Synthesis and characterization of novel bio-based benzoxazines from gallic acid with latent catalytic characteristics

Article abstract of DOI:10.1016/j.reactfunctpolym.2019.03.011

A novel bio-based main chain benzoxazine with two oxazine rings and one phenolic hydroxyl group in the same aromatic ring was synthesized and characterized. The method includes the synthesis of polymeric benzoxazine precursors from simple chemicals such a

Full text of DOI:10.1016/j.reactfunctpolym.2019.03.011

ReactiveandFunctionalPolymers139(2019)9–16
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and characterization of novel bio-based benzoxazines from gallic
acid with latent catalytic characteristics
Mustafa Arslan⁎
Department of Chemistry, Faculty of Arts and Sciences, Kirklareli University, TR-39000 Kirklareli, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel bio-based main chain benzoxazine with two oxazine rings and one phenolic hydroxyl group in the same
aromatic ring was synthesized and characterized. The method includes the synthesis of polymeric benzoxazine
precursors from simple chemicals such as gallic acid, gallic amide, 4,7,10-trioxa-1,13-tridecanediamine and
formaldehyde by using traditional main chain synthesis methodology. The precursors were successfully char-
acterized by the spectral and thermal investigations using 1H NMR, FTIR, GPC, DSC and TGA. The results de-
monstrated that phenolic hydroxyl groups in the benzene ring which are adjacent to the two oxazine rings have a
great effect to reduce ROP temperature of benzoxazines. The clear reduction in ROP temperature was demon-
strated by tracking exotherm in DSC analysis with an onset value at 126 °C. Moreover, thermal stability of the
final products were investigated by TGA and high char yields observed.
Polybenzoxazines
Ring-opening polymerization
Curing temperature
Gallic acid
Bio-based benzoxazine
1. Introduction
[6–8], vanillin [9,10], guaiacol [11,12], eugenol [13–15], resorcinol
[16], stearylamine [11], furfurylamine [17], chavicol [18], arbutin
Among commonly used thermosetting resin systems (such as phe-
nolics, epoxies, bismaleimides, cyanate esters, vinyl esters, and poly-
imides), polybenzoxazines (PBZs) has recently attracted much interest.
PBZs are a class of polyphenolic thermoset resins and can be obtained
by ring-opening polymerization (ROP) of the benzoxazine monomers
without using any catalyst. These thermosetting materials have at-
tracted interest in various scientific and industrial areas due to their
superior properties such as high char yield, low shrinkage and humidity
uptake, non-catalytic synthesis, limited by-product formation during
curing and high glass transition temperatures. Most of the exclusive
properties of PBZs are due to chain structure; the Mannich base bridges
-CH2-N(R)-CH2- and the intra- and inter molecular hydrogen bonds
between amine and the phenolic -OH groups. Polybenzoxazines can
basically be synthesized by ROP of 1,3-benzoxazine monomers by
thermal curing in the range of 150–250 °C depending on the benzox-
azine monomer structure without using a catalyst [1] (Scheme 1).
Great versatility of monomer molecular design allows to synthesize
novel monomers with various properties and specific functionalities
[2–5]. Moreover, bio-based benzoxazine materials have taken excessive
attention from the academy and industry for the development of less
toxic and renewable polymers. Various benzoxazine polymers have
been prepared with the properties of classical benzoxazine thermosets
from inexpensive phenolic and amine derivatives such as cardanol
[19] and phthalonitrile [20–24]. In order to keep the interest to use a
bio-based monomer, gallic acid (3,4,5-trihydroxybenzoic acid) can be
used as a naturally occurring phenol. Gallic acid is a water-soluble
phenolic structure which is existing in grapes and in the leaves of many
plants and it is a well-known natural antioxidant that is a secondary
polyphenolic metabolite [25–28]. A significant source of gallic acid is
also tea which contains 4.5 g/kg weight in tea leaves [29,30].
The polymerization of the benzoxazine monomers is a cationic ring-
opening process which is thermally catalyzed at high temperatures
generally over 220 °C [31]. These curing temperatures are very high for
industrial applications and can be reduced via some catalyst systems
[32–36]. Liu described a new thermal latent curing strategy that sta-
bilizes reactive benzoxazine-amine monomer blends by reaction equi-
librium. The system provides long storage time and also short gel time
at low temperature [37]. Yagci and co-workers investigated the effect of
amine HCl salts as a catalyst and it was demonstrated that these salts
cause sharp reduction in ring-opening polymerization temperature.
Recently, the influence of nucleophilicity of counterions of amines HX
salts was investigated and significant reduction detected for the curing
temperature with a trend as I−, Br−, Cl− [38]. A naphthoxazine based
latent catalyst was also described to reduce the ROP temperature. Ba-
sically, the system uses naphthoxazines as a curing-promoters which
has a phenolic hydroxyl group with adjacent electron-withdrawing
⁎ Corresponding author.
E-mail address: marslan@klu.edu.tr.
https://doi.org/10.1016/j.reactfunctpolym.2019.03.011
Received 16 January 2019; Received in revised form 28 February 2019; Accepted 9 March 2019
Availableonline12March2019
1381-5148/©2019ElsevierB.V.Allrightsreserved.

M. Arslan
ReactiveandFunctionalPolymers139(2019)9–16
trioxa-1,13-tridecanediamine and formaldehyde were used to synthe-
size bio-based main-chain benzoxazine and the polymerization beha-
vior was investigated in detail.
2. Experimental
2.1. Materials
Scheme 1. Synthesis of 1,3-benzoxazines and corresponding PBZs by ring-
opening polymerization.
3,4,5-Trihydroxybenzoic acid (gallic acid) (Aldrich, 99%), 4,7,10-
trioxa-1,13-tridecanediamine (Aldrich, ≥98%), paraformaldehyde
(Acros, 96%), p-toluidine (Aldrich, 99.6%), sodium hydroxide (Aldrich,
≥97%), ethanol (Aldrich, ≥99.5%), toluene (Carlo Erba, 99.5%),
chloroform (Acros, 99 + %), hexane (Aldrich, 95%), p-xylene (Aldrich,
≥99%), celite (Aldrich), 1,4-dioxane (Riedel-de Haen, 99.5%), diethyl
ether (≥98%, Aldrich), deuterium oxide (Aldrich, D2O, 99.9 atom% D),
tetrahydrofuran (THF, VWR Chemicals, 99.7%), N,N-dimethylforma-
mide (DMF, Merck), dichloromethane (DCM, VWR Chemicals, 99.9%).
groups and the high acidity of structure catalyze ROP of monomers
[39,40]. The autocatalytic thermal polymerization behavior of ben-
zoxazine monomers containing carboxylic acid functionalities was in-
vestigated by Ishida and Ronda [41–43]. Adding small amounts of
carboxylic acid containing benzoxazine comonomers effectively cata-
lyzes the thermal curing and decrease to the ring opening poly-
merization temperature.
The intra- and inter-molecular hydrogen bonds of oxazine rings
provide unique and characteristic features to the benzoxazine mono-
mers by reducing ROP temperatures [44]. Agag et al. developed a smart
polybenzoxazine, known as ortho-amide functional benzoxazine. Inter-
estingly, exceptional features of this class of benzoxazines was ob-
served. The cationic ROP was performed at lower temperatures among
traditional benzoxazines without adding initiator or catalyst. They as-
sumed that the existence of intramolecular hydrogen bonding between
an amide linkage and the adjacent oxazine unit acts as an internal in-
centive to trigger ROP. It behaves like a self-complementary initiator
and reduce ROP temperature in a smart way. A o-trifluoroacetamide
functional benzoxazine was synthesized to benefit its superior proper-
ties of ortho-functional benzoxazine to meta- and para-counterparts.
ROP temperature was below 200 °C without mixing any catalyst and
initiator and also benzoxazole formation occurred at extraordinarily
low temperatures below 270 °C [45]. A series of ortho-, meta-, or para-
methylol-functional benzoxazine isomers are synthesized and ROP
properties investigated. The presence of methylol groups reduce ROP
temperature. However, the highest reactivity is attributed to the ortho-
functional monomer for the catalytic effect of the methylol group
[46–48]. The effect of position (ortho-, meta-, or para-) of oxazine ring
in the benzoxazine spine was investigated by Endo and they found that
when the benzoxazine monomer substituted with another benzoxazine
unit as an alternative of functional group, meta-positioning has pro-
mising properties relative to their para and ortho counterparts (meta
(225 °C) < ortho (239 °C) < para (251 °C)). They proposed a different
ROP mechanism for lowering curing temperature in meta position
which involves an intramolecular electrophilic substitution of iminium
ion, resulting in aza-cyclic rings along with phenolic Mannich bridges
[49,50]. Hartwig and co-workers prepared a resorcinol-based poly-
benzoxazine by a cationic polymerization with thermo latent super
acids and the curing temperature of the system was impressive [51].
More recently, Zhu et al. synthesized pyrogallol-based difunctional
benzoxazines (PG-FA and PG-A) with a phenolic hydroxyl between two
oxazine rings appending to the same benzene ring. The phenolic hy-
droxyl exhibited an important role on ROP. The intra- and inter-mole-
cular hydrogen bonds of hydroxyls converted into hydroxyl-pi in-
tramolecular interactions, which formed the free phenolic hydroxyls.
The catalytic activity of the hydroxyls accelerates the opening of ox-
azine ring and the attack of the carbenium ions to active position of the
pyrogallol forms polybenzoxazines [52].
2.2. Measurements
The 1H NMR spectra of the monomer and polymers were measured
at room temperature in CDCl3 or DMSO-d6 with Si(CH3)4 using a
500 MHz NMR (Agilent NMR System VNMRS). Thermal gravity analysis
was performed on Setaram Sensys Evo TG-DSC at a heating rate of
10 °C/min from 30 to 900 °C under nitrogen. Differential scanning ca-
lorimetry (DSC) measurements were taken using
a PerkinElmer
Diamond DSC and Setaram Sensys Evo TG-DSC under a scanning rate of
10 °C/min, covering temperatures of 30–320 °C. The Fourier transform
infrared (FTIR) spectroscopy measurements were recorded as 4 scans
using a PerkinElmer FTIR Spectrum One spectrometer. Gel permeation
chromatography (GPC) measurements were performed on a TOSOH
EcoSEC GPC system equipped with an auto sampler system, a tem-
perature-controlled pump, a column oven, a refractive index (RI) de-
tector, a purge and degasser unit and TSK gel superhZ2000, 4.6 mm
ID × 15 cm × 2 cm column. Tetrahydrofuran was used as an eluent at
flow rate of 1.0 mL min−1 at 40 °C. Refractive index detector was cali-
brated with polystyrene standards having narrow molecular-weight
distributions. Data were analyzed using Eco-SEC Analysis software.
2.3. Syntheses
2.3.1. Synthesis of gallic acid-based main chain benzoxazine (GA-Bz)
The synthesis was performed according to the literature with a few
modifications [52]. In
a 100 mL round flask, paraformaldehyde
(47 mmol, 1.412 g) and H2O (5.5 g) were added and the pH of the
medium was adjusted to 8–9 with NaOH solution. The colloidal mixture
was heated and stirred until to obtain a clear phase. 4,7,10-Trioxa-1,13-
tridecanediamine (11,7 mmol, 2.590 g) was dissolved in 1,4-dioxane
and added into the flask at room temperature and left to mix for 1 h.
Then, 3,4,5-trihydroxybenzoic acid was (11.7 mmol, 2 g) put into the
mixture and stirred at 70 °C for 80 h. The formed solid was removed by
filtering. The solvent was completely evaporated using a rotary eva-
porator. The remaining product dissolved in a tiny CHCl3 and pre-
cipitated by the dropwise addition into the excess diethyl ether
(200 mL). The precipitation process was performed for two times.
Precipitated polymers were collected by decantation and dried in va-
cuum at room temperature.
In these functional benzoxazine structures, hydroxyl of the phe-
nolics have a significant status as an autocatalytic group and decrease
ROP temperature. As we mentioned above, phenolic nucleophiles have
a significant effect on ROP and among them pyrogallol is one of the best
structures [36,52]. In the light of this information, first time main-chain
type benzoxazine with a phenolic hydroxyl group between two oxazine
ring could be obtained and gallic acid is the best candidate being as a
bio-based molecule. By this means, gallic acid, gallic amide, 4,7,10-
2.3.2. Synthesis of gallic amide monomer (G-Amd)
Gallic acid-based amide synthesis was achieved according to the
literature [53]. In a 50 mL round flask, 3,4,5-trihydroxybenzoic acid
(5.88 mmol, 1 g) and p-toluidine (5.88 mmol, 0.629 g) dissolved in
20 mL p-xylene. The mixture was refluxed for 24 h. Xylene was removed
using a rotary evaporator. The product was dissolved in CH2Cl2 and
filtrated through a pad of celite. Solvent evaporated and the product
was dried in vacuum at room temperature.
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M. Arslan
ReactiveandFunctionalPolymers139(2019)9–16
2.3.3. Synthesis of gallic amide-based main chain benzoxazine (G-Amd-Bz)
In a 100 mL round flask, G-Amd (1.93 mmol, 0.5 g) monomer, par-
aformaldehyde (7.72 mmol, 0,232 g) and 4,7,10-trioxa-1,13-tridecane-
diamine (1.93 mmol, 0.425 g) were added and dissolved in 50 mL 1,4-
dioxane. The flask was refluxed for 4 h. The formed solid was removed
by filtering. The solvent was evaporated using a rotary evaporator. The
product was concentrated and precipitated by the dropwise addition
into the 100 mL excess diethyl ether. The solid was collected after de-
cantation and dried at room temperature under vacuum overnight.
3. Results and discussion
Among high performance polybenzoxazine thermosets, o-amide
functional benzoxazines known as a smart class of such materials. The
precursors could be synthesized by various combinations of phenolics
and amine derivatives to form monomers, oligomers, main-, side- and
end-chain-type polymers [54,55]. ROP of benzoxazines commence by
the cleavage of methylene bridge on the oxazine ring to form a cationic
intermediate by thermal activation. These cationic species can then
attack N-, O- and aryl groups to yield polybenzoxazines. Commonly,
high curing temperatures are required to start ROP. Interestingly, o-
amide functional benzoxazines show ROP at lower temperatures
without adding any initiators and/or catalyst. Intramolecular hydrogen
bonding between amide functionality and neighboring oxazine ring
make the structure a self-complementary initiator by acting as an in-
ternal stimulus [55]. Therefore, a novel bio-based amide-functional
main chain benzoxazine polymer precursors were synthesized with the
appropriate structures.
Fig. 1. 1H NMR spectra of 3,4,5-trihydroxybenzoic acid, 4,7,10-trioxa-1,13-
tridecanediamine and GA-Bz.
The first strategy was established by the synthesis of gallic acid-
based benzoxazine with the reaction of 3,4,5-trihydroxybenzoic acid,
4,7,10-trioxa-1,13-tridecanediamine and paraformaldehyde (Scheme
2).
3.1. Preparation of GA-Bz and structural characterization
The chemical structure of the polymer was confirmed by spectral
and thermal analysis. As it can be seen from the 1H NMR shifts (Fig. 1),
the signals emerged at 4.74 (O-CH2-N) and 4.27 (Ar-CH2-N) ppm are
attributed to oxazine ring of GA-Bz. The chemical shifts of gallic acid
aromatic protons located at 6.91 ppm was completely disappeared after
reaction which supports to the formation of di-oxazine ring. The re-
sonance signals of the 4,7,10-trioxa-1,13-tridecanediamine methylene
protons also observed at 1.66 (C-CH2-C), 2.55 (N-CH2-C) and 3.43 (O-
CH2-C) ppm, respectively. DMSO-d6 would form strong interaction with
the phenolic hydroxyl and acid proton of GA-Bz. While the NMR shifts
may be visible, it is not integrable [40]. Phenolic and carboxyl acid
hydroxyl resonances are not detected or very slight because of inter-
and intra-molecular hydrogen bonding.
Fig. 2. FT-IR spectra of 3,4,5-trihydroxybenzoic acid (a), 4,7,10-trioxa-1,13-
tridecanediamine (b) and GA-Bz (c).
plane bending of the benzene to which the oxazine ring is adjacent. The
symmetric stretching vibrations of oxazine group (Ar-O-C) overlapped
with Ar-OH and C-O-C vibrations at 1102 cm−1. The broad vibration
band between 2000 and 3710 cm−1 is corresponds to the intra- and
intermolecular hydrogen bonds of carboxyls and phenolics with O and
N atoms. After synthesis, the formation of new bands clearly confirms
the success of the reaction.
As shown in Fig. 2, the characteristic bands of the benzoxazines
emerged at 1325 cm−1 (CH2 wagging), 1102 cm−1 (asymmetric
stretching vibration of Ar-O-C) and 936 cm−1 (out of plane distortion of
oxazine unit). The band at 962 cm−1 is allocated to the C − H out-of-
The ROP temperature of benzoxazines are mostly lay between 160
and 260 °C which is depending to the functionalities present on the
structure. The functional groups such as carboxylic acid, alcohol and
phenolic hydroxyl would decrease the ROP temperature by showing a
lower exothermic peak in DSC [56–58]. Phenolic hydroxyl groups have
a significant role as a catalyst due to the activation effect. Moreover,
orto-amide functional benzoxazines polymerize at much lower tem-
peratures without added initiators and/or catalyst [45,55,59]. There-
fore, the hydrogen bonding system which contains a five-membered-
ring with the amide linkage and the attached oxazine ring would be act
a “reinforced incentive” to encourage the ring opening polymerization.
In our system, GA-Bz is probably forming an intra- and intermolecular
Scheme 2. Synthesis of gallic acid-based main chain benzoxazine (GA-Bz).
11

M. Arslan
ReactiveandFunctionalPolymers139(2019)9–16
3.2. Preparation of G-Amd-Bz and structural characterization
The chemical structure of G-Amd monomer (Scheme 3) was con-
firmed by spectral analysis. As can be seen from the NMR spectra
(Fig. 4) characteristic carboxyl acid proton shifts at 12.18 ppm dis-
appeared after reaction. The amide proton around 9.10 ppm overlapped
with the hydroxyl protons of gallic acid with an increment of the peak
intense. Moreover, new resonance signals attributed to the p-toluidine
methyl and aromatic protons were located at 2.07 and 6.41–6.71 ppm,
respectively.
As shown in Fig. 5, the characteristic amide carbonyl and amide II
band emerged at 1725 and 1500 cm−1, respectively [60]. The OH
stretch of COOH dimer hydrogen band causes a very broad absorption.
The antisymmetric coupled OH stretching at 3270 cm−1 and C]O
stretch overtone at 3484 cm−1 were disappeared after amide formation.
In addition, the characteristic overtone pattern of the COOH group
around 2600 cm−1
, , stretch/bend at
bend/stretch at 1485 cm−1
1250 cm−1 and out-of-plane wag in acid dimer at 910 cm−1 were dis-
appeared after amide formation. However, N–H stretching vibrations of
the primary amine was not observed at 3360 cm−1. Consequently, both
NMR and IR results indicate the formation of G-Amd.
Fig. 3. DSC thermographs of GA-Bz (a) and cured GA-Bz (b).
1H NMR, FTIR, DSC and GPC analysis were performed to identify
the chemical structure of G-Amd-Bz (Scheme 4). The protons resonating
at 4.78 (b) (O-CH2-N) and 4.18 (c) ppm (Ar-CH2-N) is a clear evidence
for the formation of oxazine ring. Furthermore, the hydroxyl peaks of G-
Amd monomers around 9.12 ppm are disappeared because of oxazine
ring formation. Amide proton resonance signals shift to 8.30 ppm.
Aromatic protons and methyl protons are also visible in the NMR
spectra (Fig. 6). However, the resulting product contains small amount
of impurities, despite efforts to eliminate the starting materials.
As shown in overlaid FT-IR spectra (Fig. 7), the characteristic bands
of the benzoxazines emerged at 1410 (CH2 wagging), 1104 (asymmetric
stretching of Ar-O-C) and 946 cm−1 (out of plane vibration of oxazine
unit). The symmetric stretching vibrations of oxazine group (Ar-O-C)
overlapped with Ar-OH and C-O-C vibrations at 1104 cm−1. The ob-
vious decrement was observed between 2000 and 3710 cm−1 which is
belong to phenolic hydroxyls. The characteristic carbonyl and amide
band were also visible at 1725 and 1500 cm−1, respectively. The in-
tensity of the amide carbonyl band was reduced as a result of polymeric
structure.
hydrogen-bonding system involving the phenolic hydroxyl at each site
of the gallic acid monomer. As you see from the Fig. 3, this fact is
demonstrated by observing lower ROP temperature of the GA-Bz. GA-Bz
has an exotherm with a maximum of 169 °C, which is related to con-
tribution of the phenolic and acid groups. The similar catalytic effect
was also observed on the on-set and end-set temperatures.
Main-chain polymer formation was monitored by using a gel per-
meation chromatography (GPC). The number average molecular weight
of GA-Bz was measured as 4950 Da (Mw/Mn: 1.30) (Table 1). According
to the GPC results each GA-Bz chains were modified by 11 gallic acid
and diamine monomer after reaction. This data corresponds to the so-
luble part of the polymer precursor in THF. The monomodal narrow
molecular weight distribution and the clear shift were observed as a
result of the characteristics of step-growth polymerization and also it
can be inferred that no significant side reactions accorded.
Main-chain benzoxazines could be easily synthesized through using
a diphenol and diamine-based structures by means of the one-step
condensation of reaction components such as phenols, amines and
formaldehyde. On the other hand, direct reaction of amine and for-
maldehyde causes to the formation of insoluble triazine network.
Concurrently, carboxyl groups can react with the primary amines to
produce an amide structure. Two hydroxyl and one carboxyl func-
tionality of the gallic acid monomer force to behave a crosslinker. It
effects the reaction yield. Therefore, another strategy was applied to
synthesis main-chain benzoxazine. Among many methods, the simplest
way is that amides can be synthesized by the reaction of amines with
acid chlorides and carboxyl acids. Thus, gallic acid monomer was re-
acted with p-toluidine to obtain an amide functional monomer and
subsequently, it was reacted with 4,7,10-trioxa-1,13-tridecanediamine
and paraformaldehyde to form benzoxazine polymer. Hence, we could
synthesize benzoxazine with high yields by eliminating side reactions of
carboxyl groups during the classical benzoxazine condensation reac-
tions.
The number average molecular weight of G-Amd-Bz polymer was
measured as 4420 Da (Mw/Mn: 1.23) via GPC analysis (Table 1). Each
G-Amd-Bz chains were modified by nearly 9 gallic amid and diamine
monomer according to the GPC thermogram. The monomodal narrow
molecular weight distribution and the clear shift were observed as a
result of the characteristics of step-growth polymerization.
3.3. Curing behavior of GA-Bz and G-Amd-Bz
Depending on the functional groups, the ring-opening polymeriza-
tion temperatures of the benzoxazines range between 160 and 260 °C
and can be monitored as an exotherm. Therefore, ring-opening poly-
merization temperatures of GA-Bz and G-Amd-Bz were investigated
using DSC under N2 atmosphere (Fig. 8). G-Amd-Bz displayed an exo-
therm peak with an onset at 126 °C and an end-set at 167 °C. The curing
maximum was also detected as 151 °C. The overall curing exotherm
quantity is 55 J/g. Following curing of G-Amd-Bz, the second run did
not display any curing exotherm. It demonstrates the depletion of the
oxazine units in the first process during thermal treatment.
The polymerization kinetics of GA-Bz and G-Amd-Bz have been
examined and followed with a heat evaluation in DSC at different
heating rates of 2, 5, 10, 15 and 20 °C/min as shown in Fig. 9. As given
in Table 2, the maximum curing temperature peak Tmax at all the
heating rates follows the sequence of 129, 140, 151, 155, 160 °C, re-
spectively. These exotherms were lower than classic benzoxazine
monomers and the characteristics are mainly due to the gallic unit,
Table 1
Molecular weight characteristics of the polymers.
a
Run
Polymer
Mn
PDIa
(GPC)
1
2
GA-Bz
GA-Amd-Bz
4950
4420
1.30
1.23
a
Determined by GPC, according to polystyrene standards.
12

M. Arslan
ReactiveandFunctionalPolymers139(2019)9–16
Scheme 3. Synthesis of gallic amide monomer (G-
Amd).
explored using thermo-gravimetric analysis (TGA) under N2 atmo-
sphere. The TGA traces of the polymers are shown in Fig. 10 and related
results are collected in Table 3. The cured modified GA-Bz and G-Amd-
Bz polymers obviously showed higher char yields. Pristine 4,7,10-
trioxa-1,13-tridecanediamine has 0% char yield at 900 °C and has
nearly vaporized at 900 °C. GA-Bz and G-Amd-Bz displayed char yields
as 36.5% and 39%, respectively. G-Amd-Bz exhibits rather higher char
yield most possibly due to the large amount of benzoxazine component
per polymer chain. TGA results clearly disclose that integration of
benzoxazines into polymer considerably improved the char yield. The
expected product structure is a phenoxy type polybenzoxazine. Basi-
cally, Friedel–Crafts reaction takes place through the reaction of
oxygen, nitrogen and the unoccupied benzene ortho or para position to
produce a phenoxy or phenolic structure. In this study, benzoxazine
aromatic ring is all-substituted. For this reason, polymerization can
proceed through the oxygen atom to produce a phenoxy type poly-
benzoxazine structure.
Fig. 4. 1H NMR spectra of gallic acid and G-Amd.
4. Conclusion
A novel gallic acid-based main chain bio benzoxazines with two
oxazine rings and one phenolic hydroxyl group in the same benzene
ring was synthesized with different methods. The synthesized main-
chain polymers indicate thermally stimulated curing at much lower
than that of conventional benzoxazines. The results demonstrated that
it is possible to reduce the ROP temperature of main chain benzoxazine
polymers if the design is accomplished carefully. In the strategy, gallic
acid based benzoxazine were synthesized through classic main chain
benzoxazine precursor synthesis approach using gallic acid, gallic
amide, diamine and formaldehyde. The reaction yield of gallic acid-
based monomer was found low to produce a main chain polymer; thus,
this compound converted to amide derivative and then used for the
benzoxazine synthesis. Fortunately, the results indicated that free
phenolic hydroxyls have excellent catalytic effect on the ring opening
polymerization temperatures. The latent catalyst quality of hydroxyls
was studied by tracking exotherm peak in DSC thermogram. Thermal
analysis revealed the reduced maximum ROP temperatures 129, 140,
151, 155 and 160 °C at different heating rates of 2, 5, 10, 15 and 20 °C/
min, respectively. Hydrogen bond interactions (intra- and inter-) of the
free phenolic hydroxyl displayed a significant role on the ring-opening
polymerization temperature. Apart from catalytic benefits, cured GA-Bz
and G-Amd-Bz revealed high char yield as much as 39% at 900 °C.
Consequently, free phenolic hydroxyl bearing benzoxazines have no-
table potential to reduce ROP temperature of benzoxazines acting as
latent catalyst type, and the process is effective and cheap to extend
industrial applications of high performance thermosets.
Fig. 5. FT-IR spectra of 3,4,5-trihydroxybenzoic acid (b) and G-Amd (a).
which has a free phenolic hydroxyl and carbonyl group and the dif-
ferences in the reactivity. Moreover, DSC heat flow is proportional to
the reaction rate according to the non-isothermal DSC kinetic analysis.
The activation energy for the polymerization was studied using the
well-known Kissinger and Ozawa methods by different research groups
[58,61–63]. The naphtoxazine, amide and imide functional benzox-
azines has showed lower activation energy and curing temperature
[40,59]. The thermograms represent that the exothermic ROP peaks
shift to a lower temperature with the decrease of heating rate as ex-
pected. The low ROP temperatures suggest that G-Amd-Bz is easy to be
activated to polymerize and it is compatible with the literature
[40,52,59].
Data availability
The raw/processed data required to reproduce these findings cannot
be shared at this time as the data also forms part of an ongoing study.
Acknowledgment
3.4. Thermal properties of GA-Bz and G-Amd-Bz
The author would like to thank Kirklareli University, Research
Thermal stability of the GA-Bz and G-Amd-Bz polymers was
Fund, for financial support (KLUBAP-129).
13

M. Arslan
ReactiveandFunctionalPolymers139(2019)9–16
Scheme 4. Synthesis of gallic amide-based main chain benzoxazine (G-Amd-Bz).
Fig. 8. DSC thermographs of G-Amd-Bz (b) and cured G-Amd-Bz (a).
Fig. 6. 1H NMR spectra of 4,7,10-trioxa-1,13-tridecanediamine (c), G-Amd (b)
and G-Amd-Bz (a).
Fig. 9. DSC thermographs of G-Amd-Bz at various heating rates.
Fig. 7. FT-IR spectra of G-Amd (b) and G-Amd-Bz (a).
14

M. Arslan
ReactiveandFunctionalPolymers139(2019)9–16
Table 2
78071–78080.
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curing (°C) curing (°C)
End-set of
Maximum
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ΔH (J/g)
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2
5
98
143
156
167
173
178
149
210
198
216
213
129
140
151
155
160
145
150
170
171
174
−68
−62
−55
−47
−53
−9
−53
−20
−18
−12
119
126
132
134
142
130
152
122
131
G-Amd-Bz 10
15
20
2
5
GA-Bz
a
10
15
20
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Polymer
T5% (°C)
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GA-Bz
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277
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