Synthesis and thermally induced structural transformation of phthalimide and nitrile-functionalized benzoxazine: toward smart ortho-benzoxazine chemistry for low flammability thermosets

Article abstract of DOI:10.1039/C8RA10009H

A novel ortho-phthalimide-functionalized benzoxazine monomer containing an ortho-nitrile group has been synthesized in order to further systematically evaluate the thermally induced structural transformation from benzoxazine resin to cross-linked polybenzoxazole. The chemical structure of the synthesized monomer has been confirmed by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Also supporting the detailed structure is ¹H-¹³C heteronuclear multiple quantum coherence (HMQC), which identifies the local proton-carbon proximities. The polymerization behaviors, including the ring-opening polymerization of the oxazine rings and the cyclotrimerization of the nitrile functionalities, are studied by differential scanning calorimetry (DSC) and in situ FT-IR. In addition, the subsequent benzoxazole formation after polymerization has also been analyzed using thermogravimetric analysis (TGA) and magic-angle spinning (MAS) solid-state ¹³C NMR. The resulting cross-linked polybenzoxazole derived from the benzoxazine monomer exhibits exceptionally high thermal stability and low flammability, with an extremely high T_d5 temperature (550 °C), a high char yield value (70%) and an extraordinarily low total heat release (THR of 7.6 kJ g^?1).

Full text of DOI:10.1039/C8RA10009H

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis and thermally induced structural
transformation of phthalimide and nitrile-
functionalized benzoxazine: toward smart ortho-
benzoxazine chemistry for low flammability
thermosets†
Cite this: RSC Adv., 2019, 9, 1526
a
a
b
a
b
*
*
Yuqi Liu, Lu Han, Jinyun Wang and Hatsuo Ishida
Kan Zhang,
A novel ortho-phthalimide-functionalized benzoxazine monomer containing an ortho-nitrile group has
been synthesized in order to further systematically evaluate the thermally induced structural
transformation from benzoxazine resin to cross-linked polybenzoxazole. The chemical structure of the
synthesized monomer has been confirmed by 1H and 13C nuclear magnetic resonance (NMR)
spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Also supporting the detailed structure
1
is H–13C heteronuclear multiple quantum coherence (HMQC), which identifies the local proton-carbon
proximities. The polymerization behaviors, including the ring-opening polymerization of the oxazine
rings and the cyclotrimerization of the nitrile functionalities, are studied by differential scanning
calorimetry (DSC) and in situ FT-IR. In addition, the subsequent benzoxazole formation after
polymerization has also been analyzed using thermogravimetric analysis (TGA) and magic-angle spinning
(MAS) solid-state 13C NMR. The resulting cross-linked polybenzoxazole derived from the benzoxazine
monomer exhibits exceptionally high thermal stability and low flammability, with an extremely high Td5
temperature (550 ꢀC), a high char yield value (70%) and an extraordinarily low total heat release (THR of
7.6 kJ gꢁ1).
Received 5th December 2018
Accepted 26th December 2018
DOI: 10.1039/c8ra10009h
rsc.li/rsc-advances
degradation of the benzoxazole moiety when exposed to mois-
ture at moderate to elevated temperatures.5 The second method
Introduction
for the synthesis PBO consists of two steps; rst is a condensa-
tion reaction of an aromatic diacid chloride with a bis(o-ami-
nophenol) in an organic solvent at room temperature to form
a poly(o-hydroxyamide), followed by a sequential thermal
intramolecular cyclodehydration reaction in the bulk state or in
solution with the loss of water to yield the nal aromatic poly-
benzoxazole.6–9 In addition, some researchers also reported the
possible thermally induced structural transformation of o-
hydroxypolyimides into polybenzoxazoles wiꢀth the loss of
carbon dioxide at temperatures of around 400 C.10–14
There is growing interest in aromatic polymers with excellent
mechanical and thermal properties to meet the crucial
requirements of aerospace and electronic technology. Among
the many polymeric materials, aromatic polybenzoxazoles
(PBOs) are a class of heterocyclic polymers possessing superb
thermal stability, extremely high tensile strength, and excellent
chemical resistance.1 In general, there are two traditional
methods for the synthesis of aromatic PBOs. The rst method is
one-step synthesis, in which a direct polycondensation takes
place between bis(o-aminophenol)s and aromatic diacids using
poly(phosphoric acid) (PPA),2 phosphorus pentoxide/
methanesulfonic acid (PPMA),3 or trimethylsilyl poly-
phosphate (PPSE)/o-dichlorobenzene4 as the reaction medium.
This approach is normally applied for producing PBO bers.
However, the residual acid in PBO bers easily causes hydrolytic
Although the highly rigid molecular structure of PBOs brings
outstanding performance, it also causes difficulties in synthesis
and fabrication. For example, the wholly aromatic PBOs are
soluble only in harsh acids. Besides, no glass transition
temperature (Tg) can be observed before their decomposition
temperature, which impedes the melt processing of PBOs.
Furthermore, bis(o-aminophenol)s used as the main starting
materials for most synthesis approaches of PBOs are expen-
sive.15 All of the above shortcomings result in the limited
application of this class of high-performance polymers.
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, USA. E-mail: hxi3@cwru.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra10009h
Polybenzoxazines,16–18 as a novel type of thermoset, offer
outstanding advantages such as high thermal stability,19 high
1526 | RSC Adv., 2019, 9, 1526–1535
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
char yield,20 high Tg,21 near-zero volumetric change during according to the literature with slight modication (see ESI,
polymerization,22 good mechanical23 and dielectric properties,24 Fig. S1–S4†).19
low water absorption25 and low ammability.26 Various forms of
hydrogen-bonding within the nal network structure are Synthesis of 2-(8-(1,3-dioxoisoindolin-2-yl)-2H-benzo[e][1,3]
observed in polybenzoxazines, contributing greatly to their oxazin-3(4H)-yl)benzonitrile (oPP-an)
unique properties.27 Besides, the potential for rich molecular
In a 150 mL round ask were mixed 50 mL of xylenes, anthra-
design exibility is one of the most attractive properties of this
nilonitrile (1.77 g, 0.015 mol), o-PP (3.59 g, 0.015 mol), and
class of polymers.
paraformaldehyde (0.96 g, 0.032 mol). The mixture was stirred
at 120 ꢀC for 8 h. Aerwards, the reaction solution was cooled to
room temperature and precipitated into 100 mL of methanol.
A series of benzoxazines with imide functionality have been
developed by taking advantage of the molecular design exi-
bility of benzoxazine chemistry.19,28–30 One of the particularly
interesting features of these imide-functionalized benzoxazines
is their ability to be the precursor for further structural trans-
Removal of the solvent by ltering afforded a white powder
(yield ca. 91%, mp 251 ꢀC). 1H NMR (DMSO), ppm: d ¼ 4.80 (s,
Ar–CH2–N, oxazine), 5.40 (s, O–CH2–N, oxazine), 6.91–7.99
formation to polybenzoxazoles, which exhibit very high thermal
(11H, Ar). IR spectra (KBr), cmꢁ1: 2221 (C^N stretching of
stability with a Td5 temperature as high as 505 ꢀC in a nitrogen
nitrile group) 1774, 1720 (imide I), 1492 (stretching of trisub-
stituted benzene ring), 1385 (imide II), 1229 (C–O–C asymmetric
stretching), 1168 (C–N–C asymmetric stretching), 938 (oxazine
ring related mode).
atmosphere.19 Additionally, the ortho-imide-functionalized
benzoxazines show great advantages in the synthesis of poly-
benzoxazoles, since no harsh acids are required. The research of
ortho-imide benzoxazines suggests the possibility to form high
performance thermosets from simple low molecular weight
compounds.
Preparation of polybenzoxazine (poly(oPP-an)) and cross-
linked polybenzoxazole (PBO-oPP-an)
Previously, benzoxazine resins containing nitrile moieties
A solution containing 30% solid of the monomer in DMF was
have been widely studied since incorporation of the nitrile
prepared in a ask at room temperature. Then, the solution was
cast over a dichlorodimethylsilane-pretreated steel plate. The
lm was dried in an air circulating oven at 120 ꢀC for 2 days to
remove the solvent completely. The lm as xed to the plate was
ꢀ
polymerized stepwise at 160, 180, 200, 220, 240 and 260 C for
1 h each to obtain poly(oPP-an). Heat treatment of poly(oPP-an)
functionality results in an increased char yield, thereby leading
to low ammability.31,32 Interestingly, it was reported that the
polybenzoxazine derived from monofunctional benzoxazine
containing a nitrile functionality at the ortho position with
respect to the nitrogen group in the oxazine ring showed the
best thermal stability when compared with polybenzoxazines
was further carried out in a tube furnace under a steady ow of
polymerized form para- and meta-nitrile functional benzoxazine
ꢀ
N2 at 400 C for 1 h to obtain PBO-oPP-an.
analogues.31
Inspired by the smart ortho approach for developing high
performance thermosets in benzoxazine chemistry, a novel fully
Characterization
ortho-functionalized benzoxazine monomer containing phtha- A Bruker AVANCE II nuclear magnetic resonance (NMR) spec-
limide and nitrile functionalities at ortho positions with respect trometer was used to obtain 1H and 13C NMR spectra at a proton
to the oxygen and nitrogen in the oxazine ring, respectively, has frequency of 400 MHz, in DMSO-d6 using tetramethyl silane as
1
been synthesized in this study. In continuation of our previous an internal standard. The average number of transients for H
studies on the ortho-imide-functionalized benzoxazines, the and 13C NMR measurement was 64 and 1024, respectively.
aim of this study is to further develop a very high thermal 1H–13C heteronuclear multiple quantum coherence (HMQC)
stability thermoset with ultra-low ammability by incorporating was also carried out. Cross-polarization magic-angle spinning
an ortho-nitrile group. Furthermore, thermally induced struc- (CP-MAS) solid-state 13C NMR spectra were collected using
tural transformation, including ring-opening polymerization, a Bruker Avance III 400 MHz instrument, using adamantane as
cyclotrimerization and benzoxazole formation, is also system- a reference. Powder samples were packed into a 4 mm zirconia
atically investigated.
rotor with a spinning rate of 10 000 Hz, and the number of
transients was 6000 scans. FTIR measurements were recorded
on a Nicolet Nexus 670 Fourier transform infrared (FTIR)
spectrophotometer in the range of 4000–400 cmꢁ1. A NETZSCH
DSC model 204F1 was used with a temperature ramp rate of
10 ꢀC minꢁ1 under a nitrogen atmosphere, and the ow rate of
Experimental
Materials
o-Aminophenol, anthranilonitrile, phthalic anhydride, and nitrogen was 60 mL minꢁ1 for the differential scanning calori-
paraformaldehyde (99%) were used as received from Sigma- metric (DSC) study. In the analyses to determine the activation
Aldrich. Acetic acid, methanol, N,N-dimethylformamide (DMF) energy of polymerization, the samples (2.0 ꢂ 0.5 mg) were
ꢁ1
ꢀ
and xylenes were kindly supplied by East Instrument Chemical scanned at different heating rates of 2, 5, 10, 15, 20 C min
.
Glass Co., Ltd., China and used as received. The starting Thermogravimetric analyses (TGA) were conducted on
phenol, 2-(2-hydroxyphenyl)-isoindoline-1,3-dione (o-PP), and a NETZSCH STA449C Thermogravimetric Analyzer. Nitrogen at
the benzoxazine monomer, 2-(3-phenyl-3,4-dihydro-2H-benzo[e] a ow rate of 40 mL minꢁ1 was purged. For the isothermal TGA,
[1,3]oxazin-8-yl)-isoindoline-1,3-dione (oPP-a), were synthesized the temperature program was room temperature to 800 ꢀC at the
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 1526–1535 | 1527

View Article Online
RSC Advances
Paper
heating rate of 10 ꢀC minꢁ1 under nitrogen at aꢀow rate of 40
mL minꢁ1 and holding the temperature at 400 C for 1 h. The
specic heat release rate (HRR, W gꢁ1), heat release capacity
(HRC, J (g K)ꢁ1) and total heat release (THR, kJ gꢁ1) were
measured on a microscale combustion calorimeter (MCC) from
PHINIX Co., Ltd. MCC was carried out from 100 to 900 ꢀC at
a heating rate of 1 ꢀC sꢁ1 in an 80 cm3 minꢁ1 stream of nitrogen.
The anaerobic thermal degradation products in the nitrogen
gas stream were mixed with a 20 cm3 minꢁ1 stream of oxygen
prior to entering the combustion furnace (900 ꢀC). Heat release
is quantied by standard oxygen consumption,33 and HRR is
obtained by dividing dQ/dt at each time interval by the initial
sample mass. Moreover, HRC can be obtained by dividing the
maximum value of HRR by the heating rate.
Results and discussion
Synthesis of the ortho-phthalimide-functionalized
benzoxazine monomer containing a nitrile group
Fig. 1 1H NMR spectrum of oPP-an.
The successful synthesis of a fully ortho-functionalized ben-
zoxazine monomer containing a phthalimide and nitrile group
has been achieved using anthranilonitrile, formaldehyde, and
ortho-phthalimide-functionalized phenol, as shown in Scheme
1. The obtained benzoxazine in this study exhibited advantages
in synthesis from the viewpoints of short reaction time (8 h),
high yield (91%) and ease of purication, which supports the
superior properties of ortho-benzoxazine resins.
structure. For example, the characteristic band at 2221 cmꢁ1 is
due to the C^N stretching of the nitrile group. The character-
istic doublet peaks at 1784 cmꢁ1 and 1720 cmꢁ1 are the typical
bands for imide, and are attributed to the imide C–C(]O)–C
antisymmetric and symmetric stretching, respectively.34 In
addition to these bands, the presence of imide is seen by the
other characteristic band at 1385 cmꢁ1, which is due to the axial
stretching of C–N bonds.35,36 The band characteristic of anti-
symmetric trisubstituted benzene that appears at 1492 cmꢁ1
conrms the incorporation of an imide group into the benzox-
azine monomer. Besides, the presence of the benzoxazine ring
aromatic ether in the monomer is indicated by the band
centered at 1229 cmꢁ1 due to the C–O–C antisymmetric
The structure of the monomer, oPP-an, has been conrmed
1
using H NMR spectra, as depicted in Fig. 1. The typical reso-
nances attributed to the benzoxazine structure, Ar–CH2–N– and
–O–CH2–N– for oPP-an are observed at 4.80 and 5.40 ppm,
respectively. In addition, the 13C NMR spectrum shown in Fig. 2
is applied to conrm the characteristic signal of the oxazine ring
and nitrile group. The characteristic carbon resonances of the
oxazine ring are assigned at 51.78 and 80.53 ppm for Ar–CH2–
N– and –O–CH2–N–, respectively. Moreover, the 13C NMR
spectrum of oPP-an shows a characteristic resonance at
106.48 ppm corresponding to the nitrile carbon of –C^N. The
above assignments for the oxazine protons and carbons, and
stretching modes.37 Furthermore, the characteristic oxazine-
related mode is located at 938 cmꢁ1
.
The FT-IR spectrum
38
further supports the structure of the oPP-an obtained.
Polymerization behavior of the benzoxazine monomer
nitrile carbon in the structure of oPP-an are supported by the The polymerization behavior of oPP-an was studied by DSC, and
1H–13C HMQC NMR analysis (Fig. S5†).
the corresponding thermogram is depicted in Fig. 4. Ordinarily,
A number of infrared absorption bands are highlighted as the characteristic thermogram for a benzoxazine monomer
shown in Fig. 3, which are used to further verify the existence of consists of an endothermic peak as well as an exothermic peak
the oxazine ring and nitrile functionality in the molecular due to the melting and ring-opening polymerization behaviors.
Scheme 1 Synthesis of the ortho-phthalimide functional benzoxazine monomer containing a nitrile group.
1528 | RSC Adv., 2019, 9, 1526–1535
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 4 DSC polymerization behavior of a mono-functionalized ben-
zoxazine monomer.
Fig. 2 13C NMR spectrum of oPP-an.
and properties of polymers. Polymers with highly rigid struc-
tures normally possess superior properties, but suffer difficul-
ties during the synthesis and processing procedures. For
example, a highly polar solvent or high temperature is always
required for the preparation of polyimides or polybenzoxazoles.
However, the synthesis process for oPP-an with high rigidity
only requires a low polarity solvent (xylenes) and short reaction
time (8 h), showing the advantages of the synthesis of high
performance polymers via ortho-phthalimide-functionalized
benzoxazines. Additionally, the multiple exotherms of oPP-an
can be considered as two different cross-linking processes,
namely, the ring-opening polymerization of the oxazine as well
as the cyclotrimerization of the nitrile group, which occur
separately. However, the enthalpy for each polymerization of
oPP-an cannot be detected since the multiple thermal behaviors
occur in a narrow temperature range.
Fig. 3 FTIR spectrum of the benzoxazine monomer.
In order to qualitatively study the structural evolution during
polymerization, in situ FT-IR analyses were carried out and the
spectra for oPP-an are displayed in Fig. 5. As shown in Fig. 5, no
obvious changes can be observed for all of the absorption bands
For example, the ortho-phthalimide-functionalized benzoxazine
without a nitrile functionality, oPP-a, was reported to show an
endothermic peak at 209 ꢀC and an exothermic peak at 234 ꢀC.39
Herein, aer incorporating the nitrile functionality, the ther-
mogram of oPP-an shows a very sharp endothermic peak at
a temperature as high as 251 ꢀC. This sharp melting peak
further indicates the excellent purity of oPP-an. Subsequently,
as can be seen in the gure, there are two exothermic peaks with
maxima at 263 and 279 ꢀC, respectively. The high melting
temperature causes a partial overlap between the melting and
polymerization behaviors. To the best of our knowledge, this
unique DSC thermogram with such a high temperature and very
sharp endothermic peak was observed for the rst time for
a benzoxazine monomer. The rigidity, originating from the
combination of the imide structure with an aromatic structure
and nitrile group, led to oPP-an having a higher melting
temperature compared with other traditional benzoxazine
monomers. In general, a trade-off exists between the synthesis
ꢀ
of oPP-an before 200 C. This result is in agreement with the
DSC prole, suggesting that no types of polymerization take
place before the melting behavior. The characteristic absorp-
tion bands at 1229 cmꢁ1 (C–O–C asymmetric stretching modes)
and 938 cmꢁ1 (the oxazine ring related mode) gradually disap-
pear from 200 to 220 ꢀC. Meanwhile, a broad band of –OH
between 3500 and 3300 cmꢁ1 gradually appears, indicating the
completion of the ring-opening polymerization of the oxazine
rings. Additionally, as shown in Fig. 6, an intensity decrease of
the band at 2221 cmꢁ1 (C^N stretching) is observed, and new
bands appear gradually at 1621 cmꢁ1 (C]N stretching) and
1381 cmꢁ1 (triazine related mode) from 220 C, suggesting the
ꢀ
cyclotrimerization of the nitrile functionality. Furthermore, the
characteristic absorption band for nitrile at around 2221 cmꢁ1
mostly disappeared aer the thermal treatment at 260 ꢀC for
1 h. Both Fig. 5 and 6 provide reliable information on the order
of these two events taking place. From these results, it is
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 1526–1535 | 1529

View Article Online
RSC Advances
Paper
Fig. 5 FT-IR spectra of oPP-an after various thermal treatments at each designed temperature stage for 1 h.
determined that the ring-opening polymerization of the oxazine where C is a constant.
ring takes place rst at 220 ꢀC, while the nitrile functionality
Fig. 8 and 9 show the plot of ln(b/Tp2) or ln(b) against 1/Tp for
subsequently cyclotrimerizes at a relatively higher temperature. oPP-an according to the Kissinger and Ozawa methods. The
Therefore, the orders of the two exothermic peaks from the activation energies calculated from the slopes of the Kissinger
above DSC results can be clearly established, namely benzox- and Ozawa plots of the rst exotherm are 146.4 and
azine ring-opening polymerization rst, and then the cyclo- 147.2 kJ molꢁ1. Besides, those from the second exotherm are
trimerization of the nitrile group.
estimated to be 111.5 and 114.6 kJ molꢁ1, respectively, as shown
Fig. 7 shows the DSC proles of oPP-an at heating rates of 2, in Table 1. The activation energies obtained for the rst exo-
5, 10, 15 and 20 ꢀC minꢁ1, respectively. The thermograms of therm are much higher than that of oPP-a, which was estimated
oPP-an show that the endothermic peaks and exothermic peaks to be 86.0 kJ molꢁ1 (Kissinger) and 89.5 kJ molꢁ1 (Ozawa)
shi to higher temperatures with an increase of heating rate. (Fig. S6, S7 and Table S1†). The only difference between oPP-
The activation energy of the polymerization process was deter- a and oPP-an is the existence of a nitrile functionality at the
mined using the well-known Kissinger and Ozawa methods.40,41 ortho position with respect to the nitrogen in the oxazine ring in
According to the Kissinger method, the activation energy can be oPP-an, by considering their molecular structures. Thus, the
calculated using eqn (1) as follows:
high activation energy for the rst exotherm of the ring-opening
polymerization of the oxazine ring is proposed to be caused by
incorporating a nitrile functionality into the benzoxazine
structure, leading to higher rigidity of the molecule itself.
However, the estimated activation energy for both exotherms is
lower than that of some other reported amide-functionalized or
imide-functionalized benzoxazines, which are regarded as
precursors for high-performance materials.39 Therefore, oPP-an
can be easily activated to polymerize in terms of benzoxazine
resins for high-performance applications. Additionally, the
apparent activation energies for both exotherms of oPP-an re-
ported above are an average value of those processes, and thus
should be considered only as a guide since the maxima of the
two exotherms heavily overlap.
!
ꢀ
ꢁ
b
AR
Ea
Ea
ln
¼ ln
ꢁ
(1)
2
RTp
Tp
where b is the heating rate, A is the frequency factor, Tp is the
temperature of the exothermic peak, Ea is the activation energy,
and R is the gas constant. If the plot of ln(b/Tp2) against 1/Tp is
linear, Ea can be obtained from the slope of the corresponding
straight line.
Another theoretical treatment, namely, the Ozawa method,
can also be applied to the thermal data using eqn (2) as follows:
Ea
ln b ¼ ꢁ1:052
þ C
(2)
RTp
1530 | RSC Adv., 2019, 9, 1526–1535
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 6 Expansion of the FT-IR spectra for oPP-an after various thermal treatments in the range of 2300 to 1350 cmꢁ1
.
the structural evolution of oPP-an during heating, TGA and
solid-state 13C NMR analyses were carried out.
TGA curves of a representative sample of poly(oPP-an) are
shown in Fig. 10. Poly(oPP-an) shows a bimodal degradation
prole in which the rst maximum weight-loss rate is at 381 ꢀC,
which corresponds to the typical benzoxazole formation
temperature. Besides, the second maximum weight loss rate is
located at 640 ꢀC, which is in agreement with the maximum
degradation weight-loss rate of benzoxazole groups. Marked on
Fig. 7 DSC curves of oPP-an at different heating rates.
Thermally induced structural transformation
It has been established that the ortho-imide polybenzoxazines
undergo thermally induced structural transformation by further
thermal treatment into another class of polymer, specically,
cross-linked polybenzoxazoles. However, benzoxazole forma-
tion from ortho-imide benzoxazines was only supported by
evidence from FT-IR analyses.19 In order to qualitatively study
Fig. 8 Representations of the Kissinger method for the calculation of
the activation energy for oPP-an.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 1526–1535 | 1531

View Article Online
RSC Advances
Paper
oPP-an during polymerization, which likely results in the low
degree of crosslinking, generating terminal defect structures.
Thus, the additional weight-loss during benzoxazole formation
originates from the degradation of the defect structures of
polybenzoxazine networks.
The structural conversion from benzoxazine to poly-
benzoxazole was further conrmed by solid-state 13C NMR with
cross-polarization magic-angle spinning (CP-MAS) as shown in
Fig. 12. In accordance with the 13C NMR spectrum of oPP-an in
solution, the characteristic solid 13C resonances of the oxazine
ring are assigned at 54 and 78 ppm for Ar–CH2–N– and –O–CH2–
N–, respectively. Besides, the characteristic resonance of the
nitrile carbon of –C^N is centered at 105 ppm. Moreover, the
spectrum for PBO-oPP-an showed drastic differences compared
with that of oPP-an as expected. The carbon resonances of the
oxazine ring and nitrile of oPP-an completely disappeared, and
a broad signal corresponding to the –CH2– of the Mannich
bridge around 41 ppm as well as a characteristic resonance of
triazine at 167 ppm can be observed in the spectrum of PBO-
oPP-an. These variations suggest the completion of ring-
opening polymerization and cyclotrimerization for oPP-an
aer the designed procedure of thermal treatments. In addi-
tion, the new carbon resonances at 161, 148 and 142 ppm,
which can be assigned to the typical carbon resonances of
benzoxazole groups, are well resolved.42 This difference between
the benzoxazine monomer and the nal obtained polymer
strongly supports the proposed benzoxazole formation from
ortho-phthalimide-functionalized benzoxazine and is in agree-
ment with other evidence of FTIR and TGA analyses.19 As
a result, the proposed structural transformation mechanism of
oPP-an can be described as Scheme 2.
Fig. 9 Representations of the Ozawa method for the calculation of
the activation energy for oPP-an.
Table 1 The activation energy of oPP-an obtained by the Kissinger
and Ozawa methods
Kissinger
Ozawa
Ea (kJ molꢁ1
)
Ea (kJ molꢁ1
)
First exotherm
Second exotherm
146.4
111.5
147.2
114.6
the plot is the weight change expected from the weight loss of
CO2 on the thermal conversion from polybenzoxazine to poly-
benzoxazole. In the present study, TGA of poly(oPP-an) with the
ꢀ
isothermal condition at 400 C for 1 h under N2 has also been
applied as shown in Fig. 11. A weight loss of 19 wt% is observed
aer thermal treatment at 400 ꢀC. The measured weight loss is
greater than expected for benzoxazole formation alone, indi-
cating that some additional degradation reactions also
occurred. The rigidity decreases the mobility of molecules of
Thermal and heat release properties of thermosets
The thermal stability of PBO-oPP-an has also been studied by
TGA and the result is shown in Fig. 13. The initial decomposi-
tion temperatures of 5% and 10% weight losses (Td5 and Td10
for PBO-oPP-an under an inert atmosphere are as high as 550
)
Fig. 10 Thermogravimetric analysis of poly(oPP-an). The rectangular
box indicates the theoretical weight change upon loss of CO2 during Fig. 11 Thermogravimetric analysis of poly(oPP-an). At 400 ꢀC,
benzoxazole formation.
isothermal heating was applied for 1 h.
1532 | RSC Adv., 2019, 9, 1526–1535
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 13 Thermogravimetric analysis of PBO-oPP-an.
Fig. 12 Solid-state 13C NMR spectra of oPP-an and PBO-oPP-an.
predictors of the ammability of a material,45 and was ob-
tained from microscale combustion calorimetry (MCC) in this
study. Milligram size samples of poly(oPP-an) and PBO-oPP-an
were tested at a constant heating rate of 1 K sꢁ1 over the
temperature range 100–900 ꢀC. As shown in Fig. 14, MCC
characterization of poly(oPP-an) and PBO-oPP-an reveals HRC
values of 67.3 and 30.1 J gꢁ1 Kꢁ1, respectively. Besides, poly(-
oPP-an) exhibits a THR value of 14.8 kJ gꢁ1, while PBO-oPP-
a shows a much lower THR value of 7.6 kJ gꢁ1. In general,
the lower the values of HRC and THR, the higher the ame
resistance. The HRC values of both thermosets based on oPP-
an were much lower than those of the other 47 polymers re-
ported by Lyon et al.45 Furthermore, this new class of ther-
mosets is better than the benzoxazole resins derived from
and 591 ꢀC, respectively, as a consequence of the oxazole
formation. Besides, PBO-oPP-an shows a degradation prole in
which the maximum weight-loss rate occurs at 637 ꢀC, which is
consistent with the degradation temperature of poly-
benzoxazoles. Furthermore, PBO-oPP-an exhibits a very high
char yield (Yc) value of 70% at 800 ꢀC. The thermal property data
of PBO-oPP-an are summarized in Table 2, which demonstrates
the exceptionally high thermal stability of PBO-oPP-an.
The LOI parameter of PBO-oPP-an can be determined from
the Yc values based on TGA results by using the Van Krevelen
equation,43 from which the LOI value of PBO-oPP-an at 800 C
was found to be 45.5. PBO-oPP-an has a LOI value in the self-
extinguishing region (LOI > 28),44 suggesting that the cross-
linked polybenzoxazole derived from the ortho-imide-
ꢀ
ortho-amide benzoxazines, which show HRC values of ꢃ92 J
gꢁ1 Kꢁ1
.
Thus, introduction of triazine or benzoxazole
42
functionalities into a polybenzoxazine network leads to
a substantial HRC reduction. Therefore, this newly developed
ortho-phthalimide-functionalized benzoxazine monomer con-
taining an ortho-nitrile group opens up opportunities for
applications that benet from low ammability resins.
functionalized benzoxazine containing
a good candidate for applications such as re resistant and
electronic encapsulation materials.
The heat release capacity (HRC) is an effective evaluation
parameter of thermal combustion and is one of the best single
a nitrile group is
Scheme
2 Polymerization of oPP-an generating polybenzoxazine and subsequent thermally induced structural transformation into
polybenzoxazole.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 1526–1535 | 1533

View Article Online
RSC Advances
Paper
Table 2 Thermal and heat release properties of poly(oPP-an) and PBO-oPP-a
Sample
Td5 (ꢀC)
Td10 (ꢀC)
Yc (wt%)
HRC (J gꢁ1 Kꢁ1
)
THR (kJ gꢁ1
)
Poly(oPP-an)
PBO-oPP-an
362
550
388
591
52
70
67.3
30.1
14.8
7.6
References
1 G. A. Holmes, K. Rice and C. R. Snyder, J. Mater. Sci., 2006,
41, 4105–4116.
2 M. H. Dotrong, R. C. Evers and G. Moore, Polymer Prepr.,
1990, 31, 675–676.
3 M. Ueda, H. Sugita and M. Sato, J. Polym. Sci., Polym. Chem.
Ed., 1986, 24, 1019–1026.
4 B. A. Reinhardt, Polym. Commun., 1990, 31, 453–454.
5 P. J. Walsh, X. Hu, P. Cunniff and A. J. Lesser, J. Appl. Polym.
Sci., 2006, 102, 3517–3525.
6 T. Kubota and R. Nakanishi, J. Polym. Sci., Part B: Polym. Lett.,
1964, 2, 655–659.
7 W. W. Moyer, C. Cole and T. Anyos, J. Polym. Sci., Part A: Gen.
Pap., 1965, 3, 2107–2121.
8 Y. Imai, K. Uno and Y. Iwakura, Makromol. Chem., 1965, 83,
179–187.
9 M. Ueda, K. Ebara and Y. Shibasaki, J. Photopolym. Sci.
Technol., 2003, 16, 237–242.
Fig. 14 Heat release rate (HRR) versus temperature for poly(oPP-an)
and PBO-oPP-an.
10 T. Okabe and A. Morikawa, High Perform. Polym., 2008, 20,
53–66.
Conclusions
11 H. B. Park, C. H. Jung, Y. M. Lee, A. J. Hill, S. J. Pas,
S. T. Mudie, E. V. Wagner, B. D. Freeman and
D. J. Cookson, Science, 2007, 318, 254–258.
12 E. Schab-Balcezak, M. Jikei and M. Kamimoto, Polym. J.,
2003, 35, 208–212.
13 D. Guzman-Lucero and D. Likhatcher, Polym. Bull., 2002, 48,
261–269.
14 B. K. Chen, Y. J. Tsai and S. Y. Tsay, Polym. Int., 2006, 55, 93–
100.
15 S. S. Kim and E. M. Pearce, Makromol. Chem., Suppl., 1989,
15, 187–218.
16 S. Ohashi, D. Iguchi, T. R. Heyl, P. Froimowicz and H. Ishida,
Polym. Chem., 2018, 9, 4194–4204.
A new ortho-phthalimide-functionalized benzoxazine monomer
containing an ortho-nitrile group has been successfully obtained
via Mannich condensation. This benzoxazine resin was observed
to undergo a special thermally induced structural transformation
including ring-opening polymerization, cyclotrimerization and
benzoxazole formation. The resulting cross-linked poly-
benzoxazole derived from the benzoxazine monomer showed
exceptionally high thermal stability with a high Td5 temperature
ꢀ
(550 C) and high char yield value (70%) and extraordinarily low
ammability with a low heat release capacity (HRC of 30.1 J gꢁ1
Kꢁ1) as well as a very low total heat release (THR of 7.6 kJ gꢁ1).
Thus, the combination of the easy synthesis of such a high rigidity
compound and the outstanding thermal properties exhibited by
thermosets obtained from the same benzoxazine monomer makes
oPP-an a highly attractive resin for high-performance elds.
17 H. Ishida, and P. Froimowicz, Advanced and Emerging
Polybenzoxazine
Science
and
Technology,
Elsevier,
Amsterdam, 2017.
18 N. N. Ghosh, B. Kiskan and Y. Yagci, Prog. Polym. Sci., 2007,
32, 1344–1391.
19 K. Zhang, J. Liu, S. Ohashi, X. Liu, Z. Han and H. Ishida, J.
Polym. Sci., Part A: Polym. Chem., 2015, 53, 1330–1338.
20 Z. Deliballi, B. Kiskan and Y. Yagci, Polym. Chem., 2018, 9,
178–183.
21 K. Zhang and X. Yu, Macromolecules, 2018, 51, 6524–6533.
22 H. Ishida and H. Y. Low, Macromolecules, 1997, 30, 1099–
1106.
23 S. B. Shen and H. Ishida, Polym. Compos., 1996, 17, 710–719.
24 J. Wu, Y. Xi, G. T. McCandless, Y. Xie, R. Menon, Y. Patel,
D. J. Yang, S. T. Iacono and R. M. Novak, Macromolecules,
2015, 48, 6087–6095.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors express their gratitude to the National Natural
Science Foundation of China (NSFC) for their nancial support
(No. 51603093). This project was also supported by the Science
and Technology Agency of Jiangsu Province (No. BK20160515),
and the China Postdoctoral Science Foundation (No.
2016M600369 and No. 2018T110451).
1534 | RSC Adv., 2019, 9, 1526–1535
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
25 H. Ishida and D. Allan, J. Polym. Sci., Part B: Polym. Phys., 35 H. Ishida, S. T. Wellinghoff, E. Baer and J. L. Koenig,
1996, 34, 1019–1030. Macromolecules, 1980, 13, 826–834.
26 M. W. Wang, C. H. Lin and T. Y. Juang, Macromolecules, 36 B. T. Low, Y. Xiao, T. S. Chung and Y. Liu, Macromolecules,
2013, 46, 8853–8863. 2008, 41, 1297–1309.
27 H. D. Kim and H. Ishida, J. Phys. Chem. A, 2002, 106, 3271– 37 T. Agag and T. Takeichi, Macromolecules, 2003, 36, 6010–
3280. 6017.
28 M. W. Wang, C. H. Lin and T. Y. Juang, Macromolecules, 38 L. Han, D. Iguchi, P. Gil, T. R. Heyl, V. M. Sedwick, C. R. Arza,
2013, 46, 8853–8863.
29 K. C. Chen, H. T. Li, W. B. Chen, C. H. Liao, K. W. Sun and
F. C. Chang, Polym. Int., 2011, 60, 432–442.
S. Ohashi, D. J. Lacks and H. Ishida, J. Phys. Chem. A, 2017,
121, 6269–6282.
39 K. Zhang and H. Ishida, Polym. Chem., 2015, 6, 2541–2550.
30 A. F. M. El-Mahdy and S. W. Kuo, Polym. Chem., 2018, 9, 40 H. E. Kissinger, Anal. Chem., 1957, 29, 1702–1706.
1815–1826. 41 T. Ozawa, J. Therm. Anal., 1970, 2, 301–324.
31 Z. Brunovska and H. Ishida, J. Appl. Polym. Sci., 1999, 73, 42 T. Agag, J. Liu, R. Graf, H. W. Spiess and H. Ishida,
2937–2949. Macromolecules, 2012, 45, 8991–8997.
32 Z. Brunovska, R. Lyon and H. Ishida, Thermochim. Acta, 2000, 43 D. W. Van Krevelen, Polymer, 1975, 16, 615–620.
357, 195–203.
44 S. Mallakpour and V. Behranvand, Colloid Polym. Sci., 2015,
293, 333–339.
45 R. E. Lyon and M. L. Janssens, Polymer Flammability, U.S.
33 R. E. Lyon and R. N. Walters, J. Anal. Appl. Pyrolysis, 2004, 71,
27–46.
34 K. P. Pramoda, S. L. Liu and T. S. Chung, Macromol. Mater.
Eng., 2002, 287, 931–937.
Department
Administration, Report #DOT/FAA/AR-05/14.
of
Transportation,
Federal
Aviation
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 1526–1535 | 1535