 (4H,
ANACH2AOA), and 6.69–7.96 (15H, Aromatic H). 13C NMR
(DMSO-d6, ppm): d ¼ 48.9 and 50.7 (ANHACH2AArA), 78.3
and 80.9 (ANACH2AOA), and 112–162 (Ar). FTIR (KBr,
cmꢁ1): 945 (oxazine ring), 1056 (ArAOAC symmetric
stretch), 1227 (ArAOAC asymmetric stretch), 1662 (vibra-
tion of benzimidazole), and 3408 (NAH of benzimidazole
stretch). C29H24N4O2: Calcd. C 75.65 %, H 5.22%, N 12.17%,
O 6.96%. Found C 75.54%, H 5.34%, N 12.31%, and O
6.81%.
ꢀ
phere at a heating rate of 10 C/min without special instruc-
tions. DMA was performed with a TA Instruments DMA
Q800 at 1 Hz at a heating rate of 5 ꢀC/min in three-point
bending model under nitrogen. TGA was performed with a
TA Instruments’ High Resolution Q600 thermogravꢀimetric
analyzer under nitrogen atmosphere from 40 to 800 C at a
ꢀ
heating rate of 10 C/min.
CONCLUSIONS
In this work, we synthesized a novel benzoxazine-containing
benzimidazole moiety with low-polymerization temperature
and good thermal properties. During thermal polymerization,
this benzoxazine formed N,O-acetal-type structure first and
then transformed into Mannich-type structure. Adding this
benzoxazine into other benzoxazines can effectively decrease
the processing temperature, and the polymers have better
Tgs and enhanced thermal stability. We believe that the ben-
zoxazine-containing benzimidazole moiety can satisfy the
demands of decreasing the polymerization temperature of
benzoxazines.
Preparation of the Blends of P-PABZ and P-MDA
In the blends of P-PABZ and P-MDA, the weight ratios of
P-PABZ to P-MDA were 1/9, 3/7, and 5/5, respectively. The
procedure was as follows: in a 50-mL flask equipped with
beater, P-PABZ and P-MDA were introduced with the ratios.
Then, 5 mL of acetone was added, and the system was
stirred at 65 ꢀC until both P-PABZ and P-MDA melted com-
pletely. After removing the solvent, transparent light yellow
products were obtained, which were named as B10, B30,
and B50, respectively.
Preparation of the Blend of Imidazole and P-MDA
The blend of imidazole and P-MDA was prepared in compari-
son with the blends of P-PABZ and P-MDA. This blend was
prepared as follows: imidazole (0.3 g), P-MDA (10 g), and ac-
etone (5 mL) were introduced into a 50-mL flask. The mix-
ture was stirred at 65 ꢀC until imidazole and P-MDA melt
completely. After removing the solvent, we obtained a trans-
parent light yellow product named as EMI.
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