Facile, one-pot synthesis of aromatic diamine-based benzoxazines and their advantages over diamines as epoxy hardeners

Article abstract of DOI:10.1002/pola.24013

Three aromatic diamine-based benzoxazines were successfully prepared by a facile, clean, one-pot procedure from 1,4-phenylenediamine (1), 4,4'-diaminodiphenyl ether (2), and 4,4′-diaminodiphenyl methane (3), respectively. Their structures were confirmed b

Full text of DOI:10.1002/pola.24013

Facile, One-Pot Synthesis of Aromatic Diamine-Based Benzoxazines
and Their Advantages Over Diamines as Epoxy Hardeners
SHENG LUNG CHANG, CHING HSUAN LIN
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
Received 17 December 2009; accepted 26 February 2010
DOI: 10.1002/pola.24013
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Three aromatic diamine-based benzoxazines were
successfully prepared by a facile, clean, one-pot procedure from
1,4-phenylenediamine (1), 4,40-diaminodiphenyl ether (2), and
4,40-diaminodiphenyl methane (3), respectively. Their structures
were confirmed by NMR spectra and single crystal diffractogram.
The effect of the reactivity of diamines on the purity of the result-
ant benzoxazines was discussed. The resultant benzoxazines
were applied as hardeners for cresol novolac epoxy (CNE). The
processing window, the latent curing characteristic, and the mis-
cibility of benzoxazine/CNE systems were discussed. Compared
with diamines (1 and 3), (1 and 3)-based benzoxazines show latent
curing characteristic as epoxy hardeners, and wide processing
windows can be obtained. Compared with diamine (2) which is
immiscible with CNE in the molten state, (2)-based benzoxazine
shows good miscibility with CNE. Dynamic mechanical analysis
shows the Tgs of the benzoxazine/CNE thermosets are as high as
242–243 ꢀC. Thermogravimetric analysis shows the outstanding
C
V
thermal stability of the resultant thermosets. 2010 Wiley Period-
icals, Inc. J Polym Sci Part A: Polym Chem 48: 2430–2437, 2010
KEYWORDS: crosslinking; ring-opening polymerization; thermo-
sets
INTRODUCTION Benzoxazines are resins that can be poly-
merized to thermosets via thermally activated cationic ring-
opening reactions. Thermosets with low water absorption,
superior electrical properties,1 and low surface energy2 can
be obtained after curing. These properties have led to
renewed interest in this field in recent years.3–9 According to
related research, most difunctional benzoxazines are synthe-
sized from aromatic biphenols, monoamines, and formalde-
hyde. The large varieties of aromatic biphenols and mono-
amines allow for considerable molecule-design flexibility of
benzoxazines. Some special functional groups can be intro-
duced via biphenols or monoamines to provide certain
desired properties. For example, Agig and Takeichi prepared
phenyl propargyl ether-based10 and allylamine-based ben-
zoxazines11 to increase Tg and thermal stability. Ishida and
coworkers synthesized acetylene,12,13 maleimide,14 and phe-
nylphosphine oxide15-containing benzoxazines to increase Tg,
char yield, and flame retardancy. Kimura et al. prepared a
terpenediphenol-based benzoxazine to reduce water absorp-
tion and dielectric constant.16 Su and Chang incorporated
trifluoromethyl groups into benzoxazines to reduce the
dielectric constant.17 Recently, linear polymers containing
benzoxazine structures have been reported by Takeichi
et al.,18 Yagci and coworkers,19,20 Ishida and coworkers,21–23
and Endo et al.24,25
gelation occurs during the preparation of most aromatic
diamine-based benzoxazines. As a result, benzoxazines based
on difunctional18,27 and multifunctional aromatic amines or
their derivatives28 have seldom been reported. Recently,
researchers at Matsushita Electric Works reported on the
manufacturing of benzoxazines from aromatic diamines and
paraformaldehyde using solvents such as toluene in which
the aromatic diamine and paraformaldehyde have limited
solubility.29 The poor solubility of aromatic diamines and
paraformaldehyde in the solvent reduced the formation of
the triazine network and the condensations among methylol-
methylol, methylol-amino, or methylol-ortho hydrogen, while
the formation of benzoxazine was not retarded due to the
high concentration of phenol in the solvent. Very recently,
Ishida and coworkers prepared the 4,40-diaminodiphenyl sul-
fone-based benzxaozine in xylene using the traditional one-
step procedure.30 However, to our knowledge, only limited
aromatic diamine-based benzoxazines can be prepared by
the traditional one-step procedure.
To prevent the gelation that results from the reaction of aro-
matic diamine and formaldehyde, the direct contact between
formaldehyde and aromatic diamines, especially diamine
with high reactivity, should be avoided. Recently, we pro-
posed a three-pot procedure to prepare aromatic diamine-
based benzoxazines.31,32 Almost at the same time, Andreu
and Ronda prepared deuterated benzoxazines to prove the
mechanism.33 In the first pot of the procedure, aromatic
However, because of the triazine networks that result from
the condensation of aromatic diamines and formaldehyde,26
Additional Supporting Information may be found in the online version of this article. Correspondence to: C. H. Lin (E-mail: linch@nchu.edu.tw)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2430–2437 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
diamines reacted with 2-hydroxybenzaldehyde, yielding
intermediate (I) with an o-hydroxy phenylimine linkage. In
the second pot, the imine linkage was reduced, yielding an
intermediate (II) with a secondary amine structure. In the
third step, formaldehyde was added to react with the sec-
ondary amine at room temperature, resulting in a presuma-
ble intermediate with a hydroxymethylamine linkage. There-
after, the reaction temperature was raised to induce the ring
closure condensation between the hydroxymethylamine and
o-hydroxy groups, forming benzoxazines. However, multi-pot
and the expensive reducing agent, sodium borohydride,
make the procedure unattractive for industrial application.
As a result, simplifying the three-pot produce and using
inexpensive reducing agent may make the procedure attrac-
tive. In this study, three aromatic diamine-based benzoxa-
zines (4–6), were prepared by an economic, one-pot proce-
dure using 1,4-phenylenediamine (1), 4,40-diaminodiphenyl
ether (2), and 4,40-diaminodiphenyl methane (3), respec-
tively, as starting materials. The structures of the benzoxa-
zines (4–6) were confirmed by NMR spectra and single crys-
tal diffractogram. Benzoxazines (4–6) served as epoxy
hardeners. We also studied the structure–property relation-
ship of the (4–6)/CNE thermosets.
400–4000 cmꢂ1 by a Perkin-Elmer RX1 infrared spectropho-
tometer. High resolution mass spectra were obtained by a
Finnigan/Thermo Quest MAT 95XL mass spectrometer. Single
crystal diffractogram was obtained by a Bruker AXS SMART-
1000 X-Ray diffractometer.
Preparation and Characterization of (4)
2-Hydroxybenzaldehyde 4.96
g (40.7 mmole), (1) 2 g
(18.5 mmole), DMAc 30 mL, and the catalyst Pd/C 0.06 g
were charged into a high pressure reactor (Parr Instrumen-
tal Co., United States) and reacted at ambient temperature
under a hydrogen atmosphere. The reaction was completed
when the indicator of the manometer stopped moving.
Then, formaldehyde (37%) 3.3 g (40.7 mmole) was added
into the reactor. The mixture solution was heated to 70 ꢀC
and maintained at that temperature for 12 h and then, the
mixture was filtered to remove Pd/C and the filtrate was
poured into methanol/water solution and stirred, yielding a
yellow powder. The precipitate was filtered and dried in
the vacuum oven. Yellow powder 4.97 g (78% yield) with a
melting point of 180 ꢀC (by DSC) and a delta enthalpy of
118 J/g was obtained.
HR-MS(FABþ) m/z: Calcd. for C22H20N2O2 344.1525; Anal.,
344.1526 for C22H21N2O2. Elem. Anal. Calcd for C22H20N2O2:
C, 76.72%; H, 5.85%; N, 8.13%. Found: C, 76.86%; H, 5.94%;
N, 8.17%. 1H NMR (DMSO-D6), d ¼ 4.54(4H, H3), 5.33(4H,
H10), 6.67(2H, H8), 6.83(2H, H6), 6.99(4H, H1 ), 7.05(4H, H5
and H7). 13C NMR (DMSO-D6), d ¼ 49.27(C3), 79.47(C10),
116.08(C8), 120.29(C6), 118.84(C1), 127.08(C5), 127.54(C7),
127.54(C4), 141.97(C2), 153.88(C9). Single crystal data:
EXPERIMENTAL
Materials
Cresol novolac epoxy (CNE) with EEW 200 g/eq was kindly
supplied by Chang Chun Plastics, Taiwan under the trade
name of CNE-200ELD. 2-Hydroxybenzaldehyde (Showa),
1,4-phenylenediamine (1, Acros), 4,40-diamino diphenyl ether
(2, ChrisKev), 4,40-diaminodiphenyl methane (3, ChrisKev),
and 4,40-diaminodipheyl sulfone (ChrisKev) were used as
received. N,N-dimethylacetamide (DMAc) was purchased
from TEDIA, purified by distillation under reduced pressure
over calcium hydride (Acros), and stored over molecular
sieves. Methanol (99.5%) was purchased from various com-
mercial sources and used without further purification.
C
22H20N2O2, 0.49 ꢁ 0.18 ꢁ 0.16 mm3, monoclinic with a ¼
9.2181(8) Å, b ¼ 8.8299(8) Å, c ¼ 11.3588(10) Å, a ¼ 90ꢀ,
b ¼ 113.8880(10)ꢀ, c ¼ 90ꢀ with Dc ¼ 1.353 mg/m3 for
Z ¼ 2, V ¼ 845.35(13) Å3, T ¼ 297 K, k ¼ 0.71073 Å,
F(000) ¼ 364, final R indices: R1 ¼ 0.0447, WR2 ¼ 0.1400.
Characterization
Thermal gravimetric analysis (TGA) was performed using a
Perkin–Elmer TGA 7 at a heating ratꢀe of 20 ꢀC/min in a
nitrogen atmosphere from 100 to 800 C. Dynamic mechani-
cal analysis (DMA) was performed with a Perkin–Elmer Pyris
Diamond DMA with a sample size of 5.0 cm ꢁ 1.0 cm ꢁ 0.2
cm. The storage modulus E0 and tan d were determined as
the sample was subjected to the temperature scan mode at a
programmed heating rate of 5 ꢀC/min at a frequency of
1 Hz. The test was performed by the bending mode with am-
plitude of 5 lm. Thermal mechanical analysis (TMA) was
performed with a Seiko TMA/SS6100 at a heating rate of 5
ꢀC/min. The coefficient of thermal ꢀ expansion (CTE) was
measured in the range of 50 to 150 C. NMR measurements
were performed using a Varian Inova 600 NMR in DMSO-d6,
and the chemical shift was calibrated by setting the chemical
shift of DMSO-d6 as 2.49 ppm. Elemental analysis was per-
formed with a Heraeous CHN-O Rapid Elemental Analyzer
using acetanilide as a standard. IR Spectra were obtained
from at least 32 scans in the standard wavenumber range of
Preparation and Characterization of (5)
Compound 5 was synthesized in a similar procedure with 4
using 2 as starting material. White powder (76% yield) with
a melting point of 128 ꢀC (by DSC) and a delta enthalpy of
77 J/g was obtained.
HR-MS(FABþ) m/z : Calcd. for C28H24N2O3 436.1787; Anal.,
436.1781 for C28H25N2O3. Elem. Anal. Calcd for C28H24N2O3:
C, 77.04%; H, 5.45%; N, 6.42%. Found: C, 76.94%; H, 5.62%;
N, 6.44%. 1H NMR (DMSO-D6), d ¼ 4.59(4H, H5), 5.38(4H,
H12), 6.73(2H, H10), 6.84(4H, H3), 6.86(2H, H8), 7.08(2H, H9),
7.09(4H, H2), 7.10(2H, H7). 13C NMR (DMSO-D6), d ¼
49.37(C5), 79.18(C12), 116.18(C10), 119–119.14(C2 and C3),
120.41(C8), 121.15(C6), 127.12(C9), 127.61(C7), 143.58(C4),
151.11(C1), 153.9(C11).
ONE-POT SYNTHESIS OF DIAMINE-BASED BENZOXAZINES, CHANG AND LIN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 One-pot synthesis of
benzoxazines (4–6).
simultaneously. After the hydrogen was introduced, the
imine linkage resulting from the condensation of aromatic
diamines and 2-hydroxybenzaldehyde was reduced by hydro-
gen in the presence of Pd/C, forming an intermediate with a
secondary amine linkage. Then, formaldehyde was added
and the temperature was raised to induce ring closure con-
densation, yielding benzoxazines (4–6). Generally, there are
many reducing agents, such as NaBH4, LiAlH4, hydrogen/Pd-C,
hydrazine hydrate/Pd-C. Among them, hydrogen is a relatively
clean and inexpensive. To reduce the preparation cost, hydro-
gen/Pd-C was applied to replace sodium borohydride, which
has been used in the three-pot procedure.24,25 In general, the
carbonyl groups of 2-hydroxybenzaldehyde are also reduced
by hydrogen/Pd-C. However, the reduction rate is much
slower than the formation rate of imine due to the high reac-
tivity of (1–3) in which the amino groups are activated by the
conjugation of a para-position amino group, ether, and meth-
ylene, respectively. This speculation can be supported by the
unsuccessful preparation (poor purity) of benzoxazine based
on a less-reactive diamine, such as 4,40-diaminodipheyl sul-
fone. In such a case, the formation rate of imine is reduced
due to the low reactivity of 4,40-diaminodipheyl sulfone. As a
result, the carbonyl groups of 2-hydroxybenzaldehyde are
more likely to be reduced by hydrogen/Pd-C. Therefore, to
ensure high purity of resulting benzoxazine, the one-pot
approach in this study is limited to diamines with high
reactivity.
Preparation and Characterization of (6)
Compound (6) was prepared in a similar synthesis route
with (4) using (3) as starting material. Light yellow powder
(72% yield) with a melting point of 123 ꢀC (by DSC) and a
delta enthalpy of 72 J/g was obtained.
MS(FABþ) m/z: Calcd. for C29H26N2O2 434.1994; Anal.,
436.1987 for C29H27N2O2. Elem. Anal. Calcd for C29H26N2O2:
C, 80.16%; H, 6.03%; N, 6.45%. Found: C, 80.12%; H, 6.12%;
N, 6.41%. 1H NMR (DMSO-D6), d ¼ 3.70(2H, H1), 4.58(4H,
H6), 5.37(4H, H13), 6.69(4H, H4), 6.84(2H, H9), 7.01(2H, H10),
7.02(2H, H11), 7.04(2H, H8), 7.07(4H, H3). 13C NMR (DMSO-
D6),
d
¼
39.92(C1), 49.01(C6), 78.87(C13), 116.14(C4),
117.57(C10), 120.33(C9), 121.25(C7), 127.10(C3), 127.56(C8),
129.15(C11), 133.80(C2), 145.84(C5), 153.89(C12).
Figure 1 shows the 1H and 13C NMR spectra of (4). The
characteristic peaks at 4.54 and 5.33 ppm, which correspond
to the OACH2AN and phACH2AN of oxazine, respectively,
verify the formation of benzoxazines. The peaks of Ar-H and
Ar-C marked on Figure 1 also support the structure and pu-
rity of (4). Figure 2 shows the single crystal diffractogram of
(4), further confirming the structure of (4). Benzoxaiznes
(5–6) were also characterized by 1H and 13C NMR spectra
(Supporting Information Figures S1–S2), and the assignment
of each peaks supports the structure and purity. One thing
that has to be emphasized in this approach is that except for
pouring the reaction mixture to methanol/water solution, no
extra purification procedures like extraction, recrystallization,
or column chromatography are required to obtain benzoxa-
zine with good purity. Figure 3 shows the DSC thermograms
Preparation of Polybenzoxazines
Mixtures of (1–3)/CNE and (4–6)/CNE with equal equiva-
lency were melted, stirred, and transferred to an aluminum
mold, and then cured at 180, 200, 220, and 240 ꢀC for 2 h
each in an air-circulating oven. Thereafter, samples were
allowed to cool slowly to room temperature to prevent
cracking.
RESULTS AND DISCUSSION
Synthesis and Characterization of Monomers
Benzoxazine (4–6) were synthesized by a one-pot produce
(Scheme 1), in which aromatic diamines (1–3), 2-hydroxy-
benzaldehyde, and Pd/C were charge into Parr reactors
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ARTICLE
FIGURE 1 (a) 1H and (b) 13C NMR spectra of (4).
of as-prepared (4) and single crystal (4). The melting point
of as-prepared (4) is similar to that of single crystal (4), fur-
ther demonstrating the purity of this approach.
showing a wide processing window. For the (4)/CNE curing
system, the melting endothermic peak temperature is
166 ꢀC, and the exothermic peak occurred at 258 ꢀC, also
DSC Thermograms of Benzoxazines
Figure 4 shows the DSC thermograms of (4), (4)/CNE, and
(1)/CNE mixtures. The melting point of benzoxazine (4)ꢀ is
ꢀ
180 C, whereas the exothermal peak temperature is 260 C,
FIGURE 2 Single crystal diffractogram of (4). [Color figure can
be viewed in the online issue, which is available at
www.interscience.wiley.com.]
FIGURE 3 DSC thermograms of as-prepared (4) and single
crystal (4). [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]
ONE-POT SYNTHESIS OF DIAMINE-BASED BENZOXAZINES, CHANG AND LIN
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ꢀ
(Fig. 5). The Tgs of P(4–6) are 220, 194, and 200 C, respec-
tively, which are much higher than that of the bisphenol A
and aniline-based polybenzoxazine ((P(B-a)), about 160 ꢀC)
reported by Ishida and Allen.1 As in Scheme 2, instead of
bonding to a phenyl pendant, the nitrogen linkage of P(4–6)
is bonded to the other repeating unit, leading to a small seg-
mental mobility and explaining the higher Tg of P(4–6).
Instead of enhancing the Tg value of polybenzoxazines by
incorporating curable propargyl ether,10 allyl,11 and malei-
mide functional groups,14,34,35 which may decrease the
toughness of polybenzoxazines due to the higher crosslinking
density, this study demonstrates the power of the molecule-
approach to Tg enhancement. The order of Tg is P(4) > P(6)
> P(5), which is related to the rigidity and crosslinking den-
sity of the polybenzoxaszines. P(5) with flexible ether link-
age between two phenylene structures shows lower Tg than
P(6) with methylene linkage between two phenylene struc-
tures. P(4) with the smallest molecular weight between
crosslinking points shows the highest Tg. For the (4)/CNE
thermoset, only one tan d peak was observed, indicating that
a homogeneous copolymer was obtained. As in Figure 5, the
Tg of the (4)/CNE thermoset is as high as 243 ꢀC, which is
23 ꢀC higher than that of neat P(4). This indicates that the
crosslinking density of P(4) was enhanced by the incorpora-
tion of CNE. It is thought that the phenolic groups, resulting
from ring-opening polymerization of benzoxazine, reacted
with the oxirane of CNE and thus tighten the network struc-
ture.36 This speculation can be confirmed by the DMA curves
of P(4) and the (4)/CNE thermoset. The height of tan d for
the (4)/CNE thermoset is much lower than that of P(4), con-
firming the higher crosslinking density of the (4)/CNE ther-
moset. In addition to latent curing characteristic, one partic-
ular benefit of using benzoxazine as a curing agent is its
miscibility with epoxy resin. Generally, 4,40-diaminophenyl
ether (2) is immiscible with CNE, and an obvious two-phase
morphology was observed for the (2)/CNE system in the
molten state. However, the (2)-based benzoxazine, (5), is
miscible with CNE in the molten state. As shown in Figure 5,
FIGURE 4 DSC thermograms of (4), (4)/CNE, and (1)/CNE mix-
tures. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
showing a wide processing window. In contrast, the exother-
mic reaction of 1/CNE occurs at a relatively low tempera-
ture, suggesting the poor processability and short pot-life of
the system. From Figure 4, we can confirm the latent curing
characteristic of (4) as an epoxy curing agent. This observa-
tion can be supported by the gel time test (Table 1). At
ꢀ
190 C, the gel time of (4)/CNE is 3.19 min, whereas the gel
time of (1)/CNE is only 0.12 min, also demonstrating the
latent curing characteristic of (4). The rapid curing of the
(1)/CNE system makes it difficult to prepare a void-free
sample for property measurement. In contrast, a void-free
thermoset can easily be prepared for the (4)/CNE system
due to the wide processing window. As listed in Table 1,
(5)/CNE and (6)/CNE also show reasonable gel time for
processing. The longer gel time makes the (4–6)/CNE system
attractive for IC encapsulation in which low viscosity during
mold transfer is important.
Thermal Properties of Polybenzoxazines
Dynamic mechanical analyzer was applied to measure the
dynamic mechanical properties of the resultant thermosets
only one tan
d peak was observed for the (5)/CNE
TABLE 1 Thermal Properties of P(4–6) and (3–6)/CNE Thermosets
Thermosets
Based on
Gel Time at
190 ꢀC (min)
E0 at
50 ꢀC (GPa)
Tg Form
DMA (ꢀC)
Tg Form
TMA (ꢀC)
CTE
(ppm/ꢀC)
Char
Yieldb (%)
a
Td (ꢀC)
(4)
2.71
3.19
0.12
4.94
6.87
–
2.61
3.30
–
220
243
–
193
54
51
–
372
378
–
53
38
–
(4)/CNE
(1)/CNEc
(5)
220
–
1.61
2.53
–
194
242
–
185
57
51
–
376
385
–
52
37
–
(5)/CNE
(2)/CNEd
(6)
215
–
3.27
3.68
0.26
2.28
3.19
2.10
200
243
272
183
57
54
–
413
406
385
46
40
16
(6)/CNE
(3)/CNE
a
225
–
5% Decomposition temperature (oC) in a nitrogen atmosphere.
Residual weight percentage at 800 ꢀC in
Gel time of (1)/CNE is too short to prepare a void-free and homogene-
ous sample.
c
b
a
nitrogen atmo-
d
sphere.
(2) and CNE are immiscible in the molten state.
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ARTICLE
FIGURE 6 TMA curves of (6)/CNE thermoset.
FIGURE 5 DMA curves of P(4–6) and (4–6)/CNE thermosets.
P(6) > P(5) > P(4). The trend is consistent with the inverse
order of the electron density of nitrogen in P(4-6). As a
result, it is speculated that the stability is related to the
thermoset, indicating that a homogeneous copolymer, not
two immiscible IPN networks, was obtained. The Tg value of
(5)/CNE thermoset is as high as 242 ꢀC, which is 48 ꢀC
higher than that of P(5). In addition, Tg as high as 243 ꢀC
was also observed for the (6)/CNE thermoset, which is
ꢀ
43 C higher than that of P(6). The high Tg characteristic can
also be confirmed by TMA measurements. Figure 6 shows a
typical TMAꢀcurve of the (6)/CNE thermoꢀset. A onset transi-
tion at 225 C was observed, which is 42 C higher than that
of P(6). A CTE of 54 ppm/ꢀC was obtained, which is slightly
smaller than that of P(6). Similar results were observed in
other systems, and the results are listed in Table 1.
Figure 7 shows the TGA thermograms of P(4–6) and the
(4–6)/CNE thermosets. The 5% degradation temperatures of
P(4-6) are 372, 376, and 413 ꢀC, respectively, which are
much higher than that of P(B-a). According to previous stud-
ies, the 5% degradation temperature of conventional poly-
benzoxazines rarely exceeds 350 ꢀC, and the release of ani-
line fragments is responsible for the low thermal stability
of polybenzoxazines.37–40 Therefore, the introduction of a
crosslinkable site into aniline, such as ethylnyl,40,41 propargyl
ether,10 and nitrile group13 has been proven to be an effec-
tive way to enhance thermal stability of polybenzoxazines.
However, an enhanced crosslinking density also indicates a
sacrificial toughness. As in Scheme 2, the nitrogen linkage in
P(4–6) is bonded to the other repeating units, making the
release of an aniline fragment difficult and causing P(4–6) to
be thermally more stable than P(B-a). The order of Td is
FIGURE 7 TGA curves of (a) P(4–6) and (b) (4–6)/CNE
thermosets.
SCHEME 2 Proposed ideal structure of P(4–6).
ONE-POT SYNTHESIS OF DIAMINE-BASED BENZOXAZINES, CHANG AND LIN
2435

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
3 Velez-Herrera, P.; Ishida, H. J Polym Sci Part A: Polym Chem
electron density of the electron density of nitrogen. For the
2009, 47, 5871–5881.
(4–6)/CNE thermosets, the 5% degradation temperatures
are 378, 385, and 406 C, respectively. The order is the same
ꢀ
4 Kiskan, B.; Aydogan, B.; Yagci, Y. J Polym Sci Part A: Polym
as that in P(4-6). Detailed investigation of the decomposition
mechanism by TGA-Mass is under investigation to confirm
the observation. As listed in Table 1, the 5% decomposition
temperature and char yield at 800 ꢀC of the (6)/CNE ther-
moset are 406 ꢀC and 40%, respectively, which are higher
than those of the (3)/CNE thermoset (385 ꢀC and 16%).
These results demonstrate the (4–6)/CNE thermosets have
better thermal stability than the (1–3)/CNE thermosets.
Chem 2009, 47, 804–811.
5 Sponto´ n, M.; Larrechi, M. S.; Ronda, J. C.; Galia`, M.; Ca´diz,
V. J Polym Sci Part A: Polym Chem 2008, 46, 7162–7172.
6 Kiskan, B.; Demirel, A. L.; Kamer, O.; Yagci, Y. J Polym Sci
Part A: Polym Chem 2008, 46, 6780–6788.
7 Chou, C. I.; Liu, Y. L. J Polym Sci Part A: Polym Chem 2008,
46, 6509–6571.
8 Andreu, R.; Reina, J. A.; Ronda, J. C. J Polym Sci Part A:
CONCLUSIONS
Polym Chem 2008, 46, 6091–6101.
Three aromatic diamine-based benzoxazines (4–6) were suc-
cessfully prepared by a one-pot procedure. Because of the
competition between the imine formation and the carbonyl
reduction, the reactivity of diamine is critical to the purity of
resultant benzoxazine. Benzoxazines based on diamines with
high reactivity (1–3) can be successfully prepared by this
one-pot procedure. In contrast, benzoxazine prepared by dia-
mine with low reactivity such as 4,40-diaminodiphyl sulfone
lead to poor purity.42 The clean and inexpensive hydrogen
reduction makes this one-pot attractive for industrial appli-
cations. Compared with diamines (1–3), diamine-based ben-
zoxazines (4–6) show some advantages as epoxy hardeners.
For example, the gel time of the (1)/CNE system is too short
to prepare a void-free sample for DMA measurement. In con-
trast, the gel time of the (4)/CNE system is long enough to
process a void-free sample. In addition to allowing for better
processing, the longer gel time makes the (4)/CNE system
attractive for IC encapsulation, in which low viscosity during
mold transfer is important. In addition to latent curing char-
acteristic, one particular benefit of using benzoxazine as an
epoxy hardener is its miscibility with epoxy resin. For exam-
ple, (2) and CNE are immiscible in the molten state, whereas
(5)/CNE is miscible and a single Tg in the DMA measure-
ment was observed for the (5)/CNE thermoset. Tgs of the
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23–48 ꢀC higher than those of P(4–6). According to the
height of tan d in the DMA measurement, the (4–6)/CNE
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explaining the high Tg characteristic of (4–6) as CNE harden-
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The authors would like to thank the National Science Council of
the Republic of China, Taiwan, for financial support. Partial
sponsorship by the Green Chemistry Project (NCHU), funded by
the Ministry of Education, is also gratefully acknowledged.
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