Synthesis of a benzoxazine with precisely two phenolic OH linkages and the properties of its high-performance copolymers

Article abstract of DOI:10.1002/pola.26661

A phenolic OH-containing benzoxazine (F-ap), which cannot be directly synthesized from the condensation of bisphenol F, aminophenol, and formaldehyde by traditional procedures, has been successfully prepared in our alternative synthetic approach. F-ap was

Full text of DOI:10.1002/pola.26661

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Synthesis of a Benzoxazine with Precisely Two Phenolic OH Linkages and
the Properties of Its High-Performance Copolymers
Ching Hsuan Lin,1 Yu Ren Feng,1 Kang Hong Dai,1 Hou Chien Chang,1 Tzong Yuan Juang2
1Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan
2Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
Correspondence to: C. H. Lin (E-mail: linch@nchu.edu.tw) or H. C. Chang (E-mail: hchang@dragon.nchu.edu.tw)
Received 28 December 2012; accepted 23 February 2013; published online 29 March 2013
DOI: 10.1002/pola.26661
ABSTRACT: A phenolic OH-containing benzoxazine (F-ap), which
cannot be directly synthesized from the condensation of bisphe-
nol F, aminophenol, and formaldehyde by traditional procedures,
has been successfully prepared in our alternative synthetic
approach. F-ap was prepared by three steps including (a) con-
densation of 4-aminophenol and 5,5’-methylenebis(2-hydroxy-
benzaldehyde) (1), (b) reduction of the resulting imine linkage by
sodium borohydride, and (c) ring closure condensation by form-
aldehyde. The key starting material, (1), was prepared from
2-hydroxybenzaldehyde and s-trioxane in the presence of sulfuric
acid. F-ap is structurally similar to bis(3,4-dihydro-2H-3-phenyl-
1,3-benzoxazinyl)methane (F-a, a commercial benzoxazine based
on bisphenol F/aniline/formaldehyde) except for two phenolic
OHs. The phenolic OHs can provide reaction sites with epoxy
and 1,1’-(methylenedi-p-phenylene)bismaleimide (BMI). The
structure–property relationships between the thermosets of
F-ap/epoxy, F-a/epoxy, F-ap/BMI, and F-a/BMI were discussed.
Experimental data showed that thermosets based on F-ap/ep-
oxy and F-ap/BMI provided much better thermal properties
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than those based on F-a/epoxy and F-a/BMI.
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KEYWORDS: high performance polymers; synthesis; thermosets
It has been reported that incorporation of the maleimide
group into benzoxazine can improve the thermal properties of
the resulting thermosets.20–22 Takeichi et al. reported a ther-
mal reaction between the phenolic OH (resulting from the
ring-opened polybenzoxazine) and maleimide, as supported
by the model reaction of phenol and N-phenylmaleimide.23
Based on their finding, they successfully incorporated bisma-
leimide into a polybenzoxazine matrix to achieve high-per-
formance alloys. We suspect that a benzoxazine with phenolic
OHs can provide extra reaction sites with maleimide linkages
of 1,1’-(methylenedi-p-phenylene)bismaleimide (BMI). In addi-
tion, it is known that phenolic OH can also provide a reaction
site with epoxy. Therefore, high-performance benzoxazine/
BMI and benzoxazine/epoxy thermosets can be expected to
result from a phenolic OH-containing benzoxazine.
INTRODUCTION Benzoxazines are resins that can be polymer-
ized to thermosets by means of thermally activated ring-open-
ing reactions.1 Thermosets with low water absorption,
superior electrical properties,2 and low surface energy3 can
be obtained after curing. Bifunctional benzoxazine monomers
are generally prepared from bisphenols, mono primary amine,
and formaldehyde at a molar ratio of 1:2:4. The wide variety
of aromatic bisphenols and monoamines allow for consider-
able flexibility in the molecule-design of benzoxazines. Some
special functional groups, such as acetylene,4,5 furan,6 allyl,7
propargyl,8 vinyl,9 amide,10 phenol,11 maleimide,12 carboxylic
acid,13 methacrylol,14 thymine,15 and amine16 have been
incorporated into benzoxazines to provide the desired physi-
cal properties. Recently, Endo et al. reported that polymer
bearing 1,3-benzoxazine moiety in the side chain was synthe-
sized based on a stepwise strategy.17 However, precise syn-
thesis of a bifunctional benzoxazine monomer with phenolic
OHs in its structure is a challenge in benzoxazine synthesis.18
For example, it is difficult to prepare a phenolic OH-contain-
ing benzoxazine from bisphenol F, aminophenol, and formal-
dehyde because the Mannich-type polycondensation of
aminophenol and formaldehyde lead to polybenzoxazine pre-
cursors with Mw ranging from 1300 to 4500 g/mol.19
In this study, we provide a strategy for preparing a phenolic
OH-containing benzoxazine (F-ap) (In this facile nomenclature
of benzoxazine, “F” stands for bisphenol F, and “ap” stands for
aminophenol). F-ap is structurally similar to bis(3,4-dihydro-
2H-3-phenyl-1,3-benzoxazinyl)methane (F-a,
a commercial
benzoxazine based on bisphenol F/aniline/formaldehyde)
except for two phenolic OHs. We prepare thermosets based
on F-ap/epoxy, F-a/epoxy, F-ap/BMI, and F-a/BMI. This study
Additional Supporting Information may be found in the online version of this article.
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provides a detailed synthetic strategy and the structure–prop-
erty relationships of the resulting thermosets.
EXPERIMENTAL
Synthesis of (2)
Five grams of (1) (19.5 mmol), 4-aminophenol 4.26 g (39
mol), and DMAc 50 mL were introduced into a 250-mL glass
flask equipped with a hydrogen balloon and a magnetic stir-
rer. The mixture was reacted at 25ꢀC for 12 h. NaBH4 0.775
g (20.48 mmol) was added every 1 h. After NaBH4 was
added three times (total 62.4 mmol), the reaction mixture
was further stirred at room temperature for 12 h. The mix-
ture was then poured into water. The precipitate was filtered
and dried in a vacuum oven at 105ꢀC. Yellow powder (85%
yield) was obtained.
Materials
s-Trioxane (from Acros), 2-hydroxybenzaldehyde (from
Showa), sulfuric acid (from Scharlau), acetic acid (from Schar-
lau), 4-aminophenol (from Aldrich), paraformaldehyde (from
TCI), 1,1’-(methylenedi-p-phenylene)bismaleimide (BMI, from
Acros), and sodium borohydride (NaBH4, from Acros) were
used as received. Diglycidyl ether of bisphenol A (DGEBA)
with an epoxy equivalent weight (EEW) of 188 g/eq was
kindly supplied by Chang Chun Plastics, Taiwan. Solvents were
purchased from TEDIA and used without further purification.
1H NMR (DMSO-d6), d 5 3.61(s, 2H, H1), 4.04(d, 4H, H5),
5.18(t, 2H, NH), 6.45(br, 4H, H7), 6.52(br, 4H, H8), 6.67(br,
2H, H11), 6.77(br, 2H, H12), 7.03(s, 2H, H3), 8.39(s, 2H, OH),
9.27(s, 2H, OH). 13C NMR (DMSO-d6), d 5 39.5 (C1), 43.2
(C5), 114.8 (C7), 114.8 (C11), 115.4 (C8), 125.8 (C4), 127.4
(C12), 128.8 (C3), 131.9 (C2), 141.7 (C6), 148.4 (C10), 153.2
(C9). FTIR (KBr): 1244 cm21 (CAN stretch), 3290 cm21 (NH
stretch), 3400 cm21 (OH stretch).
Characterization
Differential scanning calorimetry (DSC) scans were obtained
using a Perkin-Elmer DSC 7 in a nitrogen atmosphere at a
heating rate of 20ꢀC/min. Thermal gravimetric analysis (TGA)
was performed with a Perkin-Elmer Pyris1 at a heating rate of
20ꢀC/min in an atmosphere of nitrogen or air. Dynamic mechan-
ical analysis (DMA) was performed with a Perkin-Elmer Pyris
Diamond DMA with a sample size of 5.0 3 1.0 3 0.2 cm3. The
storage modulus E’ and tan d were determined as the sample
was subjected to a temperature scan mode at a programmed
heating rate of 5ꢀC/min at a frequency of 1 Hz. The test was per-
formed by a bending mode with an amplitude of 5 lm. Thermal
mechanical analysis (TMA) was performed by
a
SII
TMA/SS6100 at a heating rate of 5 oC/min. 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 at 2.49 ppm. IR Spectra were obtained from at least 32
scans in the standard wavenumber range of 400–4000 cm21
using a Perkin-Elmer RX1 infrared spectrophotometer.
Synthesis of F-ap. (2)
Five grams (11.3 mmol), paraformaldehyde 1.09 g (36.16
mmol), and toluene 30 mL were introduced into a 100-mL
round-bottom glass flask equipped with a condenser and a
magnetic stirrer. The mixture was stirred at 50ꢀC for 6 h.
The precipitate was filtered, washed with n-hexane, and
dried in a vacuum oven at 105ꢀC. Light yellow powder (93%
yield) with a melting point of 143ꢀC (DSC) and an exother-
mic peak temperature of 213ꢀC was obtained.
Synthesis of (1)
s-Trioxane 10.0 g (0.111 mol), 2-hydroxybenzaldehyde 89.46
g (0.732 mol), acetic acid 100 g, and sulfuric acid 3.09 g
were introduced into a 250-mL glass flask equipped with a
nitrogen inlet and a magnetic stirrer. The mixture was
reacted at 85ꢀC for 22 h. After the reaction solution was
cooled to room temperature, the precipitate was filtered, dis-
solved in methanol, and poured into water. The precipitate
was filtrated and then dried in a vacuum oven at 105ꢀC. Off-
white powder (44 % yield) was obtained.
1H NMR (DMSO-d6), d 5 3.68(s, 2H, H1), 4.52(s, 4H, H5),
5.24(s, 4H, H10), 6.61(br, 2H, H12), 6.63(br, 2H, H7), 6.86(s,
2H, H3), 6.91(br, 4H, H13), 6.92(br, 2H, H8), 8.96(s, 2H, OH).
13C NMR (DMSO-d6), d 5 39.9 (C1), 49.9(C5), 79.9(C10),
115.5(C7), 116.0(C12), 119.7 (C8), 121.1(C4), 126.9 (C3),
127.8 (C13) 133.4(C2), 140.4 (C6), 152.2 (C9), 151.9 (C11),
FTIR (KBr):954 cm21, 1040 cm21, 1240 cm21, 1370 cm21
.
1H NMR (DMSO-d6), d 5 3.84(s, 2H, H1), 6.93(d, 2H,H7),
7.37(d, 2H, H8), 7.46(s, 2H, H3), 10.22(s, 2H, HAC@O),
10.57(s, 2H,OH), 13C NMR (DMSO-d6), d 5 38.6 (C1),
117.5(C7), 122.1 (C6), 128.3 (C3), 132.3 (C2), 136.9 (C8),
159.2(C4), 191.3 (C5). FTIR (KBr):1655 cm21 (C@O stretch),
3425 cm21 (OH stretch)
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SCHEME 1 Synthesis of F-ap.
RESULTS AND DISCUSSION
Preparation of Thermosets
A powder mixture of F-ap/BMI with equal equivalency was
stirred at 160ꢀC in an aluminum mold and cured for 2 h at
180, 200, 220, and 240ꢀC in an air-circulating oven (Note
that four phenolic OHs are generated after the ring-opening
of each F-ap molecule, so the equivalency of F-ap is 116 g/eq).
Thereafter, the F-ap/BMI thermoset was allowed to cool slowly
to room temperature to prevent cracking. Thermosets of F-a/
BMI, F-ap/DGEBA, and F-a/DGEBA were prepared using the
same procedure as that used for the F-ap/BMI thermoset.
(Note that two phenolic OHs are generated after the ring-
opening of each F-a molecule, so the equivalency of F-a is
217 g/eq).
Synthesis of F-ap
A phenolic OH-containing benzoxazine (F-ap), which cannot
be directly prepared from bisphenol F/aminophenol/formal-
dehyde by traditional procedures, has been successfully pre-
pared by a three-step procedure (Scheme 1).24,25 The key
starting material, 5,5’-methylenebis(2-hydroxybenzaldehyde)
(1), was prepared from 2-hydroxybenzodehyde and s-triox-
ane in the presence of sulfuric acid.26 Supporting Informa-
tion Figure S1 shows the 1H NMR spectrum of (1).
Methylene at 3.84 ppm, aldehyde (H5) at 10.2 ppm, and phe-
nolic OH at 10.6 ppm were clearly observed. An enlarged
NMR spectrum shows that H7 is a doublet, H8 is a doublet-
FIGURE 1 1H NMR spectrum of F-ap in DMSO-d6.
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FIGURE 2 13C NMR spectrum of F-ap in DMSO-d6.
doublet, and H3 is a doublet, further confirming the structure
of (1). In the 13C NMR spectrum (Supporting Information
Figure S2), the methylene (C1) at 38.6 ppm, and aldehyde at
191.3 ppm were clearly evident.
Supporting Information Figure S3 shows the 1H NMR spec-
trum of (2). Two signals of phenolic OH at 8.4 and 9.3 ppm
were observed. Moreover, two new peaks appeared at 5.2
and 4.0 ppm, standing for CH2ANH and CH2ANH, respec-
tively. In the 13C NMR spectrum (Supporting Information
Figure S4), the methylene (C5) at 43.2 ppm, and two Ar-OH
carbons (C9 and C10) at 148.4 and 153.2 ppm were clearly
evident.
In the second step, (1) reacted with aminophenol, yielding a
bis(o-hydroxyphenylimine) linkage. Sodium borohydride was
then employed to reduce the imine linkage, yielding (2).
FIGURE 3 DSC thermograms of F-ap and F-a.
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TABLE 1 Thermal Properties of the Resulting Thermosets
Tg (DMA)a
Tg (DSC)b
Td5%
Char Yieldd
c
Thermoset ID
P(F-a)
173
–
162
220
155
202
162
295
314
381
365
392
375
410
49
69
41
40
50
58
P(F-ap)
F-a/DGEBA
F-ap/DGEBA
F-a/BMI
164
222
204
294
F-ap/BMI
a
The temperature corresponding to the peak temperature of tan d,
measured by DMA at a heating rate of 5ꢀC/min.
b
Tg is measured by DSC at a heating rate of 20ꢀC/min.
c
The temperature corresponding to 5% weight loss in
atmosphere, measured by TGA at a heating rate of 20ꢀC/min.
a nitrogen
d
Residual weight % at 800ꢀC in a nitrogen atmosphere.
5.2 and 4.5 ppm, respectively. No signal was observed at
around 4.0 ppm corresponding to the NACH2Aph resulting
from the ring opened benzoxazine, revealing the purity of the
synthesized benzoxazine. The signal of phenolic OH was
observed at 9.0 ppm. All signals of Ar-H (H3, H7, H8, H12, and
H13) were clearly assigned. Figure 2 shows the 13C NMR spec-
trum of F-ap. The characteristic peaks of benzoxazine at 49.9
(phACH2AN, C5) and 79.9 (OACH2AN, C10) ppm confirm the
structure of the benzoxazine. The detailed assignment of aro-
FIGURE 4 IR spectra of F-ap after accumulative curing at each
temperature for 20 min.
In the last step, paraformaldehyde was added to the toluene
solution of (2) to induce ring closure condensation. Supporting
Information Figure S5 shows the IR spectra of (2) and F-ap.
The sharp NH absorption at 3300 cm21 disappeared, and new
absorptions appeared at 945 cm21 (the out-of-plane mode of
the benzoxazine ring to which the oxazine ring is attached,
1040 (ArAOAC symmetric stretch), 1240 (ArAOAC asymmet-
1
matic peaks in Figures 2, assisted by the correlation in H-1H
COSY, and 1H-13C HETERCOR (Supporting Information Figures
S6 and S7), confirms the structure of F-ap.
1
ric stretch) and 1370 cm21 (oxazine). Figure 1 shows the H
NMR spectrum of F-ap. The characteristic peaks of benzoxa-
zine, OACH2AN (H10) and phACH2AN (H5), were observed at
DSC thermograms of benzoxazines
The DSC thermograms of F-a and F-ap are shown in Figure
3. F-ap shows a melting point of 143ꢀC, and a much lower
initial exothermic temperature than that of F-a. Endo el al.
reported that the stability of the Zwitter ion is crucial to the
ring opening of benzoxazine.27 Ronda et al. also reported
that the electron-donating substitute in the para position sta-
bilizes the Zwitter ion, and enhance the reactivity.28 In this
case, the electron-donating phenolic OH in F-ap can stabilize
the Zwritter ion through resonance. In addition, it has been
reported that phenolic OH can catalyze the ring-opening of
oxazine.20,29–31 Both factors shift the exothermic peak to a
lower temperature. The rapid curing characteristic can be
reflected in the short gel time of F-ap. The gel time of F-ap
at 170ꢀC is less than 1 min, making it difficult to make a
void-free sample for DMA measurement.
Curing of F-ap probed by IR
Benzoxazine can be thermally cured into a cross-linked poly-
benzoxazine through the ring-opening of the benzoxazine
linkages. Figure 4 shows the IR spectra of F-ap after accu-
mulative curing at each temperature for 20 min. After curing
at 180ꢀC, the ArAOAC absorptions at 1040 and 1240 cm21
,
and the oxazine absorption at 1370 cm21 disappeared. The
absorptions of the characteristic mode of benzene with an
attached oxazine ring at 945 cm21 also disappeared, suggest-
ing a rapid curing characteristic. As the intensity of the
aforementioned peaks decreased, the reduction in intensity
SCHEME 2 Ideal structure of benzoxazine thermosets, P(F-a)
and P(F-ap).
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FIGURE 5 DSC heating thermograms of F-ap/BMI mixtures.
of 1,2,4-trisubstituted benzene absorption appearing at 1499
cm21 was not obvious. The phenomena can be explained by
the compensation of a newly formed 1,2,4-trisubstituted ben-
zene linkage, as marked by a circle in Scheme 2. As shown
in Scheme 2, the nitrogen linkage in P(F-a) is linked to a
phenyl pendant, but the nitrogen linkage in P(F-a) is bonded
to the other repeating unit, leading to a higher crosslinking
density. The higher crosslinking is reflected in the Tg values
of P(F-ap) and P(F-a). DSC shows the Tg of P(F-ap) is 220ꢀC
(Supporting Information Figure S8), while the Tg of P(F-a) is
160ꢀC. However, as mentioned before, the rapid curing charac-
teristic hinders the formation of a void-free sample, so no
DMA data for P(F-ap) to further support the DSC data. The
thermal stability of P(F-ap) and P(F-a) were evaluated by TGA
in a nitrogen atmosphere, and the results are provided in Ta-
ble 1. The 5 wt% degradation temperature is 314 and 381ꢀC
for P(F-a) and P(F-ap), respectively. The char yield is 49 and
69 wt% for P(F-a) and P(F-ap), respectively. The higher cross-
linking density of P(F-ap) than P(F-a) is thought to be respon-
sible the for the better thermal stability of P(F-ap).
FIGURE 6 DSC heating thermograms of F-a/BMI mixtures.
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FIGURE 8 DMA thermograms of F-a- and F-ap-based
thermosets.
25/75 and 50/50 mixtures. However, the enthalpy measured
was 39 and 52 kJ/mol, respectively for the 25/75 and 50/50
mixtures. The larger enthalpy suggests co-reaction occurred
in addition to the hompolymerizaiton of F-ap and BMI. Fig-
ure 6 shows the DSC heating thermograms of the F-a/BMI
powder mixtures. The forward exothermic temperature was
not as obvious as that in the F-ap/BMI system. This indi-
cates that the obvious forward exothermic temperature in
Figure 5 is related to the co-reaction between phenolic OH
and maleimide. The result is consistent with the observation
of Takeichi et al.23
FIGURE 7 IR spectra of the F-ap/BMI (50/50) mixture after accu-
mulative curing at each temperature for 20 min.
DSC and IR analyses
Figure 5 shows the DSC heating thermograms of the F-ap/
BMI powder mixtures. Pristine BMI showed a melting point
of 166ꢀC, and an exothermic peak from 200 to 250ꢀC. The
exothermic peak temperature of the F-ap/BMI powder mix-
ture decreased from 235 to 185ꢀC as the ratio of F-ap
increased from 0% to 75%. In addition, the exothermic en-
thalpy was much higher than the enthalpy calculated from
linear combination. For example, the measured enthalpy was
59 and 17.3 kJ/mole for F-ap and BMI, respectively. The cal-
culated enthalpy was 27 and 38 kJ/mol, respectively, for the
Figure 7 shows the IR spectra of the F-ap/BMI (50/50) mix-
ture after accumulative curing at each temperature for 20
min. The C@CH absorption of maleimide at 3101 cm21 and
oxazine absorption at 945 cm21 decreased simultaneously,
SCHEME 3 Ideal structure of benzoxazine/BMI and benzoxazine/DGEBA thermosets.
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and disappeared after curing at 200ꢀC. An ether absorption
at 1195 cm21 appeared rapidly with the progress of curing,
indicating a phAOAC bond was formed. Similar results were
found in the 75/25 and 25/75 compositions (Supporting In-
formation Figures S9–S10). This result is consistent with
that observed by Takeichi et al.23
formed between the co-reaction of the phenolic OH and mal-
eimide. Compared to F-a, the phenolic OHs of F-ap provided
extra reaction sites with epoxy and bismaleimide. Therefore,
F-ap based thermosets show higher Tg and better thermal
stability than F-a based thermosets. The combination of the
rapid curing characteristic, high Tg, and good thermal stabil-
ity makes the F-ap attractive for electronic applications,
especially in the field of copper clad laminates.
Thermal properties of thermosets
Figure 8 shows the DMA thermograms of the F-a/BMI and
F-ap/BMI thermosets. Only one tan d peak was observed,
indicating that a homogeneous copolymer was obtained. The
Tg of F-ap/BMI thermoset was as high as 294ꢀC, which is
90ꢀC higher than that of the F-a/BMI thermoset. The result
demonstrates the high-Tg advantage of the F-ap-based ther-
mosets. The high-Tg characteristic of the F-ap based thermo-
set can also be supported by the DMA data of the
F-a/DGEBA and F-ap/DGEBA thermosets (Fig. 8). As shown
in the figure, the Tg of the F-ap/DGEBA thermoset was
222ꢀC, which is 58ꢀC higher than that of the F-a/DGEBA
thermoset. Since F-ap and F-a exhibit a similar structure
except for the phenolic OH linkages, the higher Tg of the F-
ap-based thermoset is thought to be related to the extra
crosslinking point provided by the extra phenolic OH, as
marked by a circle in Scheme 3. The smaller tan d height
supports the more rigid structure of the F-ap/BMI thermoset
than that of the F-a/BMI thermoset. The same result was
also observed in the F-ap/DGEBA and F-a/DGEBA systems.
Compared with the data with those in our previous work,32
P(F-ap) shows lower Tg than P(P-bapf), but the phenolic
OHs of F-ap make the Tg can be significant enhanced
through copolymerization with BMI.
ACKNOWLEDGMENT
The authors thank the National Science Council of the Republic
of China, Taiwan, for financial support.
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The 5% degradation temperature of the F-ap/BMI thermoset
was 410ꢀC, which is higher than that of the F-a/BMI thermo-
set (375ꢀC) (Supporting Information Figure S11, Table 1).
The larger number of crosslinking sites between F-ap and
BMI might be responsible for the better thermal stability of
the F-ap/BMI thermoset. A similar result was observed for
the F-ap/DGEBA and F-a/DGEBA thermosets. The 5% degra-
dation temperature of the F-ap/DGEBA thermoset was
392ꢀC, which was higher than that (365ꢀC) of the F-a/
DGEBA thermoset (Supporting Information Figure S12).
According to the literature, the release of aniline fragments
is responsible for the low thermal stability of biphenol and
aniline-based polybenzoxazines.33 In contrast, the nitrogen
linkage in the F-ap-based thermoset is bonded to the other
repeating unit, as shown by the circled structure in Scheme
3. This makes the release of aniline fragments difficult, and
causes the F-ap-based thermoset to be thermally more stable
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