A deep insight into polybenzoxazole formation in the heterocycle-containing polybenzoxazine: An enlightening thought for smarter precursor design

Article abstract of DOI:10.1016/j.polymer.2021.123789

When artificially introducing an ortho-amide group into benzoxazine monomer, the conversion of polybenzoxazine (PBZ) into polybenzoxazole (PBO) could happen at higher temperature. However, the mechanism investigation for this reaction is far from enough,

Full text of DOI:10.1016/j.polymer.2021.123789

Polymer 226 (2021) 123789
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Polymer
journal homepage: www.elsevier.com/locate/polymer
A deep insight into polybenzoxazole formation in the
heterocycle-containing polybenzoxazine: An enlightening thought for
smarter precursor design
a,
c
a,
b
a,
b
a,
c
a,b,*
Jingkai Liu , Lijun Cao , Jinyue Dai , Yunyan Peng , Xiaoqing Liu
a
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang, 315201, China
b
Key Laboratory of Marine Materials and Related Technologies, Chinese Academy of Sciences, Ningbo, Zhejiang, 315201, China
c
University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
When artificially introducing an ortho-amide group into benzoxazine monomer, the conversion of poly-
benzoxazine (PBZ) into polybenzoxazole (PBO) could happen at higher temperature. However, the mechanism
investigation for this reaction is far from enough, which is very significant for the PBO fabrication from PBZ. In
this work, we designed three kinds of benzoxazines (PIC-abz, 4-MEP-abz and 6-MEP-abz) containing both pyridyl
and amide group with similar structures. The existence of intramolecular N–H⋯N H-bonding involved with
amide group and the N atom in pyridine ring, as well as the N–H⋯O H-bonding between the amide N–H and the
Polybenzoxazine
Polybenzoxazole
Hydrogen bonding
Mechanism
1
1
O atom in oxazine ring, were confirmed by H– H nuclear overhauled effect spectroscopy (NOESY) and con-
centration dependent 1H NMR spectra. In addition, the respective strength of these H-bonding in different
precursors was calculated by non-covalent interaction (NCI) simulation. Results showed that besides the per-
formance of resulted PBZ, the conversion temperature of PBZ into PBO was significantly influenced by the
N–H⋯N hydrogen bonding involved with pyridyl and amide groups. Based on which, the mechanism of PBO
formation was proposed and systematically investigated. The outcome of this work will help us to obtain PBO in
a more efficient and reasonable manner starting from PBZ.
1
. Introduction
Polybenzoxazine (PBZ) is a relatively new type of phenolic resin,
In last two decades, the functional groups containing different hetero
atoms, including oxygen (hydroxyl [9], carbonyl [11], carboxyl [12],
alkoxy [13], furan [14], ether [15]), nitrogen (amide [16], azo [17],
pyridyl [18], cyanuric [19]), fluorine atoms [20,21], and sulfur [22]
have been extensively introduced into the benzoxazine monomers to
adjust their performances. Among them, the aromatic heterocycles
pyridine can not only provide strong hydrogen bonding acceptors, but
also rigid aromatic structure, which endowed the resin with high ther-
mal and mechanical properties [12]. In Takeichi’s group, the catalytic
effect on curing reaction caused by the basicity of pyridyl was revealed,
and they confirmed that the H-bonding involved with pyridyl ring was
which has received considerable attention in recent years, due to its
flexible molecular design, high glass transition temperature, excellent
thermal stability, low water absorption, good chemical-corrosion resis-
tance, as well as zero shrinkage and no by-product released upon curing
[
1]. Up to now, a large quantity of studies has proved that most of these
impressive properties were attributed to the inbuilt hydrogen bonds (i.
+
e., O–H⋯O, O–H⋯
π
, O–H⋯N and O⋯H N) [2–7]. For example, PBZ
possesses very low moisture absorption, which thanks to the presence of
abundant H-bonds shielding the interaction between Ar-OH and H
3]. The H-bonding can promote the benzoxazine ring-opening reaction,
2
O
g
responsible for a higher glass transition temperature (T ) [23]. Kuo et al.
[
[12] also reported the pyridine involved intermolecular H-bonding and
studied its effect on the wettability of PBZ. In addition, amide is a
frequently-used group to improve the H-bonding systems in benzoxazine
as well. The introduction of ortho-amide group can establish an intra-
molecular H-bonding associating with oxygen in oxazine, which
lower the curing temperature and limit its volatility before polymeri-
zation [8–10]. Therefore, designing and introducing extra hydrogen
bonding donors or acceptors to enrich the H-bonding systems in PBZ is a
promising strategy for its performance improvement.
*
Corresponding author. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang, 315201, China.
E-mail address: liuxq@nimte.ac.cn (X. Liu).
https://doi.org/10.1016/j.polymer.2021.123789
Received 25 February 2021; Received in revised form 12 April 2021; Accepted 16 April 2021
Available online 29 April 2021
0
032-3861/© 2021 Elsevier Ltd. All rights reserved.

J. Liu et al.
Polymer 226 (2021) 123789
resulted in a remarkable decrease of curing temperature [9]. Besides, an
additional, but important advantage of ortho-amide containing PBZ was
that they were thought to be the latent intermediate of polybenzoxazole
2. Experimental section
2.1. Materials
(
PBO). In 2012, for the first time, Ishida [16] pointed out that the PBZ
containing o-hydroxyamide moiety could be converted into PBO at
50–300 ◦C, indicating its potential as a PBO precursor. Since then, a
Picolinic acid (99%) and 4-Methylpyridine-2-Carboxylic Acid (99%)
were purchased from Adamas-beta reagent co., Ltd., China. 6-Methyl-
pyridine-2-Carboxylic Acid (99%) was obtained from Struchem, Co.,
2
series of benzoxazine monomers bearing o-amides and o-maleimide
have been developed [24–29]. And the conversion of PBZ into PBO has
been widely recognized. However, the specific mechanism for this re-
action and the influencing factors were rarely reported. On the one
hand, the occurrence of this transformation usually requires a much high
temperature (250–400 ◦C) [26–29], which will lead to the thermal
decomposition of PBZ. On the other hand, the contradiction between the
high performance of PBO [30] and the harsh fabrication process makes
easy synthesized PBZ an attractive PBO precursor [31–34]. Therefore, it
is necessary to explore the mechanism of oxazole structure formation in
PBZ system, to find out the influencing factors and to regulate its con-
version temperature.
2
Ltd., China. Triethylanmine (99%), sulfoxide chloride (SOCl ) (99%),
dimethylformamide (DMF) (99.5%), o-aminophenol (98%), 1,4-dioxane
(99%), n-hexane (99%) and paraformaldehyde (99%) were received
3
from Aladdin Reagent, Co., Ltd., China. CHCl (AR grade) and ethanol
absolute (AR grade) were purchased from Sinopharm Chemical Reagent
Co., Ltd. (Shanghai, China). All the chemicals were used as received
without treatment.
2
.2. Measurement
1H and 13C nuclear magnetic resonance (NMR) and 1H– H nuclear
1
In our previous study [35], it was found that the o-amide PBZ con-
taining different aromatic heterocyclic bridging groups showed different
PBO formation temperatures, which inspired us to investigate the
interaction between heterocyclic group and amides as well as their ef-
fects on the oxazole formation. In this work, we designed three kinds of
benzoxazine monomers containing both pyridyl and amide groups with
similar structures (PIC-abz, 4-MEP-abz and 6-MEP-abz, shown in
Scheme 1). By chemically changing the methyl substitution on pyridine
ring and taking advantages of their different induction and hindering
effects, the N atoms with various nucleophilicity were created, which
would produce diverse interactions with the adjacent amide groups, and
their effects on PBO formation were investigated. The interaction be-
tween N atom and amide group was identified as the N–H⋯N
overhauled effect spectroscopy (NOESY) spectra were conducted on a
4
3
00 MHz Bruker AVANCE III spectrometer using CDCl as solvent with
tetramethyl silane as internal standard reagent, in which the NOESY
spectra was conducted with the follow parameters: 2048 data points
2 1
along the f dimension, 256 free induction decays for the f dimension,
2
s relaxation delay and mixing time of 700 ms. Fourier transform
infrared (FT-IR) spectra were collected on a Thermo Nicolet 6700
Fourier transform infrared spectrometer (Thermo-Fisher Scientific) at
the transmission mode. The sample used for FT-IR was pressed into disk
by grounding with KBr powder. Spectra were collected from 400 to
ꢀ 1
4
000 cm with 32 scans. The in-situ non-isothermal FT-IR measure-
ment was conducted at various temperatures (25, 160, 170, 180, and
◦
◦
ꢀ 1
1
90 C) with a heating rate of 10 C min , while the isothermal FT-IR
1
H-bonding, whose existence was further confirmed by H NMR and
◦
C
for different samples was carried out at 25 and 250 respectively. Mass
1
1
H– H nuclear overhauser effect spectroscopy (NOESY). The different
spectra (MS) were measured with a LC-Q-TOF (AB Sciex, America) at
strength of involved H-bonding was determined by NRI calculations,
curing kinetic and splitting FT-IR. Furthermore, combining DSC and
non-isothermal FT-IR, significant differences in thermal stability and
oxazole formation temperature were observed. With the help of theo-
retical calculation, we further proposed the possible mechanism of
oxazole formation. This paper will not only lay a theoretical foundation
for developing smarter benzoxazine precursor, but also provide refer-
ential significance for optimizing the condition for PBO formation from
PBZ.
◦
5
00 C with an ionization voltage of 5500 V. Elementary analysis was
recorded on Elementar (Elementar Germany) showed as averaging data
from three times test respectively for each sample. Differential scanning
calorimetric (DSC) measurements were conducted with a METTLER
TOLEDO-TGA/DSC I under a nitrogen atmosphere flow rate of 20 mL
ꢀ 1
◦
min . The monomer was scanned from 50 to 300 C using various
◦
ꢀ 1
heating rates of 5, 10, 15 and 20 C min , while the cured resin was
◦
◦
ꢀ 1
scanned from 50 to 350 C at the heating rate of 20 C min , as well as
c. Thermogravimetric analysis (TGA) was performed on a Mettler-
Toledo TGA/DSC1 thermogravimetric analyzer (METTLER TOLEDO,
ꢀ 1
Switzerland) with high purity nitrogen as purge gas (50 mL min ) at a
◦
ꢀ 1
◦
scanning rate of 20 C min from 100 to 800 C, the differential thermal
gravity (DTG) results was directly obtained from software calculation of
TGA data. Thermogravimetric infrared gas chromatograph/mass
Scheme 1. Synthetic route of precursors and benzoxazine monomers.
2

J. Liu et al.
Polymer 226 (2021) 123789
spectrometer (TG-IR-GC/MS) was carried out by a PerkinElmer TGA
7.93 (td, J = 7.7, 1.7 Hz, 1H), 7.51 (ddd, J = 7.6, 4.8, 1.2 Hz, 1H),
7.36–7.27 (m, 4H), 7.22–7.15 (m, 2H), 6.99 (t, J = 7.7 Hz, 2H), 6.85 (dd,
J = 7.7, 1.5 Hz, 1H), 5.57 (s, 2H), 4.71 (s, 2H), 3.71 (s, Dioxane).
8
000-specturm Two-Clarus SQ8T with the same conditions used for TGA
testing, in which MS only recorded the intensity of water and the IR
ꢀ 1
ꢀ 1
spectra were scanned from 400 to 4000 cm
.
FT-IR (KBr, cm ): 3332, 1678, 1589, 1224, 1071, and 941.
17 3 2
Anal. Calcd for C20H N O : C, 72.49; H, 5.17; N, 12.68; O, 9.66.
2
2
.3. Synthesis of precursors and monomers
Found: C, 73.06; H, 5.13; N, 12.80; O, 9.03.
.3.1. N-(2-hydroxyphenyl)picolinamide (Abbreviated as PIC)
2.3.5. 4-methyl-N-(3-phenyl-3,4-dihydro-2H-benzo[e] [1,3]oxazin-8-yl)
picolinamide (Abbreviated as 4-MEP-abz)
The synthesis of PIC is shown in Scheme 1. Picolinic acid (4.92 g, 40
mmol), NEt
drops) were dissolved in CHCl
Then SOCl (3 mL, 41 mmol) was added dropwise and the solution was
3
(4.15 g, 41 mmol) and catalytic amounts of DMF (~15
4-MEP-abz was synthesized from 4-MEP (6.91 g, 20 mmol), aniline
(1.86 g, 20 mmol) and paraformaldehyde (1.80 g, 60 mmol) with similar
method as PIC-abz. Faint yellow powder (yield: 33%).
3
(100 mL) under an atmosphere of argon.
2
1
stirred at room temperature for 4 h. After that, the mixture was slowly
H NMR (400 MHz, Chloroform-d) δ 10.44 (s, 1H), 8.54–8.42 (m,
transferred into a solution of o-aminophenol (4.62 mg, 40 mmol) and
3H), 8.12 (dd, J = 8.7 Hz, 1H), 7.33–7.24 (m, 2H), 7.22–7.10 (m, 2H),
6.96 (t, J = 7.7 Hz, 2H), 6.81 (dd, J = 7.7, 1.5 Hz, 1H), 5.53 (s, 2H), 4.67
(s, 2H), 3.71 (s, Dioxane), 2.46 (s, 3H).
◦
NEt
3
(4.15 g, 41 mmol) dissolved in CHCl
3
(100 mL) at 0 C. The mixture
was stirred for another 20 h and then poured into distilled water. At last,
◦
ꢀ 1
the precipitate was dried in a vacuum oven at 60 C for 6 h (Yield: 81%)
FT-IR (KBr, cm ): 3333, 1679, 1592, 1223, 1066, and 939.
1
H NMR (400 MHz, Chloroform-d) δ 10.20 (s, 1H), 9.46 (s, 1H), 8.62
19 3 2
Anal. Calcd for C21H N O : C, 73.03; H, 5.54; N, 12.17; O, 9.26.
(
d, J = 4.8, 1.3 Hz, 1H), 8.27 (d, J = 7.8, 1.2 Hz, 1H), 7.91 (dd, J = 7.7,
.7 Hz, 1H), 7.50 (dd, J = 7.6, 4.8, 1.2 Hz, 1H), 7.18–7.09 (td, 2H), 7.04
d, J = 8.5, 1.5 Hz, 1H), 6.88 (dd, J = 7.5, 1.5 Hz, 1H).
Found: C, 73.14; H, 5.52; N, 12.03; O, 9.31.
1
(
2.3.6. 6-methyl-N-(3-phenyl-3,4-dihydro-2H-benzo[e] [1,3]oxazin-8-yl)
picolinamide (Abbreviated as 6-MEP-abz)
ꢀ
1
FT-IR (KBr, cm ): 3308, 3133, 2965, 1656, 1612, 1283, and 741.
Anal. Calcd for C12 : C, 67.28; H, 4.71; N, 13.08; O, 14.94.
Found: C, 68.31; H, 4.70; N, 12.83; O, 15.16.
H
10
N
2
O
2
6-MEP-abz was obtained from the reaction with aniline (1.86 g, 20
mmol) and paraformaldehyde (1.80 g, 60 mmol), according to the
similar method above. Light brown powder (yield: 27%).
1
2
.3.2. N-(2-hydroxyphenyl)-4-methylpicolinamide (Abbreviated as 4-
H NMR (400 MHz, Chloroform-d) δ 10.47 (s, 1H), 8.44 (d, J = 8.0
MEP)
Hz, 1H), 8.08 (d, J = 7.5 Hz, 1H), 7.76 (t, J = 7.7 Hz, 1H), 7.34–7.24 (m,
4H), 7.18–7.11 (m, 2H), 6.96 (td, J = 7.6, 3.3 Hz, 2H), 6.81 (d, J = 7.7
Hz, 1H), 5.52 (s, 2H), 4.66 (s, 2H), 3.71 (s, Dioxane), 2.66 (s, 3H).
4
-Methylpyridine-2-Carboxylic Acid (5.48 g, 40 mmol) reacted with
3
o-aminophenol (4.62 mg, 40 mmol), companied with NEt (4.15 g, 41
ꢀ 1
mmol), CHCl
3
as solvent and catalytic amounts of DMF (~15 drops),
FT-IR (KBr, cm ): 3326, 1678, 1591, 1225, 1060, and 921.
Anal. Calcd for C21 : C, 73.03; H, 5.54; N, 12.17; O, 9.26.
following a method similar to that used for PIC (Yield: 71%)
19 3 2
H N O
1
H NMR (400 MHz, Chloroform-d) δ 10.23 (s, 1H), 9.53 (s, 1H), 8.46
Found: C, 73.21; H, 5.49; N, 12.43; O, 8.87.
(
d, J = 4.9 Hz, 1H), 8.09 (s, 1H), 7.3 (d, J = 5.1 Hz, 1H), 7.2–7.1 (m, 2H),
7
.04 (dd, J = 8.0, 1.5 Hz, 1H), 6.92–6.83 (m, 1H), 2.45 (s, 3H).
2.4. Preparation of polybenzoxazine and polybenzoxazole
ꢀ
1
FT-IR (KBr, cm ): 3309, 3125, 2965, 1657, 1612, 1283, and 741.
Anal. Calcd for C13 : C, 68.41; H, 5.30; N, 12.27; O, 14.02.
Found: C, 68.62; H, 5.36; O, 14.09; N, 11.93.
H
12
N
2
O
2
Benzoxazine monomers (PIC-abz, 4-MEP-abz, and 6-MEP-abz) were
filled into a stainless-steel mold, which was cured by a programmed
◦
◦
◦
temperature rising procedure: 1 h at 140 C, 2 h at 160 C, 2 h at 180 C,
◦
2
.3.3. N-(2-hydroxyphenyl)-6-methylpicolinamide (Abbreviated as 6-
and 1 h at 200 C under certain pressure. After that, the materials were
MEP)
cooled down to room temperature slowly, and the related poly-
6
-Methylpyridine-2-Carboxylic Acid (5.48 g, 40 mmol) reacted with
benzoxazines were obtained. The polybenzoxazole (PBO) were prepared
◦
o-aminophenol (4.62 mg, 40 mmol), companied with NEt
3
(4.15 g, 41
by direct heat treating polybenzoxazine at 300 C for 1 h.
3
mmol), CHCl as solvent and catalytic amounts of DMF (~15 drops),
following a method similar to that used for PIC (Yield: 73%)
2.5. Calculation details
1
H NMR (400 MHz, Chloroform-d) δ 10.26 (s, 1H), 9.58 (s, 1H), 8.07
(
d, J = 7.6 Hz, 1H), 7.78 (t, J = 7.7 Hz, 1H), 7.34 (d, J = 7.6 Hz, 1H),
2.5.1. Non-covalent interaction (NCI)
7
2
.14 (dd, J = 8.0, 6.2 Hz, 2H), 7.08–7.01 (m, 1H), 6.92–6.83 (m, 1H),
.62 (s, 3H).
Selecting the molecular geometry and orbital composition optimized
by gaussian 09 [36] at B3LYP [37,38]-D3 [39]/6-31G**level, based on
FT-IR (KBr, cm 1): 3296, 3118, 2965, 1656, 1612, 1283, and 741.
ꢀ
the research method of weak interaction proposed by Yang’s research
2
Anal. Calcd for C13
H
12
N
2
O
2
: C, 68.41; H, 5.30; N, 12.27; O, 14.02.
group [40], the reduced density gradient (RDG)-sign(λ )
ρ
diagram was
Found: C, 68.54; H, 5.31; O, 14.30; N, 11.85.
obtained by using NCIplot [41] and Multiwfn [42], and visual molecular
dynamics (VMD) was used to visualize the NCI results.
2
.3.4. N-(3-phenyl-3,4-dihydro-2H-benzo[e] [1,3]oxazin-8-yl)
picolinamide (Abbreviated as PIC-abz)
2.5.2. Density functional theory (DFT)
PIC (6.62 g, 20 mmol), aniline (1.86 g, 20 mmol) and para-
formaldehyde (1.80 g, 60 mmol) were added into a single flask with
All structures were optimized using the B3LYP [37,38]-D3
[39]/6–311++G(d,p). At the level of B3LYP-D3/6–311++G(d,p), the
electronic energy is calculated using the optimized structure. The fre-
quency of optimized structure was further analyzed by employing
M06/6-31G(d,p) method under the condition of 523.15 k and 1 atm,
from which the Gibbs free energy correction value was calculated.
Finally, the Gibbs free energy is obtained by adding the correction
values of Gibbs free energy and electronic energy.
◦
chloroform as solvent, the mixture was refluxed at 80 C and maintained
at this temperature for 20 h with constant stirring. The obtained solution
was washed by sodium hydroxide (4% w/w), and the organic phase was
dried over anhydrous magnesium sulfate. Then the filtrated organic
phase was further dropped into n-hexane and the precipitate was
collected, followed by recrystallization in ethanol. At last, the solid was
filtered to afford filtration final product which was dried in vacuum oven
to obtain the pink powder (yield: 35%).
1
H NMR (400 MHz, Chloroform-d) δ 10.47 (s, 1H), 8.70 (dt, J = 4.8,
1
.4 Hz, 1H), 8.49 (dd, J = 8.2, 1.5 Hz, 1H), 8.32 (dt, J = 7.9, 1.1 Hz, 1H),
3

J. Liu et al.
Polymer 226 (2021) 123789
showing at 690-710 cm 1, which is attributed to the 1,2,3-trisubstituted
benzene ring. Besides these evidences, the elemental analysis results
(Experimental Section) and Molecular mass spectra (Fig. S4) also
implied the successful synthesis of target benzoxazine monomers.
ꢀ
3
. Results and discussion
3
.1. Synthesis and characterization of precursors
The synthetic process of PIC-abz, 4-MEP-abz and 6-MEP-abz are
illustrated in Scheme 1. The reaction underwent two steps, namely
amidation and Mannich condensation. In the first step, pyridine acid
could selectively react with the amino groups to form 2-amidephenol
structure with high yield, due to the higher activity of primary
amines. As for the second step, the combination reaction between ani-
line, paraformaldehyde and different precursor with the feeding ratio of
3
.2. Hydrogen bonding identification in precursors
The N–H⋯N H-bonding investigation in PIC, 4-MEP and 6-MEP is of
great significance, because the groups dedicated to form H-bonding
(
pyridine and amides) were unwavering after the formation of benzox-
azine monomer. Considering the chemical similarities between PIC, 4-
1
:3:1 readily took place.
1H NMR, 13C NMR, and FT-IR were employed to confirm the chem-
MEP and 6-MEP, the 1H– H nuclear overhauser effect spectroscopy
1
(
NOESY) was employed to identify the intramolecular H-bonding be-
ical structures of precursors PIC, 4-MEP and 6-MEP (Supporting infor-
mation, Figs. S1-S3 and Fig. 1d, e, and f), all of which prove their high
tween the N atom in pyridine and N–H in amide group. In Fig. S5a, b and
c, the signals marked by blue circle pointed out the presence of N–H⋯N
H-bonding interaction, which can be detected due to that the chemical
environment of proton adjacent to H-bonding acceptor or donor will be
changed after the formation of H-bonding [35]. In addition, once an
intramolecular H-bonding was formed, the chemical shift of involved
proton will not be influenced by the varying solution concentration, and
this fact was frequently reported in previous work [9,35,43,44]. In
Fig. S5d, the chemical shifts belonging to N–H were almost unchanged
as the solution concentration increased, which further proved the exis-
tence of N–H⋯N intramolecular H-bonding in PIC, 4-MEP and 6-MEP.
Theoretically, as an electron-donating group, methyl will slightly
increase the electron cloud density of connecting aromatic rings, then
improve the nucleophilicity of N atom in pyridine. That is the reason for
most methyl-substituted pyridine chemicals demonstrate certain regu-
larity in basicity. For example, 4-methylpyridine possesses a higher pKa
than 2-methylpyridine, while 2,4-Lutidine exhibits a better solubility
than 2,6-Lutidine in water. In this work, for 6-MEP, the methyl located at
ortho-position of the N atom in pyridine could contribute more to
alkalinity than the para-positioned methyl in 4-MEP. Besides, there is
additional steric hindrance between the lone-pair electron of N atom in
1
13
purities very well. Fig. 1a, b, and c shows the H and C NMR spectra of
the target benzoxazines (PIC-abz, 4-MEP-abz and 6-MEP-abz), and their
mass spectrometry were recorded in Fig. S4. In 1H NMR spectra, the
characteristic signals at 4.7 (s, Ar-CH
2
-N) and 5.6 ppm (s, O–CH –N) are
2
due to successful oxazines ring formation. The signals appeared at 3.65
ppm are attributed to the dioxane for assisting dissolution. The peak
integration for different protons, i.e., amide hydrogen, Ar-CH
O–CH –N, and methyl on pyridine ring, were also marked in Fig. 1. For
PIC-abz, the integral area ratio of peaks for amide hydrogen, Ar-CH -N
and O–CH –N was found to be 1: 2: 2. While for 4-MEP-abz and 6-MEP-
2
-N,
2
2
2
abz, the ratio is 1: 2: 2: 3, due to the pendant methyl on pyridine rings. In
Fig. 1d, e, and f, compared with the precursors (PIC, 4-MEP, 6-MEP), the
disappearance of hydroxyl groups can be clearly observed. And the
characteristic bonds for amide and methyl groups still remained except
ꢀ
1
ꢀ 1
for the slight red shift of N–H (~3300 cm ) and amide I (~1660 cm ),
hinting that their chemical environment changed after oxazine forma-
tion. In addition, not only did the characteristic bands for oxazine ring
ꢀ 1
ꢀ 1
occur at ~930 and ~1065 cm , but also the peak near 741 cm
standing for 1, 2-bisubstituted benzene was replaced by the signal
1
13
Fig. 1. H and C NMR spectra for PIC-abz (a), 4-MEP-abz (b) and 6-MEP-abz (c); FT-IR curves of different precursors and monomers (d).
4

J. Liu et al.
Polymer 226 (2021) 123789
pyridine and methyl in 6-MEP. Therefore, in 4-MEP, the 4-methyl will
pose more positive effect than the 6-methyl in 6-MEP on N–H⋯N H-
bonding strength enhancement. Based on these facts, it is easy to turn
out that the order of N–H⋯N H-bonding strength is: 4-MEP > 6-MEP >
PIC, which was displayed in Scheme 2 and the different color was used
only to distinguish the strength instead of their quantitative
relationships.
owing to the basicity of N atom in pyridine of monomers is different, it is
inevitable that there will be various proportions of N–H⋯N and N–H⋯O
in different monomers. The frequently used methods, 1H– H nuclear
overhauser effect spectroscopy (NOESY) was firstly employed to confirm
the existence of the two kinds of intramolecular H-bonding [48,49]. As
shown in Fig. 3a, in PIC-abz, the through-space interactions signal be-
tween amide proton and N in pyridine ring labeled by close letter a and b
prove the presence of N–H⋯N H-bonding, while the signal marked by
letter c illustrate the formation of N–H⋯O H-bonding. Similar results
can be observed in Fig. 3b as well. Notably, in 6-MEP-abz, other than the
letter c represented for N–H⋯O H-bonding, the through-space interac-
tion between amide proton to the pyridine shows only one signal peak,
whereas a signal peak derived from the interaction between methyl and
amide at (2.66, 10.61) instead (letter b), which was attributed to the
transmission made by N–H⋯N H-bonding as well (Fig. 3c). In addition,
1
In order to further make sure above conclusion, the Non-Covalent
Interaction (NCI) calculation was taken to figure out the inner second-
ary bonds in PIC, 4-MEP and 6-MEP. The results were exhibited as sign
2
(
λ )ρ
-Reduced Density Gradient (RDG) scatter diagram in Fig. S6. When
RDG was defined as 0.5 (empirical value) [40,45], the visualized results
for NCI by Visual Molecular Dynamic (VMD) are shown in Fig. 2a as an
example, in which the sign marked by red circle serve as the interaction
between the amide N–H and N atom in pyridine. Based on NCI theory,
2
1
the sign(λ )
ρ
value of this interaction can be read from the scale in the
H NMR in variable concentration was used to identified the presence of
lower left corner (varied color). From which, the repulsion interaction
the intramolecular hydrogen bonding system (N–H⋯N or N–H⋯O), in
which the chemical shifts affiliated with the sharing donor (amide
protons) in Fig. 3d do not change with concentration. Both results prove
the presence of the mentioned H-bonding system. Importantly, the
chemical environment of the proton involved in H-bonding (mainly
amide) in monomers would be different from that in precursors, which
corresponds well to the phenomena of red shift in FT-IR results.
Due to the existence of N–H⋯N and N–H⋯O intramolecular H-
bonding, where the two heteroatoms (N in pyridine and O in oxazine
ring) with different electronegativity shared one proton (amide N–H)
simultaneously, the competition between the two H-bonding acceptors
is predictable. Generally speaking, hydrogen bond acceptor with higher
electronegativity is more capable of attracting protons, leading to a
stronger H-bonding. Thus, the strength of N–H⋯O H-bonding in PIC-
abz, 4-MEP-abz and 6-MEP-abz would be different from each other,
caused by N–H⋯N H-bonding with different strength. Based on the re-
sults shown in Scheme 2, the strength of N–H⋯O H-bonding in ben-
zoxazine monomers should follow the sequence of PIC-abz > 6-MEP-abz
> 4-MEP-abz (Scheme 3b), which is just the opposite of the order of
N–H⋯N H-bonding strength in PIC, 4-MEP and 6-MEP. And the strength
of N–H⋯N H-bonding, both in benzoxazine monomers (PIC-abz, 4-MEP-
abz and 6-MEP-abz) and their precursors (PIC, 4-MEP and 6-MEP),
follow the same orders.
2
can be confirmed when the value of sign(λ )
ρ
is greater than zero, while
less than zero implies attraction interaction [40,45]. It is note that, the
symbol surrounded by the red circle simultaneously exhibit both the
attraction (green part) and repulsion (red part) interaction. It is easy to
propose that the attraction interaction was attributed to the formation of
N–H⋯N H-bonding, and the repulsion was caused by the five-membered
ring tension. Reading the value for green part in the marked symbol
2
corresponding to the ruler, the sign(λ )
ρ
value was found to be about
ꢀ
0.025. Therefore, the coordinate of N–H⋯N H-bonding is around
2
(
ꢀ 0.025, 0.05) in the sign(λ )
ρ
-RDG graph. In Fig. 2b–d, the sign
2
(
λ )
ρ
-RDG results of PIC, 4-MEP and 6-MEP were overlapped with each
other for better comparison, where the region stood for N–H⋯N
H-bonding was all labeled by a red oval. As we know, the higher the
2
absolute value of sign(λ )
ρ
for the scattered points implies the stronger
the non-covalent force [46,47]. In Fig. 2b, the superposition diagram
2
between PIC and 4-MEP shows that, the sign(λ )
scatter point is more negative in the square, indicating the stronger
ρ
belonged to the blue
N–H⋯N H-bonding interaction in 4-MEP. Similarly, this interaction in
6
-MEP was also observed to be stronger than that in PIC, as indicated by
Fig. 2c. As for the comparison between 4-MEP and 6-MEP, the blue
scatters for 4-MEP are on the left of those for 6-MEP (Fig. 2d), which
meant that the N–H⋯N H-bonding in 4-MEP were stronger than that in
6
-MEP. Therefore, the calculated order regarding the strength of
To our knowledge, the intramolecular H-bonding involves the O
atom in oxazine ring can catalyze the ring-opening reaction of benzox-
azine, stimulating the event occurs at a lower temperature. And the
stronger the H-bonding, the lower the activation energy required [35,
N–H⋯N H-bonding in PIC, 4-MEP and 6-MEP is the same as our
conjecture based on above structure analysis. And the pyridine N in
monomers do have different alkalinity.
4
9]. Therefore, the investigation of benzoxazine monomers’ curing be-
haviors serve as another effective and reliable method to discern the
3
.3. Hydrogen bonding identification in benzoxazine monomers
strength of N–H⋯O hydrogen bonds in PIC-abz, 4-MEP-abz and
6
-MEP-abz, and the curing reactivity of them will be negatively corre-
After the introduction of oxazine ring with O atom as H-bonding
lated with the strength of N–H⋯N H-bonding in them.
Both FT-IR measurement and DSC test was carried out to study the
curing behaviors of PIC-abz, 4-MEP-abz and 6-MEP-abz. Before curing,
the TGA test of different monomers was carried out, in which the similar
acceptor, protons in amides (N–H) can either form hydrogen bond with
pyridinic N (N–H⋯N) or have interactions with oxygen in oxazine ring
(
N–H⋯O), in which the NH proton has different attraction competition
between the involved two electron-donor (Scheme 3a). Nevertheless,
Scheme 2. Strength comparison of N–H⋯N H-bonding in PIC, 4-MEP and 6-MEP.
5

J. Liu et al.
Polymer 226 (2021) 123789
2
Fig. 2. NCI calculation results: visualization structure of PIC based on RDG = 0.5 (a); sign(λ )
ρ-RDG diagrams of PIC (b), 4-MEP (c) and 6-MEP (d).
Scheme 3. The hydrogen bonding system involved in the as-synthesized monomers (a); the predicted strength order in different monomers (b).
curves with two-steps of weight loss were observed (Fig. S7), suggesting
molecules with analogous structure and no obvious impurities in sam-
ples. In Fig. 4a, b and c, the non-isothermal FT-IR spectrum of different
monomers were portrayed, in which the blue dashed lines highlight the
abz possess the highest one. Based on which, we speculated that the
strengthen of N–H⋯O H-bonding in the benzoxazine monomers follows
the order of PIC-abz > 6-MEP-abz > 4-MEP-abz, and the strength of
N–H⋯N H-bonding in them showed the opposite sequence: PIC-abz < 6-
MEP-abz < 4-MEP-abz.
ꢀ 1
characteristic signals of oxazine ring (925-931 cm ). It was observed
that although different monomers underwent the same curing proced-
ures, the characteristic bonds regarding oxazine showed various
Besides, DSC measurement was also employed to investigate the
curing behaviors. In general, under the same curing conditions, the peak
or onset curing temperature is related to its intrinsic curing reactivity.
Namely, the lower onset curing temperature usually means higher
curing reactivity. As shown in Fig. 4d, PIC-abz exhibit the lowest onset
ꢀ 1
extinction times. In PIC-abz, the bond showing at 921 cm almost
◦
disappeared at 170 C. As for 4-MEP-abz, there was still a small bond
ꢀ
1
◦
◦
ꢀ 1
showing at 939 cm at 180 C. While in Fig. 4c, the signal at 941 cm
◦
almost disappeared at 180 C for 6-MEP-abz. These results roughly
indicate that 4-MEP-abz have the lowest ring-opening activity and PIC-
curing temperature at 152.6 C. And for 6-MEP-abz and 4-MEP-abz, they
◦
are at 158.4 and 160.2 C, respectively. By calculating the activation
6

J. Liu et al.
Polymer 226 (2021) 123789
1
1
1
Fig. 3. H– H NOESY spectrum of PIC-abz (a), 4-MEP-abz (b) and 6-MEP-abz (c); Concentration dependence H NMR measurement of N–H in different pre-
cursors (d).
◦
ꢀ 1
Fig. 4. FT-IR spectra with various heat treatment for PIC-abz (a), 4-MEP-abz (b) and 6-MEP-abz (c); DSC curves at a heating rate of 5 C min for different
monomers (d); Linear fitting results based on Kissinger (e) and Ozawa methods.
ꢀ
ꢁ
ꢀ
ꢁ
energy (E
In this work, the curing E
respectively obtained from Kissinger (1) and modified Ozawa equation
a
), the curing reaction activities can be intuitively quantified.
AE
R
a
E
a
for PIC-abz, 4-MEP-abz and 6-MEP-abz was
ln q = ln
ꢀ ln g(
α
) ꢀ 5.331 ꢀ 1.052
(2)
a
RT
p
(
2) [50,51], based on the DSC curves showing in Fig. S8.
p
where T was the peak curing temperature (K), q was the DSC heating
(
)
o
ꢀ 1
ꢀ
ꢁ
rate ( C/min), A served as the pre-exponential factor (min ) and R
q
AR
E
a
.
ln
= ln
ꢀ
RT
p
(1)
represented the universal gas constant (8.314 J/mol K). And in eq (2),
2
T
E
a
q
α
∫
d
α
g(
α
) =
= ꢀ ln(1 ꢀ
f(α)
a
α). The E (J/mol) based on Kissinger equation
0
7

J. Liu et al.
Polymer 226 (2021) 123789
ꢀ
2) and T
ꢀ 1
p
(Fig. 4e),
was figured out by linear fitting logarithm of (qT
p
hydroxyl groups and protons attached to N atom (i.e., dehydration), and
the N–H⋯N H-bonding was completely destroyed. Meanwhile, in
different PBZ systems, this process underwent different energy changes,
which mainly depended on the strength of N–H⋯N H-bonding, that was,
the more stable the H-bonding interaction, the greater energy con-
sumption required. Combined with the statement above regarding the
respective H-bonding strength in different PBZs, the difficulty of forming
oxazole structure in the involved system was also proposed and repre-
sented by illustration (Scheme 4b).
while the E
a
based on Ozawa formulate was obtained by calculating the
ꢀ
1
slope of ln q against T
p
a
(Fig. 4f). Theoretically, higher E implies lower
curing activity. Therefore, based on the results collected in Table 1, we
easily summarized the curing activity order of the three monomers as:
PIC-abz is highest followed by 6-MEP-abz, and 4-MEP-abz is lowest.
a
Notably, the E of synthesized monomers in this work was lower than
some monofunctional benzoxazine resins without intramolecular H-
bonding, and even lower than those of some typical bifunctional
monomers [25,48,52–56]”. From another point of view, these results
once again indicated the order of H-bonding strength in PIC-abz,
To verify the conjecture, at first, it was necessary to demonstrate that
the N–H⋯N H-bonding still existed at high temperatures, at least with a
high proportion. Therefore, the ratio of diverse H-bonding in the cured
systems (poly(PIC-abz), poly(4-MEP-abz) and poly(6-MEP-abz)) were
surveyed by splitting FT-IR spectra both at room temperature and
4
-MEP-abz and 6-MEP-abz. Namely, the strength of N–H⋯O
H-bonding in them follows the order of PIC-abz > 6-MEP-abz >
-MEP-abz, and their N–H⋯N H-bonding strength shows an exact
opposite sequence. Now, we could draw a definite conclusion that the
manipulation of H-bonding systems with different strength by properly
changing the substituent groups and their positions in pyridine rings was
successfully achieved, which was supported by above NCI and experi-
mental characterization.
4
◦
250 C. Based on previous studies [35,57,58], the region involving
ꢀ 1
hydrogen bonds (3700–2650 cm ) were divided into four fitting peaks
(the R2 values of the fitting curves are higher than 0.9) by OMNIC 8.2
using the Lorentzianꢀ Gaussian method [59], they were respectively
ꢀ
1
standing for intra- O–H⋯
π
H-bonding (3500 cm ), inter-/intra-
ꢀ 1
ꢀ 1
O–H⋯O and inter- O–H⋯N (3320 cm ), intra- O–H⋯N (3100 cm ),
and intra- N–H⋯O and N–H⋯N (2925 cm ), as shown in Fig. 5a, b and
c. The ratio and evolution of diverse H-bonding can be more clearly
represented by the histogram of their proportion in Fig. 5d and Table S1.
ꢀ 1
3
.4. Influence of hydrogen bonding on benzoxazole conversion
As demonstrated in Scheme 4a, after the benzoxazine monomers
From which, we can see that after the temperature was increased to
(
PIC-abz, 4-MEP-abz and 6-MEP-abz) were thermally cured, the ortho-
◦
2
50 C, all kinds of the H-bonding had been reduced to a certain extent,
amide substituted phenolic hydroxyl group was formed, which would
further transform into polybenzoxazole (PBO) at an elevated tempera-
ture (260–400 ◦C) accompanied with dehydration [26,35]. Looking
deep into the chemical structures (poly(PIC-abz), poly(4-MEP-abz) and
poly(6-MEP-abz)), the moiety marked by red circle was the core site
taking part into the formation of oxazole when the matrix was heated
especially the intra- O–H⋯
π
was completely disappeared. Although, the
ꢀ 1
proportion of the splitting peak at 2925 cm occupied a relatively small
proportion in the cured resins H-bonding system, a large percentage of
them could be remained under the high temperature environment,
hinting that the N–H⋯N intramolecular H-bonding possessed relatively
high stability. In addition, among them, the proportion of intra- N–H⋯O
and N–H⋯N in poly(PIC-abz) decreased the most (from 14.70% to
◦
over 260 C, in which the presence of N–H⋯N H-bonding was consid-
ered to have a very significant effect on PBO formation. We speculated
that the existence of N–H⋯N H-bonding may hinder the rotation of the
reactive group and affect the structure of the intermediate, both of
which will related with the strength of N–H⋯N H-bonding until the
dehydration is completed due to the vanishment of donor and acceptor
of H-bonding. In Scheme 4b, the attack mechanism of intramolecular
cyclization involving an intermediate state containing twisted N–H⋯N
H-bonding was proposed (marked by yellow), at which time the dehy-
dration do not happen. In detail, in the early stage, upon the related
groups preparing for the next intramolecular cyclization after thermal
activation, the chemical bonds involved were rotated to the position
suitable for attack, in which the N–H⋯N H-bonds started to twist due to
the change of the relative position. Subsequently, the transition state
gradually appeared, and the heterocycles containing N and O atoms
initially formed, of which the ring tension would lead to further
distortion of the implicated H-bonding. As for the closing stage, the final
formation of oxazole ring was accompanied by the disappearance of
9
.07%), implying their weakest strength, as shown in Table S1. In
contrast, poly(4-ME-abz) exhibited the lowest decreasing amplitude
(
(
from 15.55% to 11.48%) due to the highest stability, while poly
4-ME-abz) showed medium change (from 15.70% to 11.12%), which
well supported our analysis above. However, the PBO conversion re-
quires the participation of amide, most of which was still in the state of
H-bonding association, we could scarcely deny that the N–H⋯N
H-bonding would have effects on the oxazole formation reaction.
In order to investigate the PBO generation reaction in detail, DSC and
TG-IR-GC/MS tests were performed. As we know, the conversion of PBZ
into PBO is an endothermic reaction, therefore the degree of difficulty
for this reaction can be preliminarily estimated by DSC measurement. In
Fig. 6a, it is found that all the PBZ resins began to show an endothermic
◦
peak above 280 C, which indicated the formation of oxazole structure.
Meanwhile, the onset temperatures followed the order of poly(PIC-abz)
◦
◦
◦
(
287.1 C) < poly(6-MEP-abz) (290.1 C) < poly(4-MEP-abz) (291.2 C),
as well as the same trend was observed in their peak temperature, which
◦
was found to be as high as over 320 C. The trigger time of benzoxazole
Table 1
reaction could be also confirmed by TG-IR-GC/MS, due to the occur-
rence of oxazole formation will be accompanied by the production of
water. The MS could record the appearance time of the fragments with
Data of curing dynamic calculation.
ꢀ
1
a
Samples
PIC-abz
Heating rate
q)
T
p
(K)
Ea (kJ mol
Kissinger
83.47
)
ln A
(
2
molecular weight of 18 (H O). From Fig. 6b, we could see that the time
Ozawa
84.18
Kissinger
22.21
Ozawa
22.96
sequence for water generation was same as the DSC results. In addition,
the thermal stability of PBZ could be further improved once the PBO was
formed. At the same time, this reaction could happen only at high
temperature, the accompanied thermal decomposition of polymeric
segment can hardly be avoided. Therefore, the earlier the oxazole
structure is produced, the less the resin is decomposed, and the higher
the carbon residue will be [26,60]. As could be seen from Fig. 6c, the
5
1
1
2
5
436.4
447.4
456.3
461.7
438.3
447.6
456.6
462.6
441.4
452.6
458.4
465.5
0
5
0
6
4
-MEP-
abz
87.40
92.78
90.21
95.37
23.23
24.47
24.58
25.84
10
1
2
5
5
0
◦
first weight-loss stage (~325 C) occurred earlier in poly(4-MEP-abz)
-MEP-
abz
10
and poly(6-MEP-abz) system than that in poly(PIC-abz), because of
not only the production of oxazole was accompanied by dehydration,
but also some fragment was more likely to be decomposed when the
conversion of PBO delayed. As a result, the second weight-loss stage
1
2
5
0
a
ꢀ 1
The dimension of A is min , and
α
was 0.5.
8

J. Liu et al.
Polymer 226 (2021) 123789
Scheme 4. Polymerization reaction and polybenzoxazole formation under heat (a); The proposed evolution process from polymers to PBO (b).
◦
Fig. 5. Fitting curves of FT-IR spectra at 25 and 250 C of poly(PIC-abz) (a), poly(4-MEP-abz) (b) and poly(6-MEP-abz) (c); Proportion of various hydrogen bonding
of the cured resins(d).
◦
(
~486 C) was observed in the lower temperature range for poly
PIC-abz), while for poly(4-MEP-abz) and poly(6-MEP-abz), they
Additionally, owing to the higher oxazole reaction temperature
accompanied by segment decomposition, the PBOs derived from poly
(4-MEP-abz) and poly(6-MEP-abz) exhibited a slightly lower glass
(
showed
a
relatively higher second weight-loss temperature.
9

J. Liu et al.
Polymer 226 (2021) 123789
◦
Fig. 6. DSC curves for polybenzoxazines (a), appearance time of fragment that molecular weight of 18 (b), TGA (c), and FT-IR spectra of the fragment at 300 C (d).
transition temperature than poly(PIC-abz) (Fig. S9). The debris from the
3.5. Theoretical calculation
thermal decomposition of poly(PIC-abz), poly(4-MEP-abz) and poly
◦
(
6-MEP-abz) at 300 C were also detected by FT-IR. In Fig. 6d, the
To further seek the definite evidences, the theoretical calculation
was used to simulate the transformation process. We took the moiety
marked by red circle in Scheme 4a as the model compound. It should be
noted that the environment of this simulation was set to high temper-
area stood for water showed different intensity responsible for different
extent of PBO reaction. Meanwhile, the emergence of alkyl and carbon
dioxide peaks meant carbon-containing groups fractured. It was also
found that the peak related to N–H was found in the lines, indicating that
a portion of amide groups failed to form benzoxazole structure. How-
ever, the intensity of N–H in poly(6-MEP-abz) was observed to be the
lowest, that was to say, the earliest formation of benzoxazole conferred
the resins higher thermal stability. From these results, it was easy to
conclude that the stronger the N–H⋯N hydrogen bonding, the later
formation of PBO, as well as the less fragment released at higher tem-
perature. Compared with previous literatures [16,20,24,61–63], the
temperature required for PBO formation in this work was higher. For
example, Ishida et al. [16] first monitored the production of oxazole
◦
ature over 250 C and absence of additional chemicals. After optimizing
this model chemical, a majorizing structure could be obtained and
shown in Fig. 7. In which both the distance between pyridine nitrogen
and proton of 0.211 nm as well as the angle of N–H⋯N H-bonding
◦
112.21 for ground state were figured out by measuring their geometric
properties. According to Desiraju and Sreiner’s theory [64], we can
confirm the presence of this H-bonding and classified it into a strong
hydrogen bonding according to its nature. When the oxazole formation
reaction was initiated, the intramolecular cyclization took place and a
transition state (TS) was formed, where the N–H⋯N H-bonding became
a distorted state with transitional geometric properties (distance of
1
3
using C CP MAS, and found that the diagnostic peak of PBO appeared
◦
◦
after 250 C treatment for 1 h. The similar temperatures for this thermal
0.220 nm and angle of 110.12 ). At this point, this hydrogen bond still
event were also observed in the PBZ containing an electron-withdrawing
existed but the strength became weaker. The data for three model
compounds both in ground and transition states were summarized in
Table S2, in which not only the twisted H-bonding are exhibited in every
transition states, but also the sequence of bond lengths or angles agree
well with our above data regarding the strength of N–H⋯N H-bonding.
Subsequently, the intermediate was finally dehydrated to form an oxa-
zole ring (final state, FS), accompanied with the complete disappearance
of N–H⋯N H-bonding due to the leaving of both donor and acceptor.
The calculated energy required for this reaction for poly(PIC-abz), poly
(4-MEP-abz) and poly(6-MEP-abz) were summarized in Table 2.
Relating the calculated data to different cured resins, we concluded that
poly(PIC-abz) required the least energy while the system with 4-methyl
substitution need the most energy.
group, different bridge groups, and ortho-amide [20,24,63]. In present
study, the endothermic peak for water generation in poly(PIC-abz), poly
◦
(
4-MEP-abz) and poly(6-MEP-abz) all took place over 320 C. And the
only difference between ortho-amide PBZ in this work and the previously
reported ones might be the intentionally designed N–H⋯N H-bonding.
Therefore, it was easy and reasonable to correlate the PBO conversion
reaction with N–H⋯N H-bonding. That was to say, the N–H⋯N
H-bonding would significantly affect the conversion temperature of
oxazole, and the higher H-bonding strength would lead to the higher
temperature for this transformation, based on above results.
1
0

J. Liu et al.
Polymer 226 (2021) 123789
Fig. 7. Simulating process of benzoxazole conversion, where the model compounds correspond to poly(PIC-abz).
high purities were confirmed. Through NCI calculation and experi-
mental investigation, the manipulation of H-bonding systems with
different strength by properly changing the substituent groups and their
positions in pyridine rings was successfully achieved. The strengthen of
N–H⋯O H-bonding in the synthesized benzoxazine monomers followed
the order of PIC-abz > 6-MEP-abz > 4-MEP-abz, while the strength of
N–H⋯N H-bonding in them showed the opposite sequence: PIC-abz < 6-
MEP-abz < 4-MEP-abz. After corelating the H-bonding features with the
formation of PBO moieties in poly(PIC-abz), poly(4-MEP-abz) and poly
Table 2
Calculated energy required to complete the reaction.
Model compound
Non-
4- substituted
6- substituted
substituted
Corresponding polymer
poly(PIC-abz)
19.36
poly(4-MEP-
abz)
poly(6-MEP-
abz)
Reaction potential (Kcal/
mol)
20.42
20.39
(
6-MEP-abz), the mechanism regarding intramolecular cyclization and
the influencing factors of oxazole formation related to the N–H⋯N H-
bonding strength were proposed and proved. Based on the attack
mechanism shown in Scheme 4, the N–H⋯N H-bonding with varied
strength would hinder the rotation of reactive group (carbonyl) in
different degrees, and then a five-membered ring intermediate state
constructed by the N–H⋯N H-bonding with different degree of distor-
tion was formed, which resulted in the varied PBO formation tempera-
tures. The results in this work will inspire us a new thought for exploring
a more efficient way to control the PBO formation temperature from
PBZ. Based on which, a more efficient strategy for the synthesis of PBO
might be developed in the future.
Throughout the whole reaction process, on one hand, the reaction
involves the dissociation of strong hydrogen bonding, on the other hand,
there is no difference in the chemical groups participating in the reaction
in diverse systems. Therefore, we have adequate reasons to believe that
the intramolecular cyclization and dehydration is not the main factor
affecting the reaction energy, but the strength of N–H⋯N H-bonding.
According to above measurements and calculations towards the N–H⋯N
H-bonding in different systems, the N–H⋯N H-bonding in 4-MEP-abz
was the strongest, and the highest energy was required for poly(4-
MEP-abz) conversion into PBO. As for PIC-abz, it had the weakest
N–H⋯N linkage, which led to the lowest conversion temperature from
poly(PIC-abz) to PBO moieties. As exactly proposed in Scheme 4, the
fragments containing ortho-amide substituted phenolic hydroxyl group
undergo intramolecular cyclization at higher temperature, and finally
dehydrate to form PBO accompanied by the disappearance of N–H⋯N
H-bonding. In the whole process, due to the varied strength of same H-
bonding in different systems, different energies were required for
destructing it, resulting in different temperatures for the conversion of
PBZ into PBO. In one word, the proposed mechanism for oxazole for-
mation in PBZ and its relationship with N–H⋯N H-bonding strength are
reasonable and acceptable.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors are grateful for the financial support from Zhejiang
Provincial Natural Science Foundation of China (LR20E030001), Chi-
nese Academy of Sciences (KFZD-SW-439), National Natural Science
Foundation of China (U1909220), “Science and Technology Innovation
2025” Major Project of Ningbo (2018B10013), National Ten Thousand
Talent Program for Young Top-notch Talents, Ten Thousand Talent
Program for Young Top-notch Talents of Zhejiang Province.
4
. Conclusions
Three kinds of benzoxazine monomers containing both pyridyl and
amide groups, PIC-abz, 4-MEP-abz and 6-MEP-abz were successfully
prepared. Based on NMR, MS and FT-IR, their chemical structures and
1
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J. Liu et al.
Polymer 226 (2021) 123789
Appendix A. Supplementary data
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