Unique self-catalyzed cationic ring-opening polymerization of a high performance deoxybenzoin-based 1,3-benzoxazine monomer

Article abstract of DOI:10.1016/j.polymer.2019.01.089

An asymmetric bis(1,3-benzoxazine) monomer based on deoxybenzoin has been synthesized through Mannich condensation. The chemical structure of this deoxybenzoin-based monomer has been confirmed using ¹H NMR, ¹³C NMR, FT-IR and element

Full text of DOI:10.1016/j.polymer.2019.01.089

Accepted Manuscript
Unique Self-Catalyzed Cationic Ring-Opening Polymerization of a High
Performance Deoxybenzoin-based 1,3-Benzoxazine Monomer
Kan Zhang, Xiaoxiao Tan, Yuting Wang, Hatsuo Ishida
PII:
S0032-3861(19)30119-3
10.1016/j.polymer.2019.01.089
JPOL 21238
DOI:
Reference:
To appear in:
Polymer
Received Date:
Accepted Date:
25 November 2018
30 January 2019
Please cite this article as: Kan Zhang, Xiaoxiao Tan, Yuting Wang, Hatsuo Ishida, Unique Self-
Catalyzed Cationic Ring-Opening Polymerization of a High Performance Deoxybenzoin-based 1,3-
Benzoxazine Monomer, Polymer (2019), doi: 10.1016/j.polymer.2019.01.089
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ACCEPTED MANUSCRIPT
A deoxybenzoin-based 1,3-benzoxazine resin with unique catalyzed cationic
polymerization mechanism was developed to afford high flame-retardant thermosets.

ACCEPTED MANUSCRIPT
Unique Self-Catalyzed Cationic Ring-Opening Polymerization of a High
Performance Deoxybenzoin-based 1,3-Benzoxazine Monomer
Kan Zhang,*a Xiaoxiao Tan,a Yuting Wang,a and Hatsuo Ishida*b
aSchool of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China. E-mail:
zhangkan@ujs.edu.cn
bDepartment of Macromolecular Science and Engineering, Case Western Reserve University,
Cleveland, OH, 44106, United States. E-mail: hxi3@cwru.edu
ABSTRACT: An asymmetric bis(1,3-benzoxazine) monomer based on deoxybenzoin
has been synthesized through Mannich condensation. The chemical structure of this
deoxybenzoin-based monomer has been confirmed using 1H NMR, 13C NMR, FT-IR and
elemental analysis. The assignments for the oxazine protons and carbon atoms in the
asymmetric structure have also been confirmed using two-dimensional (2D) NMR
1
techniques, including H-1H nuclear Overhauser effect spectroscopy (NOESY) and
1H-13C heteronuclear multiple quantum coherence spectroscopy (HMQC). In addition,
the effect of incorporation of deoxybenzoin on the mode of polymerization of
benzoxazine has been investigated using differential scanning calorimetry (DSC), in situ
1
FT-IR and H NMR spectroscopy. This deoxybenzoin containing bis(benzoxazine)
exhibits a low polymerization temperature resulting from a unique self-catalyzed cationic
polymerization mechanism. Furthermore, the thermal stability of the resulting
polybenzoxazine has been studied using thermogravimetric analysis (TGA). This
deoxybenzoin-based polybenzoxazine exhibits high thermal stability and low
o
flammability, with a Td5 temperature of 403 C, a high char yield value (68%) and low
total heat release (THR of 11.4 KJ/g).
Keyword: Benzoxazine; deoxybenzoin; Polybenzoxazine; low flammability.
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1. Introduction
Composites based on polymeric matrices are useful for a wide range of industrial
applications that require high performance but light-weight, such as in construction,
transportation and electronic materials. However, the inherent flammability of many
polymeric materials poses a societal threat, especially in enclosed or isolated spaces.
Halogenated molecules, such as brominated aromatics, are widely used as flame
retardants by inhibiting gas phase combustion [1]. Unfortunately, while such
halogen-containing organic structures are effective flame retardants, they have also been
revealed as unfriendly and harmful to human health [2-4]. Nonhalogenated flame
retardants, such as phosphorus-containing compounds and hydrated metal oxides, inhibit
flammability through various mechanisms [5-9]. In general, the trade-off between
polymer flammability and flame retardant toxicity has to be addressed, and halogen-free
polymers that combine the desirable properties of excellent processability and low
flammability are desired.
Benzoxazine resin, as a relatively new type of thermoset, has attracted strong attention
from both academia and industry in recent years [10-15]. Specifically, this class of
polymers offers significant advantages compared with other traditional thermosets, such
as high thermal resistance [16,17], good mechanical properties [18], low dielectric
constant [19-21], low surface free energy [22-24], and low flammability [25]. These
desirable properties originate from the excellent flexible molecular structure of
benzoxazines. By taking advantage of molecular design flexibility, various
bis(benzoxazine) monomers synthesized from bisphenols, primary monoamines and
formaldehyde have been widely reported since 1994 (Scheme 1) [26-28].
Scheme 1. Synthesis of bis(benzoxazine)s through a one-pot modified Mannich reaction
based on the flexible molecular design capability of benzoxazines.
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The introduction of deoxybenzoin into polymers has been reported to significantly
decrease the possibility to undergo combustion [29-32]. The first use of
4,4’-bishydroxydeoxybenzoin (BHDB) in polycondensation chemistry has been
described. Calorimetric methods has been used to demonstrate exceedingly low heat
release for BHDB-containing polymers undergoing combustion [33,34]. Since
BPA-based polymers are moderately flammable and used with flame-retardant additives,
BHDB could be used as a replacement for BPA. Surprisingly, deoxybenzoin-based
bis(benzoxazine) monomer has not been reported. The synthesis and properties of main
chain type bishydroxydeoxybenzoin-based benzoxazines has recently been reported
[35].However, the structure of most benzoxazine polymers is poorly defined. A detailed
structural characterization for these materials has not been reported. In addition, the mode
of polymerization for deoxybenzoin-based benzoxazines has not been detailed.
An asymmetric bis(benzoxazine) monomer based on deoxybenzoin from BHDB,
aniline and paraformaldehyde has been prepared. The temperature for ring-opening
polymerization of deoxybenzoin-based benzoxazine is significantly lower than that for
traditional bis(benzoxazine)s. Furthermore, the polybenzoxazine derived from this new
benzoxazine monomer shows higher thermal stability and lower flammability than many
traditional benzoxazine resins. Unexpectedly, the thermal properties of the BHDB based
polybenzoxazine is comparable to those of BHDB based main chain type benzoxazines.
This reveals an exception since it is well-known that thermosets derived from main chain
type benzoxazines shows higher thermal stability than ones from monomers. Detailed
synthetic methodology, a proposed polymerization mechanism and the kinetics of
deoxybenzoin-based benzoxazine monomer, and the properties of the resulting
polybenzoxazine are presented.
2. Experimental
2.1. Materials
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Desoxyanisoin, pyridine hydrochloride, and paraformaldehyde (99%), were used as
received from Sigma-Aldrich. Aniline was purchased from Aldrich and purified by
distillation. Hexanes, xylenes, ethyl acetate, and sodium sulfate were obtained from
Fisher scientific and used as received. Bisphenol-A and aniline derived benzoxazine
monomer,
6,6'-(propane-2,2-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)
(hereinafter abbreviated as BA-a), was synthesized according to a previously reported
procedure [11].
2.2. Characterization
1
One dimensional H NMR spectra were recorded using a Bruker AVANCE II 400 MHz
nuclear magnetic resonance (NMR) spectrometer at a proton frequency of 400 MHz and
solutions in CDCl3/DMSO-d6 containing tetramethylsilane as an internal reference. Two
1
1
dimensional H-1H nuclear Overhauser effect spectroscopy (NOESY) and H-13C
Heteronuclear multiple quantum coherence (HMQC) were also used. A Fourier transform
infrared (FT-IR) spectrophotometer (Bomem, model Michelson MB 110) was used to
acquire FT-IR spectra. The spectrometer was equipped with a deuterated triglycine
sulfate (DTGS) detector and dry air purge unit and was operated at a resolution of 4 cm-1
in the frequency range of 4000-400 cm-1. A TA Instruments differential scanning
calorimeter (DSC) model 2920 was used with a temperature ramp rate of 10°C/min and a
nitrogen flow rate of 60 mL/min for all tests of DSC study. All samples were sealed in
hermetic aluminum pans with lids. Thermomechanical analysis (TMA) was measured by
a NETZSCH TMA/402F4 at a heating rate of 5°C/min, and the coefficient of thermal
expansion was calculated in the range from 50 to 250°C. Dynamic mechanical analysis
(DMA) was conducted on a NETZSCH DMA/242E applying controlled strain tension
mode with amplitude of 10 µm and a temperature ramp rate of 3oC/min from room
temperature to 400oC. Thermogravimetric analyses (TGA) were performed on a TA
Instruments Q500 thermogravimetric analyzer that was purged with nitrogen at a flow
rate of 40 mL/min. A heating rate of 10°C/min was applied. Specific heat release rate
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(HRR, W/g), heat release capacity (HRC, J/g.K) and total heat release (THR, kJ/g) were
measured on a microscale combustion calorimeter (MCC). MCC thermograms were
measured from 100 to 750oC at a heating rate of 1oC/s in an 80 mL/min stream of
nitrogen. The anaerobic thermal degradation products in the nitrogen gas stream were
mixed with a 20 mL/min stream of oxygen prior to entering the combustion furnace
(900oC). Heat release is quantified by standard oxygen consumption [36], and HRR is
obtained by dividing dQ/dt at each time interval, by the initial sample mass, and HRC is
obtained by dividing the maximum value of HRR by the heating rate.
2.3. Methods
Synthesis of 4,4’-bishydroxydeoxybenzoin (abbreviated as BHDB).
4,4’-Bishydroxydeoxybenzoin (BHDB) was prepared by demethylation of desoxyanisoin
(1,2-bis(4-methoxyphenyl)ethanone), as previously described [30]. Desoxyanisoin (5.0 g,
19.5mmol) and pyridine hydrochloride (9.0 g, 78.1mmol) were added to a round-bottom
flask equipped with a condenser. The mixture was kept stirring for 5 h at 200oC, then
cooled to room temperature, and poured into water. The precipitate was filtered and
1
recrystallized from acetic acid to give a yellow crystal (yield ca. 81%). H NMR (400
MHz, DMSO-d6), ppm: δ = 4.10 (s, 2H, Ar-CH2-CO-Ar), 6.67-7.92 (8H, Ar-H), 9.27 (s,
OH-Ar-CH2), 10.38 (s, OH-Ar-CO). FT-IR spectra (KBr), cm-1: 3457 and 3344 (O-H
stretching), 1666 (carbonyl stretching), 759 (C=O bending).
Synthesis of 1,2-bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)ethanone
(abbreviated as BHDB-a).
Into a 100 mL round-bottom flask were added 35 mL of xylenes, aniline (1.09 g, 0.012
mol), BHDB (1.33 g, 0.006 mol), and paraformaldehyde (0.76 g, 0.025 mol). The mixture
was stirred at 120oC for 8 h. After cooling to room temperature, the reaction solution was
washed with deionized water for three times. Then column chromatography was used
afterward to purify benzoxazine monomer with mixed solvents of hexane and ethyl
acetate in 4:1 ratio. The purified products were further recrystallized from
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acetone/toluene mixtures (1:1). After recrystallization, needle like crystals were obtained
1
(yield ca. 72%). H NMR (400 MHz, CDCl3), ppm: δ = 4.12 (s, 2H, Ar-CH2-CO-Ar),
4.62-4.68 (d, 2H, Ar-CH2-N, oxazine), 5.36-5.44 (d, 2H, O-CH2-N, oxazine), 6.79-7.83
(16H, Ar). FT-IR spectra (KBr), cm-1: 1676 (carbonyl stretching), 1498 (stretching of
trisubstituted benzene ring), 1238 (C-O-C asymmetric stretching), 924 (benzoxazine
related band), 781 (C=O bending). Anal. Calcd. for C30H26N2O3: C, 77.90; H, 5.67; N,
6.06. Found: C, 77.84%; H, 5.70%; N, 6.02%.
Polymerization of benzoxazine monomers.
Solution of 30% solid content of BHDB-a and BA-a in DMF was prepared, respectively.
Then, each solution was cast on a dichlorodimethylsilane-pretreated glass plate. The
films were dried in an air-circulating oven at 100oC for 2 days to remove the solvent. At
last, the films of BHDB-a and BA-a as coated on the glass plates were polymerized
stepwise at 160, 180, 200, 220 and 240oC for 1 h each to obtain poly(BHDB-a) and
poly(BA-a), respectively.
3. Results and discussion
3.1. Synthesis of Deoxybenzoin-Based Benzoxazine Monomer.
The new deoxybenzoin-based benzoxazine monomer (BHDB-a) was synthesized
successfully by the traditional Mannich condensation reaction between
4,4’-bishydroxydeoxybenzoin (BHDB), aniline and paraformaldehyde using xylenes at
120oC as shown in Scheme 2. Similar to the most reported bis(benzoxazine)s, a further
purification, such as column chromatography and/or recrystallization, has to be applied
for obtaining pure compound due to the existence of phenolic byproducts in the synthesis
of bis(benzoxazine) monomers.
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Scheme 2. Synthesis of deoxybenzoin-based benzoxazine monomer.
The structure of BHDB-a was confirmed by 1H NMR, 13C NMR, and FT-IR analyses.
1
In general, the characteristic benzoxazine H NMR resonance pair for O-CH2-N and the
Ar-CH2-N consists of singlets with frequency separation of around 0.8-0.9 ppm [10,11].
However, as seen in Fig. 1, the characteristic proton resonance due to the O-CH2-N and
the Ar-CH2-N groups in the oxazine ring of BHDB-a exhibits two sets of doublets at 5.36
and 5.44 ppm and 4.62 and 4.68 ppm, respectively. Furthermore, two sets of doublets at
79.42 and 80.22 ppm and 50.43 and 50.54 ppm for the characteristic carbon resonances
of the O-CH2-N and the Ar-CH2-N groups can also be observed as shown in Fig. 2. One
1
goal of the current study is to assign H and 13C NMR resonances of BHDB-a as this
asymmetric monomer is much more complex compared with symmetric benzoxazine
monomers. Before attempting the assignments of the two sets of doublets from oxazine
ring, it must be noticed that there is no isomerism involved in BHDB-a due to the
characteristic resonance of the methylene in -OC-CH2- bridge exhibiting a singlet at 4.12
ppm as shown in Fig. 1. In addition, Fig. 2 also shows a singlet at 44.23 ppm for the
corresponding carbon resonance of -OC-CH2- group.
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Fig. 1. 1H NMR spectrum of BHDB-a.
Fig. 2. 13C NMR spectrum of BHDB-a.
Complementary spectroscopic studies were then performed. Fig. 3a shows the 2D
1H-1H nuclear Overhauser effect spectroscopy (NOESY) of BHDB-a. There are many
possible manners and different order to follow the NOE interactions to analyze the
spectrum. Nevertheless, the most important information is obtained from the NOE
interaction observed by HD. HD is a characteristic aromatic proton resonance with singlet
peak as assigned in Fig. 3a. Clearly, only one interaction with a proton HA’ is detected for
HD. Since HA’ is much closer through space to HD than HA, it strongly suggests that the
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HA’ should be in the oxazine ring attached to the HD containing benzene ring. Therefore,
the proton signals could now be assigned unambiguously as shown in the inset structure
in Fig. 3a. In addition, the assignments for the carbons of the oxazine ring and methylene
bridge in the structure of BHDB-a are supported by the 1H-13C HMQC spectrum as seen
in Fig. 3b.
Fig. 3. (a) 2D 1H-1H NOESY NMR spectrum of BHDB-a and NOE interactions
observed. (b) 2D 1H-13C HMQC NMR spectrum of BHDB-a.
FT-IR spectroscopy was further used to monitor the effectiveness of the reaction to
form benzoxazine. There are a number of infrared absorption bands, highlighted in Fig. 4,
that are used to verify the existence of deoxybenzoin and oxazine ring formation in
BHDB-a. The observed strong band at 1676 cm-1, attributed to the C=O stretching, shows
that the deoxybenzoin nuclei is present in the molecule. Besides, the band for
tri-substituted benzene that appears at 1495 cm-1 indicates the combination of
deoxybenzoin with benzoxazine structures. Moreover, the presence of the benzoxazine
ring aromatic ether in the monomer is indicated by the band centered at 1238 cm-1, which
is due to the C-O-C antisymmetric stretching modes [37]. The characteristic oxazine
related mode, which includes the vibration of O-C2 vibration with a minor contribution
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from the phenolic ring is located at 924 cm-1 [38]. All the above information is consistent
with the successful synthesis of the target bis(benzoxazine) compound.
Fig. 4. FT-IR spectrum of BHDB-a.
3.2. Thermal Behavior and Ring-Opening Polymerization Mechanism of BHDB-a.
The polymerization behavior of BHDB-a was studied by DSC as depicted in Fig. 5. The
thermogram of BHDB-a exhibits two obvious thermal events. The first one is a very
sharp endothermic process with its minimum at 134oC, which is associated with the
melting of the bis(benzoxazine) resin’s needle-like crystal. This sharp endothermic peak
further indicates the high purity of BHDB-a. Additionally, an exotherm, attributed to the
ring-opening polymerization, is observed with its maximum at 217°C and an onset
temperature at 198°C. In order to better understand the polymerization behavior of
BHDB-a, the DSC thermogram of another bis(benzoxazine) monomer, BA-a, has also
been performed as shown in Fig. 5. Similarly, BA-a exhibits the melting endotherm with
its minimum at 111oC and the polymerization exotherm with its maximum at 258oC. The
pure BA-a crystalizes and the onset of exotherm becomes very high. Obviously, BHDB-a
shows a much lower polymerization temperature compared with BA-a, although the
structural difference is simply in the middle bridge structure of both compounds. These
DSC result implies the existence of the self-catalytic polymerization possibly caused by
the interaction of deoxybenzoin and oxazine ring in BHDB-a.
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Fig. 5. DSC thermogram of BHDB-a.
Fig. 6 shows a comparison of TGA and DSC thermograms of BHDB-a. As shown in
Fig. 6, the initial weight loss of BHDB-a occurs around 180oC, which reflects that almost
no evaporation can take place before its polymerization. A further ~5.3% of weight loss
can be observed during the polymerization at the range from the onset temperature to
300oC. This weight-loss stage has been determined to be the cleavage of the zwitterionic
intermediate, forming unstable phenolic species and N-methyleneaniline during the
ring-opening polymerization of benzoxazines [16,39]. The degradation in this
temperature range mostly occurs for the monomer-based polybenzoxazines rather than
from main-chain type polybenzoxazines, due to the degradation from the chain ends
and/or side groups, rather than from the middle of the main chain [40]. Additionally, the
weight-loss during the polymerization of BHDB-a is much lower compared with other
reported bis(benzoxazine)s [16,41]. The unique polymerization behavior resulting from
the interaction of oxazine ring and deoxybenzoin seems to significantly improve the
thermal stability of BHDB-a during the polymerization process.
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Fig. 6. TGA vs DSC plots of of BHDB-a (heating rate: 10 oC/min, under a N2
atmosphere).
To gain deeper understanding, the qualitative study of the polymerization behavior for
BHDB-a was performed by in situ FT-IR analyses as shown in Fig. 7. The characteristic
absorption bands around 1238 cm-1 (C-O-C antisymmetric stretching mode) and 924 cm-1
(benzoxazine related mode) can be used to study the ring-opening polymerization of the
oxazine ring in BHDB-a by observing the gradual decrease of those bands as a function
of the temperature and finally complete disappearance at 240oC. This result suggests that
BHDB-a completes polymerization by heating stepwise at 160, 180, 200, 220 and 240 oC
for 1 h each. In addition, the characteristic band for carbonyl absorption at 1676 cm-1
gradually decreases as the ring-opening polymerization of oxazine ring proceeds, and
meanwhile the multiple broadband of -OH around 3400 cm-1, gradually appears. The very
broad background ranging typically from 3400 to 2200 cm-1 is due to the so-called
chelated phenolic OH groups that are forming intramolecular 6-membered hydrogen
bonding with the nitrogen atom of the main chain [10,11]. However, the polybenzoxazine
derived from deoxybenzoin-based monomer shows additional rather strong and broad
band in the region between 3600 and 3200 cm-1, implying different source of hydroxyl
groups as proposed in a later section that discusses the polymerization mechanism. This
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variation from in situ FT-IR spectra strongly suggests the structural evolution of
deoxybenzoin in BHDB-a should be a key factor for lowering the polymerization
temperature of benzoxazine.
Fig. 7. FT-IR spectra of BHDB-a after various thermal treatments at the designated
temperature for 1 h.
In order to further investigate the structural evolution of BHDB-a during the
polymerization, the 1H NMR analyses in deuterated DMSO (DMSO-d6) were also carried
1
out. Fig. 8 shows the H NMR spectra of the thermally treated BHDB-a at different
1
temperatures, where H NMR spectrum of BHDB-a after polymerization at 180oC
presents complicated signals due to the different types of -CH2- units in polybenzoxazine
chains. Doublet resonances apart from ones around 5.4 and 4.6 ppm attributed to the
benzoxazine structure may be ascribed to the residual oxazine rings at the chain ends of
some oligomers or polybenzoxazines with low molecular weight. In addition, a broad
signal with multiple resonances between 4.5 and 3.7 ppm is probably due to the presence
of -CH2- in deoxybenzoin and phenolic -CH2- groups, which are associated with the
Mannich bridge after ring-opening polymerization. Moreover, a new signal at 5.0 ppm
can also be observed at the 1H NMR spectrum of BHDB-a after polymerization at 180oC,
which seems to be the presence of new –OH group.
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Fig. 8. 1H NMR spectra of BHDB-a in DMSO-d6 after various thermal treatments.
The above DSC, in situ FTIR and 1H NMR analyses show that the deoxybenzoin group
exhibits additional autocatalytic ring-opening polymerization of benzoxazine beyond the
usual autocatalytic activities caused by the formed phenolic OH groups upon oxazine ring
opening. The remarkable difference in reactivity of BHDB-a over BA-a suggests a
mechanism where the deoxybenzoin has an important role to promote the ring-opening
polymerization. The proposed polymerization mechanisms for BA-a and BHDB-a are
shown in Scheme 3. As the purity of both BA-a and BHDB-a synthesized in this work is
high, both bis(benzoxazine)s do undergo polymerization induced only by heat without
any impurity participating in or during the reaction, which is consistent with the reported
study on intrinsic ring opening polymerization of oxazine ring [42]. However, BHDB-a
shows a polymerization exotherm maximum at 217oC, which is rather unusual for
ordinary benzoxazine monomers with such high purity without added initiators. We,
therefore, hypothesize a self-catalyzed polymerization mechanism. The deoxybenzoin
structure in BHDB-a can be activated to form enolic structure by the electron transfer
reaction as shown in Scheme 3b. Afterward, the ionization of enolic structure leads to the
polymerization of BHDB-a into a traditional cationic ring-opening polymerization
mechanism. This built-in intramolecular catalytic feature can be considered a smart
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feature of this monomer.
Scheme 3. Proposed polymerization mechanism of benzoxazine monomers.
Thermal behavior of products obtained by the polymerization of BHDB-a was also
studied by DSC as shown in Fig. 9 after cumulative thermal treatments at 160, 180, 200,
220 and 240oC for 1 h at each temperature. Interestingly, BHDB-a can be made into a
more processing-friendly state simply by partially polymerized at 160oC for 1 h. After
this simple thermal treatment, the endothermic peak of BHDB-a can be eliminated and
the exothermic maximum appears at a lower temperature (206°C). Besides, it is observed
that the exothermic peak temperature gradually decreases after each thermal treatment.
After the treatment at 240oC, completion of the ring-opening polymerization has been
observed. Moreover, a glass transition temperature (Tg) as high as 298°C was observed
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after the end of the heat cycle at 240oC of BHDB-a, which indicates the excellent thermal
property of the final thermoset.
Fig. 9. DSC thermograms of BHDB-a after various thermal treatments.
3.3. Activation Energy of Polymerization of BHDB-a.
The activation energy of polymerization of BHDB-a was also investigated with
nonisothermal DSC at different heating rates of 2, 5, 10, 15 and 20 °C/min. Fig. 10 shows
the DSC thermograms of BHDB-a at different heating rates, respectively.
Fig. 10. DSC curves of BHDB-a at various heating rates.
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The apparent activation energy of polymerization process was determined using the
well-known Kissinger and Ozawa methods [43,44]. Based on the Kissinger method, the
activation energy can be calculated by Eq. (1) as follows:
Ea

AR
Ea
ln( )  ln(
) 
(1)
Tp2
RTP
where the heating rate is represented by β, the frequency factor is represented by A.
Besides, Tp is the temperature of the exothermic peak, Ea is the activation energy, and R
is the gas constant. Another theoretical treatment, namely, the modified Ozawa method,
can also be applied to the thermal data using Eq. (2) as follows:
Ea
ln   1.052
 C
(2)
RTP
where C is a constant.
2
Alternatively, if the plot of ln(β/Tp ) and ln(β) against 1/Tp are linear, a single process
can be assumed, and the corresponding Ea can be obtained from the slope of the straight
2
lines. Fig. 11 shows the plot of ln(β/Tp ) or ln(β) against 1/Tp for BHDB-a according to
the Kissinger and Ozawa methods. The Ea value was estimated to be as low as 91.2
(Kissinger) and 93.1 (Ozawa) kJ/mol. Generally, the activation energy reported for
difunctional benzoxazines showed higher values compared with monofunctional
benzoxazines [45]. However, the Ea value of BHDB-a is even lower than those reported
values for monobenzoxazines [46]. This result suggests that BHDB-a is easy to be
activated to polymerize possibly due to the existence of self-catalyzed polymerization
system.
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Fig. 11. Plots generated by the Kissinger and Ozawa methods for determination of the
activation energy of BHDB-a.
3.4. Thermal Properties of polybenzoxazines.
The thermomechanical properties of poly(BHDB-a) were studied by TMA. Fig. 12
shows the TMA curve of poly(BHDB-a) thermosetting film. The coefficient of thermal
expansion (CTE) is obtained from TMA as 48.4 ppm/oC which compares favorably with
the reported thermal expansion coefficient of bisphenol A epoxy resin of 70 ppm/oC [47].
Besides, a Tg of 301oC is observed, which is in accordance with the result from DSC.
Notably, poly(BHDB-a) shows a much higher Tg temperature compared with poly(BA-a),
which was reported as around 152oC [27]. Moreover, dynamic mechanical analysis
(DMA) was utilized to further confirm the Tg value of poly(BHDB-a). As shown in Fig.
o
13, the Tg determined by the peak temperature of tan δ is as high as 311 C, which is in
relatively good agreement with that measured by DSC and TMA with the expected
discrepancy amongst the three different techniques.
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Fig. 12. Thermomechanical analysis of thermoset film of poly(BHDB-a).
Fig. 13. Dynamic mechanical spectra of poly(BHDB-a).
The thermal stability of polybenzoxazines has been studied by TGA under nitrogen
atmosphere and the result is shown in Fig. 14. The decomposition temperatures of 5 %
and 10% weight losses (Td5 and Td10) for poly(BHDB-a) and poly(BA-a) are 403 and
440oC and 302 and 324oC, respectively. As anticipated, the incorporation of
deoxybenzoin into the benzoxazine backbone structure significantly improved thermal
stability as compared to typical BA-a resin. Besides, poly(BHDB-a) exhibits a high char
yield (Yc) value of 68 % at 800oC. Particularly noteworthy is the nearly complete lacking
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of the low temperature degradation events, as seen around 300 and 400oC for poly(BA-a).
The high thermal stability of poly(BHDB-a) is thought to be attributed to the formation of
diphenylacetylene as other BHDB related polymers [35]. Much more detailed study is
required to verify this hypothesis, though it is not the main goal of the current paper. The
thermal property data of poly(BHDB-a) and poly(BA-a) are summarized in Table 1,
which demonstrates a high thermal stability of poly(BHDB-a).
Fig. 14. Thermogravimetric analysis of poly(BHDB-a) and poly(BA-a).
Table 1 Thermal and heat release properties of poly(BHDB-a) and poly(BA-a).
Td5
(°C)
403
302
Td10
(°C)
440
324
Yc
(wt.%)
68
HRC
(Jg-1K-1)
96
THR
(KJg-1)
11.4
Sample
poly(BHDB-a)
poly(BA-a)
32
328
35.3
Microscale combustion calorimetry (MCC) was also applied to quantitatively evaluate
the flammability of polybenzoxazines. Typically, the specific heat release rate (HRR) is
obtained by dividing dQ/Dt at each time interval, by the initial sample mass. In addition,
the heat release capacity (HRC) can be obtained by dividing the maximum value of HRR
by the heating rate. In General, HRC is regarded as an effective evaluation of thermal
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combustion that is one of the best single predictors of the flammability of a material [48].
Herein, milligram size samples of poly(BHDB-a) and poly(BA-a) were tested at a
constant heating rate of 1 K/s over the temperature range 100-750C. As shown in Fig. 15,
MCC characterization of poly(BHDB-a) and poly(BA-a) reveals HRC values of 96 and
328 Jg-1K-1, respectively. Besides, poly(BHDB-a) shows a low THR value of 11.4 KJg-1
whereas poly(BA-a) exhibits that of 35.3 KJg-1. In general, the lower the values of HRC
and THR, the higher the flame resistance. The HRC less than 300 J/g.K is considered
self-extinguishing whereas materials with values less than 100 J/g.K are considered
non-ignitable [49]. Therefore, introduction of deoxybenzoin into polybenzoxazine
network leads to a substantial HRC reduction and improved flame retardancy, which
makes this material attractive for application in high thermal requirement without the
need of flame-retardant additives.
Fig. 15. Microscale combustion calorimetric curves of poly(BHDB-a) and poly(BA-a).
4. Conclusion
A deoxybenzoin based asymmetric bis(benzoxazine) monomer has been successfully
synthesized, and the chemical structures of this monomer have been analyzed in detail by
1D and 2D NMR techniques. The benzoxazine resin was shown to undergo a unique
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self-catalyzed cationic ring-opening polymerization without adding any initiators or
catalysts and formed cross-linked polymeric networks. This self-catalyzed reaction takes
place beyond the usual autocatalytic reaction caused by the phenolic OH formed as a
result of oxazine polymerization. The activation energy for the polymerization was
measured to be as low as 91.2 (Kissinger) and 93.1 (Ozawa) kJ/mol. Moreover, the
polybenzoxazine derived from bis(benzoxazine) monomer exhibited high thermal
o
stability with the glass transition temperature of 301 C and a char yield of 68%.
Furthermore, the polybenzoxazine also showed low heat release capacity (HRC of 96
Jg-1K-1) as well as the low total heat release (THR of 11.4 KJg-1). The thermal properties
of the deoxybenzoin-based bis(benzoxazine) resin is comparable to deoxybenzoin based
main-chain type benzoxazine polymers. Therefore, the combination of above excellent
properties makes this new deoxybenzoin based benzoxazine resin as an attractive
candidate for high-performance applications.
Acknowledgements
Financial support for this work was provided by the National Natural Science Foundation
of China (No. 51603093), the Science and Technology Agency of Jiangsu Province (No.
BK 20160515) and China Postdoctoral Science Foundation (No. 2016M600369 and No.
2018T110451).
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Highlights
1. A novel asymmetric bis-benzoxazine monomer based on deoxybenzoin has been
successfully synthesized.
2. This deoxybenzoin-based benzoxazine shows a unique self-catalyzed cationic
ring-opening polymerization mechanism.
3. Deoxybenzoin-based benzoxazine possesses much higher thermal stability
compared with BA-a resin.
4. The polybenzoxazine derived from deoxybenzoin-based benzoxazine also exhibits
low flammability.