A study on the co-reaction of benzoxazine and triazine through a triazine-containing benzoxazine

Article abstract of DOI:10.1039/c5ra23760b

To study the co-reaction of benzoxazine and triazine, a triazine-containing benzoxazine (P-tta) was prepared through nucleophilic substitution of 4-(2H-benzo[e][1,3]oxazin-3(4H)-yl)phenol (P-ap) with 2,4,6-trichloro-1,3,5-triazine. DSC thermograms show th

Full text of DOI:10.1039/c5ra23760b

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A study on the co-reaction of benzoxazine and
triazine through a triazine-containing
Cite this: RSC Adv., 2016, 6, 17539
benzoxazine†
a
Ching Hsuan Lin, Yu-Chun Chou,a Meng Wei Wangb and Ru Jong Jengb
*
To study the co-reaction of benzoxazine and triazine, a triazine-containing benzoxazine (P-tta) was
prepared through nucleophilic substitution of 4-(2H-benzo[e][1,3]oxazin-3(4H)-yl)phenol (P-ap) with
2,4,6-trichloro-1,3,5-triazine. DSC thermograms show that the exothermic temperature of P-tta is lower
than that of other benzoxazines with a similar structure except for the triazine structure, so we speculate
that the forward polymerization is related to the existence of the triazine structure. Through monitoring
the curing process using IR, we propose that the curing reactions of P-tta include a concerted co-
reaction between the triazine and benzoxazine, and a self-polymerization of benzoxazine. A thermoset
with a high Tg (279 ꢀC, DMA data), a low thermal expansion coefficient (32 ppm per ꢀC), and high
thermal stability (Td5% 417 ꢀC) can be obtained through the curing of P-tta.
Received 10th November 2015
Accepted 29th January 2016
DOI: 10.1039/c5ra23760b
www.rsc.org/advances
indicated that the phenol resulting from the ring-opening poly-
merization of benzoxazine catalyzes the cyclotrimerization.12,13,16
Introduction
Gu et al. reported that the fundamental catalyst for the cyclo-
trimerization of cyanate ester is not the phenolic hydroxyl but the
oxygen anion.17 Recently, we unexpectedly observed that gelation
occurred in a methyl ethyl ketone solution of a P-oda/BACY (1/1
Fully cured cyanate esters exhibit high glass transition
temperatures, high thermal stabilities, and low dielectric
constants.1–11 They have been used in electronics, especially in
printed circuit boards for high frequency communications.
Based on the excellent thermal properties of cured cyanate
esters, incorporating a cyanate ester into benzoxazine is ex-
pected to improve the properties of polybenzoxazines.
Blends of benzoxazine and cyanate ester have been actively
studied by multiple research groups such as Nair et al.,12 Kimura
et al.,13 Gu et al.,14 and Lin et al.15 Different curing reactions have
been reported, but they all conclude that the blends are miscible,
and the cyclotrimerization of cyanate ester is accelerated in the
presence of benzoxazine. To discuss the miscibility, Nair et al. and
Kimura et al. proposed that the phenolic OH group resulting from
ring opening reaction of benzoxazine co-reacted with cyanate
ester to form polycyanurate as part of the polybenzoxazine
matrix.12,13 Gu et al. suggested that the opened oxazine rings could
insert into triazine rings, and then, some of the triazine iso-
merized to isocyanurate.14 In our previous work, we reported
a concerted reaction mechanism for the co-reaction of benzox-
azine and triazine that resulted from the cyclotrimerization of
cyanate ester.15 Furthermore, to discuss the origin of the rapid
cyclotrimerization of cyanate ester, Nair et al. and Kimura et al.
mol molꢁ1) blend aer 24 h at 30 C, in which P-oda is a 4,40-
ꢀ
oxydianiline/phenol-based benzoxazine and BACY is a dicyanate
ester of bisphenol A (Fig. 1). We proposed that the unpaired
nitrogen electrons of benzoxazine catalyzed the trimerization of
cyanate ester to form a triazine structure.18 Based on the above
literature, the triazine structure resulting from cyclotrimerization
of cyanate ester plays an important role in the co-reaction. To the
best of our knowledge, all of the research has focused on the co-
reaction of cyanate ester with benzoxazine but no research has
independently discussed the co-reaction of triazine with benzox-
azine. In this work, to study the co-reaction of benzoxazine and
triazine, we synthesized a triazine-containing benzoxazine (P-tta).
An advantage can be achieved in this system, the built-in triazine
structure makes analysis of the curing reaction easier since it
avoids interference from benzoxazine-catalyzed cyclo-
trimerization of cyanate ester in the benzoxazine/cyanate ester
blend. The ring-opening mechanism of benzoxazine in the pres-
ence of the triazine structure could be evaluated clearly. In
addition to the curing mechanism, the thermal properties of the
resulting thermoset were studied in this work.
aDepartment of Chemical Engineering, National Chung Hsing University, Taichung,
Taiwan. E-mail: linch@nchu.edu.tw
bInstitute of Polymer Science and Engineering, National Taiwan University, Taipei,
Experimental
Materials
Taiwan
2-Hydroxybenzaldehyde, 4-aminophenol, and sodium borohy-
dride (NaBH4) were purchased from Alfa. Paraformaldehyde,
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra23760b
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NMR (DMSO-d6), d ¼ 4.11 (s, 2H, ph–CH2–N), 5.33 (s, 1H, –NH),
7.20–6.40 (m, 8H), 8.40 (s, 1H, –OH), 9.50 (s, 1H, –OH).
2-(((4-Hydroxyphenyl)amino)methyl)phenol, 1.0
g
(4.6
mmol), paraformaldehyde, 0.1524 g (5.1 mmol), and 25 mL of
1,4-dioxane were introduced into a 100 mL round bottom glass
ask equipped with a condenser and a magnetic stirrer. The
mixture was stirred at 90 ꢀC for 16 h. Aer that, 1,4-dioxane was
removed using a rotary evaporator. A light red viscous liquid, 4-
(2H-benzo[e][1,3]oxazin-3(4H)-yl)phenol (P-ap), was obtained.
1H-NMR (DMSO-d6), d ¼ 4.50 (2H, H4), 5.29 (2H, H11), 6.64 (d,
2H, H2), 6.70 (d, 1H, H9), 6.84 (t, 1H, H7), 6.93 (t, 2H, H3), 7.06 (t,
1H, H6), 7.07 (t, 1H, H8), 8.96 (s, 1H, OH). 13C-NMR (DMSO-d6),
d ¼ 49.80 (C4), 80.08 (C11), 115.52 (C2), 116.10 (C9), 119.80 (C3),
120.28 (C7), 121.35 (C5), 127.07 (C6), 127.53 (C8), 140.35 (C12),
151.99 (C10), 154.04 (C1).
Fig. 1 The structures of P-tta, P-oda, and BACY.
2,4,6-trichloro-1,3,5-triazine, and triethylamine were purchased
from Acros. 4,40-Diamino diphenyl ether/phenol-based ben-
zoxazine (P-oda) was prepared in our lab, according to a previ-
ously reported procedure.19 All solvents (HPLC grade) were
purchased from various commercial sources and used without
further purication.
Characterization
Differential scanning calorimetry (DSC) scans were obtained
using a Perkin-Elmer DSC 7 in a nitrogen atmosphere at
a heating rate of 10 ꢀC minꢁ1. Thermogravimetric analysis
(TGA) was performed with a Perkin-Elmer Pyris 1 at a heating
rate of 20 ꢀC minꢁ1 in an atmosphere of nitrogen or air.
Dynamic mechanical analysis (DMA) was performed with
a Perkin-Elmer Pyris Diamond DMA with a sample size of 5.0 cm
ꢂ 1.0 cm ꢂ 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 minꢁ1 at
a frequency of 1 Hz. The test was performed through a bending
mode with an amplitude of 5 mm. Thermomechanical analysis
(TMA) was performed with a Perkin-Elmer Pyris Diamond TMA
at a heating rate of 5 ꢀC minꢁ1 from 50 ꢀC to 240 ꢀC. The
coefficientꢀof thermal expansion (CTE) in the temperature range
of 50–150 C was recorded. The sample pellet for IR measure-
ment was prepared through blending the sample with KBr salt
with a weight ratio of 1 : 100. IR spectra were obtained from at
least 32 scans in the standard wavenumber range of 400–4000
cmꢁ1 using a Perkin-Elmer RX1 infrared spectrophotometer.
Synthesis of P-tta
P-tta was prepared through a nucleophilic substitution of 4-(2H-
benzo[e][1,3]oxazin-3(4H)-yl)phenol (P-ap) with 2,4,6-trichloro-
1,3,5-triazine. P-ap, 1.0056 g (4.6 mmol), triethylamine, 0.4701
g (4.6 mmol), and 20 mL of acetone were introduced into a 100
mL round bottom glass ask equipped with a condenser and
ꢀ
a magnetic stirrer. The solution was stirred at 3–5 C for 1 h.
Cyanuric chloride 0.2770 g (1.5 mmol) dissolved in 10 mL of
acetone was added drop-wisely. The solution was stirred at
room temperature for 1 h, then reuxed for another 1.5 h. Aer
that, the solution was poured into water. The white precipitate
was ltered, washed with acetone, and dried at 60 ꢀC. The yield
was 77%. 1H-NMR (DMSO-d6), d ¼ 4.64 (s, 2H, H13), 5.43 (s, 2H,
H6), 6.72 (d, 1H, H8), 6.86 (t, 1H, H10), 7.06 (t, 1H, H9), 7.08 (d,
2H, H4), 7.11 (d, 1H, H11), 7.13 (d, 2H, H3). 13C-NMR (DMSO-d6),
d ¼ 40.09 (C13), 78.97 (C6), 116.22 (C8), 118.26 (C3), 120.50 (C10),
121.12 (C12), 121.92 (C4), 127.19 (C11), 127.68 (C9), 144.99 (C5),
145.75 (C7), 153.84 (C2), 173.26 (C1).
Synthesis of 4-(2H-benzo[e][1,3]oxazin-3(4H)-yl)phenol (P-ap)
P-ap was prepared from 2-hydroxybenzaldehyde and 4-amino-
phenol using a three-step procedure.19 2-hydroxybenzaldehyde
5 g (40.9 mmol), 4-aminophenol 4.468 g (40.9 mmol) and
ethanol 50 mL were introduced into a 500 mL round bottom
glass ask equipped with a nitrogen inlet and a magnetic
stirrer. The reaction mixture was stirred at room temperature
for 12 h. NaBH4, 0.52 g (13.7 mmol), was added every hour. Aer
NaBH4 was added three times (13.7 ꢂ 3 mmol), the reaction
mixture was further stirred at room temperature for 12 h. The
mixture was then poured into water (500 mL) and stirred. The
yellow precipitate was ltered and dried at 60 ꢀC. 2-(((4-
Sample preparation and curing procedure
Hydroxyphenyl)amino)methyl)phenol with a melting point of P-tta was heated on a hot plate at about 150 ꢀC with continuous
131 ꢀC (DSC) and a delta enthalpy of 149 J gꢁ1 was obtained. 1H- stirring, then poured into aluminum modes with dimensions of
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5.0 cm ꢂ 1.0 cm ꢂ 0.3 cm (for DMA measurement) and 1.0 cm
ꢂ 1.0 cm ꢂ 0.3 cm (for TMA measurement), and cured at 180 ꢀC
(2 h), followed by 200 ꢀC (2 h) and 220 ꢀC (2 h). A further curing
at 240 ꢀC (2 h), or 240 ꢀC (2 h) followed by 260 ꢀC (2 h) was
applied for property comparison. Aer that, the samples were
allowed to cool slowly to room temperature to prevent cracking.
The thermoset of P-tta is named P(P-tta)-X, in which X is the
nal curing temperature. For example, the nal curing
ꢀ
temperature and period is 240 C (2 h) for P(P-tta)-240.
Results and discussion
Scheme 2 Synthesis of P-ap and P-tta.
Synthesis of benzoxazine P-tta
We initially attempted to synthesize P-tta through a Mannich
condensation of phenol, paraformaldehyde and a triazine-
containing triamine, 4,40,400-((1,3,5-triazine-2,4,6-triyl)tris(oxy))
trianiline (tta) (Scheme 1). In this strategy,
a triazine-
containing trinitro(2,4,6-tris(4-nitrophenoxy)-1,3,5-triazine,
tnt) was rst synthesized through a nucleophilic substitution
of 4-nitrophenol with 2,4,6-trichloro-1,3,5-triazine in the pres-
ence of sodium hydroxide (1H-NMR spectrum, Fig. S1†). To
reduce tnt to tta, we initially chose hydrazine hydrate as
1
a reducing agent. Fig. S2† shows the H-NMR spectrum of the
reduction product. The characteristic peak of phenolic OH
appeared at 8.3 ppm. This result explains that the ether bond of
tnt might be broken in the presence of hydrazine hydrate. To
avoid this phenomena, we then used a high pressure procedure
using hydrogen as a reducing agent. However, the hydrogena-
tion led to undesired byproducts under the various conditions
that we applied, so the synthetic route could not be carried out
in this work.
We then redesigned our strategy and prepared P-tta through
a nucleophilic substitution of 4-(2H-benzo[e][1,3]oxazin-3(4H)-
yl)phenol (P-ap) with 2,4,6-trichloro-1,3,5-triazine in the pres-
ence of triethylamine (Scheme 2). The precursor, P-ap, was
prepared using a three-step procedure from 4-aminophenol and
2-hydroxybenzaldehyde.19 Fig. 2 shows the 1H-NMR spectra of P-
Fig. 2 1H-NMR spectra of P-ap and P-tta.
ap and P-tta. The disappearance of the signal for phenolic OH at
8.96 ppm and the shi in the characteristic peaks for benzox-
azine from 5.29 to 5.43 ppm, and from 4.50 to 4.64 ppm support
the nucleophilic substitution. No signal at around 3.80 ppm
corresponding to N–CH2–ph (resulting from the ring opening of
benzoxazine) was observed,20 revealing the purity of the
synthesized benzoxazines. Fig. 3 shows the 13C-NMR spectra of
P-ap and P-tta. The characteristic peaks of benzoxazine shi
from 80.08 to 78.97 ppm, and from 49.80 to 49.09 ppm, sup-
porting the nucleophilic substitution. The combination of the
1H and 13C-NMR spectra conrm the structure of P-tta.
Microstructure
Fig. 4 shows the DSC thermograms of P-oda and P-tta at
a heating rate of 10 ꢀC minꢁ1. The exothermic temperature of P-
tta is lower than that of P-oda, showing a forward polymeriza-
tion. P-tta and P-oda have the same structure (O–ph–oxazine,
Fig. 1) except for the triazine structure, so we speculate that the
forward polymerization is related to the existence of the triazine
structure. IR was used to monitor the curing reactions, and
explain the exothermic peaks.
Scheme 1 Attempted synthesis of P-tta through Mannich conden-
sation of phenol, paraformaldehyde and a triazine-containing triamine
(tta).
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Fig. 5 FT-IR spectra of P-tta after accumulative curing at each stage
for 20 min.
Fig. 3 13C-NMR spectra of P-ap and P-tta.
that the aryl cyanurate structure is stable, and does not rear-
range to aryl isocyanurate. Therefore, the direct rearrangement
of aryl cyanurate to aryl isocyanurate in Scheme 3 is speculated
not to occur. The key point is how to explain the reduction in the
triazine absorption (at 1371 cmꢁ1) and the occurrence of an
isocyanurate absorption (at 1682 cmꢁ1) in Fig. 5. As shown in
Fig. 4, DSC thermograms show that the ring-opening polymer-
ization of benzoxazine in P-tta occurs at lower temperature than
that in P-oda, suggesting the ring-opening polymerization is
related to the existence of the triazine structure. In our previous
work, we reported a concerted reaction between triazine and
oxazine to explain the miscibility of the benzoxazine/cyanate
ester blend, and to explain the reduction in triazine absorp-
tion and the formation of an isocyanurate linkage.15 In that
proposed one-step mechanism, the electron-rich oxygen (CH2–
O–ph) attacks the aromatic carbon next to the oxygen. Then, the
C]N double bond opens, and attacks the electron-decient
methylene (O–CH2–N) of benzoxazine, forming isocyanurate.
Studying the rearrangement of cyanurate with epoxy, we
proposed a modied co-reaction mechanism for benzoxazine
and triazine in P-tta (path a of Scheme 4). In the rst step,
a zwitterion from the breakup of oxazine was formed at
temperatures around 180 ꢀC (since the onset of isocyanurate
absorption is at 180 ꢀC, as shown in Fig. 5). The phenolate
attacks the aromatic carbon next to oxygen (note that the carbon
Fig. 4 DSC thermograms of P-oda and P-tta at a heating rate of 10 ꢀC
minꢁ1
.
Fig. 5 shows the IR spectra of P-tta aer accumulative curing
at each stage for 20 min. The triazine absorptions at 1570 and
1371 cmꢁ1 decrease gradually with the curing temperature.
However, from the IR spectra of the dicyanate ester of bisphenol
A (BACY) aer accumulative curing at each stage for 20 min
(Fig. S3†), the intensity of the triazine absorptions (1367 and
1569 cmꢁ1) was maintained even aer curing at 240 ꢀC. The
spectra suggests that the triazine structure of cured BACY is
stable as the curing progressed. Therefore, the instability of
triazine in P-tta shown in Fig. 5 is speculated with the existence
of the oxazine structure. As shown in Fig. 5, the benzoxazine-
related absorptions of N–CH2–O at 947 cmꢁ1 and ph–O–C at
1222 cmꢁ1 decreased with the progress of the curing, and dis-
appeared aer curing at 200 ꢀC. It has been reported that triallyl
cyanurate can thermally rearrange to triallyl isocyanurate.21,22
Ueda et al. reported the alkyl–aryl cyanurate will rearrange to
alkyl–aryl isocyanurate.23,24 Hamerton discussed the reaction
between the cyanate ester and epoxy.25
The aryl cyanurate (triazine) can react with epoxy, forming
Scheme 3 The rearrangement of aryl cyanurate to aryl isocyanurate.
alkyl cyanurate, which can rearrange to alkyl isocyanurate. Note Note that the rearrangement is speculated not to occur.
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aer accumulative thermal treatment of P-tta at 140, 160, and
ꢀ
180 C for 20 min. As shown in Fig. S4,† there was no reaction
aer thermal treatment at 140 ꢀC. Only slight ring-opening
polymerization of the oxazine of P-taa occurred aer thermal
treatment at 160 ꢀC. However, a further thermal treatment at
180 ꢀC led to the reaction mixture being only partially soluble in
DMSO-d6. Therefore, we cannot elucidate the proposed mech-
anism using NMR spectra. A new design of molecule that
contains only one oxazine and triazine linkage and is soluble
aer thermal treatment is required to prove this mechanism in
the future.
Storage stability
In our previous work, we observed that gelation occurred in
a methyl ethyl ketone solution of a P-oda/BACY (1/1 mol molꢁ1
)
ꢀ
Scheme 4 Proposed curing reactions of P-tta.
blend aer 24 h at 30 C. The gel was also insoluble in tetra-
hydrofuran and dimethyl sulfoxide, suggesting that a cross-
linking structure was present. Through IR and DSC analysis, we
concluded that the tertiary amine of benzoxazine catalyzes the
cyclotrimerization of cyanate ester, leading to gelation. Fig. S5†
shows pictures of a dioxane solution of P-tta before and aer
thermal treatment. A homogeneous solution can be obtained
is slightly electron-decient due to the electron-withdrawing
character of the oxygen and triazine structure). Then, the Ar–
O bond breaks, and the oxygen anion attacks the carbocation of
the zwitterion, producing an alkyl substituted cyanurate. This
reaction is similar to the insertion of a glycidyl ether into an aryl
cyanurate.25 Furthermore, the alkyl substituted cyanurate rear-
ranged to alkyl isocyanurate in the second step (note that the
resulting structures are the same as those proposed in our
previous work).15,26 Theoretically, one triazine structure can
react with three oxazines in the concerted reaction, so the
stoichiometric ratio of triazine to oxazine is one in P-tta.
However, self-polymerization of oxazine can occur in the curing
process (path b in Scheme 4), which is supported by the pres-
ence of phenolic OH signals at 3389 cmꢁ1 in Fig. 6. This leads to
no stoichiometric benzoxazine to react with triazine, and
explains the co-existent triazine and isocyanurate absorptions
in Fig. 5. Therefore, the structure of P(P-tta) should include
isocyanurate, triazine, and ring-opened benzoxazine structures.
Since the proposed reaction mechanism cannot be elucidated
by using only FTIR spectra, we have performed 1H NMR analysis
ꢀ
aer thermal treatment at 60 C for 96 h. The result suggests
that the built-in triazine structure can avoid the gelation
resulting from the cyclotrimerization of cyanate ester in the
benzoxazine/cyanate ester blend. Therefore, the one-
component P-tta can avoid gelation in the two-component
benzoxazine/cyanate ester system when dissolved in a solvent
to act as a varnish in preparing a copper clad laminate. This
phenomenon implies that P-tta is a potential material for
making high-performance copper clad laminates.
Thermal properties of poly(P-tta)
Fig. 7 shows DMA thermograms of P(P-tta)-X, in which X is the
nal curing temperature. The Tg taken from the peak temper-
ature of tan d increased with the curing temperature, ranging
from 247–279 ꢀC. The value of 279 ꢀC is even higher than that of
all the thermosets of the P-oda/BACY blends reported in our
previous work.15 In addition, the peak intensity of tan d
Fig. 6 Enlarged FT-IR spectra of P-tta after accumulative curing at
each stage for 20 min.
Fig. 7 DMA thermograms of P(P-tta)-X.
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include a concerted co-reaction between the triazine and ben-
zoxazine, and a self-polymerization of benzoxazine. The co-
reaction of triazine and benzoxazine generates isocyanurate
linkages, as supported by an increase in the isocyanurate
absorption at 1682 cmꢁ1, and a simultaneous decrease in the
triazine absorption at 1367 cmꢁ1 and oxazine absorption at 947
cmꢁ1. In contrast to the instability of the benzoxazine/cyanate
ester blend in a solution state, P-tta is stable in a solution
state. A solubility test shows that the built-in triazine structure
can avoid the gelation resulting from the cyclotrimerization of
cyanate ester in the benzoxazine/cyanate ester blend, making P-
tta
a potential candidate material for making a high-
performance copper clad laminate. Aer thermal curing at
ꢀ
260 C, the resulting thermoset shows high-performance char-
ꢀ
ꢀ
Fig. 8 TMA thermograms of P(P-tta)-X.
acteristics, with Tg values of 279 C (DMA) and 265 C (TMA),
a coefficient of thermal expansion of 32 ppm per ꢀC, a 5%
decomposition temperature of 417 ꢀC, and a char yield at 800 ꢀC
ꢀ
of 68%. The value of 279 C is even higher than that of all the
thermosets of the P-oda/BACY blends reported in our previous
work.15
Acknowledgements
Financial support of this work from the Ministry of Science and
Technology (MOST 104-2622-8-007-001), Taiwan is highly
appreciated.
Notes and references
Fig. 9 TGA thermogram of P(P-tta)-260 in a nitrogen atmosphere.
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