Synthesis and Characterization of Cyanate Ester Functional Benzoxazine and Its Polymer

Article abstract of DOI:10.1021/acs.macromol.5b02285

The first benzoxazine incorporating cyanate ester group in its structure has been synthesized. The polymerization process investigated by differential scanning calometry (DSC) exhibits two clearly separated exotherm maxima, corresponding to cyanate ester

Full text of DOI:10.1021/acs.macromol.5b02285

Article
pubs.acs.org/Macromolecules
Synthesis and Characterization of Cyanate Ester Functional
Benzoxazine and Its Polymer
Seishi Ohashi, John Kilbane, Tyler Heyl, and Hatsuo Ishida*
Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United
States
ABSTRACT: The first benzoxazine incorporating cyanate
ester group in its structure has been synthesized. The
polymerization process investigated by differential scanning
calometry (DSC) exhibits two clearly separated exotherm
maxima, corresponding to cyanate ester trimerization and
benzoxazine, respectively. The nature of each exotherm is
studied by Fourier transform infrared spectroscopy (FT-IR).
The activation energies for cyanate ester trimerization and benzoxazine ring-opening polymerization are determined by both
Kissinger’s and Ozawa’s methods. Furthermore, both exothermic temperatures observed are lower than those of the published
values of benzoxazine and dicyanate ester blends. Thermal properties such as glass transition temperature and char yield are
studied by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Those properties are found to be
outstanding compared to the reported properties of general benzoxazine resins and polymerized blends of benzoxazine and
dicyanate ester, offering a polymeric matrix for composite satisfying many requirements for such a material.
lowered.11 The blend of benzoxazine with cyanate ester is an
1. INTRODUCTION
active research subject in the polybenzoxazine field12−16 after a
Benzoxazine polymer is a recently developed class of thermoset
which is receiving strong attention in recent years because they
possess significant advantages compared with other typical
thermosetting resins, such as excellent mechanical and thermal
properties, near-zero shrinkage upon polymerization, remark-
able molecular design flexibility, high char yield, and surface
report that showed significant synergism for polymerization of
benzoxazine in the presence of cyanate ester groups. Their
synergetic effects in thermal and mechanical properties were
reported while its reaction mechanism is still not fully
understood and is continuously discussed. Nair12 and Kimura
et al.13 proposed that the network of benzoxazine and cyanate
ester group with iminocarbonate intermediate is produced by
the reaction of cyanate ester group with the phenolic hydroxyl
group that resulted from the benzoxazine ring-opening. This
iminocarbonate intermediate is proposed to further induce
polymerization reaction of more molecules of cyanate ester to
form polycyanulate. Gu et al.14 proposed that hydroxyl group of
ring-opened benzoxazine catalyzes cyclotrimerization of cyanate
ester groups and the ring-opened benzoxazine inserted into
triazine structures, resulting in the coexistence of cyanurate and
isocyanurate. On the other hand, Lin et al. proposed the
benzoxazine ring itself, rather than the phenolic hydroxyl group
as a result of oxazine ring-opening, catalyzes cyanate ester
trimerization and cyanurate can be converted into isocyanurate
with benzoxazine as it is being heated.16 However, no
benzoxazine containing cyanate ester group on the same
molecule has yet been prepared and reported to date.
energy lower than polytetrafluoroethylene.1−3 Among those
benzoxazine resins, benzoxazines functionalized with malei-
mide,4 amide,5 and acetylene6 were synthesized and found to
exhibit very high thermal resistant. Incorporation of functional
group with benzoxazine structure is not only simple to prepare
but also compatibility to other compounds to improve their
properties. Furthermore, good processability is expected due to
their structural simplicity. Additionally, functional groups other
than benzoxazine structure will add new polymerization
mechanism in addition to the cationic ring-opening polymer-
ization of benzoxazine group, allowing adding more flexibility in
processing conditions and wider property ranges.
Cyanate ester resin is also known as a high performance
polymer whose properties favorably compare with polyimide,
bismaleimide, and high performance epoxy resins for the
electronic and aerospace applications7,8 since it possesses
excellent properties, including high thermal resistance, low
dielectric constant, and radiation resistance.9 The cyanate ester
group undergoes thermal cyclotrimerization to form a stable
triazine network under proper reaction conditions. Unfortu-
nately, elevated temperature is needed to trimerize cyante ester
groups,10 and it is the main drawback of cyanate ester resins to
consume high energy in addition to the sensitivity of the
cyanate ester group to moisture. In order to solve this issue, the
blend of cyanate ester with other polymers, such as epoxy
resins, has been studied, and the trimerization temperature was
For a high performance matrix used in composite materials, a
number of required properties must simultaneously be satisfied.
For example, the polymerization temperature is desired to be
rather low in order to save energy and reduce expenses for the
heating devices. Yet, too low a polymerization temperature
Received: October 16, 2015
Revised: November 13, 2015
© XXXX American Chemical Society
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Scheme 1. p-Cyanate Ester Functional Benzoxazine (PH-acy) Synthesis
was cooled to 0 °C under a nitrogen atmosphere, and 5 mL of acetone
solution of triethylamine (0.45 g, 4.40 mmol) was added dropwise into
the solution and stirred at 0 °C for 30 min. The precipitates were
filtered, and the filtrate was dried at room temperature. The resulting
compound was redissolved in ethyl acetate and washed with water
three times. The solution was dried under vacuum to afford white
crystal. Yield: 0.60 g, 54%; mp 77 °C. 1H NMR (300 MHz, DMSO-d6,
ppm): δ 4.66 (s, 6H, C−CH2−NPh), 5.44 (s, 6H, O−CH2−NPh),
6.72 (d, 1H, ArH), 6.86 (t, 1H, ArH), 7.09 (m, 2H, ArH), 7.27 (q, 4H,
ArH). 13C NMR (600 MHz, DMSO-d6, ppm): δ 79.1, 109.9, 116.9,
117.1, 119.5, 121.3, 121.6, 127.9, 128.4, 147.1, 147.4, 154.4. FTIR
(KBr), cm−1: 2271 (CN stretching of cyanate ester), 2237 (CN
stretching of cyanate ester), 1498 (stretching of substituted benzene
ring), 1226 (asymmetric stretching of C−O−C), 1165 (asymmetric
stretching of C−N−C), and 950 (out-of-plane C−H of benzene ring
of benzoxazine). Anal. Calcd for C15H12N2O2: C, 71.42; H, 4.79; N,
11.10. Found: C, 70.44; H, 4.64; N, 11.10.
approaching room temperature often results in poor shelf life
and requires cryogenic storage, which adds extra cost. Typically
a trade-off exists between polymerization temperature and shelf
life. Another desired property is the high thermal properties,
such as high glass transition and degradation temperatures and
high char yield. In general, precursors for high performance
polymers possessing these properties are expensive and difficult
to process. Thus, rather inexpensive raw material cost and low
monomer or precursor viscosity are rare but highly desirable. It
is a goal of the current project to develop a material that
exhibits as many these seemingly conflicting properties as
possible.
In this paper, a novel, anilinic cyanate-ester-functional
benzoxazine (hereinafter appreviated as PH-acy) was synthe-
1
sized, and the structure was studied by H NMR, 13C NMR,
differential scanning calorimetry (DSC), and Fourier transform
infrared spectroscopy (FT-IR). Its polymer was characterized
by thermogravimetric analysis (TGA) and dynamic mechanical
analysis (DMA). It is further the interest of this paper to
compare the reaction of benzoxazine/cyanate ester blends and
the benzoxazine that contains cyanate ester in itself.
2.3. Preparation of Poly(PH-acy). The molten PH-acy was cast
over a glass plate at 80 °C. The resulting film was placed in convection
oven and polymerized at 220 °C for 2 h. It was cooled to room
temperature, and the dark brown film was obtained.
2.4. Characterization. 1H NMR spectra were acquired in
deuterated dimethyl sulfoxide (DMSO-d6) with tetramethylsilane as
an internal standard on a Varian Oxford AS300 at a proton frequency
of 300 MHz. The 13C NMR spectrum was acquired in deuterated
dimethyl sulfoxide (DMSO-d6) on a Varian Oxford AS600 at a carbon
frequency of 150.864 MHz (proton frequency equivalence of 600
MHz). The average number of transients for 1H and 13C NMR was 16
and 1024, respectively. A relaxation time of 10 s was used for the
integrated intensity determination of 1H NMR spectra for quantitative
analysis. Support to molecular studies by NMR was done by Fourier
transform infrared spectroscopy (FT-IR) using a Bomem Michaelson
MB 110 spectrophotometer which is equipped with a deuterated
triglycine sulfate detector. Thirty-two scans were coadded per
spectrum at a resolution of 4 cm−1 after purging the spectrometer
with dry air. Samples were ground with KBr powder for analysis and
compressed into a 13 mm pellet or coated on a 25 mm KBr circular
disk. A TA Instruments DSC model 2920 was used with a heating rate
of 10 °C/min and a nitrogen flow rate of 60 mL/min for the tests of
differential scanning calorimetric (DSC) study. Thermogravimetric
analysis (TGA) was carried out on a TA Instruments Q500 TGA with
a heating rate of 10 °C/min under nitrogen at a flow rate of 60 mL/
min. Dynamic mechanical analyses (DMA) were done on a TA
Instruments Q800 DMA, applying a controlled strain tension mode
with an amplitude of 10 μm and a temperature ramp rate of 3 °C/min
at a frequency of 1.0 Hz.
2. EXPERIMENTAL SECTION
2.1. Materials. Salicylaldehyde (98%), 4-aminophenol (97.5%),
sodium borohydride (98%), paraformaldehyde (96%), cyanogen
bromide (98%), and triethylamine (99%) were used as purchased
from Sigma-Aldrich. Dimethylacetamide (99%), 1,4-dioxane (99%),
and anhydrous acetone (99%) were from Fisher Scientific.
2.2. Monomer Synthesis. 2.2.1. Synthesis of 2-((4-
Hydroxyphenylamino)methyl)phenol. Synthesis of 2-((4-hydroxy-
phenylamino)methyl)phenol adopted the basic approach used by
Ronda17 and Lin18 et al. but modified to suit the special nature of
bisphenolic structure where one of the phenolic group is intra-
molecularly hydrogen bonded and the other intermoleularly hydrogen
bonded. Salicylaldehyde (4.76 g, 0.04 mmol), 4-aminophenol (4.26 g,
0.04 mmol), and dimethylacetamide (DMAc, 50 mL) were added into
a flask and stirred at ambient temperature for 24 h. Sodium
borohydride (0.775 g, 0.02 mmol) was added in the solution every
1 h three times and continuously stirred at ambient temperature for
more than 12 h. After the reaction, the resulting solution was poured
into 500 mL of water. The precipitate was washed several times with
water and dried overnight under vacuum at 50 °C. Yield: 4.79 g, white
1
powder, 39%. H NMR (300 MHz, DMSO-d6, ppm): δ 4.08 (d, 2H,
ArCH2NH), 5.36 (s, 1H, ArCH2NH), 6.44 (q, 4H, ArH), 6.69 (t, 1H,
ArH), 6.76 (d, 1H, ArH), 7.00 (t, 1H, ArH), 7.15 (d, 1H, ArH), 8.38
(s, 1H, OH), 9.46 (s, 1H, OH).
2.2.2. Synthesis of 3-(4-Cyanatophenyl)-3,4-dihydro-2H-benzo-
[e][1,3]oxazine (PH-acy). 2-((4-Hydroxyphenylamino)methyl)phenol
(0.95 g, 4.40 mmol) and paraformaldehyde (0.14 g, 4.80 mmol) in 1,4-
dioxane (25 mL) were stirred at 100 °C for 18 h. After the reaction,
the solvent was dried under vacuum, and cyanogen bromide (0.46 g,
4.40 mmol) and anhydrous acetone (25 mL) were added. The solution
3. RESULTS AND DISCUSSION
The synthesis of cyanate ester functional benzoxazine is shown
in Scheme 1. Incorporation of cyanate ester onto a benzoxazine
molecule was achieved first by closing the oxazine ring while
maintaining one of the two phenolic hydroxyl groups
unreacted. Phenolic hydroxyl group-containing benzoxazine
cannot be simply synthesized with one-pot Mannich reaction of
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phenol, 4-aminophenol, and formaldehyde due to the presence
of two different hydroxyl groups. However, synthesis of a
phenolic hydroxyl group-containing benzoxazine was reported
by Andreu and Ronda17 and Lin18 et al. using the approach to
close the intramolecularly hydrogen-bonded phenolic hydroxyl
group and 2-hydroxybenzylamine into a benzoxazine moiety.
This 2-hydroxylbenzylamine was prepared from the reduction
of the Schiff base that was obtained from the reaction of
salicylaldehyde and 4-aminophenol. The cyanate ester func-
tional benzoxazine, abbreviated as PH-acy, was prepared from
cyanogen bromide in the presence of triethylamine. The
1
structure of PH-acy was verified by H NMR and 13C NMR
Figure 3. DSC thermograms of (a) PH-acy and (b) PH-a.
ester and ring-opening polymerization of benzoxazine, which
occur separately. It is noteworthy that both exotherms of PH-
acy appeared in lower temperature ranges than those of the
benzoxazine/cyanate ester blends reported in the literature
while similar multiple exothermic events were reported in the
blend of dicyanate ester and diamine-based benzoxazine.12−16
This significant reduction of the polymerization temperature is
not due to the phenolic impurities caused by the presence of
oligomers as the PH-acy studied was quite pure and sharp
crystal melting was also observed. Therefore, it is very likely
that cyanate ester reaction promotes benzoxazine cationic ring-
opening polymerization as proposed by the studies on
benzoxazine/cyanate ester blends but more efficiently in the
case where cyanate ester is part of the benzoxazine molecule.
The molecular structure was also confirmed by FT-IR
spectra. The characteristic bands at 2271 and 2237 cm−1 are
typical OCN stretches. The band at 1226 cm−1 corresponds to
the antisymmetric C−O−C stretch of benzoxazine ring,
whereas the 950 cm−1 band is assigned to the out-of-plane
mode of benzene to which oxazine ring is attached. These
spectral features are all consistent with the published
benzoxazine and cyanate ester frequencies. In the isothermal
FT-IR studies at various temperatures, multiple polymerization
processes which are observed by the DSC thermogram in
Figure 3 can be simultaneously and independently followed on
the molecular level. Heating at 120 °C without added initiator
and/or catalyst, cyanate ester stretch mostly disappeared after 2
h, and triazine absorption derived from trimerization of cyanate
esters appeared at 1368 cm−1. Additionally, a significant
absorption band at 1683 cm−1 due to isocyanurate, which is
probably formed by the rearrangement of alkyl group
substituted triazine ring, was also observed. From this result,
it is concluded that cyanate esters can trimerize at first at 120
°C while benzoxazine ring subsequently opens as signified by
the reduction of the band located at 950 cm−1. Spectrum b also
provides important information on the order of these two
events happening. While the signature of cyanate ester almost
completely disappeared at 120 °C after 2 h, the intensity of the
950 cm−1 band that represents unreacted benzoxazine group
mostly remains albeit some reduction in intensity. Thus, this
clearly establishes the order of these two events, namely cyanate
ester trimerizes first. After heating at 170 °C for 2 h, both bands
of cyanate ester at 2271 and 2237 cm−1 and benzoxazine at 950
1
Figure 1. H NMR spectrum of PH-acy in DMSO-d6.
Figure 2. 13C NMR spectrum of PH-acy in DMSO-d6.
spectra which are shown in Figures 1 and 2, respectively. In the
1H NMR spectrum, there are two characteristic resonances at
4.66 and 5.44 ppm, which are assigned to Ar−CH2*−N− and
−O−CH2*−N− protons, confirming the formation of the
benzoxazine group. The separation of these two resonances,
0.78 ppm, is within the typical range of many benzoxazine
compounds.19 The further evidence of benzoxazine ring
formation was confirmed by the resonances at 49.8 and 79.2
ppm in 13C NMR. The characteristic carbon signal of the
cyanate ester −OC*N was also observed at 109.9 ppm.
The cross-linking process was studied by DSC as shown in
Figure 3. There are two exothermic maxima at 213 and 229 °C,
and the onset of the first peak is around 160 °C in PH-acy.
These multiple exotherms can be considered as two different
cross-linking processes, namely, cyclotrimerization of cyanate
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cm−1 disappeared. As increasing temperature for further cross-
linking, there are almost no significant changes in the FT-IR
spectra, and triazine and isocyanurate bands still coexist even
heating at 220 °C for 2 h. There are various hypotheses13−15
reported on the polymerization mechanism of the blend of
benzoxazine and cyanate ester. However, none of them can
completely explain the polymerization behavior of this cyanate
ester incorporated benzoxazine. A further detailed study on the
cyanate ester functional benzoxazine will be discussed to clarify
the polymerization mechanism. However, important conse-
quence of the current study is that this novel benzoxazine offers
possibility of polymerizing at significantly lower temperature
than the majority of benzoxazines reported in the literature. As
lowering the polymerization temperature of benzoxazine resins
is one of few very strong interests in the benzoxazine research
community, the observation reported in this paper contributes
toward this goal..
Figure 6. Kissinger plot for determination of the activation energy of
PH-acy.
Figure 7. Ozawa plot for determination of the activation energy of
PH-acy.
are about 71 and 79 kJ/mol. For the second exotherm are 91
and 100 kJ/mol, respectively, as shown in Table 1. The
Figure 4. FT-IR spectra of PH-acy (a) monomer, (b) polymerized at
120 °C/2 h, (c) 170 °C/2 h, and (d) 220 °C/2 h.
Table 1. Activation Energy of PH-acy Calculated by
Kissinger’s and Ozawa’s Methods
The polymerization kinetic study of PH-acy can be
performed with nonisotheral DSC scans at different heating
rates, 5, 10, 15, 20, and 25 °C/min, to determine the apparent
activation energy of the polymerization process. The Kis-
singer20 and Ozawa21 methods have been used to determine
the activation energy based on these DSC results shown in
Figures 5, 6, and 7. The activation energies calculated from the
slope of the Kissinger and Ozawa’s plots of the first exotherm
activtion energy (Ea, kJ/mol)
Kissinger’s plot
Ozawa’s plot
first exotherm
71
91
79
second exotherm
100
activation energies obtained for the first exotherm agrees with
that for the bisphenol A dicyanate, which was reported to be 71
kJ/mol,22 and the second exotherm is also within the range of
activation energy for bifunctional benzoxazines, 81−117 kJ/
mol.23,24 These data strongly support that trimerization of
cyanate ester groups occurs first followed by benzoxazine
polymerization, which were hypothesized from the FT-IR
spectra shown in Figure 4. It is interesting to note that as the
heating rate is reduced, the complexity of first exotherm
becomes more visible. It appears that overlapped peaks exist in
the 170−200 °C range. Broad maxima at approximately 175
and 187 °C are heavily overlapped, whereas a weak but visible
small peak also appears around 196 °C. Contrary to these
complex cyanate ester exotherms, the peak due to the
benzoxazine polymerization seems to show a single process.
Therefore, the apparent activation energy for the cyanate ester
polymerization reported above is an average value of those
processes and thus should be considered only as a guide.
Figure 5. DSC thermograms of PH-acy for several heating rates.
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The TGA thermogram of PH-acy polymerized at 220 °C for
2 h is shown in Figure 8. Poly(PH-acy) has 65% char yield at
Figure 9. DMA thermogram of poly(PH-acy).
Figure 8. TGA thermogram of poly(PH-a) (a) and poly(PH-acy) (b)
polymerized at 220 °C/2 h.
where E′ is the storage modulus (dyn/cm2) of the polymer at
temperature T, R is the gas constant, and γ is Poisson’s ratio
which is assumed to be 0.5 for incompressible networks.
d
cross‑link is the cross-link density (mol/cm3) of the polymer. The
800 °C, which is significantly higher than the polybenzoxazine
without cyanate ester, poly(PH-a). This value is higher than
that of any blends of benzoxazine and cyanate ester which are
thus far reported or typical dicyanate ester polymers which
show char yields lower than 60%.25 For thermal degradation of
polybenzoxazine, evaporation of amine portion should be
considered and is already studied by Ishida26 and Hacaloglu27
et al. The cyanate ester group helps the amine moiety to
covalently attach to the cross-linked networks and can prevent
from decomposition or evaporation of aniline portion during
heating. From the summary of TGA thermogram shown in
Table 2, significant enhancement of thermal stability with
cyanate ester as much as over 80 °C for both 5 and 10% weight
loss temperatures, Td5 and Td10, has been observed.
The DMA thermogram of PH-acy polymerized at 220 °C for
2 h is shown in Figure 9. The glass transition temperature, Tg, is
determined by the peak temperature of the loss modulus, E″;
however, the Tg is frequently determined by the peak position
of tan δ for polybenzoxazine because of difficulty of recognizing
the precise peak position in the E″ spectrum. For this polymer,
we can observe the Tg on both loss modulus and tan delta
curves, 284 and 265 °C, respectively. In order to explain these
rather high Tg of poly(PH-acy), the following equation for
rubbery plateau storage modulus at Tg + 40 °C was used to
calculate the cross-link density.28
cross-link density of the poly(PH-acy) can be calculated to be
3.51 × 10−3 mol/cm3, which is significantly higher than
polybenzoxazine derived from bisphenol A and aniline that is
reported to be 1.1 × 10−3 mol/cm3.2 Thus, its high cross-link
density of poly(PH-acy) contributes to its high glass transition
temperature. It should be noted that since the rubbery plateau
slightly increases because of the small amount of residual
reactive groups as the temperature increases beyond the Tg, the
cross-link density reported here should only be considered as a
guide, although the error is expected to be relatively small.
4. CONCLUSIONS
Monofunctional benzoxazine that incorporates cyanate ester
functional group resulted in significant decrease in the
polymerization temperature without any added initiator or
catalysts. Its polymerization temperature was significantly
reduced from the ordinary benzoxazine resins and even lower
than the blends of benzoxazine and dicyanate ester while its
polymer has high char yield above 60% and glass transition
temperature over 250 °C for monofunctional benzoxazine
polymer and even the blends of dicynate ester. Thus, this
material is highly attractive for the industrial use, especially for
RTM (resin transfer molding) due to its simplicity, high
properties, and low-energy consumption because the polymers
for RTM often have to sacrifice thermal or/and mechanical
properties to accomplish their higher processability. Addition-
dcross‐link = E′/2(1 + γ)RT
Table 2. Summary of TGA Thermogram Results
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ally, even blends of this benzoxazine with other polymers are
promising in terms of reinforcement of processability and
properties.
AUTHOR INFORMATION
Corresponding Author
*E-mail hxi3@cwru.edu (H.I.).
■
Notes
The authors declare no competing financial interest.
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