Synthesis and characterization of novel bio-based benzoxazines from eugenol

Article abstract of DOI:10.1039/c3ra46582a

Polybenzoxazines are phenolic-like materials that possess dimensional and thermal stability, and they release no by-products during their polymerization. In this study, a new class of benzoxazine-containing monomers has been prepared from renewable resource (eugenol, a natural phenol obtained from clove oil), paraformaldehyde and various aromatic diamines. The structures of the monomers were supported by Fourier Transform Infrared (FT-IR), ¹H-NMR and ¹³C-NMR spectral analysis, proving the existence of the benzoxazine ring containing eugenol moiety in its molecular structure. The monomers were polymerized/cured via ring-opening polymerization by heating as indicated by FT-IR and Differential Scanning Calorimetry (DSC). This is confirmed by the disappearance of the peaks due to oxazine ring (942 cm^-1). The exothermic peak associated with curing was observed from 170°C to 250°C. The thermal and mechanical properties of the polybenzoxazines were investigated through thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The temperatures corresponding to 5% and 10% weight loss is from 240 to 295°C and from 290 to 340°C, respectively. The completely cured materials could achieve char yields up to 36.5% at 800°C in nitrogen atmosphere. DMA revealed that the glass transition temperatures of PBz-SUL and PBz-PHE were higher than that of PBz-DDS and PBz-OXY.

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SYNTHESIS AND CHARACTERIZATION OF NOVEL BIO-BASED
BENZOXAZINES FROM EUGENOL
Introduction
Benzoxazine is a molecule having a benzene ring fused to another six-membered
heterocycle containing one oxygen atom and a nitrogen atom. There are a number of possible
isomeric benzoxazines depending on the relative positions of the two heteroatoms of the
oxazine ring system, and 1, 3-benzoxazine is the kind of isomer used for polymerization1.
This is relatively a new family of phenolic resin which combines the thermal properties and
flame retardance of phenolics, with the mechanical performance and design flexibility of
epoxies2,3. Benzoxazines are generated by the Mannich-like condensation of a phenol,
formaldehyde and an amine4.
The chemistry of benzoxazines is responsible for a number of inherent processing
advantages, including low melt viscosity, no harsh catalyst required, and no volatile release
during cure and minimal cure shrinkage5. Shrinkage does not occur during curing, which is a
striking feature of benzoxazine-based polymers6. They also have good thermal stability that is
demonstrated by their high glass transition temperature (170-3400C), degradation temperature
(350-4000C), and char yield (28-66%)7. Though, polybenzoxazines offer a variety of
advantages, pure polybenzoxazine-based polymers also suffer from a number of
disadvantages, in terms of (i) high curing temperature (ii) difficulty in processing and (iii)
poor mechanical strength [brittleness]. To properly address these issues and overcome the
associated disadvantages, various strategies such as (i) preparation of modified monomers
with additional functionality (ii) synthesis of novel polymeric precursors and (iii) blending
with a high-performance polymer or filler and fiber may be adopted6.

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Eugenol is a member of the phenylpropanoids class of chemical compounds. The
name is derived from the scientific name for clove, Eugenia aromaticum or Eugenia
caryophyllata. Clove is commercially cultivated in India, Madagascar, Sri Lanka, Indonesia
and the south of China. Eugenol is responsible for the aroma of cloves. It is the main
component in the essential oil extracted from cloves, comprising 72-90% of the total. It is a
clear to pale yellow oily liquid extracted from certain essential oils especially from clove oil,
nutmeg, cinnamon, basil and bay leaf. It is slightly soluble in water and soluble in organic
solvents. It has a spicy, clove-like aroma. Clove oil is widely used for flavouring pastry,
special sauces and condiments. It is also used in medicine, especially in the preparations for
gum and teeth8,9. Table 1 shows the relative percentages of all the components in clove leaf
and bud oil.
The development of new polymers, especially those based on renewable organic raw
materials deserve the attention of both academic and industrial research10. In this study, we
focus on the effect of incorporation of eugenol in benzoxazine-based phenolic resins on the
thermal properties. Hence, we report the synthesis of benzoxazine-based phenolic resin using
eugenol, aromatic diamine and paraformaldehyde as raw materials. The synthesis,
characterization, thermal polymerization and the properties of these new benzoxazines are
described in this paper.
Experimental
Materials
Eugenol was purchased from Rajkart Enterprises, India; paraformaldehyde, N,N-
dimethylformamide (DMF) and ethanol were purchased from Merck Limited, India;
potassium carbonate (K2CO3) was purchased from s d fine – chem limited, Inida; bis (4-
aminophenyl)sulphone and Bis(4-hydroxyphenyl)sulphone were purchased from Alfa Aesar

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(Johnson Mathew Company), India, Pvt. Ltd.; hydrazine monohydrate was purchased from
Spectrochem Pvt. Ltd., India; hydroquinone was purchased from Fisher Scientific (a part of
Thermo Fisher Scientific, India Pvt. Ltd.);
p-nitrophenol, p-chloronitrobenzene, ,
dimethylsulphoxide (DMSO) and 10% Pd/C from Sisco Research Laboratories, Pvt. Ltd.,
India. All chemicals and solvents were used without further purification.
Characterization methods
Fourier transform infrared (FTIR) spectra of the samples were obtained using an ABB
Bomen (Model MB 3000) Spectrometer. The samples were ground with spectroscopy grade
KBr and made into pellets. 1H (500 mHz) & 13C (125 mHz) nuclear magnetic resonance
(NMR) spectra were recorded on a Joel Spectrometer with tetramethylsilane (TMS) as the
internal standard. Solutions were prepared in DMSO-d6. Differential Scanning Calorimetry
was performed in a TA instrument Q10 model using 5-10 mg of the sample at a heating rate of
10oC/min in nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed using a
TA Q 600 thermal analyzer. Cured samples were analyzed in open silicon pan at a heating
rate of 20oC/min in N2 atmosphere, up to a maximum temperature of 800oC.
1. Synthesis of diamines
1.1 Synthesis of Bis (4-aminophenyl) ether [BAPE]
p-Nitrophenol (0.1m, 13.9g) and p-chloronitro benzene (0.1m, 15.7g) were taken in a
500 mL three necked round bottomed flask fitted with a reflux condenser and magnetic
stirrer. 150 mL of DMF and 31.8g (0.1m) of potassium carbonate were added to this mixture
and heated to 145-150oC under N2 atmosphere while stirring. The mixture was maintained at
this temperature for 8h, cooled and then poured into 300 mL of water. The yellow solid [bis
(4-nitrophenyl) ether] obtained was separated by filtration and dried at 70oC. Yield: 80%.

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Bis (4-nitrophenyl) ether (0.1m, 26g), ferric chloride hexahydrate (1.5g), activated
carbon (1.5g) and ethanol (300 mL) were taken in a 500 mL three necked round bottomed
flask fitted with a reflux condenser and magnetic stirrer. To this mixture hydrazine hydrate
(60 mL of 85%) was added dropwise over a period of 1h at reflux temperature. The reaction
was continued for about 5h under reflux conditions. The solution was then filtered to remove
the catalysts, cooled to room temperature, and then filtered to obtain white crystals of [bis (4-
aminophenyl) ether] [BAPE]. Yield: 75%, m.p. 190oC.
FT-IR (KBr, cm-1) 3040 (C-H, aromatic), 1512 (C=C, aromatic), 1319 (C-O-C), 3387
and 3325 (N-H, asymmetric and symmetric stretching vibrations); 1H-NMR (DMSO-d6, ppm)
3.5 (s, -NH2) 6.5 (d, -CH=C-N) 6.7 (d, -CH=C-O); 13C-NMR (DMSO-d6, ppm) 145 (C=*C-
NH2) 115 (*C=C-NH2) 119 (*C=C-O) 147 (C=*C-O).
1.2 Synthesis of 1, 4 [bis (4’-aminophenoxy) benzene] [BAPB]
BAPB was synthesized using a procedure similar to that of BAPE using hydroquinone
and p-chloronitrobenzene as starting materials. White crystals (85% yield, m.p. 192oC) of
BAPB were obtained.
IR (KBr, cm-1) 3040 (C-H), 1497 (C=C, aromatic), 1273 (C-O-C), 3310 (N-H,
1
asymmetry), 3225 (N-H, symmetry); H NMR (DMSO-d6, ppm) 4.5 (s, -NH2) 7.4 (d, -
CH=C-N) 7.8 (d, -CH=C-O) 8.0 (d, -CH=C-O); 13C NMR (DMSO-d6, ppm) 125 (C=*C-NH2)
100 (*C=C-NH2) 114 (*C=C-O) 144 (C=*C-O).
1.3 Synthesis of 1,4 [bis (4’-aminophenoxy) phenyl sulfone] [BAPPS]
BAPPS was synthesized using a procedure similar to that of BAPE using bis(4-
hydroxyphenyl)sulphone and p-chloronitro benzene as starting materials. White crystals
(83% yield, m.p. 195oC) of BAPPS were obtained.

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FT-IR (KBr, cm-1) 3040 (C-H, aromatic), 1582 (C=C, aromatic), 1288 (C-O-C), 3456
and 3371 (N-H, asymmetric and symmetric stretching vibrations), 1412 and 1242 (O=S=O,
1
asymmetric and symmetric stretching vibrations); H-NMR (DMSO-d6, ppm) 3.6 (s, -NH2),
6.5 (d, -CH=C-N), 6.8 (d, -CH=C-O), 7.1 (d, -CH=C-O), 7.9 (d, -CH=C-S); 13C-NMR
(DMSO-d6, ppm) 145 (C=*C-NH2), 115 (*C=C-NH2) 119 (*C=C-O), 147 (C=*C-O), 160
(C=*C-O), 130 (*C=C-S), 133 (C=*C-S).
2. Synthesis of benzoxazine monomers
2.1 Synthesis of bis(6-allyl-8-methoxy-3-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl)oxane
[Bz-OXY]
In a 500 mL round bottomed three necked flask bis (4-aminophenyl) ether [BAPE]
(0.05m, 10g), eugenol (0.1m, 16.4g) and 50 mL of DMSO were taken and stirred well to get
a homogenous solution. Paraformaldehyde (0.3m, 12g) was added to the solution and the
mixture was heated at 1300C for 5h in an oil bath, cooled and finally poured into 1L of 1N
aqueous sodium hydroxide solution. The precipitate formed was collected by filtration,
washed several times with water. After drying at 60oC in a vacuum oven, pale yellow
powder (82% yield, m.p.140oC) was obtained. Since the benzoxazine monomer was prepared
from BAPE, we refer to it as Bz-OXY 2.
FT-IR (KBr, cm-1) 3044 (C-H), 1597 (C=C), 1324 (CH2), 939 (oxazine ring), 1109,
1227 and 1365 (C-O-C), 1186 (C-N-C); 1H-NMR (DMSO-d6, ppm) 4.5 (s, Ar-CH2-O), 5.4 (s,
*
O-CH2-N), 3.7 (s, -OCH3), 3.2 (d, -CH2 -CH=CH2), 5.9 (m, -CH2-CH*=CH2), 5.0 (d, -CH2-
*
CH=CH2 ), 6.5 – 7.5(m, aromatic protons);13C-NMR (DMSO-d6, ppm) 49.5 (Ar-CH2-O),
79.9 (O-CH2-N), 55.8 (-OCH3), 115.6 (-*CH2-CH=CH2), 138.2 (-CH2-*CH=CH2), 40.5 (-
CH2-CH=*CH2).

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2.2
Synthesis
of
bis(6-allyl-8-methoxy-3-phenyloxy-3,4-dihydro-2H-1,3-
benzoxazinyl)oxane [Bz-PHE]
Bz-PHE was synthesized using a procedure similar to that of Bz-OXY. Pale yellow
powder (81% yield, m.p. 90oC) was obtained. Since the benzoxazine monomer was prepared
from BAPB, we refer to it as Bz-PHE.
FT-IR (KBr, cm-1) 3044 (C-H), 1596 (C=C), 1327 (CH2), 939 (oxazine ring), 1107,
1224 and 1327 (C-O-C), 1193 (C-N-C); 1H-NMR (DMSO-d6, ppm) 4.5 (s, Ar-CH2-O), 5.3 (s,
*
O-CH2-N), 3.7 (s, -OCH3), 3.2 (d, -CH2 -CH=CH2), 5.9 (m, -CH2-CH*=CH2), 5.0 (d, -CH2-
*
CH=CH2 ), 6.5 – 7.5(m, aromatic protons);13C-NMR (DMSO-d6, ppm) 49.8 (Ar-CH2-O),
79.5 (O-CH2-N), 56.0 (-OCH3), 116.0 (-*CH2-CH=CH2), 138.2 (-CH2-*CH=CH2), 39.9 (-
CH2-CH=*CH2).
2.3
Synthesis
of
bis(6-allyl-8-methoxy-3-phenyl-3,4-dihydro-2H-1,3-
benzoxazinyl)sulfone [Bz-DDS]
Bz-DDS was synthesized using a procedure similar to that of Bz-OXY. Pale yellow
powder (78% yield, m.p. 130oC) was obtained. Since the benzoxazine monomer was
prepared from DDS, we refer to it as Bz-DDS.
FT-IR (KBr, cm-1) 3040 (C-H), 1593 (C=C), 1376 (CH2), 942 (oxazine ring), 1103
1
and 1233 (C-O-C), 1191 (C-N-C), 1144 (O=S=O); H-NMR (DMSO-d6, ppm) 4.6 (s, Ar-
*
CH2-O), 5.4 (s, O-CH2-N), 3.6 (s, -OCH3), 3.2 (d, -CH2 -CH=CH2), 5.9 (m, -CH2-CH*=CH2),
*
5.0 (d, -CH2-CH=CH2 ), 6.5 – 7.5(m, aromatic protons);13C-NMR (DMSO-d6, ppm) 49.5
(Ar-CH2-O), 79.1 (O-CH2-N), 55.9 (-OCH3), 115.8 (-*CH2-CH=CH2), 138.2 (-CH2-
*CH=CH2), 40.4 (-CH2-CH=*CH2).
2.4
Synthesis
of
bis(6-allyl-8-methoxy-3-phenyloxy-3,4-dihydro-2H-1,3-
benzoxazinyl)diphenylsulfone [Bz-SUL]

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Bz-SUL was synthesized using a procedure similar to that of Bz-OXY. Pale yellow
powder (81% yield, m.p. 100oC) was obtained. Since the benzoxazine monomer was
prepared from BAPB, we refer to it as Bz-SUL.
FT-IR (KBr, cm-1) 3068 (C-H), 1587 (C=C), 1322 (CH2), 942 (oxazine ring), 1105,
1
1242 and 1383 (C-O-C), 1166 (C-N-C), 1147 (O=S=O); H-NMR (DMSO-d6, ppm) 4.6 (s,
*
Ar-CH2-O), 5.4 (s, O-CH2-N), 3.7 (s, -OCH3), 3.2 (d, -CH2 -CH=CH2), 5.9 (m, -CH2-
CH*=CH2), 5.0 (d, -CH2-CH=CH2 ), 6.5 – 8.0 (m, aromatic protons); 13C-NMR (DMSO-d6,
*
ppm) 49.4 (Ar-CH2-O), 79.3 (O-CH2-N), 56.1 (-OCH3), 116.0 (-*CH2-CH=CH2), 138.5 (-
CH2-*CH=CH2), 39.9 (-CH2-CH=*CH2).
Results and Discussion
Chemical structure of the monomers was confirmed by FT-IR, 1H NMR & 13C NMR
spectral methods and elemental analysis.
Eugenol based benzoxazine monomer (Bz-DDS) was synthesised by the condensation
reaction of eugenol with formaldehyde and amine as shown in Scheme 2. The FT-IR
spectrum (Fig.2) of this monomer shows characteristic absorption bands at 1233 cm-1
(asymmetric stretching of C-O-C), 1029 cm-1 (symmetric stretching of C-O-C) and 942 cm-1
which is typical of benzoxazine ring structure12. The spectrum also shows a band at 1376 cm-
1 due to tetra substituted benzene ring. Other aromatic vibrations are observed at 1593 and
1504 cm-1. C-H stretching vibration of benzene ring appears at 3068 cm-1 and the peaks at
2934 and 2899 cm-1 are owing to the asymmetric and symmetric stretching vibrations of the
methylene group of the oxazine ring as well as alkyl side chain of eugenol13,14
Fig.3 Shows the 1H-NMR spectra of all the benzoxazine monomers. The
characteristic oxazine ring protons appear as two singlets at 4.6 and 5.4 ppm which are
assigned to C-CH2-N and O-CH2-N respectively15. The singlet at 3.7 ppm is due to the –

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OCH3 protons. The doublets at 3.3 and 5.1 ppm are assigned to [-CH2=CH-CH2-] allyl
protons. The multiplets between 6.5 and 7.5 ppm are assigned to the aromatic protons16.
Fig.4 shows the 13C-NMR spectra of all the monomers. The characteristic carbon
resonances of the oxazine ring are found at 49.5 and 79.1 ppm for Ar-CH2-N and N-CH2-O
respectively17. The peak at 55.9 ppm is attributed to the OCH3 carbon and the peaks at 115.8,
138.2 and 40.4 ppm are assigned to CH2=CH-CH2- allyl carbons.
The polymerization behaviour of all the Eugenol based benzoxazine monomers were
studied using DSC at a heating rate of 100C/min under nitrogen atmosphere from 250C to
3250C. Figure 5 shows the DSC thermograms of all the monomers and the results are
summarized in Table 2. The endotherm peaks centered around 90 - 1300C are attributed to
the melting point of the monomers
The onset of curing is around 195 and 2050C for Bz-DDS and Bz-SUL, whereas it is
around 190 and 2250C for Bz-OXY and Bz-PHE, respectively. The values are almost 10 to
350C lower than that of the monomers containing Bz-SUL and Bz-PHE structures. This is
because the oxygen and sulphur linkages allow the molecule to rotate easily and increase the
flexibility in Bz-SUL and Bz-PHE, whereas the incorporation of aromatic rings in Bz-DDS
and Bz-OXY adds rigidity to the molecule. The exothermic maxima are observed at 255 and
2500C for Bz-DDS and Bz-SUL, and at 225 and 2500C for Bz-OXY and Bz-PHE,
respectively. The temperature difference between melting point and onset of polymerization
is defined as the processing window. The processing window for the monomers Bz-DDS,
Bz-SUL, Bz-OXY & Bz-PHE is 65, 105, 50, 1350C, respectively. All the four monomers
have wide processing window, indicating good process-ability for the monomers16.
The curing reactions of the monomers were further confirmed by FTIR analysis.
Figures 6 (a-d) show the FTIR spectra of the monomers after heating at 100, 150, 200 and
250oC for 1 h at each temperature.

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For Bz-DDS monomer, the dramatic decrease of intensity of peaks around 942, 1029,
1233 & 1376 cm-1 shows the curing of the benzoxazine ring components in the monomer17.
During the curing process, the oxazine ring is opened by the breakage of C-O bond. The
disappearance of the band at 1686 cm-1 typical of the allylic stretching present in the eugenol
part of the benzoxazine monomer confirms the reactivity of the double bonds18. This occurs
at high temperature (2000C) after the thermal treatment of benzoxazine with the possible
production of a network of cross linked material. The formation of hydroxyl groups was
observed during the thermal cure of the monomer which was justified by the presence of
broad band in the range of 3400 to 3500 cm-1. This result implies that the curing reaction
takes place by the ring opening of benzoxazine and addition reaction of allyl group of the
eugenol component as shown in the Scheme 3 (curing). Similar curing behaviour was
observed in other three benzoxazine monomers19.
The thermal properties of the polybenzoxazines crosslinked by heating upto 2500C
were studied by thermo gravimetric analysis (TGA) (Fig.7). The data obtained are tabulated
in Table 3. It can be seen that the polybenzoxazines show fairly good thermal stability. For all
the polymers studied, , the 5% and 10% weight loss temperatures (T5 and T10) varies from
240 to 2950C and 290 to 3400C. Obviously, the thermal stability of Bz-DDS based polymer
is superior to that of the other three polymers. This may be attributed to the fact that the
sulfonyl group incorporated in Bz-DDS based polymer had a significant effect on improving
the thermal stability, whereas the flexible oxygen linkages in the other three polymers
decreased the thermal stability. The char yield at 8000C in N2 atmosphere varies from 23% to
36% for all the cross linked polybenzoxazines. The highest char yield (36%) of Bz-PHE is
due to high aromatic content in its structure compared with the other three polymers.
The limiting oxygen index (LOI) values, which can be taken as an indicator to
evaluate the polymer’s flame retardancy, were measured and shown in Table 3. Char yield of

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a material can be used to estimate LOI according to Van Krevelen and Hoftyzer equation20.
The char yield of the polybenzoxazines was found to be high in the range of 23.4-36.5%
indicating high flame retardancy, which is expressed in terms of their LOI value. The LOI
values of the polymers should be above the threshold value of 26, to render them self-
extinguishing and for their qualification for many applications requiring good flame
resistance. It was found that the LOI increases with increasing char yield as expected. The
prepared polybenzoxazines show LOI value greater than 26 confirming their good flame
retardant properties.
Conclusion
Bio-based benzoxazine monomers (Bz-OXY, Bz-PHE, Bz-DDS and Bz-SUL) have
been successfully synthesized from Eugenol, a well known renewable organic resources and
by-product of clove oil and polymerized to produce different polybenzoxazines. The
1
structure of the monomers supported by FT-IR, H NMR & 13C NMR spectra show the
existence of reactive benzoxazine ring, methoxy and allyl groups in its molecular structure.
DSC scans of all the benzoxazine monomers indicate good process-ability showing wide
processing window. The ring-opening polymerization of monomers is confirmed by cure
cycle of FT-IR spectrum. The allyl moiety of benzoxazine also participates during the curing
process which improves the cross-linking density of the polymer. This leads to enhancement
in thermal stability (340oC) and char yield (36.5% at 800oC). Furthermore, all the polymers
possess good flame retardancy as shown by their LOI values. From the obtained results, it
can be seen that eugenol can be considered as a very attractive renewable bio-source for use
in eco-friendly processes.

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Acknowledgement
The authors acknowledge the Department of Science and Technology, New Delhi and
The council of Scientific and Industrial Research for funding this project. The authors also
acknowledge DST (FIST) and UGC (SAP) for the financial support extended to procure instrumental
facilities.
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S.No. Monomer Tm (0C)
Tonset
(0C)
195
Tmax Tprocessing
(0C) window(0C)
1.
2.
3.
4.
Bz-DDS
Bz-SUL
Bz-OXY
Bz-PHE
130
100
140
90
255
250
225
250
65
105
50
205
190
225
135

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S.No.
Polymer
T5%
T10%
Char yield
(%)
32.0
LOI
values
30.3
1.
2.
3.
4.
PBz-DDS
PBz-SUL
PBz-OXY
PBz-PHE
295
275
240
260
340
325
290
310
23.4
30.1
36.5
26.9
29.4
32.1

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Fig.1 A clove plant with leaves and buds
Fig. 2 FTIR spectrum of (a)Bz-DDS, (b)Bz-SUL, (c)Bz-OXY and (d)Bz-PHE monomer
Fig. 3 1H NMR spectra of the monomers
Fig. 4 13C NMR spectra of the monomers
Fig. 5 DSC curves of (a)Bz-DDS, (b)Bz-SUL, (c)Bz-OXY and (d)Bz-PHE
Fig. 6 FTIR spectra of (a)Bz-DDS, (b)Bz-SUL, (c)Bz-OXY and (d) Bz-PHE at different
curing temperature
Fig. 7 TGA curves of (a)PBz-DDS, (b)PBz-SUL, (c)PBz-OXY and (d)PBz-PHE
Fig. 8 DMA spectra showing strorage modulus of (a)PBz-DDS, (b)PBz-SUL, (c)PBz-OXY
and (d)PBz-PHE
Fig. 9 DMA spectra showing loss modulus of (a)PBz-DDS, (b)PBz-SUL, (c)PBz-OXY and
(d)PBz-PHE
Scheme 1 Synthesis of diamines
Scheme 2 Synthesis of benzoxazine monomers
Scheme 3 Schematic representation of thermal curing of benzoxazine monomer
Table 1 DSC data of benzoxazine monomers
Table 2 Thermal properties of polybenzoxazines

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