Benzoxazine Atropisomers: Intrinsic Atropisomerization Mechanism and Conversion to High Performance Thermosets

Article abstract of DOI:10.1021/acs.macromol.8b01924

Atropisomers have inspired chemists and biologists for decades due to their chiral structures and associated biological properties. However, most of atropisomers reported so far arise in highly substituted biaryls and related compounds, and other types have been rarely observed. Here we report a new type of atropisomerism in ortho-tetrahydrophthalimide functional 1,3-benzoxazine family, where the atropisomerism is evident from NMR spectra, with the branching ratio of the atropisomeric configurations invariant with the measurement temperatures. Density functional theory calculations suggested that the reaction intermediate, ortho-tetrahydrophthalimide phenol, is key to the atropisomerism, which creates a large energy barrier after deprotonation and thus determines the branching ratios of the benzoxazine atropisomers. In addition, the ring-opening polymerization of benzoxazine atropisomers has also been investigated. The benzoxazine atropisomers bearing acetylene exhibit unexpectedly low polymerization temperature in the absence of catalysts, suggesting a self-catalyzed polymerization process. Despite the absence of antiflammable additives, the corresponding polybenzoxazine deriving from benzoxazine atropisomers containing acetylene shows exceptionally low heat release capacity (67.2 J g^1-K^-1) and excellent char residue value (62%). With this work we demonstrate atropisomerism in the 1,3-benzoxazine family for the first time, and provide molecular-level insights to the mechanism, which can open up possibilities for new applications of atropisomers spanning from the microelectronic to the aerospace industries.

Full text of DOI:10.1021/acs.macromol.8b01924

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Benzoxazine Atropisomers: Intrinsic Atropisomerization Mechanism
and Conversion to High Performance Thermosets
Kan Zhang,† Zhikun Shang,† Corey J. Evans,‡ Lu Han,§ Hatsuo Ishida,*,§ and Shengfu Yang*,‡
†School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
‡Department of Chemistry, University of Leicester, Leicester LE1 7RH, United Kingdom
§Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
S
* Supporting Information
ABSTRACT: Atropisomers have inspired chemists and biologists for decades due to their chiral structures and associated
biological properties. However, most of atropisomers reported so far arise in highly substituted biaryls and related compounds,
and other types have been rarely observed. Here we report a new type of atropisomerism in ortho-tetrahydrophthalimide
functional 1,3-benzoxazine family, where the atropisomerism is evident from NMR spectra, with the branching ratio of the
atropisomeric configurations invariant with the measurement temperatures. Density functional theory calculations suggested
that the reaction intermediate, ortho-tetrahydrophthalimide phenol, is key to the atropisomerism, which creates a large energy
barrier after deprotonation and thus determines the branching ratios of the benzoxazine atropisomers. In addition, the ring-
opening polymerization of benzoxazine atropisomers has also been investigated. The benzoxazine atropisomers bearing
acetylene exhibit unexpectedly low polymerization temperature in the absence of catalysts, suggesting a self-catalyzed
polymerization process. Despite the absence of antiflammable additives, the corresponding polybenzoxazine deriving from
benzoxazine atropisomers containing acetylene shows exceptionally low heat release capacity (67.2 J g1−K−1) and excellent char
residue value (62%). With this work we demonstrate atropisomerism in the 1,3-benzoxazine family for the first time, and
provide molecular-level insights to the mechanism, which can open up possibilities for new applications of atropisomers
spanning from the microelectronic to the aerospace industries.
are strongly dependent on the temperature, the steric demand
of the substituents, and the length and rigidity of bridges. In
addition to a merely physical rotation about the axis, the
atropisomerism in biaryls can also result from the photo-
chemical or chemically induced processes.2 However, so far the
molecular-level interpretation for the atropisomerization
mechanism in biaryls is still lacking, which poses a great
challenge to developing atropselective approach to single
isomers. Moreover, atropisomerism has rarely been reported in
INTRODUCTION
■
Axial chirality, as a stereogenic element in rotationally hindered
biaryl compounds, has been extensively investigated and
reviewed.1−6 This special type of stereochemistry featured by
hindered rotation is termed as atropisomerism,7 where the
restricted rotation about a single chemical bond results in
stereoisomers. Since the first observation of atropisomerism in
1922,8 natural products equipped with a rotationally hindered
biaryl axis have been found to be widespread and structurally
diverse, which can exhibit unique bioactivities9,10 and can
enable biological stereoselectivity for molecular targets.11,12 In
general, the atropisomerism in axially chiral biaryl compounds
Received: September 6, 2018
© XXXX American Chemical Society
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magic-angle spinning (CP-MAS) solid-state 13C NMR spectra were
collected using a Bruker Avance III 400 MHz instrument, using
adamantance as reference. Powder samples were packed into a 4 mm
zirconia rotor with a spinning rate of 10 000 Hz, and the number of
transients was 6000 scans. Fourier transform infrared (FT-IR)
spectrophotometer was used to acquire FT-IR spectra. The
spectrometer was equipped with a deuterated triglycine sulfate
(DTGS) detector and dry air purge unit and was operated at a
resolution of 4 cm−1 in the frequency range of 4000−400 cm−1.
Elemental analysis was characterized by an elementary analyzer
(Elementar Vario EL-III). A TA Instruments differential scanning
calorimeter (DSC) model 2920 was used with a temperature ramp
rate of 10 °C/min and a nitrogen flow rate of 60 mL/min for all tests
of DSC study. All samples were sealed in hermetic aluminum pans
with lids. Viscosity was tested by Anton Paar GmbH Rheometer MCR
301 with constant shear rate at 10 s−1 with a temperature ramp rate of
5 °C/min. Thermogravimetric analyses (TGA) were performed on a
TA Instruments Q500 thermogravimetric analyzer that was purged
with nitrogen at a flow rate of 40 mL/min. A heating rate of 10 °C/
min was applied. Dynamic mechanical analysis (DMA) was
conducted on a TA Instruments Model Q800 DMA applying
controlled strain tension mode with amplitude of 10 μm and a
temperature ramp rate of 3 °C/min. Strain sweep was first performed
to determine the linear viscoelastic limit. Specific heat release rate
(HRR, W/g), heat release capacity (HRC, J/(gK)) and total heat
release (THR, kJ/g) were carried out on a microscale combustion
calorimeter (MCC) from PHINIX. MCC was measured from 100 to
900 °C at a heating rate of 1 °C/s in an 80 cm3/min stream of
nitrogen. The anaerobic thermal degradation products in the nitrogen
gas stream were mixed with a 20 cm3/min stream of oxygen prior to
entering the combustion furnace (900 °C). Heat release is quantified
by standard oxygen consumption,31 and HRR is obtained by dividing
dQ/dt at each time interval, by the initial sample mass, and HRC is
obtained by dividing the maximum value of HRR by the heating rate.
Computational Methods. Computational chemistry calculations
were carried out using both the Gaussian03 and CASTEP suite of
programs. For the G03 calculations the B3LYP hybrid function was
employed using a 6-31+G(d) basis set, with tight SCF convergence
and an ultrafine pruning grid.32 Structures were then optimized and
the harmonic vibrational frequencies determined to show that a true
structural minima had been found. For the 13C NMR calculation step
the B3LYP/6-31+G(d) optimized structures were used but the
shielding tensors were calculated at the MPW1PW91/6-311+G(2d,p)
level of theory which has been found to give accurate values of the 13C
chemical shift.33,34 The infrared spectrum were generated by
performing frequency calculations.
other types of chemical structures beyond biaryls and related
compounds.13,14
Benzoxazine has been widely investigated and reported due
to its outstanding properties for applications.15−21 Benzoxazine
resin is known to have the highest molecular-design flexibility
among all the polymeric materials and can be tailored to
desired properties. Besides, some unique stereochemistry can
also be observed in benzoxazine monomers.22−24 1,3-
Benzoxazine compounds consisting of the ortho-substituted
phenolic components possess significant advantages over the
para-phenolic counterparts in terms of monomer synthesis, as
well as the mechanical and physical properties of the
corresponding thermosets, due to intramolecular hydrogen
bonding in phenolic precursor and further reactions of the
ortho-substituted phenolic structure after ring-opening poly-
merization. The superior properties of the ortho-benzoxazines
to their para-substituted counterparts were unexpected since
all nonbenzoxazine polymers reported to data exhibit the
opposite trend.25−30
Ortho-imide substituted 1,3-benzoxazine molecules contain
three oxygen atoms in the oxazine ring and imide
functionalities, which are expected to be negatively charged.
If the two groups rotate about the C−N bond so the oxygen
atoms approach each other, then strong ionic repulsion will
occur, which may hinder the rotation and thus deliver
atropisomerism for these molecules. To prove the concept,
in this work we synthesized a few ortho-tetrahydrophthalimide
substituted 1,3-benzoxazines and investigated the chemical
structures using NMR techniques, including correlated spec-
troscopy (COSY) and heteronuclear multiple quantum
coherence spectroscopy (HMQC). Density functional theory
(DFT) calculations were then performed to interpret the
experimental observations. In addition, we also investigated the
ring-opening polymerization of this class of benzoxazine
atropisomers. To our knowledge, there are no previous reports
on the polymerization of atropisomers, neither on the
properties of their resulting polymers. Thus, another purpose
of this paper is to develop novel thermosets based on
benzoxazine atropisomers, aiming to achieve very high
performance thermosets with high thermally stability and low
flammability from mono-oxazine benzoxazines, despite the
majority of mono-oxazine benzoxazines cannot exhibit
excellent thermal properties due to their inability to form
large molecular weight polymers which is caused by
termination of the propagating species by the competing side
reactions.
Synthesis of 2-(2-Hydroxyphenyl)-3a,4,7,7a-tetrahydro-1H-iso-
indole-1,3(2H)-dione (Abbreviated as oHTI). Into a 250 mL round
flask were placed cis-1,2,3,6-tetrahydrophthalic anhydride (4.92 g,
0.03 mol), o-aminophenol (3.27 g, 0.03 mol), and 80 mL of acetic
acid. The mixture was stirred and refluxed for 6 h. After cooling to
room temperature, the precipitate was filtered and washed with 500
mL of deionized water. Removal of water by evaporation afforded a
white powder. The resulting white product was recrystallized from
EXPERIMENTAL SECTION
■
Materials. o-Aminophenol (>98%), p-aminophenol (>98%), cis-
1,2,3,6-tetrahydrophthalic anhydride, 4-chloroaniline (98%), 4-
ethynylaniline (98%), 4-aminobenzonitrile (98%), and paraformalde-
hyde (99%) were used as-received from Sigma-Aldrich. Aniline was
purchased from Aldrich and purified by distillation. Acetic acid,
hexanes, and xylenes were obtained from Fisher Scientific and used as-
received.
1
isopropanol. (yield ca. 88%). H NMR (400 MHz, DMSO-d6), ppm:
δ = 2.26−2.29 (d, 2H, −CH2−), 2.42−2.45 (d, 2H, −CH2−), 3.29−
3.40 (d, 2H), 5.94 (s, 2H, C−H), 6.83−7.27 (4H, Ar), 9.74 (s, 1H,
−OH). Anal. Calcd. for C14H13NO3: C, 69.12; H, 5.39; N, 5.76.
Found: C, 69.08; H, 5.42; N, 5.75.
Synthesis of 2-(3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-8-
yl)-3a,4,7,7a-tetrahydro-1H-isoindole-1,3(2H)-dione (Abbreviated
as oHTI-a). Into a 100 mL round flask were added 30 mL of xylenes,
aniline (0.93 g, 0.01 mol), oHTI (2.55 g, 0.01 mol), and
paraformaldehyde (0.61 g, 0.02 mol). The mixture was stirred at
120 °C for 6 h. The mixture was cooled to room temperature and
white crystal was obtained. (yield ca. 96%). 1H NMR (CDCl3), ppm:
δ = 2.31−2.43 (m, 2H, −CH2−), 2.68−2.74 (m, 2H, −CH2−), 3.24−
3.32 (m, 2H), 4.64−4.66 (d, 2H, Ar−CH2−N, oxazine), 5.31−5.35
(d, 2H, O−CH2−N, oxazine), 6.00−6.06 (m, 2H, C−H), 6.91−
Characterization. 1D 1H NMR spectra were recorded on a
Bruker AVANCE II 400 MHz nuclear magnetic resonance (NMR)
spectrometer at a proton frequency of 400 MHz in CDCl3/DMSO-d6
using tetramethyl silane as internal standard. 2D H−1H Correlated
1
1
spectroscopy (COSY) and H−13C Heteronuclear multiple quantum
coherence (HMQC) were also examined. In order to elucidate the
difference in isomerism, NMR spectra were also acquired using a
Varian Oxford AS300 (300 MHz), a Bruker Ascend III 500 MHz, and
a Varian Inova 600 MHz NMR sepctrometers. Cross-polarization
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Scheme 1. Synthesis of ortho-Tetrahydrophthalimide Benzoxazine Monomers
Figure 1. 1H-13C HMQC NMR spectra of oHTI-a in CDCl3. The expanded view shows that the oxazine protons of oHTI-a are correlated to four
different carbon resonances.
7.31 (8H, Ar). Anal. Calcd. for C22H20N2O3: C, 73.32; H, 5.59; N,
7.77. Found: C, 73.29%; H, 5.61%; N, 7.75%.
Synthesis of 4-(8-(1,3-Dioxo-3a,4-dihydro-1H-isoindol-2-
(3H,7H,7aH)-yl)-2H-benzo[e][1,3]oxazin-3(4H)-yl)benzonitrile (Ab-
breviated as oHTI-cy). Into a 100 mL round flask were added 40
mL of xylenes, 4-aminobenzonitrile (1.18 g, 0.01 mol), oHTI (2.55 g,
0.01 mol), and paraformaldehyde (0.60 g, 0.02 mol). The mixture was
stirred at 120 °C for 12 h. The mixture was cooled to room
temperature and white crystal was obtained. (yield ca. 91%). 1H NMR
(CDCl3), ppm: δ = 2.28−2.43 (m, 2H, −CH2−), 2.65−2.74 (m, 2H,
−CH2−), 3.24−3.35 (m, 2H), 4.69−4.71 (d, 2H, Ar−CH2−N,
oxazine), 5.32−5.36 (d, 2H, O−CH2−N, oxazine), 5.99−6.03 (m,
2H, C−H), 6.93−7.58 (7H, Ar). Anal. Calcd. for C23H19N3O3: C,
71.67; H, 4.97; N, 10.90. Found: C, 71.61%; H, 5.01%; N, 10.87%.
Synthesis of 3-(4-Ethynylphenyl)-3,4-dihydro-2H-benzo[e][1,3]-
oxazine (Abbreviated as Ph-ac). Into a 100 mL round flask were
added 30 mL of toluene, 4-ethynylaniline (1.17 g, 0.01 mol), phenol
(0.94 g, 0.01 mol), and paraformaldehyde (0.60 g, 0.02 mol). The
mixture was stirred at 100 °C for 8 h. Column chromatography was
used afterward to purify benzoxazine monomer with mixed solvents of
hexane and ethyl acetate in 4:1 ratio. The purified products were
further recrystallized from acetone/toluene mixtures (1:1). After
recrystallization, needle like crystals were obtained. (yield ca. 38%).
1H NMR (CDCl3), ppm: δ = 3.01 (d, 1H, CH), 4.67 (s, 2H, Ar−
Synthesis of 2-(3-(4-Ethynylphenyl)-3,4-dihydro-2H-benzo[e]-
[1,3]oxazin-8-yl)-3a,4,7,7a-tetrahydro-1H-isoindole-1,3(2H)-dione
(Abbreviated as oHTI-ac). Into a 100 mL round flask were added 40
mL of xylenes, 4-ethynylaniline (1.17 g, 0.01 mol), oHTI (2.43 g, 0.01
mol), and paraformaldehyde (0.60 g, 0.02 mol). The mixture was
stirred at 120 °C for 8 h. The mixture was cooled to room
temperature and white crystal was obtained. (yield ca. 92%). 1H NMR
(CDCl3), ppm: δ = 2.26−2.43 (m, 2H, −CH2−), 2.63−2.75 (m, 2H,
−CH2−), 3.01 (d, 1H, CH), 3.23−3.35 (m, 2H), 4.65−4.67 (d,
2H, Ar−CH2−N, oxazine), 5.30−5.34 (d, 2H, O−CH2−N, oxazine),
5.99−6.03 (m, 2H, C−H), 6.91−7.43 (7H, Ar). Anal. Calcd. for
C24H20N2O3: C, 74.98; H, 5.24; N, 7.29. Found: C, 74.92%; H,
5.26%; N, 7.24%.
Synthesis of 2-(3-(4-Chlorophenyl)-3,4-dihydro-2H-benzo[e]-
[1,3]oxazin-8-yl)-3a,4,7,7a-tetrahydro-1H-isoindole-1,3(2H)-dione
(Abbreviated as oHTI-ch). Into a 100 mL round flask were added 40
mL of xylenes, 4-chloroaniline (1.28 g, 0.01 mol), oHTI (2.43 g, 0.01
mol), and paraformaldehyde (0.60 g, 0.02 mol). The mixture was
stirred at 120 °C for 6 h. The mixture was cooled to room
temperature and white crystal was obtained. (yield ca. 96%). 1H NMR
(CDCl3), ppm: δ = 2.30−2.43 (m, 2H, −CH2−), 2.66−2.75 (m, 2H,
−CH2−), 3.23−3.34 (m, 2H), 4.60−4.62 (d, 2H, Ar−CH2−N,
oxazine), 5.27−5.31 (d, 2H, O−CH2−N, oxazine), 5.97−6.04 (m,
2H, C−H), 6.92−7.31 (7H, Ar). Anal. Calcd. for C22H19ClN2O3:
C, 66.92; H, 4.85; N, 7.09. Found: C, 66.89%; H, 4.88%; N, 7.08%.
CH2−N, oxazine), 5.38 (s, 2H, O−CH2−N, oxazine), 6.83−7.43
(8H, Ar). Anal. Calcd. for C16H13NO: C, 81.68; H, 5.57; N, 5.95.
Found: C, 81.61%; H, 5.59%; N, 5.94%.
Polymerization of Benzoxazine Monomers. Each benzoxazine
monomer was polymerized using different polymerization cycles.
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Polymerization of oHTI-ac was carried out by heating at 160 °C, 180
°C, 200 °C, and 220 °C for 2 h each step, thus obtaining poly(oHTI-
ac). Polymerizations of oHTI-a and oHTI-ch were carried out by
heating at 160 °C, 180 °C, 200 °C, 220 °C, and 240 °C for 2 h each
step, thus obtaining poly(oHTI-a) and poly(oHTI-ch). In addition,
polymerization of oHTI-cy was done by heating at 160 °C, 180 °C,
200 °C, 220 °C, 240 °C, and 260 °C for 2 h each step, thus forming
poly(oHTI-cy).
RESULTS AND DISCUSSION
■
Synthesis and Atropisomerism Analysis of Benzox-
azine Atropisomers. 1,3-Benzoxazine bearing tetrahydroph-
thalimide group as ortho-functionality, 2-(3-phenyl-3,4-dihy-
dro-2H-benzo[e][1,3]oxazin-8-yl)-3a,4,7,7a-tetrahydro-1H-iso-
indole-1,3(2H)-dione, denoted as oHTI-a (Scheme 1), was
synthesized using primary amine (aniline), formaldehyde, and
ortho-tetrahydrophthalimide functional phenol (oHTI). 1,3-
Benzoxazine compounds are expected to have two equal
intensity singlet resonances in 1H NMR spectra due to the two
methylenes in the oxazine ring.35,36 However, as seen in Figure
1, the characteristic proton resonance due to the O−CH2−N
and the Ar−CH2−N groups in the oxazine ring of oHTI-a
exhibits two sets of doublets at 5.31 and 5.35 ppm and 4.64
and 4.66 ppm, respectively. In addition, the oxazine protons of
oHTI-a are correlated to four different carbon resonances in
the 1H−13C HMQC spectrum, suggesting the distinct
subspectra for benzoxazine compounds. All these observations
suggest the coexistence of two isomers. The branching ratio
can be integrated as 1:1.72 for the two products according to
the relative intensities. Further measurements were performed
at different temperatures (15 °C, 20 °C, 25 °C, 30 °C, and 35
°C) and at different frequencies (300, 400, 500, and 600 MHz)
but no change in chemical shifts, line widths, J-couplings,
integration values of each proton in both products and the
branching ratio was observed (Figure S1 of the Supporting
Information, SI). Furthermore, the ratio persists over a period
of three months and even after multiple rounds of drying or
thermal treatments in vacuo (30 °C, 60 °C, 90 °C, and 120
°C). These observations confirm that the two structures are
not conformers under thermal equilibrium, but are stereo-
chemical isomers.
To provide molecular level interpretation to the isomerism
in 1,3-benzoxazine, DFT calculations were performed using
B3LYP/6-31+G(d) method. For oHTI-a, two configurations
were identified (Figure 2a). In both structures, the oxazine ring
is perpendicular to the plane of imide group; while the
cyclohexene end-cap is located on either side of oxazine ring,
resulting in two different configurations (denoted as oHTI-a-A
and oHTI-a-B, respectively). Besides, the interconversion
barrier between oHTI-a-A and oHTI-a-B was calculated as
high as 106.8 kJ/mol, which makes the two configurations
thermally stable and hinders the rotation about the C−N bond.
Both experimental and calculated 13C NMR spectra are
presented in Figure 2b, which show excellent agreement (see
Table 1 for the full list of chemical shifts) and confirm that the
two isomers seen in the NMR spectra are oHTI-a-A and oHTI-
a-B, respectively. However, the energetic difference between
the two isomers is 2.4 kJ/mol, dedicating a branching ratio of
1:2.6 between oHTI-a-A and oHTI-a-B at 298 K, notably
different from the experimental ratio of 1:1.72. Together with
the large interconversion energy barrier between the two
structures, such a deviation indicates that the branching ratio is
Figure 2. Optimized stuctures and 13C NMR spectra of oHTI-a. (a)
Optimized stuctures of oHTI-a: oHTI-a-A (left) and oHTI-a-B (right)
by DFT calculations. (b) Comparison between the experimental and
calculated 13C NMR spectra of oHTI-a. The NMR spectra are first
calculated for oHTI-a-A and oHTI-a-B, respectively, and the overall
spectrum is generated by adopting the experimental branching ratio of
1:1.72.
not determined by the reaction products under thermal
equilibrium.
To unveil the determining factor for the branching ratio of
oHTI-a, we performed calculations on a reaction intermediate,
oHTI, which were found to have four conformations (denoted
as oHTI-1, oHTI-2, oHTI-3, and oHTI-4, see Figure 3). oHTI-
3 and oHTI-4 both have an intramolecular hydrogen bond,
making it difficult for deprotonation. In contrast, the phenolic
OH bond is perpendicular to the plane of imide group in
oHTI-1 and oHTI-2, and can release a proton and form
anionic products (oHTI-1* and oHTI-2*) in solution. The
energy difference between oHTI-1 and oHTI-2 was found to
be 1.84 kJ/mol in benzene, which gives rise to a branching
ratio of 1.75:1 at the reaction temperature of 393 K. Further
calculations on oHTI-1* and oHTI-2* and their transition
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Table 1. Experimental and Calculated 13C Chemical Shifts
of Each Atropisomer
reaction intermediate, ortho-tetrahydrophthalimide phenol,
accounts for the atropisomerism and determines the branching
ratio of the benzoxazine atropisomers. Potentially, the
atropisomerization mechanism discovered in this work can
be used as a generic design principle for improving drug
development, organic catalysts, molecular electronics, and
other fields wherever atropisomers can play important and
prevalent roles.
Polymerization Behavior of Benzoxazine Atro-
pisomers. The polymerization profiles of benzoxazine
atropisomers with various functionalities were studied by
DSC as depicted in Figure 4a, and the results are summarized
in Table 2. The thermograms show that the exothermic
maximum of the ring-opening polymerization for oHTI-a
centers at 252 °C, while the maxima for oHTI-ac, oHTI-ch,
and oHTI-cy are at 214 °C, 261 °C, and 273 °C, respectively.
The polymerization temperatures of benzoxazine atropisomers
are shifted due to the difference in electronegativity of
acetylene, chlorine, and nitrile functionalities.
oHTI-a exhibits an endothermic peak at 129 °C and a very
small exothermic peak at 131 °C as shown in Figure S14a,
followed by a very sharp endothermic peak at 154 °C can be
observed after the exothermic peak, which is due to the phase
changes of crystal-to-crystal transition. As shown in the DSC
profiles of Figure S14b, this phase transition disappears after
the first scanning by heating to 137 °C at a heating rate of 10
°C/min, suggesting that the monotropic characteristic between
these two crystal forms in oHTI-a according to the Burger-
Ramberger heat-of-fusion rule.37 Besides, the value of the
exothermic peak of oHTI-a shows no variations as shown in
Figure S14, suggesting that the crystal phase has no influence
on the ring-opening polymerization of benzoxazines.
Generally, one expects an increase on the polymerization
temperature due to the electron-withdrawing groups in the
para-position of the phenyl in amine substituent.38 According
to the electronegativity order −H < −CCH < −Cl < CN,
the electronegativity in the para-position of the phenyl should
follow the order oHTI-a < oHTI-ac < oHTI-ch < oHTI-cy but
the polymerization temperature for these benzoxazines has a
different order as oHTI-ac < oHTI-a < oHTI-ch < oHTI-cy.
The values for the heat of benzoxazine monomers are
summarized in Table 2. The acetylene containing monomer,
oHTI-ac, shows the highest heat of polymerization with a value
of 563 J/g, and the heat of polymerization were 226, 176, and
186 J/g for oHTI-a, oHTI-ch and oHTI-cy, respectively. The
significantly higher heat of polymerization for oHTI-ac
suggests the involvement of other events in the ring-opening
reaction. Since the presence of acetylene is the unique feature
of oHTI-ac, one possibility is the polymerization of acetylene,
which takes place in advance of oxazine ring, lowering
polymerization temperature of oHTI-ac. To compare, we
synthesized another benzoxazine monomer containing acety-
lene group, Ph-ac (a benzoxazine monomer derived from
phenol and 4-ethynylaniline), which showed the maximum of
polymerization at 226 °C. In contrast, the maximum
temperature of polymerization for oHTI-a is 38 °C higher
than that of oHTI-ac in absence of acetylene functionality.
These DSC results indicate the existence of self-catalytic
polymerization mechanism possibly caused by the interaction
between acetylene and benzoxazine when acetylene function-
ality is incorporated into benzoxazine atropisomers.
shift (ppm)
shift (ppm)
expt.
calc.
atoms
expt.
calc.
atoms
178.93
178.93
149.73
148.36
129.32
129.32
127.75
127.47
127.47
127.36
122.10
122.10
121.99
120.37
120.27
118.80
79.63
181.16
180.34
154.60
152.83
132.86
132.49
130.30
129.91
129.91
127.32
123.62
123.62
123.17
122.88
120.93
119.72
81.94
5
3
13
22
7
178.89
178.89
149.73
148.34
129.32
129.32
127.47
127.84
127.84
127.39
122.21
122.21
122.06
120.39
120.13
118.94
80.01
181.02
180.56
154.02
152.71
133.41
133.08
130.38
130.31
130.17
127.40
123.59
123.44
123.31
122.81
120.75
120.07
82.15
18
21
10
2
24
23
4
15
6
13
1
16
5
11
3
14
8
8
24
26
17
15
27
10
25
14
23
16
19
21
1
50.98
47.27
50.71
47.21
12
19
20
22
25
39.51
40.15
39.37
39.73
39.51
39.73
2
9
6
39.37
39.43
23.67
23.36
23.76
23.30
23.67
22.96
23.76
23.09
state suggested a large energy barrier between the two anionic
precursors, i.e., as high as 102.2 and 95.2 kJ/mol for oHTI-1*
and oHTI-2*, respectively, which prevents any further
interexchange between the two structures under thermal
conditions. The emergence of such an energy barrier can be
attributed to the repulsion between negatively charged oxygen
atoms in the oxazine ring and imide functionalities when they
approach the transition state structure via rotation, and a large
configurational change will be necessary for rotational switch
about the C−N bond. As a result, no conformational switch via
rotation is possible in the follow-on reactions and thus the
branching ratio maintains. The mechanism for the formation of
atropisomers of oHTI-a is illustrated in Scheme 2.
To further confirm the reaction mechanism, three ortho-
tetrahydrophthalimide benzoxazine monomers with different
functionalities in amine (Scheme 1) were also designed and
synthesized. Similar to oHTI-a, all of the three ortho-
tetrahydrophthalimide benzoxazines (oHTI-ac, oHTI-ch, and
oHTI-cy) have a very high yield (>90%). Although monomers
of these three molecules have different functionalities in amine,
the branching ratio between two atropisomers in each
monomer was found to be similar to that of oHTI-a. This
strongly supports the intrinsic atropisomerization mechanism
in ortho-tetrahydrophthalimide benzoxazines, where the
In order to qualitatively study the structural evolution of
oHTI-ac during heating, in situ FT-IR analyses were carried
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Figure 3. (a) 1H NMR spectra of oHTI-a, oHTI-ac, oHTI-ch, and oHTI-cy. (b) The expanded region between 4.5 and 5.5 ppm shows two sets of
doublet resonances belonging to −CH2− of the oxazine ring, where the well-separated doublets of −O−CH2−N− are used to integrate for
determine the branching ratio of atropisomers in each monomer.
a
Scheme 2. Atropisomeric Mechanism of ortho-Tetrahydrophthalimide Functional Benzoxazine
a
(a) 2-D (dimension) scheme for the simplified mechanism of benzoxazine formation. (b) 3-D scheme for the atropisomerism mechanism. The
energy barrier between oHTI-1* and oHTI-2* is calculated as 102.2 and 95.2 kJ/mol, respectively, making it difficult for inter-conversion of the
structures.
out. As shown in Figure 4c, the characteristic absorption bands
933 cm−1 (benzoxazine related mode) can be used to monitor
the ring-opening polymerization of the benzoxazine resin.39,40
at 1239 cm−1 (C−O−C antisymmetric stretching mode) and
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Figure 4. (a) DSC thermograms of oHTI-a, oHTI-ac, oHTI-ch, and oHTI-cy. (b) DSC comparison of oHTI-ac with oHTI-a and Ph-ac reveals the
presence of self-catalyzed polymerization between oxazine ring and acetylene in oHTI-ac. (c) FT-IR spectra of oHTI-ac after various thermal
treatments. (d) FT-IR spectra of Ph-ac after various thermal treatments.
polymerization in oHTI-ac. This cyclotrimerization is reported
to be preceded by an intermediate trimer which is formed at
much lower temperature than the cyclotrimerization temper-
ature. This intermediate structure forces three monomeric
molecules to form a rigid structure that resembling a
cyclotrimer. The intermediate structure thus increases the
likelihood of packing with neighboring cyclotrimer intermedi-
ates forcing the oxazine ring in the vicinity of each other. This
two cyclotrimer intermediates can be formed in the melt,
allowing the benzoxazine polymerization to take place at lower
temperature than the completely random orientation of the
benzoxazine monomers. Much more detailed study is required
to verify this hypothesis, though it is not the main goal of the
current paper.
The structural conversion from benzoxazine monomer to
polybenzoxazine of oHTI-ac was also investigated by solid-
state 13C NMR with cross-polarization magic-angle spinning
(CP-MAS). In accordance with the 13C NMR spectrum of
oHTI-ac in solution, the characteristic solid 13C resonances of
the oxazine ring are assigned at around 47 and 77 ppm for Ar−
CH2−N− and −O−CH2−N−, respectively (Figure 5).18
Besides, the characteristic resonances of the acetylene carbons
of CH and −C are centered at around 70 and 84 ppm. In
addition, the spectrum for poly(oHTI-ac) showed drastic
differences compared with that of oHTI-ac as expected. The
carbon resonances of oxazine ring and acetylene of oHTI-ac
Table 2. DSC Thermograms of Benzoxazine Monomers
onset temp
(°C)
max temp
(°C)
heat of polymerization
(J/g)
monomer
oHTI-a
231
179
245
246
252
214
261
273
226
563
176
186
oHTI-ac
oHTI-ch
oHTI-cy
Both bands decrease as the temperature increase, and fully
disappear at 220 °C. Meanwhile, the OH band in the region of
3500−3200 cm−1 emerges alongside the decrease of both the
characteristic bands of the internal arynes at 2100 cm−1 and
the C−H stretching vibration of the monosubstituted
acetylene at 3267 cm−1, and the broadening of the absorption
band at 1605 cm−1 attributed to the aromatic ring. These
variations are likely related to the benzene ring formation by
the cyclotrimerization of acetylenes.41 The above results
therefore suggest that the exotherm in the DSC thermogram
of oHTI-ac has two undergoing processes, including the
polymerization of acetylene and ring-opening polymerization
of oxazine group. Extra evidence is available from the complete
disappearance of the characteristic absorption bands of oxazine
ring for Ph-ac at 1239 cm−1 (C−O−C antisymmetric
stretching mode) and 933 cm−1 (benzoxazine related mode)
complete the disappearance at a higher temperature of 240 °C
(Figure 4d), further suggesting the existence of self-catalyzed
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Figure 5. Comparison among solution-state 13C NMR spectrum, solid-state 13C NMR spectrum for oHTI-ac, and solid-state CP-MAS 13C NMR
spectrum for poly(oHTI-ac).
Scheme 3. Representation of Self-Catalyzed Polymerization of oHTI-ac
completely disappeared, and a broad signal corresponding to
the −CH2− of Mannich bridge around 47 ppm can be
observed in the spectrum of poly(oHTI-ac). Moreover, a very
broad peak attributed to the −CH of benzene ring are
formed after polymerization. These spectral changes suggest
the completion of ring-opening polymerization and cyclo-
trimerization for oHTI-ac can be achieved at the temperature
as low as 220 °C, and formation of the thermoset with highly
aromatic cross-linking network. Therefore, the results of solid-
state 13C NMR analysis fully agree with in situ FTIR results,
and we can now propose a structural transformation
mechanism of oHTI-ac, as being described in Scheme 3.
Thermal and Heat Release Properties of Thermosets.
The thermal stability of poly(oHTI-a), poly(oHTI-ac), poly-
(oHTI-ch), and poly(oHTI-cy) were studied by TGA as seen
in Figure 6a. As expected, degradation in the temperature
range of 250 °C to 400 °C was observed for poly(oHTI-a),
poly(oHTI-ch), and poly(oHTI-cy), which can be attributed to
the partial decomposition of terminal groups as defects of
polybenzoxazines from monofunctional benzoxazines.42 On
the contrary, poly(oHTI-ac) exhibits excellent thermal stability
at this temperature range with the highest Td5 (5% weight loss
temperature) of 396 °C among this class of polybenzoxazines.
The highly aromatic cross-linked networks formed from the
cyclotrimerization of acetylene along with the polybenzoxazine
network significantly decrease the fraction of the terminal
defects in poly(oHTI-ac). In addition, poly(oHTI-ac) also
showed a char yield of as high as 62%. The TGA data therefore
demonstrates the high thermal stability of poly(oHTI-ac).
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Figure 6. (a) Thermogravimetric analysis of polybenzoxazines. (b) DSC thermograms of polybenzoxazines recorded under nitrogen at a heating
rate of 10 °C/min. (c) Dynamic mechanical spectra of poly(oHTI-ac). (d) Viscosity versus temperature of oHTI-ac.
Figure 6b compares DSC thermograms of polybenzoxazines.
In general, the glass transition temperature (Tg) of
polybenzoxazines based on monofunctional benzoxazine
monomers are much lower than those derived from difunc-
tional or main-chain type benzoxazines because of the small
molecular weight and/or low cross-link density of monofunc-
tional benzoxazines, despite some expected reactivity on the
aromatic ring of the phenolic and aniline moiety.43 As shown
in Figure 6b, the Tg of poly(oHTI-a), poly(oHTI-ch) and
poly(oHTI-cy) are observed at 233 °C, 198 °C, and 217 °C,
which are much lower than that of poly(oHTI-ac) (Tg = 276
°C). Moreover, dynamic mechanical analysis (DMA) was
utilized to further confirm the Tg value of poly(oHTI-ac). As
shown in Figure 6c, the Tg determined by the peak
temperature of both E″ and tan δ is as high as 298 °C,
which is in relatively good agreement with that measured by
DSC with the expected discrepancy between the two different
techniques. The significant improvement of Tg for poly(oHTI-
ac) is attributed to the formation of highly aromatic cross-
linked networks endowed by cyclotrimerization of acetylene
along with phenolic Mannich bridge networks. In general, the
required viscosity for resin transfer molding (RTM) processing
technique is roughly below 1 Pa·s at the processing
temperature. Interestingly, the viscosity of oHTI-ac is less
than 0.1 Pa.s in the temperature range between 169 °C and
200 °C at a heating rate of 5 °C/min as shown in Figure 6d,
which exhibits the advantage of being monofucntional
benzoxazine with very low viscosity. All the result above
highlights the advantage of the presence of acetylene in
benzoxazine atropisomers.
Microscale combustion calorimetry (MCC) is typically a
constant heating rate pyrolysis test in which the evolved gases
are combusted in excess oxygen and the specific heat release
rate (HRR) is determined for a milligram-sized specimen by
oxygen consumption calorimetry. HRR is obtained by dividing
dQ/Dt at each time interval, by the initial sample mass.
Besides, the heat release capacity (HRC) is obtained by
dividing the maximum value of HRR by the heating rate.
Generally, HRC is an effective evaluation of thermal
combustion that is one of the best single predictors of the
flammability of a material.44 In addition, HRC is a material
property that is rooted in the chemical structure of the polymer
and can be calculated from additive molar group contributions
for simple materials.44
Milligram size samples of poly(oHTI-a), poly(oHTI-ac),
poly(oHTI-ch), and poly(oHTI-cy) were tested at a constant
heating rate of 1 K/s over the temperature range 100−900 °C.
As shown in Figure 7, MCC characterization of poly(oHTI-a),
poly(oHTI-ac), poly(oHTI-ch), and poly(oHTI-cy) reveals
HRC values of 107.8, 67.2, 109.0, and 113.2 J g1−K−1,
respectively. Besides, poly(oHTI-a), poly(oHTI-ch), and
poly(oHTI-cy) exhibit THR values of 17.8, 17.6, and 16.4
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between experiments and calculations suggests that the
intrinsic mechanism for the formation of this new type
atropisomerism, i.e., hindering rotation about the C−N bond
by intramolecular repulsion between negatively charged
oxygen atoms in the oxazine ring and imide functionalities
and the consequent structural change needed for rotation.
Such detailed structural and mechanistic analysis delivers a
general mechanism as well as the basis for better understanding
of the factors that are critical for the atropisomerization in this
and related fields, stemming from a variety of new types of
atropisomers to be discovered utilizing the mechanism
discovered in this work. In addition, polymerization of
benzoxazine atropisomers was investigated in this study. The
benzoxazine atropisomers bearing acetylene functionality
exhibited to undergo self-catalyzed polymerization between
oxazine ring and acetylene. The resultant thermoset derived
from the acetylene containing benzoxazine atropisomers
showed exceptionally high thermal stability with high char
yield value (62%) and extraordinarily low flammability with
low heat release capacity (HRC of 67.2 J g1−K−1) as well as the
very low total heat release (THR of 11.2 kJ g−1).
Consequently, the benzoxazine atropisomers discovered in
this study also offers great potential for designing high-
performance materials with very low molecular weight
precursors.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01924.
■
S
NMR spectra, FT-IR spectra, as well as DSC scans of
benzoxazines, and structural coordinates from DFT
calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: hxi3@cwru.edu (H.I.).
*E-mail: sfy1@le.ac.uk (S.Y.).
■
Figure 7. Microscale combustion calorimetric results of poly(oHTI-
a), poly(oHTI-ac), poly(oHTI-ch), and poly(oHTI-cy) as a function
of temperature: (a) heat release rate and (b) total heat release. Heat
release rate is the same as heat release capacity at the temperature
ramp rate (1 K/s) used in this experiment.
ORCID
Hatsuo Ishida: 0000-0002-2590-2700
Shengfu Yang: 0000-0001-5684-6388
Notes
KJg−1, while poly(oHTI-ac) shows a much lower THR value of
11.2 KJg−1. Generally, the lower the values of HRC and THR,
the higher the flame resistance becomes. The HRC value of
poly(oHTI-ac) is lower than the other 47 polymers reported
by Lyon et al.45 Furthermore, the HRC value of poly(oHTI-ac)
is even lower than the benzoxazole resins derived from ortho-
amide benzoxazines, which shows HRC of ∼92 J g1−K−1.9 The
highly aromatic cross-linked networks formed from the
cyclotrimerization of acetylene along with the polybenzoxazine
network leads to a substantial HRC reduction. Therefore, this
newly developed benzoxazine atropisomers containing acety-
lene functionality opens opportunities for the applications that
benefit from low flammability.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this work was provided by the National
Natural Science Foundation of China (No. 51603093), the
Science and Technology Agency of Jiangsu Province (No. BK
20160515), China Postdoctoral Science Foundation (No.
2016M600369) and Jiangsu Postdoctoral Science Foundation
(No. 1601015A). S.Y. wishes to thank the UK Engineering and
Physical Sciences Research Council (EPSRC) and the
Leverhulme Trust for grants in support of this work.
CONCLUSIONS
■
REFERENCES
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DOI: 10.1021/acs.macromol.8b01924
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