Synthesis of a smart bisbenzoxazine with combined advantages of bismaleimide and benzoxazine resins and its unexpected formation of very high performance cross-linked polybenzoxazole

Article abstract of DOI:10.1016/j.polymer.2021.123703

We report a bisbenzoxazine thermosetting system, oHPMI-ddm, which shows combined advantages of both bismaleimide and benzoxazine resins. The chemical structure of oHPMI-ddm has been identified by FT-IR and NMR spectroscopies, and its polymerization processes have been investigated using differential scanning calorimetry (DSC) and in situ FT-IR. The resulting polybenzoxazine derived from this thermosetting system exhibits significantly higher thermal stability than thermosets polymerized from the commercialized 4,4′-diaminodiphenylmethane based bisbenzoxazine (PH-ddm) and bismaleimide (DDM-BMI). This newly obtained benzoxazine also shows a unique structural thermal rearrangement in comparison to the cross-linked polybenzoxazole in a similar manner as other reported smart ortho-benzoxazines, although it has no obvious hydrogen bond forming group and cannot form 5- or 6-membered intramolecular interaction. Such conversion from polybenzoxazine to polybenzoxazole has been confirmed by both in situ FT-IR and solid state ¹³C NMR. Furthermore, the finally obtained cross-linked polybenzoxazole possesses outstanding properties of high thermal stability, excellent flame retardancy and low dielectric constant, indicating great potential for high performance material applications.

Full text of DOI:10.1016/j.polymer.2021.123703

Polymer 223 (2021) 123703
Contents lists available at ScienceDirect
Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Synthesis of a smart bisbenzoxazine with combined advantages of
bismaleimide and benzoxazine resins and its unexpected formation of very
high performance cross-linked polybenzoxazole
a,
*
a
b,**
Kan Zhang , Boran Hao , Hatsuo Ishida
a
School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, China
b
Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH, 44106, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
We report a bisbenzoxazine thermosetting system, oHPMI-ddm, which shows combined advantages of both
bismaleimide and benzoxazine resins. The chemical structure of oHPMI-ddm has been identified by FT-IR and
NMR spectroscopies, and its polymerization processes have been investigated using differential scanning calo-
rimetry (DSC) and in situ FT-IR. The resulting polybenzoxazine derived from this thermosetting system exhibits
Benzoxazine
Bismaleimide
Polybenzoxazole
High-performance
′
significantly higher thermal stability than thermosets polymerized from the commercialized 4,4 -dia-
minodiphenylmethane based bisbenzoxazine (PH-ddm) and bismaleimide (DDM-BMI). This newly obtained
benzoxazine also shows a unique structural thermal rearrangement in comparison to the cross-linked poly-
benzoxazole in a similar manner as other reported smart ortho-benzoxazines, although it has no obvious
hydrogen bond forming group and cannot form 5- or 6-membered intramolecular interaction. Such conversion
1
3
from polybenzoxazine to polybenzoxazole has been confirmed by both in situ FT-IR and solid state C NMR.
Furthermore, the finally obtained cross-linked polybenzoxazole possesses outstanding properties of high thermal
stability, excellent flame retardancy and low dielectric constant, indicating great potential for high performance
material applications.
1
. Introduction
Bismaleimide (BMI) resins are a family of high-performance poly-
copolymerizing them with allyl [9], cyanate [10], amine [11] and
benzoxazine-related compounds [12,13].
Polybenzoxazine thermosets have attracted increasing attentions in
recent years because they exhibit many outstanding properties [14–18],
such as near-zero shrinkage upon polymerization [19], outstanding
thermal [20,21] and mechanical properties [22–25], low surface energy
[24,25] and excellent electric properties [26]. The most unique char-
acter of polybenzoxazine thermosets is the high molecular design flex-
ibility of their precursor, namely benzoxazines, which enables synthesis
of various cross-linked networks to tailor the target performance [14,
15]. Specifically, addition of smart functionalities from other
high-performance polymers in benzoxazine resins provides a useful
means to further improve already well-balanced, unique properties of
polybenzoxazine thermosets.
mers that are generally used as matrix materials for industrial applica-
tions [1] [[,2] [][][]. BMI polymers [3,4] have many attractive
properties, such as resistance to high-temperature, radiation, and
chemical degradation as well as good mechanical performance [1].
However, traditional unmodified BMI resins possess some shortcomings,
including poor solubility in common organic solvents and high melting
temperature due to its rigid and symmetrical structure. These undesir-
able properties cause difficulties in the processing of resins. In addition,
the polymerized BMI resins exhibit high brittleness because of their
highly cross-linked networks with rigid molecular backbones [5,6]. The
above shortcomings in BMI resins have evidently restricted exploration
of their further applications. In order to improve the ductility of BMI
resins, researchers have developed various approaches, including
blending them with elastomers [7] and thermoplastics [8], or
Cross-linked polybenzoxazole (cPBO) thermosets with excellent
mechanical and physical properties have been developed via ortho-
amide or ortho-imide functionalized benzoxazine resins (Scheme 1)
*
Corresponding author.
** Corresponding author.
E-mail addresses: zhangkan@ujs.edu.cn (K. Zhang), hxi3@cwru.edu (H. Ishida).
https://doi.org/10.1016/j.polymer.2021.123703
Received 18 January 2021; Received in revised form 20 March 2021; Accepted 23 March 2021
Available online 28 March 2021
0
032-3861/© 2021 Elsevier Ltd. All rights reserved.

K. Zhang et al.
Polymer 223 (2021) 123703
[
27–30]. In general, the conventional method to obtain poly-
benzoxazoles is through the use of strong acids, which can dissolve both
reaction intermediates and products [31,32]. However, using strong
acids as reaction solvent is environmentally unfriendly, and such
manufacturing approaches are unacceptable in most industries. In
addition, difficulty of eliminating the residual solvent can lead to an
aging problem at elevated temperature and humid environment via
hydrolysis of benzoxazole ring. Thus, approaches for achieving
cross-linked polybenzoxazoles (cPBOs) based on ortho-benzoxazines
show many alternative advantages, such as excellent molecular design
flexibility, requiring no strong acids, easy processability and cost effi-
ciency. In our recent work, we synthesized a series of ortho-maleimide
mono-benzoxazines, which expanded the family of known smart
ortho-benzoxazine [33]. Unfortunately, the thermal conversion of
ortho-hydroxylmaleimide into benzoxazoles was not evaluated in that
study.
Due to the success of the above findings, we were encouraged to
design a new thermosetting system, oHPMI-ddm, which presents struc-
tural features of both bisbenzoxazine and bismaleimide (Scheme 2).
Previously, we developed a series of smart benzoxazines as cPBO pre-
cursors by designing monomers with an intramolecular 5- or 6-
membered hydrogen bond group that is in the vicinity of the oxazine
ring [27–30]. However, in this paper we report the first cross-linked
polybenzoxazole based on ortho-maleimide benzoxazine. This newly
obtained benzoxazine monomer has no obvious hydrogen bond forming
group and cannot form 5- or 6-membered intramolecular interaction.
This observation was totally unexpected. In addition, the conversion
through neighboring hydroxyl and maleimide has never been evaluated.
The detailed synthetic strategy, the thermally activated structural
transformation, and the properties of the resulting thermosets will be
discussed in the current work.
Scheme 2. Chemical Structures of PH-ddm, DDM-BMI and oHPMI-ddm.
NMR spectrometer (Bruker AVANCE II). The average number of tran-
sients, 64 was used for proton NMR measurement, and 1024 was applied
for carbon NMR testing. Fourier-transform infrared (FT-IR) spectra were
recorded on a Nicolet Nexus 670 spectrometer, at a spectral resolution of
2
. Experimental
ꢀ ꢀ 1
1
4
cm (from 4000 to 400 cm ) using 64 co-added scans per spectrum.
2
.1. Materials
The spectrometer was equipped with a dry-air purging unit and
deuterated triglycine detector. The benzoxazine sample was ground and
pressed into a KBr pellet which was examined by the transmission
method. Elemental analysis for the newly synthesized benzoxazines was
carried out on an Elementar Vario EL-III analyzer. Thermograms of
differential scanning calorimeter (DSC) were recorded on a NETZSCH
DSC (Model 204f1) at different heating rates with a nitrogen flow rate of
p-Toluenesulfonic acid (p-TSA) (98%), maleic anhydride (98%), 2-
′
aminophenol (>98%) and 4,4 -bismaleimidodiphenylmethane (DDM-
BMI) were purchased from Aladdin Reagent, China, and used without
′
further purification. 4,4 -Diaminodiphenylmethane (98%), para-
formaldehyde (99%), aniline, toluene, isopropanol, dimethylformamide
(
DMF), sodium hydroxide (NaOH) and acetone were purchased from
Energy Reagent, China. 1-(2-Hydroxyphenyl)-1H-pyrrole-2,5-dione
oHPMI) was synthesized according to the procedures reported by our
group [33].
6
0 mL/min. Dynamic mechanical analysis (DMA) was performed by a
DMA analyzer (NETZSCH DMA/242E). A tension mode with an ampli-
(
tude of 10
μ
m at a frequency of 1Hz was used during the DMA testing,
◦
and the heating rate for DMA measurement was set as 3 C/min. Ther-
mogravimetric analyses (TGA) were performed on a NETZSCH STA449-
2
.2. Characterization
All NMR spectra of benzoxazine samples, including H and C NMR,
◦
C thermal analyzer at a heating rate of 10 C/min. The dielectric con-
stants (k) and dielectric loss (tan δ) of thermoset films were measured by
the capacitance method at a frequency range from 100 Hz to 1 MHz
1
13
and 2 dimensional (2-D) H–13C HMQC were recorded on a 400 MHz
1
Scheme 1. Preparation of Cross-Linked Polybenzoxazoles via Thermal Conversion from Ortho-Amide (a) and Ortho-Imide (b) Functional Benzoxazines.
2

K. Zhang et al.
Polymer 223 (2021) 123703
using a TZDM-RT-300 Dielectric Testing System Analyzer at room
temperature. Silver paste was coated onto both surfaces of thermoset
films before testing. The heat release rate (HRR, W/g) was recorded on a
ddm), respectively. Poly(oHPMI-ddm) was further treated at 300, 350
◦
and 400 C for 1 h at each step to obtain cPBO-oHPMI-ddm.
microscale combustion calorimeter (MCC, Fire Testing Technology) at a
3. Results and discussion
◦
heating rate of 1 K/s from 100 to 750 C. A stream of N
2
(flow rate of 80
mL/min) was mixed with a stream of O
2
◦
(flow rate of 20 mL/min) before
3.1. Synthesis and characterization of benzoxazine monomers
entering the combustion furnace (900 C).
In the current work, the pathway for preparing cPBO started from
ortho-maleimide functionalized phenol (oHPMI) for synthesis of ben-
zoxazine, then polymerizing and further structurally transforming to
obtain cPBO-oHPMI-ddm. The bisbenzoxazine monomer with ortho-
meleimide functionality, (oHPMI-ddm), was successfully synthesized
2
.3. Methods
Synthesis of bis(4-(2H-benzo[e] [1,3]oxazin-3(4H)-yl)phenyl)
′
methane (abbreviated as PH-ddm). 4,4 -Diaminodiphenylmethane
′
(
1.98 g, 0.01mol), phenol (1.88 g, 0.02 mol), paraformaldehyde (1.32 g,
.044 mol) and 30 mL of toluene were added into a 100 mL single-neck
flask. The mixture was magnetically stirred and then heated to reflux for
2 h. Afterwards, the crude product was washed with 1 N NaOH aqueous
using a primary amine, (4,4 -diaminodiphenylmethane), and para-
0
formaldehyde as shown in Scheme 3. In addition, a counterpart ben-
zoxazine without the maleimide group, PH-ddm, was prepared and
highly purified.
1
1
solution and distilled water 3 times on each step, and the product was
then concentrated by a rotary evaporator. Further purification of the
washed product was achieved by recrystallization in a 1:1 mixture of
acetone and toluene to generate a white crystalline needle-like product.
The structures of benzoxazine monomers were identified by H NMR,
1
3
1
C NMR, and FT-IR analyses. Fig. 1 shows the H NMR spectra of PH-
ddm and oHPMI-ddm. The characteristic resonances assigned to the
benzoxazine moiety, Ar-CH -N- and -O-CH -N- for PH-ddm are found at
2
2
◦
Next, the product was dried under vacuum at 50 C for 48 h (yield ca.
4.61 and 5.35 ppm, respectively. These signals for oHPMI-ddm are
◦
1
7
5%). mp: 125 C. H NMR (400 MHz, CDCl
3
), ppm: δ = 7.15–6.80
ꢀ NR, oxazine), 4.61 (s, 4H,
ꢀ NR, oxazine), 3.83 (s, 2H, Arꢀ CH ꢀ Ar). FT-IR spectra (KBr),
located at 4.62 and 5.31 ppm, respectively. Besides the typical reso-
(
16H, Ar), 5.35 (s, 4H, Ar-O-CH
2
2
nances attributed to Ar-CH -Ar in diaminodiphenylmethane are located
1
Arꢀ CH
2
2
at 3.83 and 3.82 ppm for PH-ddm and oHPMI-ddm, respectively. The H
NMR spectrum of oHPMI-ddm also confirms the maleimide functionality
ꢀ
1
cm : 1226 (C–O–C asymmetric stretching), 946 (oxazine ring related
–
mode). Anal. calcd for C29
H
26
2
N O
2
: C, 80.16%; H, 6.03%; N, 6.45%.
as observed by the presence of the protons of -CH CH- at 6.86 ppm.
–
Found: C, 80.07%; H, 6.05%; N, 6.42%.
Synthesis of 1,1’-(3,3’-(4,4′-methylenebis(4,1-phenylene))bis
(
3,4-dihydro-2H-benzo[e] [1,3]oxazine-8,3-diyl))bis(1H-pyrrole-
′
2
,5-dione) (abbreviated as oHPMI-ddm). 4,4 -Diaminodiphenyl-
methane (1.98 g, 0.01 mol), oHMPI (3.78 g, 0.02 mol), para-
formaldehyde (1.32 g, 0.044 mol) and toluene (80 mL) were added into
a 250 mL single-neck flask. The reaction mixture was stirred and heated
to reflux for 24 h. Afterwards the solution was washed with distilled
water for 3 times, and then the product sample was concentrated by a
rotary evaporator. The washed product was further purified by recrys-
tallization in a 1:1 of acetone-toluene to produce a light yellow crys-
◦
talline product, which was then dried in a vacuum oven at 60 C for 48 h
◦
1
(
yield ca. 89%). mp: 105 C. H NMR (400 MHz, CDCl
3
), ppm: δ =
CH-, maleimide), 5.31 (s, 4H, Ar-
ꢀ NR, oxazine), 4.62 (s, 4H, Arꢀ CH ꢀ NR, oxazine), 3.82 (s, 2H,
–
7
.08–6.93 (14H, Ar), 6.86 (s, 4H, -CH
–
O-CH
Arꢀ CH
stretching) 1709 (C
2
2
1
ꢀ
–
–
2
ꢀ Ar). FT-IR spectra (KBr), cm : 1776 (C
O asymmetrical
–
–
O symmetrical stretching), 1229 (C–O–C asym-
metric stretching), 920 (oxazine ring related mode), 836 (C–H wagging),
6
92 (out-of-plane of = C–H). Anal. calcd for C37
28 4 6
H N O : C, 71.14%; H,
4
.52%; N, 8.97%. Found: C, 71.06%; H, 4.55%; N, 8.91%.
Preparation of Thermosets Based on PH-ddm, DDM-BMI and
oHPMI-ddm. Polymerization of PH-ddm, DDM-BMI and oHPMI-ddm
◦
was performed by heating at 140, 160, 180, 200, 220 and 240 C for 1 h
1
Fig. 1. H NMR spectra of PH-ddm and oHPMI-ddm in CDCl
3
.
at each step, to obtain poly(PH-ddm), poly(DDM-BMI) and poly(oHPMI-
Scheme 3. Synthesis of PH-ddm (a) and oHPMI-ddm (b).
3

K. Zhang et al.
Polymer 223 (2021) 123703
Fig. 2 shows the 13C NMR spectra of benzoxazines, PH-ddm and
oHPMI-ddm, in CDCl . The characteristic carbon resonances of the
oxazine ring structure for Ar-CH -N- and -O-CH -N- in PH-ddm and
3
2
2
oHPMI-ddm appear at 50.56 and 79.76 ppm, and 50.97 and 80.16 ppm,
respectively. The characteristic carbon resonances of the methylene
group in PH-ddm and oHPMI-ddm are observed at 40.27 and 40.30,
respectively. In addition, the typical resonance at 134.46 ppm is
assigned to the double bond carbon in maleimide functionality in
oHPMI-ddm.
The structures of PH-ddm and oHPMI-ddm were also characterized
by FT-IR. As shown in Fig. 3, some bands are highlighted, which are used
to determine the presence of characteristic groups in PH-ddm and
oHPMI-ddm. For instance, the symmetric and antisymmetric stretching
modes of carbonyl in maleimide group in oHPMI-ddm are observed at
ꢀ
1
ꢀ
1
1
709 and 1776 cm , respectively [34]. The typical band at 836 cm is
–
CH- in maleimide group, and
the band at 692 cm is attributed to the = C–H out-of-plane bending
mode for oHPMI-ddm.34 Moreover, the aromatic ether of oxazine ring in
both benzoxazines is evidenced by the bands centered at 1226 and 1229
assigned to the CH wagging of the –CH
–
ꢀ 1
ꢀ
1
cm for PH-ddm and oHPMI-ddm, respectively, which are due to the
C–O–C antisymmetric stretching modes [35]. Furthermore, the oxazine
ring related modes for PH-ddm and oHPMI-ddm are observed at 946 and
Fig. 3. FT-IR spectra of PH-ddm and oHPMI-ddm.
ꢀ 1
9
20 cm , respectively [36]. Therefore, all the above results from the
structural analyses are consistent with the successful synthesis of the
anticipated benzoxazine monomers.
3
.2. Polymerization and thermal cyclization behaviors of the ortho-
maleimide functional bisbenzoxazine
The polymerization profile of oHPMI-ddm was investigated by DSC
as shown in Fig. 4. Herein, two counterparts, PH-ddm and DDM-BMI
were also investigated by DSC, which are introduced to gain better in-
sights into the polymerization behaviors of oHPMI-ddm. Notably,
oHPMI-ddm possesses two polymerizable structure characteristics,
including the oxazine ring in PH-ddm and the maleimide functionality in
DDM-BMI. Thus, the introduction of both counterparts in this study can
give better understanding of the role of oxazine ring and maleimide on
the polymerization processes in oHPMI-ddm.
As shown in Fig. 4, among the three monomers studied, oHPMI-ddm
◦
shows the lowest melting peak temperature at 105 C, followed by PH-
◦
ddm (125 C), and finally DDM-BMI showing the highest endothermic
◦
peak temperature at 159 C. Interestingly, the combination of oxazine
Fig. 4. DSC thermograms of PH-ddm, DDM-BMI and oHPMI-ddm.
ring and maleimide group in the same compound significantly decreases
the molecular interactions in the thermosetting system. This is of great
interest, as oHPMI-ddm exhibits excellent processability potential by
taking its low melting temperature and wide processing window
◦
(
>110 C) into consideration. Besides the polymerization behaviors of
PH-ddm, DDM-BMI and oHPMI-ddm were also tested by DSC. The
exotherm that is attributed to the polymerization of oHPMI-ddm has its
◦
maximum located at 217 C, which is much lower than that of PH-ddm
◦
(
266 C). Particularly, the incorporation of maleimide group in ben-
zoxazine greatly lowers the initial polymerization temperature. In the
◦
case of DDM-BMI, a very broad exothermic peak centered at 221 C is
observed. It should be pointed out that the initial polymerization tem-
perature of maleimide is much lower than oxazine ring as observed from
the DSC thermograms of PH-ddm and DDM-BMI. In the oHPMI-ddm
thermogram, the very sharp polymerization exotherm observed at
◦
2
66 C in PH-ddm has totally disappeared and seems to have overlapped
with the broad exotherm of maleimide. This indicates that the signifi-
cant reduction of the polymerization temperature for oHPMI-ddm
should be attributed to the low starting polymerization temperature of
1
3
3
Fig. 2. C NMR spectra of PH-ddm and oHPMI-ddm in CDCl .
4

K. Zhang et al.
Polymer 223 (2021) 123703
maleimide functionality. Moreover, oHPMI-ddm shows multiple exo-
Table 1
a
E values of PH-ddm and oHPMI-ddm calculated by the Kissinger and Ozawa
therms rather than a singlet as other reported benzoxazine monomers
[
14,15]. Quite often, benzoxazines with multiple polymerization
Theories.
mechanisms exhibit a single exothermic peak despite having different
polymerization temperatures that correspond to each mechanism. The
current system also shows a similar trend of joining two polymerization
behaviors into one general polymerization temperature range, possibly
due to the acceleration of the benzoxazine polymerization via the
exothermic heat generated by the maleimide moieties; however, there
are some complications as seen in weak, but multiple peaks heavily
Sample
Kissinger
Ozawa
Ea (kJ/mol)
Ea (kJ/mol)
PH-ddm
125.1
103.3
127.6
105.7
oHPMI-ddm
The E values calculated for oHPMI-ddm are much lower than those
a
overlapped.
of PH-ddm. In other words, oHPMI-ddm is comparatively easy to acti-
◦
DSC measurements at heating rates of 2, 5, 10, 15 and 20 C/min
vate towards its polymerization. The only structural difference between
PH-ddm and oHPMI-ddm is the existence of maleimide group at the
ortho position with respect to the oxygen in oxazine ring in oHPMI-ddm.
This is quite unexpected since the carbonyl group of the maleimide
moiety does not possess an obvious labile proton that can hydrogen
bond to the oxazine ring. Previously, all the smart ortho-functional
benzoxazine monomers that showed low polymerization temperatures
were structured with the functional groups that were capable of
hydrogen bond formation with the oxazine ring either as the 5-
membered or 6-membered intramolecular hydrogen bonds [39,40].
However, it is well-known that maleimide group can form a keto-enol
tautomer with the hydrogen atom migration [41,42]. The enol form of
the tautomer can potentially form a weak hydrogen bond as it can have
an intramolecular 7-membered ring configuration. Admittedly, the
7-membered ring is not as stable as the 6-membered ring. Yet, an
intramolecular 5-membered ring, which is smaller than the stable
6-membered ring, has been shown to form a hydrogen bond and cause
lowering of the oxazine ring polymerization temperature. Furthermore,
the 7-membered ring has been shown to form a hydrogen bond as well
a
were also carried out to calculate the activation energy (E ) of the
polymerization reaction of PH-ddm and oHPMI-ddm (Figs. S2 and S3).
The activation energy values for both benzoxazines were calculated via
the Kissinger and modified Ozawa theories [37,38], using the equations
as follows:
AR
E
RT
a
Kissinger
​
equation: ln( β2) = ln( ) ꢀ
(1)
(2)
T
E
a
P
p
E
RT
a
Modified Ozawa equation: ln β = ꢀ 1.052
+ C
P
p
Where β is the heating rate, T is the maximum of the exotherm, A
represents the frequency factor, R is the gas constant, and C is a constant.
As shown in Fig. 5, the plots exhibit straight lines based on the two
theories for both benzoxazines. The E
of Kissinger and Ozawa’s plots for PH-ddm are 125.1 and 127.6 kJ/mol,
respectively. In comparison, the E values of oHPMI-ddm are obtained to
a
values calculated from the slope
a
be 103.3 and 105.7 kJ/mol, respectively, as summarized in Table 1.
[
43,44]. Fig. S4 shows the FT-IR spectra of bismaleimide without oxa-
zine ring, DDM-BMI, and its benzoxazine counterpart, oHPMI-ddm in
ꢀ 1
the expanded region of 1800–1000 cm . Due to the symmetric
configuration of the benzene ring to which the maleimide is attached,
the band contour of the imide group is relatively symmetric. However,
when the oxazine ring is attached, due to the possible 7-membered
intramolecular interaction, one carbonyl is less conjugated and
another is more double bond nature, resulting in widening of carbonyl
frequencies. As a result, the carbonyl band is clearly separated into two
peaks. This observation is consistent with the hypothesized intra-
molecular interaction of oHPMI-ddm. All these advantages observed in
polymerization behaviors are largely due to the combination of oxazine
ring and maleimide in the same benzoxazine compound and the weak
interactions between these moieties via a 7-membered intramolecular
hydrogen bond.
In order to qualitatively investigate the structural transformation
during the polymerization of oHPMI-ddm, in situ FT-IR analyses were
then performed. Ordinarily, the bands corresponding to the C–O–C
ꢀ 1
ꢀ 1
antisymmetric stretching (~1230 cm ) and benzoxazine (~940 cm )
are used to monitor the ring-opening polymerization of benzoxazine
rings [35,36]. As can be seen in Figs. S5 and 6, both characteristic bands
decrease as the heating temperature increase, and fully disappear at
◦
◦
2
20 C for PH-ddm and 240 C for oHPMI-ddm, respectively. The bands
due to the CH wagging of the vinylene group from maleimide in oHP-
MI-ddm undergo substantial decrease at higher temperature, suggesting
the polymerization of maleimide group. These results therefore confirm
that the polymerization of oHPMI-ddm has two parallel processes, the
polymerization of maleimide and ring-opening polymerization of oxa-
zine ring.
Previous studies reported that ortho-imide polybenzoxazines can
form a benzoxazole by further thermal treatment, leading to another
class of thermosets, specifically, cPBOs.29 Taking advantage of the
remarkable molecular design flexibility of ortho-imide benzoxazine
resins, various ortho-imide functional benzoxazines have been synthe-
sized for preparing polybenzoxazoles via an intramolecular cyclization
between neighboring hydroxyl and phthalimide [45,46]. However, the
Fig. 5. Representations of the Kissinger (a) and Ozawa (b) theories for the
a
calculation of E values for PH-ddm and oHPMI-ddm.
5

K. Zhang et al.
Polymer 223 (2021) 123703
and -O-CH
carbon of –CH
oHPMI-ddm) and cPBO-oHPMI-ddm exhibited obvious differences
2
-N-, respectively. In addition, the signal of the maleimide
–
–
CH- is located around 134 ppm. The spectra for poly
(
compared with that of oHPMI-ddm. The carbon resonances of oxazine
ring and maleimide in oHPMI-ddm have fully disappeared in the spectra
of poly(oHPMI-ddm) and cPBO-oHPMI-ddm, multiple broad resonances,
which can be assigned to the carbons in methylene and methylidyne of
poly(oHPMI-ddm), appeared in the range from 20 to 60 ppm. These
variations indicate the completion of ring-opening polymerization of
oxazine ring and cross-linking of maleimide in poly(oHPMI-ddm).
Moreover, the new carbon resonances at 163, 149 and 141 ppm in the
spectrum of cPBO-oHPMI-ddm, which are assigned to the carbon reso-
nances in benzoxazole moiety, are well resolved [27,47,48]. The
assignment for benzoxazole group can be further supported since no
benzoxazole carbons can be observed for the samples of PH-ddm and
◦
DDM-BMI after thermal treatment at 400 C as shown in Figs. S6 and S7.
Meanwhile, the characteristic carbon resonance from carbonyl in poly
(
oHPMI-ddm) decreases after the further treatment for obtaining
cPBO-oHPMI-ddm. As a result, the conversion from polybenzoxazine to
polybenzoxazole is around 75% by integrating peaks A and e in the
spectrum of cPBO-oHPMI-ddm. These NMR spectral differences among
the benzoxazine monomer, polybenzoxazine and finally obtained ther-
moset are a strong evidence of the proposed benzoxazole formation from
ortho-maleimide-functional benzoxazine and in accordance with the
conclusion obtained from the FT-IR analyses. Therefore, we propose a
structural transformation behavior for oHPMI-ddm as shown in Scheme
Fig. 6. FT-IR spectra of oHPMI-ddm after various thermal treatments at the
designated temperature for 1 h.
thermal cyclization between neighboring maleimide and hydroxyl has
never been investigated. Since maleimide belongs to the imide family,
we reasonably predict that oHPMI-ddm could undergo a unique struc-
tural transformation at elevated temperatures. Thus, in situ FT-IR ana-
lyses at higher temperatures were further conducted for oHPMI-ddm. As
seen from Fig. 6, the typical carbonyl bands in maleimide occur at 1776
4
.
ꢀ
1
ꢀ 1
and 1709 cm , and the –OH band around 3400 cm
gradually
◦
decrease when heated from 240 to 400 C. Meanwhile, the typical band
ꢀ 1
–
–
at 1618 cm (C
2]. The in situ FT-IR spectra of this newly obtained ortho-maleimide--
N stretching) for benzoxazole gradually increases [31,
3
functional bisbenzoxazine is practically identical to those reported for
cPBOs achieved via the cyclization of phtalimide hydroxyl and from
ortho-Imide benzoxazines, indicating the possibility to prepare high
performance cPBPOs based on ortho-maleimide benzoxazine resins.
The structural transformation from benzoxazine, oHPMI-ddm, to
1
3
cross-linked polybenzoxazole was further supported by solid-state
C
1
3
NMR as shown in Fig. 7. In accordance with the C NMR result of
oHPMI-ddm in solution, the typical solid 13C resonances of the oxazine
2
ring can be assigned to resonances around 50 and 80 ppm for Ar-CH -N-
1
3
Fig. 7. Solid-state C NMR spectra of oHPMI-ddm, poly(oHPMI-ddm) and
cPBO-oHPMI-ddm.
Scheme 4. Polymerization of oHPMI-ddm Forming Polybenzoxazine and
Subsequent Structural Transformation into cPBO.
6

K. Zhang et al.
Polymer 223 (2021) 123703
3
.3. Thermal properties of thermosets
DMA measurements were carried out for poly(PH-ddm), poly
(
oHPMI-ddm) and cPBO-oHPMI-ddm as shown in Fig. 8. Herein, good
DMA samples of poly(DDM-BMI) were difficult to prepare due to the
brittleness of the thermosets based on pure DDM-BMI. It is not unex-
pected as many previous literatures have pointed out this shortcoming of
pure bismaleimide resin, and thereby bismaleimide thermosetting resins
are always processed by adding other additives [7–13]. Nevertheless, we
◦
found a reported T
g
value (222 C) of poly(DDM-BMI) obtained by an
unknown method, which could be used as a guideline for researchers
[
13]. As shown in the figure, poly(PH-ddm) and poly(oHPMI-ddm) show
◦
T
g
temperatures at 214 and 306 C, respectively. Obviously, the T
g
value
of poly(oHPMI-ddm) is much higher than poly(PH-ddm), poly
(
DDM-BMI) as well as the polymerization temperature of itself. The
–
–
additional cross-linking reaction based on the C C group in maleimide
group in oHPMI-ddm increases the overall cross-link density of the
networks, thereby greatly increasing the T
g
temperature of poly(oHP-
◦
MI-ddm). In addition, cPBO-oHPMI-ddm exhibits no T
g
below 400 C as
also shown in Fig. 8, indicating its potential application in a very high
temperature environment. Specifically, cPBO-oHPMI-ddm shows a sta-
◦
ble, constant storage modulus up to 300 C due to the presence of rigid
benzoxazole moiety, which is a stiffer structure than that of poly(oHP-
MI-ddm). Of particular interest is the readily processable precursor
◦
benzoxazine resin, having the melting endotherm at 105 C. It is
important to point out that this low melting temperature was observed
1
despite having the high monomer purity as shown in H NMR spectrum
in Fig. 1 and elemental analysis result without the presence of hydroxyl
impurities which in an impure sample act as effective initiators. Such
characteristics are very different from other reported cPBO precursors of
ortho-imide functional benzoxazine monomers, which all showed high
◦
melting peak temperatures greater than 200 C [29,46].
The thermal stability of poly(PH-ddm), poly(DDM-BMI), poly
(
oHPMI-ddm) and cPBO-oHPMI-ddm were studied by TGA under a N
2
atmosphere. The weight loss and derivative weight loss curves are
depicted in Fig. 9. Notably, poly(DDM-BMI) shows very high thermal
◦
stability during the thermal treatment below 450 C, while both poly-
Fig. 9. TGA thermograms (a) and derivative weigh loss curves (b) of poly(PH-
ddm), poly(DDM-BMI), poly(oHPMI-ddm) and cPBO-oHPMI-ddm.
benzoxazines, poly(PH-ddm) and poly(oHPMI-ddm), exhibit weight-loss
◦
from 300 to 450 C. The initial weight loss at this stage for poly-
benzoxazines is mainly due to the initial decomposition of defect groups
◦
weight-loss rate around 500 C. The effect of this sharp decomposition
[
49]. Hence, the thermal properties, in terms of Td5 (5% weight loss
behavior will be discussed in the next section.
temperature) and Td10 (10% weight loss temperature) for poly(oHP-
MI-ddm) manifest a trade-off between poly(PH-ddm), poly(DDM-BMI).
On the other hand, both polybenzoxazines show broad derivative peaks
as shown in Fig. 9b, while poly(DDM-BMI) exhibits a much higher
cPBO-oHPMI-ddm possesses outstanding thermal stability with very
◦
◦
high Td5 value of 482 C and Td10 of 533 C, respectively, as a conse-
quence of the benzoxazole formation. In addition, cPBO-oHPMI-ddm
shows very high Yc value of 69%. The thermal property data of all the
thermosets obtained in this study are summarized in Table 2. The data
indicates the very high thermal stability of the freshly obtained ortho-
maleimide functional bisbenzoxazine based thermosets.
3
.4. Flammability of thermosets
The heat release capacity (HRC) and total heat release (THR) values
are used to evaluate the flammability of thermosets [50,51], which are
obtained from MCC in the current study. Milligram size samples of poly
(
PH-ddm), poly(DDM-BMI), poly(oHPMI-ddm) and cPBO-oHPMI-ddm
◦
were measured at 1 K/s over the temperature range of 100–750 C. As
shown in Fig. 10, MCC spectra of poly(DDM-BMI), poly(PH-ddm), poly
(
oHPMI-ddm) and cPBO-oHPMI-ddm reveal HRC values of 169.7, 66.6,
ꢀ 1
ꢀ 1
2
2.7 and 7.3 Jg
K , respectively. The highest HRC value of bisma-
leimide based thermoset is due to the highest decomposition rate around
◦
5
00 C. Besides, poly(PH-ddm), poly(DDM-BMI), and poly(oHPMI-ddm)
ꢀ 1
exhibit THR values of 9.4, 8.4 and 5.3 KJg , while cPBO-oHPMI-ddm
ꢀ 1
shows a much lower THR value of 2.3 KJg . In general, lower the
values of HRC and THR suggest higher flame resistance. The HRC values
of bisbenzoxazine derived thermosets are much lower than the other 86
Fig. 8. Dynamic mechanical spectra of poly(PH-ddm), poly(oHPMI-ddm) and
cPBO-oHPMI-ddm.
7

K. Zhang et al.
Polymer 223 (2021) 123703
Table 2
Thermal, Fire and Dielectric-Related Properties of poly(PH-ddm), poly(DDM-BMI), poly(oHPMI-ddm) and cPBO-oHPMI-ddm.
◦
◦
◦
ꢀ 1 ꢀ 1
ꢀ 1
THR (KJg )
Sample
T
g
( C)
T
d5 ( C)
T
d10 ( C)
Y
c
(wt.%)
HRC (Jg
K
)
k (at 1 MHz)
poly(PH-ddm)
214
222
306
–
351
475
380
482
387
480
418
533
52
49
61
69
66.6
169.7
22.7
7.3
9.4
8.4
5.3
2.3
3.30
3.15
2.85
2.49
a
poly(DDM-BMI)
poly(oHPMI-ddm)
cPBO-oHPMI-ddm
a
Data was obtained from Ref. [13].
(
DDM-BMI) are significant disadvantages when considering flamma-
bility. Having a very narrow temperature range of the heat release curve
contributes to the sudden and significant increase in the flaming tem-
perature. On the other hand, the broadening of the derivative peaks for
polybenzoxazines indicates a slow decomposition rate over a wide
temperature range, which, in general, is beneficial for reduced flam-
mability [57,58]. Therefore, the advantages of combining benzoxazine
and maleimide moieties in the same benzoxazine monomer is at least
two-folds: firstly, enhancing the thermal stability of the initial
weight-loss stage though the cross-linked network formed by maleimide
group; secondly, improving the fire retardancy via the cross-linked
network contributed from oxazine ring.
3
.5. Dielectric properties of thermosets
The dielectric constants of thermoset films were calculated by the
capacitance method applying the following equation:
Cl
0
k = Ak
(3)
Herein, C is the capacitance of each thermoset, A is the surface area
of the silver coated samples, l is the thickness of the thermoset film
ꢀ
12
ꢀ 1
samples, and k
0
is the vacuum dielectric constant (8.854 × 10
F m ).
As shown in Fig. 11a, the k values at 1 MHz of the thermosets, poly
PH-ddm), poly(DDM-BMI), poly(oHPMI-ddm) and cPBO-oHPMI-ddm,
(
are 3.30, 3.15, 2.85 and 2.49 respectively. The k values of poly(oHPMI-
ddm) is much lower than poly(PH-ddm), poly(DDM-BMI). The intro-
duction of maleimide group into benzoxazine thermosetting system can
significantly reduce the dielectric constants of the corresponding ther-
moset. The additional crosslinking network may lower the dielectric
constant by increasing the free volume in thermoset structures. Addi-
tionally, the benzoxazole formation between the maleimide and hy-
droxyl group converts the polarizable phenolic hydroxyl into less
polarizable benzoxazole functionality, resulting in a lower dielectric
constant of cPBO-oHPMI-ddm. Particularly, the low dielectric constant
of ortho-maleimide based thermosets is comparable to the values of
other commercially available low-k polymeric materials, such as
cyanate esters and polyimides [59,60].
Fig. 10. Heat release rate (HRR) (a) and total heat release (THR) (b) verses
temperature for poly(PH-ddm), poly(DDM-BMI), poly(oHPMI-ddm) and cPBO-
oHPMI-ddm.
Fig. 11b shows the dielectric loss (tanδ) of poly(PH-ddm), poly(DDM-
BMI), poly(oHPMI-ddm) and cPBO-oHPMI-ddm as a function of the
frequency within the range of 100 Hz to 1 MHz. The dielectric loss was
found to vary from 0.0060 to 0.0063 for cPBO-oHPMI-ddm. In general a
relatively low value of dielectric loss is always required for the appli-
cation as interlayer dielectric materials [61]. In a modern electronic
communication, the communication frequency continues to increase
due to the need of conveying a large amount of messages. Therefore, the
results from dielectric measurements show that the newly developed
ortho-maleimide bisbenzoxazine is suitable for various applications,
such as in the advanced electronics, automotive and aerospace fields.
polymers reported by Walters and Lyon [52]. Particularly, the combi-
nation of oxazine ring and maleimide in the same molecular structure
leads to a substantial HRC reduction. Hence, ortho-maleimide functional
bisbenzoxazine based thermosets also extend opportunities for appli-
cations that benefit from low flammability. The extremely low HRC of
ꢀ
1
cPBO-oHPMI-ddm at 5.3 KJg is particularly noteworthy. Considering
all the polymers reported by Lyon and others [53–56], this polymer
showed the lowest HRC, clearly demonstrating an extremely
non-combustible nature. Polymers exhibiting a HRC less than 100 are
considered non-ignitable and will pass strict Federal Aviation Admin-
istration requirements for the heat release rate of materials used for
commercial aircraft cabins [56].
4. Conclusion
In summary, we have successfully synthesized an ortho-maleimide
functional bisbenzoxazine resin, which features advantages originating
from both benzoxazine and maleimide moieties, and further synergistic
effect of having both moieties in the same molecule at the ortho-position.
Despite having good thermal stability below the decomposition
temperature, the sharp decomposition and heat release behavior of poly
8

K. Zhang et al.
Polymer 223 (2021) 123703
(
52073125 and 51603093) for financial support.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.polymer.2021.123703.
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