Advanced Polymers from Simple Benzoxazines and Phenols by Ring-Opening Addition Reactions

Article abstract of DOI:10.1021/acs.macromol.0c00225

Simple benzoxazines were mixed and reacted with various phenolics such as phenol, p-nitrophenol, p-cresol, 1,3-dihyroxybenzene (resorcinol), 1,3,5-trihydroxybenzene (phloroglucinol), and N-(2-hydroxyphenyl)benzamide. The influence of these phenolic compou

Full text of DOI:10.1021/acs.macromol.0c00225

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Article
Advanced Polymers from Simple Benzoxazines and Phenols by
Ring-Opening Addition Reactions
Zeynep Deliballi, Baris Kiskan,* and Yusuf Yagci*
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ABSTRACT: Simple benzoxazines were mixed and reacted with various phenolics such
as phenol, p-nitrophenol, p-cresol, 1,3-dihyroxybenzene (resorcinol), 1,3,5-trihydrox-
ybenzene (phloroglucinol), and N-(2-hydroxyphenyl)benzamide. The influence of these
phenolic compounds on ring-opening polymerization temperature of benzoxazines was
examined by DSC analysis. The cresol-based benzoxazine (C-a) and phenolics yielded
polymers with molecular weight of around 1500 Da. Interestingly, for C-a/resorcinol-
and C-a/phloroglucinol-based polymers, a second GPC trace was observed that
corresponds to a few million daltons for mixing ratios of 4:1 and 5:1. Moreover, the
mixtures of C-a and N-(2-hydroxyphenyl)benzamide gave poly(benzoxazine−
benzoxazole)s through a new methodology eliminating the need of synthesis of
ortho-amide benzoxazines. The obtained polymers were soluble and characterized in detail by 1H NMR, 13C NMR, GPC, DSC, and
TGA analyses.
Scheme 1. Simplified Mechanism for Ring-Opening
Polymerization of a Benzoxazine Monomer to Produce a
Polybenzoxazine
INTRODUCTION
■
Polybenzoxazines as phenolic polymers resembling Novolac
resin have attracted significant attention from both academia
and industry because of their unique advantages. Typically,
these polymers exhibit high glass transition temperatures,
mechanical strength, and a high decomposition temperature
and are therefore well suited for high-temperature applica-
tions.1−6 Moreover, polybenzoxazines have additional useful
properties such as minor amounts of water sorption, hydrolytic
stability even in hot water, chemical resistance, low dielectric
constants, and limited shrinkage during production from their
monomers.1,7−9 Therefore, polybenzoxazines are comparable
to bismaleimides and superior to classical phenol−form-
aldehyde resins for many applications.10 On the other hand,
unmodified polybenzoxazines exhibit some deficiencies includ-
ing high curing temperature, as much as 260 °C depending on
the functionalities and purity of the corresponding benzoxazine
monomer, and low fracture toughness.11−16 These disadvan-
tages can be overcome by monomer design, synthesizing
curable polymeric benzoxazine precursors, or blending
benzoxazine monomer with toughening agents and other
resins prior to curing.17−30 In the latter approach, benzoxazine
monomers can also react with the added components forming
a network possessing whole system in the structure. Thus, the
reactive nature of benzoxazine monomers tolerates the use of
various compounds in benzoxazine resin formulations.31 The
benzoxazine monomers polymerize through a cationic pathway
containing two major stages. Initially, the ring-opening reaction
of oxazine occurs to form zwitterionic species, and then, the
carbocation, therefrom, reacts with the aromatic moiety of the
benzoxazine in a fashion of Friedel−Crafts reaction (Scheme
1).32−37 Accordingly, the generated carbocation also has the
ability to give similar Friedel−Crafts reaction with other
aromatic compounds, especially with those bearing an
activating substituent such as −OH and having free ortho-
and/or para-positions.38,39
Received: January 29, 2020
Revised: March 26, 2020
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These ring-opening addition reactions of benzoxazines with
suitable aromatic compounds are highly interesting in terms of
synthesizing novel compounds and to develop polymers
containing a high amount of phenolic −OH groups. Actually,
this approach was first reported in 1965 by Burke et al. by
reacting 1,3-benzoxazines and phenols to obtain amino-
alkylated phenols.40 And then, by applying this procedure as
an alternative method, the reaction of benzoxazine and phenol
dimers, trimers, and tetramers gave several azacalixarenes with
different ring sizes.41 This concept was extended to polymer
synthesis by Endo et al., and the ring-opening addition reaction
of a monofunctional benzoxazine with 2-methylresorcinol (2-
MR) gave an adduct containing four hydroxyls. In this
reaction, 2-MR acted as a bifunctional reagent to react with
2 equiv of the monofunctional benzoxazine. Additionally, 2-
methylresorcinol was also used as cross-linker for side-chain
polybenzoxazine precursors.42 In the case of bifunctional
benzoxazines, the same reaction produced ring-opened main-
chain polybenzoxazines having molecular weights between 0.8
and 3.9 kDa with broad dispersity (Đ).43 In another study, a
similar concept was applied by using a trifunctional
benzoxazine and different phenolics to obtain cross-linked
polymers with good thermal properties such as 78% char yield
at 500 °C.44
In the past decade, benzoxazines containing amide groups in
the structure are an emerging class of monomers as additional
properties may be imparted to the resulting networks. Several
synthetic strategies ensuing different accomplishments were
reported. Initially, primary amine functional benzoxazine
monomers were synthesized as a useful intermediate for the
production of various amide and polyamides.45,46 Alternatively,
an amide linkage containing mono- and difunctional
benzoxazine monomers and main-chain polybenzoxazine
precursor was synthesized starting from commercially available
and relatively cheap 3,4-dihydrocoumarin (DHC). Besides,
DHC was also used to synthesize a similar main-chain
polybenzoxazine in a one-pot approach.47,48 Apart from these
syntheses, ortho-amide functional benzoxazines were designed
to utilize benzoxazole formation that takes place at ca. 250 °C
between the phenolic −OH and ortho-amide moiety with
water elimination (Scheme 2). This reaction transforms
It seemed appropriate to take advantage of the reactive
nature of benzoxazines toward phenols which is expected to
provide the possibility to synthesize advanced polybenzox-
azines by selecting proper phenol additives during curing of
classical benzoxazine monomers. This way, the requirement of
complicated benzoxazine monomer synthesis could be
avoided. We herein report a simple method to obtain an
alternative poly(benzoxazine−benzoxazole) system by using
various additive phenols.
EXPERIMENTAL SECTION
■
Materials. Aniline (Merck, 99.5%), p-cresol (Sigma-Aldrich,
≥99.0%), paraformaldeyhde (Sigma-Aldrich, 95.0−100.5%), toluene
(Aldrich, 95.0−100.5%), ethanol (Sigma-Aldrich, 95.0−100.5%),
sodium hydroxide (Sigma-Aldrich, 95.0−100.5%), diethyl ether
(VWR Chemicals, ≥99.5%), resorcinol (Sigma-Aldrich, ≥99.0%),
phloroglucinol (Carlo-Erba, 98%), phenol (Sigma-Aldrich, ≥99.0%),
4-nitrophenol (Merck, ≥99.5%), 2-aminophenol (Sigma-Aldrich,
99.0%), benzoyl chloride (Sigma-Aldrich, ≥99.0%), N,N-dimethyla-
cetamide (DMAc, Sigma-Aldrich, anhydrous, 99.8%), acetone (Alfa
Aesar, 99+%), and distilled water were used as received.
Characterization. FTIR spectra were recorded on a PerkinElmer
FTIR Spectrum One spectrometer. Differential scanning calorimetry
(DSC) was performed on PerkinElmer Diamond DSC from 30 to 320
°C with a heating rate of 10 °C min−1 under nitrogen flow. A typical
DSC sample was 2−5 mg in a 30 μL aluminum pan. Thermal
gravimetric analysis (TGA) was performed on a PerkinElmer
Diamond TA/TGA with a heating rate of 10 °C min−1 under
nitrogen flow. Gel permeation chromatography (GPC) measurements
were performed on a TOSOH EcoSEC GPC system equipped with an
autosampler system, a temperature-controlled pump, a column oven, a
refractive index (RI) detector, a purge and degasser unit, and a
TSKgel superhZ2000, 4. 6 mm ID × 15 cm × 2 cm column.
Tetrahydrofuran was used as an eluent at flow rate of 1.0 mL/min at
40 °C. The refractive index detector was calibrated with polystyrene
and poly(methyl methacrylate) standards having narrow molecular
weight distributions. Data were analyzed by using Eco-SEC Analysis
software.
Synthesis of Monofunctional Benzoxazine Monomers from
p-Cresol (C-a) or Phenol (P-a). A typical solventless method was
used.56 Aniline (17.2 g, 0.185 mol), p-cresol (20.0 g, 0.185 mol) (or
phenol, 17.4 g, 0.185 mol), and paraformaldehyde (11.1 g, 0.370 mol)
were placed in a round-bottom flask, and the mixture was stirred at
100 °C for 2 h. After cooling the contents of the flask to room
temperature, the sticky product was dissolved in diethyl ether (250
mL) and extracted with 0.5 M sodium hydroxide (3 × 100 mL).
Then, the diethyl ether solution was washed with distilled water (3 ×
100 mL). The ether solution was dried with anhydrous MgSO4 and
filtered. Diethyl ether was evaporated by using a rotary evaporator.
The product was dried under vacuum at 45 °C for 24 h (yield ≈ 56%,
mp = 51 °C for C-a, and ≈49%, mp = 91 °C for P-a).
Scheme 2. Curing of ortho-Amide Benzoxazine to Produce
Polybenzoxazine and Subsequent Benzoxazole Formation
Synthesis of N-(2-Hydroxyphenyl)benzamide (2HPB). A
modified procedure was used.53 2-Aminophenol (5.41 g, 49.65
mmol) was dissolved in 70 mL of DMAc, and then the solution was
cooled to 0 °C. With vigorous stirring, benzoyl chloride (13.96 g, 99.3
mmol) was added dropwise into the solution. The reaction content
was stirred at 0 °C for 4 h and then at room temperature for 14 h.
Afterward, the mixture was poured in distilled water to precipitate the
product. After filtering, the obtained solid was washed with a copious
amount of cold water. The final product was dried under vacuum at
40 °C for 24 h, giving a light-brown solid (yield ≈ 90%, mp = 136
°C).
Polymerization Process. Benzoxazine monomer synthesized
from p-cresol (C-a) or phenol (P-a) was dissolved in acetone (the
volume of acetone was ca. 1.5 mL for 100 mg of monomer) and
mixed with resorcinol and pholoroglucinol in mole ratios of 1:1, 2:1,
3:1, 4:1, and 5:1 and also with phenol, p-cresol, 4-nitrophenol, and N-
(2-hydroxyphenyl)benzamide in a ratio of 5:1. The samples were
placed in a Teflon mold, and acetone was removed in a vacuum
polybenzoxazines into poly(benzoxazine−benzoxazole)s, re-
sulting in a high-performance thermoset with improved
mechanical performance and an apparent thermal stability
compared to classical polybenzoxazines.49−55 However, this
approach requires the synthesis of ortho-amide functional
benzoxazines, starting from the reaction of 2-aminophenols
with acid halides to obtain 2-hydroxyphenyl amides.
Subsequently, these molecules are used in classical benzoxazine
monomer synthesis by reacting with formaldehyde (or
paraformaldehyde) and a suitable primary amine that tolerates
the synthesis conditions.
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chamber at ambient temperature. Then, the molds were heated
gradually as 100 °C for 15 min, 150 °C for 15 min, and 220 °C for 1 h
in an open-air oven. After cooling, the obtained poly(benzoxazine-co-
phenol)s were removed from the molds by using a spatula. Moreover,
C-a and 2HPB were mixed in 2:1 and 5:1 mole ratios and molded;
then the same gradual heating was applied in an oven for ring-opening
polymerization, and thereafter a post-heat treatment was also applied
at 300 °C for 15 min to obtain poly(benzoxazine−benzoxazole)s.
of 4:1 and 5:1 was observed (Table 1). The observed high
molecular weight indicates the bridging role of added
Table 1. GPC Analyses of C-a/Phenolics-Based Polymers
Mn (kDa) first trace
Mn (kDa) second
trace (Đ)
a
b
c
polymer
(Đ)
ratio
C-a/p-cresol (5:1)
C-a/p-nitrophenol
(5:1)
1.099 (2.38)
1.450 (2.56)
RESULTS AND DISCUSSION
■
C-a/phenol (5:1)
C-a/phenol (3:1)
1.465 (2.51)
2.390 (5.77)
1.535 (2.55)
1.541 (2.19)
1.300 (1.88)
1.319 (2.11)
182.930 (2.62)
5472.400 (1.48)
29
Phenolic compounds are potential candidates for the ring-
opening addition reaction of benzoxazines as mentioned in the
Introduction. Several commercially available phenols, namely
p-nitrophenol, p-cresol, 1,3-dihyroxybenzene (resorcinol), and
1,3,5-trihydroxybenzene (phloroglucinol), were selected for
the purpose. N-(2-Hydroxyphenyl)benzamide was synthesized
according to a modified procedure.53 Although many
structurally different benzoxazine monomers are accessible,
two different simple monofunctional benzoxazine monomers
were synthesized from phenol and p-cresol, aniline, and
formaldehyde abbreviated as P-a and C-a (Scheme 3) and
C-a/resorcinol (5:1)
C-a/resorcinol (3:1)
C-a/resorcinol (1:1)
C-a/phloroglucinol
(5:1)
C-a/phloroglucinol
(4:1)
C-a/phloroglucinol
(3:1)
C-a/phloroglucinol
(2:1)
C-a/phloroglucinol
(1:1)
107
1180.760 (3.35)
4508.966 (1.68)
3457.028 (2.01)
574.879 (3.92)
176.028 (1.45)
9
1.523 (3.0)
1.288 (2.13)
1.102 (2.47)
8.4
13.5
32.6
41.3
1
characterized by H NMR spectral and DSC thermal analyses
1.414 (2.36)
(see the Supporting Information, Figures S1−S3).
a
b
c
Heated at 180 °C for 30 min. Dispersity index (Mw/Mn). The area
Scheme 3. Synthesis of C-a (R: −CH3) and P-a (R: −H)
Monomers
ratio of the second and first traces in GPC.
phenolics between C-a-based polybenzoxazine chains. This
effect is pronounced only at lower phenolic and higher C-a
moles. It is likely that the formation of such large structures
requires long polybenzoxazine chains that have ability to react
with a phenolic having vacant carbons at both ortho- and para-
positions (Scheme 5). This hypothesis was further confirmed
by conducting similar experiments using C-a monomer with p-
cresol and p-nitrophenol as the para-position blocked
phenolics in 4:1 and 5:1 mole ratios (see Figure S5). In
contrast to the usage of a low amount of phenolics, for higher
phenolic mole ratios, in for example 1:1, 2:1, and 3:1 C-a/
phenolics, only polymers with low molecular weights (Mn:
1090 and 1450 Da) were obtained because a higher amount of
phenolics reduced the probability of the self-reaction of C-a to
form long polybenzoxazine chains. On the other hand,
pholoroglucinol and resorsinol gave high molecular weight
polymers even at low C-a mole amounts due to their highly
reactive nature in the Friedel−Crafts reaction (Scheme 5). It is
worth mentioning that the amount of low molecular weight
poly(benzoxazine-co-phenolics) is much higher than that of
high molecular weight polymers as calculated by the related
area of the GPC traces (see Figure S6). Accordingly, the area
ratios of the bands of low and high molecular weight polymers
vary between 9 and 107 depending on the phenolics used and
mole ratios (Table 1). Moreover, the polydispersity index (Đ)
of the all soluble samples is quite high. This could be expected
since the polymerization takes place via uncontrolled multi-
stage reactions, and these Đ results are also typical for such a
polymerization process. Besides, the nature of reaction may
also lead to the formation of branched and/or hyperbranched
structures, especially when pholoroglucinol and resorsinol are
used as the phenolic reagent.
To prepare poly(benzoxazine-co-phenol)s, monofunctional
benzoxazines and phenolics (C-a or P-a/phenolics) were
mixed in various mole ratios starting from 1:1 to 5:1 and
heated (Scheme 4). The obtained polymers from C-a/
Scheme 4. Curing of Benzoxazines with Added Phenols
phenolics were soluble, therefore facilitating their spectral
(Figure S4) and molecular weight characterization. Even
though the P-a monomer is monofunctional as the C-a
monomer, cross-linked polybenzoxazine is formed due to the
vacant para-and ortho-positions on the benzene ring (see
Scheme 1). The observed solubility for the polymer from C-a
monomer is due to the presence of a methyl blocking group on
the para position of the benzene ring. The GPC analysis for
thermally treated C-a/phenolics gave molecular weights mostly
around 1500 Da. It is interesting to note that especially for C-
a/resorcinol and C-a/phloroglucinol mixtures a second GPC
trace corresponding to a few million daltons for mixing ratios
It is well established that, except for photoinitiated
polymerizations,57 the ROP temperatures (TROP) of benzox-
azines are between 180 and 260 °C, and these temperatures
are strictly dependent on the structure and the purity of the
monomer. In general, the ROP takes place without using any
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Scheme 5. Formation of High Molecular Weight Polybenzoxazines from Highly Active Phenols
catalyst and additive; thus, the process seems like noncatalytic
reaction. However, in fact, the traces of phenolic impurities
that remained from benzoxazine monomer synthesis initiate
the polymerization. Highly pure monomers have ROP
temperatures higher than 260 °C.12 Certainly, such high
curing temperatures can cause side reactions such as
degradation and destructing the hydrogen-bonding interac-
tions that may affect the properties of polybenzoxazines
adversely. Therefore, reducing the TROP is crucial in the
production of polybenzoxazine-related materials in indus-
try.58−60
It seemed therefore appropriate to study thermal behavior of
C-a and C-a/phenolics at different mixing ratios and types of
phenolics by DSC. In Figure 1 and Table S1, the catalytic
Figure 2. DSC thermograms of C-a (a) and C-a/phenol (5:1) (b), C-
a/p-cresol (5:1) (c), C-a/resorcinol (5:1) (d), C-a/phloroglucinol
(5:1) (e), and C-a/p-nitrophenol (5:1) (f).
the reactions of C-a with the phenols are exothermic, but the
activation energies differ depending on the used phenol. The
minor catalytic effect of phenol can be due to the sublimation
related evaporation of phenol before curing of benzoxazine
(Figure 2b). Although p-nitrophenol is another simple phenol,
it exhibited the best catalytic performance among the phenolics
used arising from the strong electron-withdrawing effect of the
−NO2 group that increases the acidity of the phenol. Highly
acidic phenolics as Brønsted−Lowry acids were previously
reported as good catalysts for the ROP of benzoxazines.59,61,62
The DSC studies also revealed that the increased number of
hydroxyl groups has a positive impact on the catalytic behavior
of the phenols. The better performance of the multihydroxyl
phenols observed could be due to the less evaporation and
high reactivity in electrophilic aromatic substitution reaction.
To clarify the effect of the −OH group, a control experiment
was conducted by using anisole instead of phenols. Expectedly,
anisole did not display any catalytic property, and the ROP
temperature remained the same for C-a (see Figure S8).
Moreover, C-a/anisole (5:1) gave only dimers, trimers, and so
on after curing, pointing out the necessity of −OH groups for
polymer formation in the proposed system.
Figure 1. DSC thermograms of C-a (a) and C-a/phloroglucinol (5:1)
(b), C-a/phloroglucinol (3:1) (c), and C-a/phloroglucinol (1:1) (d).
effect of phloroglucinol is clearly visible, and the TROP
temperatures are reduced with high phenolic content in the
formulations. A similar effect is also observed for resorcinol
(Figure S7). On the other hand, the type of phenol has a vast
impact on the TROP of benzoxazines as can be observed from
Figure 2 and Table S2. Accordingly, simple phenols such as
phenol and p-cresol has limited influence on maximum TROP
,
and only 10 °C reduction for phenol and 27 °C for p-cresol
were monitored. Similarly, large enthalpy values are observed
for C-a/phloroglucinol and C-a/resorcinol mixtures. The
curing enthalpy values of DSC peaks in Figure 1 and 2 are
between 88 and 370 J/g (Tables S1 and S2), indicating that
It was recently reported that polybenzoxazines from ortho-
functional benzoxazines exhibit unique physical properties
compared to para- and meta-based counterparts.52 These
findings led to the development different benzoxazines with
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Scheme 6. Synthesis of Poly(benzoxazine−benzoxazole) from Monofunctional Benzoxazines and N-(2-
Hydroxyphenyl)benzamide
amide and imide groups at the ortho-position. The existence of
intramolecular hydrogen bonding in ortho-amide benzoxazines
accelerated the ROP and lowered the curing temperatures.50
Besides, these polybenzoxazines exhibited an additional
thermal conversion after curing to form benzoxazoles from
ortho-hydroxylamides (see Scheme 2). In most examples,
benzoxazole formation took place around 300 °C for ortho-
amide benzoxazines, while a similar ring-forming reaction for
ortho-imide benzoxazines was at about 400 °C. Moreover,
converting ortho-hydroxylamides into benzoxazoles signifi-
cantly reduced the polarizability of the final polybenzoxazines,
which is beneficial to obtain materials with low dielectric
constants.51 The proposed strategy in the present work has
eliminated the need of synthesis of ortho-amide benzoxazines,
and related materials were obtained simply by mixing classical
benzoxazines with N-(2-hydroxyphenyl)benzamide (2HPB) in
various mole ratios (Scheme 6). The DSC thermogram of
2HPB (see Figure S9) reveals a large endotherm starting ca.
210 °C and ending at ca. 300 °C possibly due to water loss
1
Figure 3. H NMR spectra of C-a (a), 2HPB (b), and thermally
during benzoxazole formation supporting the proposed idea.
The polymers from C-a/2HPB were soluble, and the
molecular weights were determined to be 1400 and 1900 Da
for 2:1 and 5:1 mixing ratios, respectively. A two-step thermal
treatment was applied for those mixtures. At the first step, the
mixtures were exposed to 180 °C for ROP of benzoxazines,
and then the temperature was raised to 300 °C for benzoxazole
formation. The final soluble product was analyzed by 1H NMR
treated C-a/2HPB 5:1 (c).
1
and FTIR analyses. Accordingly, the H NMR spectra of C-a,
2HPB, and C-a/2HPB (5:1, cured at 300 °C) are overlaid in
Figure 3 to trace both amide NH and oxazine N−CH2−O
protons after the thermal treatment. The disappearance of NH
proton can be considered as an evidence for the transformation
of ortho-amide phenolic to benzoxazole structure. Moreover,
the disappearance of N−CH2−O protons clearly confirms the
complete ring-opening of benzoxazines. In addition, in the 13
C
Figure 4. FTIR spectra of 2HPB (a), C-a/2HPB (2:1) preheated at
180 °C (b), and C-a/2HPB (2:1) preheated at 300 °C (c).
NMR spectrum of the soluble polymer, the typical peak
belongs to the NC−O carbon of benzoxazole appearing at
167 ppm (see Figure S10). The comparative FTIR spectra of
the 2NHB and C-a/2NHB (2:1) sequentially heated at 180
and 300 °C presented in Figure 4 also gave evidence for the
formation of a benzoxazole ring. The stretching vibration of
the aromatic amide CO of 2NHB appearing at 1645 cm−1 is
still visible after the first curing of the mixture at 180 °C.
However, this band disappears after the second thermal
treatment at 300 °C. Besides, the CN stretching vibration
band of benzoxazole at ca. 1642 cm−1 is detectable.63
Moreover, in Figure 4b,c, the presence of broad hydroxyl
bands confirms the successful ring-opening polymerization of
benzoxazine. It is also deduced from the shape of the hydroxyl
bands that the hydroxyl groups have more hydrogen-bonding
interactions before the second thermal treatment. This was
further supported by the shift of hydroxyl bands over 3500
cm−1 as a consequence of benzoxazole ring formation.
To gain more insight into the overall mechanism of the two-
step procedure, further detailed DSC studies were conducted
on the mixtures of C-a/2HPB (5:1) and C-a/2HPB (2:1)
(Figure 5). The thermograms showed a typical ring-opening
polymerization exotherm at 214 and 219 °C, respectively. After
these exotherms, irregular endotherms at 262 and 231 °C for
C-a/2HPB (5:1) and C-a/2HPB (2:1), respectively, appeared.
As known, when ortho-amide phenols are exposed to higher
temperatures, they generate water through intramolecular
cyclization between the neighboring hydroxyl and amide
groups to form oxazole rings.64 Accordingly, these endotherms
can be attributed to the dehydration and formation of
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Figure 5. DSC thermograms of C-a/2HPB (5:1) first run (a) and
second run (a′) and of C-a/2HPB (2:1) first run (b) and second run
(b′).
Figure 7. TGA trace and derivative weight loss of C-a/2HPB (2:1)
thermally treated at 180 °C.
polybenzoxazole as presented in Scheme 6 (vide ante), in a
similar manner to that described for poly(o-hydroxyamide)-
s.65,66 To confirm that the observed thermograms were not
related to the degradation of the formed polymers, second
DSC runs were also performed. The only observed thermal
transitions in the second runs are associated with the glass
transition temperatures (Tg) of the poly(benzoxazine−
benzoxazole) polymers. The Tg value of postheated C-a/
2HPB (5:1) and C-a/2HPB (2:1) are 134 and 121 °C.
Notably, the polymer with higher molecular weight and more
hydrogen bondings exhibited a higher Tg value.
CONCLUSION
■
A polyaddition system based on the use of different phenols as
both additive and curative for benzoxazines is proposed. The
obtained polybenzoxazines were soluble after curing favored by
the selected benzoxazine monomer and therefore enabling
spectral and molecular weight analyses. It was shown that for
specific mixing ratios and phenols very high molecular weight
polybenzoxazines can be obtained. Especially, multihydroxy-
containing phenols as additives gave polymers with molecular
weights up to millions of daltons due to the high reactivity of
these phenols toward benzoxazines. This approach has also led
to synthesize poly(benzoxazine−benzoxazole) by using N-(2-
hydroxyphenyl)benzamide (2HPB) as the phenolic additive.
ortho-Amide phenols are known to transform to benzoxazole
structure under heat exposure with water release. In this work,
we combined the affinity of phenols toward benzoxazines and
benzoxazole generation from 2HPB for the fabrication of
advanced polymers by curing and subsequent thermal
treatment.
The dehydration phenomenon has also been supported by
TGA thermograms shown in Figure 6, which indicates the
The benzoxazine/phenol system described herein consti-
tutes an inventive approach to synthesize thermally stable
poly(benzoxazine−benzoxazole)s without synthesizing com-
plicated benzoxazine monomers. Also, it is anticipated that
many novel polybenzoxazole precursors can be produced by
using the large benzoxazine monomer library, which cannot be
found in the traditional concept for polybenzoxazole
formation.
Figure 6. TGA traces and derivative weight loss (%) of TGA of C-a/
2HPB (2:1) thermally treated at 180 °C (a) and thermally treated at
300 °C (a′) and of C-a/2HPB (5:1) thermally treated at 180 °C (b)
and thermally treated at 300 °C (b′).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.0c00225.
presence of weight loss at an earlier temperature from the
samples heated at 180 °C. The TGA traces and derivative of
weight loss are in accordance with the endothermic transition
temperatures observed in DSC analysis. On the contrary, early
weight loss is not detectable for the postheated samples. To
further verify the benzoxazole formation via TGA, another
analysis was devised by holding the temperature of TGA
furnace at ca. 300 °C for 10 min to amplify the dehydration
based weight loss and related derivative weight loss (Figure 7).
In the resulting TGA thermogram, the water removal can
clearly be seen approximately at the similar temperature ranges
observed in the previous DSC and TGA analyses.
NMR spectra of C-a, P-a, and C-a/phloroglucinol; DSC
thermograms of P-a, C-a, C-a/resorcinol, C-a/anisole,
and 2HPB; DSC results of C-a, C-a, and several phenol
mixtures; GPC traces of thermally treated C-a/
phloroglucinol and C-a/p-cresol (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Baris Kiskan − Department of Chemistry, Istanbul Technical
University, 34469 Istanbul, Turkey; orcid.org/0000-0001-
9476-2054; Email: kiskanb@itu.edu.tr
F
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pubs.acs.org/Macromolecules
Article
(15) Trejo-Machin, A.; Verge, P.; Puchot, L.; Quintana, R. Phloretic
Acid as an Alternative to the Phenolation of Aliphatic Hydroxyls for
the Elaboration of Polybenzoxazine. Green Chem. 2017, 19 (21),
5065−5073.
(16) Ganfoud, R.; Guigo, N.; Puchot, L.; Verge, P.; Sbirrazzuoli, N.
Investigation on the Role of the Alkyl Side Chain of Cardanol on
Benzoxazine Polymerization and Polymer Properties. Eur. Polym. J.
2019, 119, 120−129.
̀ ́
(17) Lligadas, G.; Tuzun, A.; Ronda, J. C.; Galia, M.; Cadiz, V.
̈ ̈
Polybenzoxazines: New Players in the Bio-Based Polymer Arena.
Yusuf Yagci − Department of Chemistry, Istanbul Technical
University, 34469 Istanbul, Turkey; Center of Excellence for
Advanced Materials Research (CEAMR) and Chemistry
Department, Faculty of Science, King Abdulaziz University,
Jeddah 21589, Saudi Arabia; orcid.org/0000-0001-6244-
6786; Email: yusuf@itu.edu.tr
Author
Zeynep Deliballi − Department of Chemistry, Istanbul
Technical University, 34469 Istanbul, Turkey
Polym. Chem. 2014, 5 (23), 6636−6644.
́
̀
(18) Zuniga, C.; Larrechi, M. S.; Lligadas, G.; Ronda, J. C.; Galia,
̃
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.0c00225
́
M.; Cadiz, V. Polybenzoxazines from Renewable Diphenolic Acid. J.
Polym. Sci., Part A: Polym. Chem. 2011, 49 (5), 1219−1227.
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Notes
The authors declare no competing financial interest.
(20) Andreu, R.; Ronda, J. C. Synthesis of 3,4-Dihydro-2h-1,3-
Benzoxazines by Condensation of 2-Hydroxyaldehydes and Primary
Amines: Application to the Synthesis of Hydroxy-Substituted and
Deuterium-Labeled Compounds. Synth. Commun. 2008, 38 (14),
2316−2329.
ACKNOWLEDGMENTS
■
The authors thank the Istanbul Technical University Research
Fund for financial support, and Z.D also thanks the Council of
Higher Education (YOK) for financial support.
(21) Takeichi, T.; Kano, T.; Agag, T. Synthesis and Thermal Cure of
High Molecular Weight Polybenzoxazine Precursors and the Proper-
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