A facile strategy to achieve fully bio-based epoxy thermosets from eugenol

Article abstract of DOI:10.1039/c9gc01184f

To achieve sustainability, many kinds of bio-based epoxy resins have been developed. However, to the best of our knowledge, a 100% bio-based epoxy thermoset has rarely been reported since it needs both the epoxy resin and curing agent to be bio-based. Thi

Full text of DOI:10.1039/c9gc01184f

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Page 1 of 31
Green Chemistry
A facile strategy to achieve fully bio-based epoxy thermosets from eugenol
View Article Online
DOI: 10.1039/C9GC01184F
∥§
†
*‡§
Chien-Han Chen∥§, Shih Huang Tung ,Ru Jong Jeng*∥§, Mahdi M. Abu-Omar , Ching-Hsuan Lin
§
Advanced Research Center for Green Materials Science and Technology, National Taiwan
University, Taipei, Taiwan.
∥
Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan.
†
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California,
United States.
‡
Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan.
Abstract
To achieve sustainability, many kinds of bio-based epoxy resins have been developed.
However, to the best of our knowledge, a 100 % bio-based epoxy thermoset is rarely reported since it
needs both the epoxy resin and curing agent being bio-based. This work provides a facile strategy to
achieve epoxy thermosets with 100 % bio-based content. The strategy includes the preparation of
four bio-based epoxy resins (1-4) and their thermosets through the self-curing reaction of (1-4). The
epoxy compounds (1-4) were prepared from the esterification of eugenol with succinyl, adipoyl,
suberoyl, and 2,5-furan chloride, respectively, followed by the oxidation of the allylic bond. Through
NMR, DSC, and FTIR analyses, we confirm that the self-curing reaction of (1-4) occurred through a
4-dimethylpyridine (DMAP)-catalyzed reaction of active esters and epoxides. This work successfully
provides a facile strategy to achieve fully bio-based epoxy thermosets.
Introduction
In the past few decades, polymeric materials have mostly relied on the petroleum-based
chemical. However, due to the environmental concern and the limited resources of petroleum, the
2
development of bio-based polymers has raised in recent years.1, More and more bio-based
1

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compounds have been produced from agriculture and other natural resources.3-11 Epoxy resinVsie,wdAurtiecletOonline
DOI: 10.1039/C9GC01184F
their unique properties after curing with curing agents, have been widely used in the fields of
coatings, encapsulation, composites and electronic applications.12, 13 To achieve sustainability, many
kinds of bio-based epoxy resins14-17 and curing agent18-20 have been reported. Epoxy resins derived
from Catechin,21 curcumin and resveratrol,22 resorcinol-acetone product,23 cardanol,24 vanillin,25 and
vanillin acid26 have been reviewed by David et al.27 and Koike.28 Bio-based thermosetting resins
derived from renewable plant oil, cardanol, rosin acid, lignin, glycerol, gallic acid, furan, isosorbide,
and itaconic acid have been reviewed by Liu and Zhu et al.29 Webster et al.30-32 also reported the
preparation of fully bio-based epoxy thermosets from the bio-based epoxidized sucrose soyate and
carboxylic acid compounds derived from isosorbide and maleic anhydride.
Eugenol, an oily liquid extracted from certain essential oils, is commonly used as a flavoring
agent in food products.33 Eugenol-rich cinnamon oil makes the most effective natural mosquito
repellent.34. Eugenol has been attracting much attention not only it can be produced from nature, but
also its distinctive structure. The phenolic group provides a reaction site for modification such as
etherification or esterification, and the allylic group provides a route to prepare epoxy resins through
oxidation. Zhang et al.,35 Wang et al.,36,37,38 Gu et al.,39 and Caillot et al.40 have reported the
synthesis of eugenol-based epoxy compounds. However, petrochemical-based curing agents are
applied to cure the epoxy mentioned above, remarkably reducing the bio-based content of the epoxy
thermosets. None of them dealt with the self-curing characteristic of the eugenol-based epoxy
compounds.
2

Page 3 of 31
Green Chemistry
In this work, we provide a facile strategy to achieve 100 % bio-based epoxy thermosets. The
View Article Online
DOI: 10.1039/C9GC01184F
strategy includes the preparation of four bio-based epoxy compounds (1-4) and their thermosets
through the self-curing reaction of (1-4). The epoxy compounds (1-4) were prepared from the
esterification of eugenol with succinyl, adipoyl, suberoyl, and 2,5-furan chloride, respectively,
followed by the oxidation of the allylic bond. Since the diacid chlorides were obtained from the
corresponding renewable acid,41-44 epoxy compounds (1-4) are thought to be fully bio-based, and
their self-cured thermosets are definitely fully bio-based. Through NMR, DSC, and FTIR analyses,
we confirm that the self-curing reaction of (1-4) occurred through a 4-dimethylpyridine
(DMAP)-catalyzed reaction of active esters and epoxides. Detailed synthesis, analyses, and
properties are reported in this work.
Experimental
Materials
Eugenol was purchased from Sigma-Aldrich. Succinyl and adipoyl chloride, trimethylamine (TEA),
3-chloroperoxybenzoic acid (mCPBA) and 4-dimethylpyridine (DMAP) were purchased from Acros.
Suberoyl chloride and glycidyl phenyl ether were purchased from TCI. 4,4'-Methylenedianiline
(DDM) was purchased from Chriskev. Sodium sulfite and sodium bicarbonate were purchased from
Fluka. 2,5-furandicarbonyl chloride was prepared according to literature.45 Phenyl
furan-2-carboxylate was prepared according to literature.46 The solvents were HPLC grade,
purchased from common resource and used without further purification.
Characterization
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NMR measurements were performed on a Varian Inova 600 NMR. Thermal stability of epoxy
View Article Online
DOI: 10.1039/C9GC01184F
thermosets was determined on a Perkin-Elmer Pyris1 TGA. The analysis was performed under a
nitrogen atmosphere. The heating rate was 20 oC/min. Differential scanning calorimetry
o
measurements were conducted on a Perkin-Elmer DSC 8000. The heating rate was 10 C/min. The
dynamic mechanical analysis was performed with a Perkin-Elmer Pyris Diamond DMA. The heating
rate was 5 oC/min. Dielectric properties were measured on an Agilent E4991A at 1GHz at 25 oC. IR
Spectra were obtained in the standard wavenumber range of 650-4000 cm-1 by a Perkin-Elmer RX1
infrared spectrophotometer. The gel content was measured according to the following procedure:
o
“The sample with a known weight (Wi) was immersed at CHCl3 at 25 C for 24 hours. The sample
was then dried at 80 oC in vacuum for 48 hours to achieve a constant weight (Wf).” The gel content is
given by the following equation:
. The Stress-strain curves was performed by
(푊푓/푊푖) × 100%
EZ-SX at 25 oC, the maximum force of the instrument is 100 Nt.
Synthesis of bis(4-allyl-2-methoxyphenyl) succinate (1a)
Eugenol 26.49 g, (64.52*2.5 mmol), triethylamine 16.32 g (64.52*2.5 mmol) and ethyl acetate 100
mL were introduced into a 250 mL flask equipped with a nitrogen inlet and stirrer. Succinyl chloride
10.00 g (64.52 mmol) in 50 mL ethyl acetate was added dropwise at ice bath over 1 h with vigorous
o
agitation and then stirred at 25 C for another 4 h. The white salt was filtered. The filtrate was
extracted with 1N NaOH(aq) and 1N HCl(aq), respectively. The organic layer was evaporated and the
obtained mixture was washed with hexane, the obtained precipitate was filtered and recrystallized
from ethanol, and dried at 60 oC to obtain a white crystal (1a) (yield=83%). 1H-NMR (ppm, CDCl3),
δ = 6.94 (2H, H9), 6.77 (2H, H5), 6.74 (2H, H10), 5.94 (2H, H2), 5.08 (4H, H1), 3.78 (6H, H6), 3.36
(4H, H3), 3.02 (4H, H12). 13C-NMR (ppm, CDCl3), δ=170.38 (C11), 150.74 (C8), 139.00 (C7), 137.82
4

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Green Chemistry
(C4), 136.99 (C2), 122.47 (C9), 120.60 (C10), 116.11 (C1), 112.63 (C5), 55.77 (C6), 40.05 (C3), 29.07
View Article Online
DOI: 10.1039/C9GC01184F
o
(C12). Melting point: 88.3-91.7 C (onset-offset temperature in the DSC thermogram). HR-MS
(ESI-MS) m/z:[Na+] calcd. for C24H26O6Na 433.16; anal. 433.1621. FTIR (KBr): ν (cm-1) = 1756
(C=O stretch of carbonyl group), 1638 (C=C stretch of allylic group).
6
O
O
7
12
11
O
5
4
O
8
2
O
O
9
1
10
3
Synthesis of bis(4-allyl-2-methoxyphenyl) adipate (2a)
(2a) was prepared by the same procedure as that for (1a) except for succinyl chloride being replaced
by adipoyl chloride, white crystal (2a) was obtained (yield=80%). 1H-NMR (ppm, CDCl3), δ = 6.92
(2H, H9), 6.76 (2H, H5), 6.74 (2H, H10), 5.94 (2H, H2), 5.08 (4H, H1), 3.78 (6H, H6), 3.36 (4H, H3),
13
2.63 (4H, H12), 1.90 (4H, H13). C-NMR (ppm, CDCl3), δ=171.53 (C11), 150.77 (C8), 138.88 (C7),
137.89 (C4), 137.02 (C2), 122.42 (C9), 120.59 (C10), 116.10 (C1), 112.58 (C5), 55.70 (C6), 40.05 (C3),
o
33.62 (C12), 24.33 (C13). Melting point: 97.9-100.8 C (onset-offset temperature in the DSC
thermogram). HR-MS (ESI-MS) m/z:[Na+] calcd. for C26H30O6Na 461.19; anal. 461.1929. FTIR
(KBr): ν (cm-1) = 1756 (C=O stretch of carbonyl group), 1638 (C=C stretch of allylic group).
6
O
O
7
12
11
8
O
5
4
O
13
2
O
O
9
1
10
3
Synthesis of bis(4-allyl-2-methoxyphenyl) octanedioate (3a)
(3a) was prepared by the same procedure as that for (1a) except for succinyl chloride being replaced
by suberoyl chloride, white crystal (3a) was obtained (yield=88%). 1H-NMR (ppm, CDCl3), δ = 6.92
(2H, H9), 6.76 (2H, H5), 6.74 (2H, H10), 5.94 (2H, H2), 5.08 (4H, H1), 3.78 (6H, H6), 3.36 (4H, H3),
2.57 (4H, H12), 1.78 (4H, H13), 1.48 (4H, H14). 13C-NMR (ppm, CDCl3), δ=171.87 (C11), 150.79 (C8),
138.81 (C7), 137.91 (C4), 137.02 (C2), 122.54 (C9), 120.59 (C10), 116.07 (C1), 112.59 (C5), 55.72
o
(C6), 40.04 (C3), 33.90 (C12), 26.83 (C14), 264.83 (C13). Melting point: 99.9-104.2 C (onset-offset
temperature in the DSC thermogram). HR-MS (ESI-MS) m/z:[Na+] calcd. for C28H34O6Na 489.22;
5

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Page 6 of 31
anal. 489.2238. FTIR (KBr): ν (cm-1) = 1759 (C=O stretch of carbonyl group), 1638 (C=C stretch of
View Article Online
DOI: 10.1039/C9GC01184F
allylic group).
6
O
O
7
14
12
11
O
5
4
O
8
9
13
2
O
O
1
10
3
Synthesis of bis(4-allyl-2-methoxyphenyl) furan-2,5-dicarboxylate (4a)
(4a) was prepared by the same procedure as that for (1a) except for succinyl chloride being replaced
1
by 2,5-furandicarbonyl chloride, white crystal (4a) was obtained (yield=81%). H-NMR (ppm,
CDCl3), δ = 7.44 (2H, H13), 7.05 (2H, H9), 6.81 (2H, H5), 6.79 (2H, H10), 5.96 (2H, H2), 5.09 (4H,
13
H1), 3.80 (6H, H6), 3.38 (4H, H3). C-NMR (ppm, CDCl3), δ=155.84 (C11), 150.76 (C12), 146.54
(C8), 139.55 (C7), 137.07 (C4), 136.88 (C2), 122.33 (C9), 120.68 (C10), 119.77 (C13), 116.23 (C1),
o
112.77 (C5), 55.79 (C6), 40.06 (C3). Melting point: 101.2-105.3 C (onset-offset temperature in the
DSC thermogram). HR-MS (ESI-MS) m/z:[Na+] calcd. for C26H24O7Na 471.14; anal., 471.1416.
FTIR (KBr): ν (cm-1) = 1761 and 1741 (C=O stretch of carbonyl group), 1638 (C=C stretch of allylic
group).
10
9
8
3
O
O
4
1
O
12
11
2
O
O
5
O 7
13
O
6
Synthesis of bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl) succinate (1)
(1a) 10.00 g (24.36 mmol) and ethyl acetate 200 mL were introduced into a flask equipped with a
nitrogen inlet, stirrer, and thermocouple. mCPBA 21.02 g (24.36*5 mmol) was added slowly at 0 oC.
The solution was slowly warmed to 40oC and stirred for 24 hours. The obtained mixture was
extracted three times with 10wt% of Na2SO3(aq), and three times with 5wt% of Na2CO3. The organic
layer was dried over magnesium sulfate, and ethyl acetate was evaporated with a rotary evaporator.
6

Page 7 of 31
Green Chemistry
1
A white solid was obtained (yield=69%). H-NMR (ppm, CDCl3), δ= 6.95 (2H, H9), 6.84 (V2ieHw ,ArtHicle5O),nline
DOI: 10.1039/C9GC01184F
6.79 (2H, H10), 3.77 (6H, H6), 3.12 (2H, H2), 3.00 (4H, H12), 2.81 (4H, H3), 2.77 (2H, H1), 2.52 (2H,
13
H1). C-NMR (ppm, CDCl3), δ=170.2 (C11), 150.7 (C8), 138.2 (C7), 136.2 (C4), 122.5 (C9), 120.9
o
(C10), 113.0 (C5), 55.7 (C6), 52.2 (C2), 46.7 (C1), 38.5 (C3), 28.9 (C12). Melting point: 89.4-97.9 C
(onset-offset temperature in the DSC thermogram).. HR-MS (ESI-MS) m/z:[Na+] calcd. for
C24H26O8Na 465.15; anal., 465.1515. FTIR (KBr): ν (cm-1) = 1753 (C=O stretch of carbonyl group),
915 (C-O stretch of oxirane group), 831 (C-O-C stretch of epoxide).
6
O
O
O
7
12
11
O
1
5
4
O
8
2
O
O
O
9
10
3
Synthesis of bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl) adipate (2)
(2) was prepared by the same procedure as that for (1) except for the (1a) being replaced by (2a),
white solid (2) was obtained (yield=70%). 1H-NMR (ppm, CDCl3), δ= 6.95 (2H, H9), 6.84 (2H, H5),
6.79 (2H, H10), 3.80 (6H, H6), 3.12 (2H, H2), 2.82 (4H, H3), 2.79 (2H, H1), 2.63 (4H, H12), 2.54 (2H,
H1), 1.89 (4H, H13). 13C-NMR (ppm, CDCl3), δ=171.4 (C11), 150.8 (C8), 138.4 (C7), 136.1 (C4),
122.6 (C9), 121.0 (C10), 113.0 (C5), 55.7 (C6), 52.3 (C2), 46.8 (C1), 38.6 (C3), 38.6 (C12), 24.3 (C13).
o
Melting point: 89.4-97.9 C (onset-offset temperature in the DSC thermogram). HR-MS (ESI-MS)
m/z:[Na+] calcd. for C26H30O8Na 493.18; anal., 493.1825. FTIR (KBr): ν (cm-1) = 1755 (C=O stretch
of carbonyl group), 935 (C-O stretch of oxirane group), 844 (C-O-C stretch of epoxide).
6
O
O
O
7
12
11
8
O
1
5
4
O
13
2
O
O
O
9
10
3
Synthesis of bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl) octanedioate (3)
(3) was prepared by the same procedure as that for (1) except for the (1a) being replaced by (3a),
7

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light yellow solid (3) was obtained (yield=65%).1H-NMR (ppm, CDCl3), δ= 6.93 (2H, H9), 6.84 (2H,
View Article Online
DOI: 10.1039/C9GC01184F
H5), 6.80 (2H, H10), 3.80 (6H, H6), 3.13 (2H, H2), 2.82 (4H, H3), 2.78 (2H, H1), 2.57 (4H, H12), 2.53
13
(2H, H1), 1.78 (4H, H13), 1.48 (4H, H14). C-NMR (ppm, CDCl3), δ=171.8 (C11), 150.9 (C8), 138.4
(C7), 136.0 (C4), 122.6 (C9), 121.0 (C10), 113.1 (C5), 55.8 (C6), 52.3 (C2), 46.8 (C1), 38.6 (C3), 33.9
o
(C12), 28.6 (C14), 24.8 (C13). Melting point: 100.0-104.8 C (onset-offset temperature in the DSC
thermogram). HR-MS (ESI-MS) m/z:[Na+] calcd. for C28H34O8Na 521.21; anal., 521.2139. FTIR
(KBr): ν (cm-1) = 1754 (C=O stretch of carbonyl group), 922 (C-O stretch of epoxide), 835 (C-O-C
stretch of epoxide).
6
O
O
O
7
14
12
11
O
1
5
4
O
8
9
13
2
O
O
O
10
3
Synthesis of bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl) furan-2,5-dicarboxylate (4)
(4) was prepared by the same procedure as that for (1) except for the (1a) being replaced by (4a),
transparency orange solid (4) was obtained (yield=67%). 1H-NMR (ppm, CDCl3), δ= 7.44 (2H, H13),
7.06 (2H, H9), 6.88 (2H, H5), 6.84 (2H, H10), 3.80 (6H, H6), 3.15 (2H, H2), 2.84 (4H, H3), 2.79 (2H,
H1), 2.54 (2H, H1). 13C-NMR (ppm, CDCl3), δ=155.7 (C11), 150.8 (C12), 146.4 (C8), 137.5 (C7),
136.8 (C4), 122.4 (C9), 121.0 (C10), 119.8 (C13), 113.2 (C5), 55.8 (C6), 52.2 (C2), 46.7 (C1), 38.6 (C3).
HR-MS (ESI-MS) m/z:[Na+] calcd. for C26H24O9Na 503.13; anal., 503.1312. FTIR (KBr): ν (cm-1) =
1754, 1742 (C=O stretch of carbonyl group), 956 (C-O stretch of oxirane group), 839 (C-O-C stretch
of epoxide).
10
3
O
O
9
4
5
2
O
12
13
O
O
11
O
O
8
7
1
O
O
6
Epoxy thermosets synthesis via the self-curing reaction
Self-curing of (1-4) was performed by heating the (1-4) with 0.5 wt% DMAP at a heating stage at
o
o
o
130-150 C until a homogeneous, viscous liquid was obtained. Then, cured at 160 C, 180 C, and
8

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Green Chemistry
200 oC for 2h at each temperature in a nitrogen atmosphere. The sample IDs are named SC(1)-SC(4),
View Article Online
DOI: 10.1039/C9GC01184F
respectively. SC means self-curing. In some cases, the four thermosets are expressed SC(x), x=1-4.
Epoxy thermosets synthesis via the diamine-curing reaction
In addition to self-curing of (1-4), we also investigate their curing of (1-4) with a common diamine,
diaminodiphenylmethane (DDM), for the structure-properties comparison. The curing of (1-4) with
DDM was exemplified by the curing of (1) and DDM. (1) 2.00 g, DDM 0.45 g was stirred at a
heating stage at 120-130 oC until a homogeneous, viscous liquid was obtained. Then, cured at 140 oC
o
and 160 C for 2h at each temperature. The thermosets are named (1)/DDM-(4)/DDM, respectively.
In some cases, the four thermosets are expressed (x)/DDM, x=1-4.
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Results and Discussion
View Article Online
DOI: 10.1039/C9GC01184F
Synthesis and characterization of (1-4)
Fully bio-based epoxy compounds were prepared by a two-step procedure from eugenol. The first
step is the esterification of eugenol with succinyl, adipoyl, suberoyl, and 2,5-furan chloride,
respectively, in ethyl acetate using triethylamine as a base, forming intermediated (1a-4a). The
second step is the oxidation of the allylic groups of (1a-4a) by m-CBPA in ethyl acetate, forming
epoxy compounds (1-4) (Scheme 1).
O
O
O
O
Cl
O
Cl
n
n
O
O
O
n=1 for (1a)
n=2 for (2a)
n=3 for (3a)
n=1, 2, 3
OH
O
TEA
+
ethyl acetate
O
O
O
O
O
O
Cl
Cl
O
O
O
O
(4a)
O
O
O
O
O
n
O
O
O
O
n=1 for (1)
n=2 for (2)
n=3 for (3)
m-CPBA
ethyl acetate
O
O
O
O
O
O
O
O
(4)
Scheme 1. Synthesis of fully bio-based epoxy compounds (1-4).
10

Page 11 of 31
Green Chemistry
Fig. 1 displays the 1H-NMR spectra of (1a-4a) in CDCl3. The characteristic signals of the allylic
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DOI: 10.1039/C9GC01184F
group at 3.36 ppm (H3), 5.08 ppm (H1), and 5.94 ppm (H2) were observed. The structures were
13
further confirmed by C-NMR spectra (Fig. 2). The signal of the carbonyl group was observed at
171 ppm for (1)-(3) and 155 ppm in (4). The characteristic signals of the allylic group were observed
at 40.1 ppm (C3), 116.1 ppm (C1) and 137.0 ppm (C2), confirming the integrity of the allylic
structures.
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DOI: 10.1039/C9GC01184F
Fig. 1. 1H-NMR spectra of (1a-4a).
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Green Chemistry
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DOI: 10.1039/C9GC01184F
Fig. 2. 13C-NMR spectra of (1a-4a) in CDCl3.
1
Fig. 3 displays the resulting H-NMR spectra of (1-4) in CDCl3. The signals from the allylic
group disappeared while new signals due to epoxide at 2.5-3.2 ppm appeared. Fig. 4 shows the
13C-NMR spectra of (1-4) in CDCl3. The signals of the allylic group disappeared while the signals of
1
epoxide appeared at 38.5 (C3), 46.6 (C1), and 52.2 (C2). H and 13C-NMR spectra support the
structure of (1-4).
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DOI: 10.1039/C9GC01184F
Fig. 3. 1H-NMR spectra of (1-4).
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Green Chemistry
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DOI: 10.1039/C9GC01184F
Fig. 4. 13C-NMR spectra of (1-4) in CDCl3.
Fig. S1 shows the IR spectra of (1a-4a) and (1-4). The C=C absorption of the allylic group was
observed at around 1638 cm-1 in (1a-4a) but disappeared in (1-4). The absorptions of epoxide were
observed at around 930 and 835 cm-1. The detailed information regarding the carbonyl absorption
will be discussed later.
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Model reaction
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In our previous work, we performed a model reaction of phenyl acetate and glycidylDpOhI:e10n.1y0l39e/Cth9GeCr0a11t84F
o
120 C in the presence of DMAP. We found that the reaction carried out through the reaction of
active ester and epoxide, as shown in Scheme 2(a).47 Since (1)-(3) are phenyl succinate, adipate, and
octanedioate, respectively, they exhibited structures of phenyl-O-C(=O)-R (R=C4, C6, and C8,
respectively), which are structurally similar to phenyl acetate (phenyl-O-C(=O)-CH3). Therefore, the
self-curing of (1)-(3) should occur through the reaction of the built-in active ester and epoxide,
similar to that shown in Scheme 2(a). However, (4) is a phenyl furan carboxylate, with a structure of
phenyl-O-C(=O)-furan, which is structurally different from phenyl acetate. Therefore, we performed
a model reaction of phenyl furan-2-carboxylate and glycidyl phenyl ether in the presence of DMAP
(Scheme 2b) to evaluate the reaction route
(a)
O
O
O
DMAP
DMAP
O
O
O
O
O
O
O
O
O
+
+
O
O
(b)
O
O
O
O
Scheme 2. The reaction of glycidyl phenyl ether with (a) phenyl acetate and (b) phenyl
furan-2-carboxylate in the presence of DMAP.
1
o
Fig. 5 shows the H-NMR spectra of the model reaction at 120 C for various periods of reaction
time. The signals of furan group from unreactive mixture were assigned at 8.1 (Ha), 7.6 (Hc), and 6.8
(Hb) ppm, respectively, and the signals of the epoxide were assigned at 2.7-4.3 ppm (Hg, Hh, and Hi)
in Fig. 5 (R.T.). As the reaction progresses, the signals of the epoxide at 2.7-4.3 ppm disappeared and
16

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Green Chemistry
the signals of furan upshifted to 8.0 (Ha’), 7.3 (Hc’), and 6.7 (Hb’), respectively. The siVgienwaAlrtsicleoOfnline
DOI: 10.1039/C9GC01184F
methylene and methine also appeared at 4.4 (Hg’) and 5.7 (Hh’) ppm, respectively (Fig. 5, 10-60 min).
The signal of He at 7.5 ppm disappeared after 30 min. In order to further confirm the structure, Fig. 6
illustrated the 1H-1H COSY NMR spectrum of the reaction at 60 min. The correlation at 4.4/5.7 ppm
(corresponds to Hg’/Hh’), at 6.7/7.3 ppm (corresponds to Hb’/Hc’), at 6.7/8.0 ppm (corresponds to
Hb’/Ha’) and at 7.0/7.3 ppm (corresponds to Hd’/He’) confirmed our prediction in scheme 2(b),
indicating the reaction of the phenyl furan carboxylate and epoxide. Therefore, the self-curing of (4)
should occur through the reaction of the built-in active ester and epoxide, similar to that shown in
Scheme 2(b).
Fig. 5. Time-dependent 1H-NMR spectra of the model reaction.
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Page 18 of 31
View Article Online
DOI: 10.1039/C9GC01184F
Fig. 6. 1H-1H COSY NMR spectrum of the reaction product of the model reaction at 60 min in
DMSO-d6.
DSC thermograms
Compounds (1-4) are designed to exhibit both active esters and epoxides in the structure, so,
according to chemistry in Scheme 2, we think that they would be self-curable in the presence of
DMAP. Fig. 7 displays the DSC thermograms of (1-4) without DMAP (a) and with 0.5 wt% DMAP
(b). No exothermic peaks were observed in Fig. 7(a), indicating that the reactions of active ester and
epoxide did not occur without a catalyst. In Fig. 7(b), exothermic peaks at 165, 169, 182, and 177 oC,
respectively, with exothermic enthalpies of around 140 kJ/mol were observed, indicating that (1-4)
carries out the self-curing reaction in the presence of DMAP. For comparison, the DSC thermogram
of DGEBA with 0.5 wt% DMAP was also given in Fig. 7(b). An exotherm with a peak temperature
18

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Green Chemistry
o
of 113 C and an exothermic enthalpy of 140 kJ/mol was observed. The exotherm is thought to be
View Article Online
DOI: 10.1039/C9GC01184F
related to the homopolymerization of epoxide groups.48 However, if we compare the thermograms of
(1-4) and DGEBA in Fig. 7(b), we can find that no exotherm around 113 oC for (1), (3) and (4) (note
o
that (2) is difficult to interpret due to its melting point at 110 C) was observed. That is, the
homopolymerization of epoxide in (1-4) did not occur, showing a high selectivity between active
esters and epoxides. This high selectivity is consistent with the good purity of the reaction product
shown in Fig. 6.
Fig. 7. DSC thermograms of (1-4) without DMAP (a) and with 0.5 wt% DMAP (b). DGEBA with
0.5 wt% DMAP was also included 4(b).
FTIR spectra and proposed curing mechanism
As shown in the model reaction (Scheme 2), the reaction between active esters and epoxide led
the ester group changed from an aromatic ester (phenyl-O-C(=O)-) to an aliphatic ester
(isopropyl-O-C(=O)-). In IR spectroscopy, an aliphatic ester has lower absorption wavenumber than
an aromatic ester does. Fig. 8 shows IR spectra of (1-4) before and after curing. The carbonyl
absorption upshifted from 1757 cm-1 to 1727 cm-1 for (1-3) after curing. The carbonyl absorption
19

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Page 20 of 31
upshifted from 1759 and 1742 cm-1 (asymmetric and symmetric stretch) to 1740 and 1718 Vciemw A-r1ticfleoOrnline
DOI: 10.1039/C9GC01184F
(4). This result indicates that the ester group has changed from an aromatic ester to an aliphatic ester,
further supporting the self-curing reaction of (1-4). The self-curing reaction of (x) to SC(x) was
expressed in Scheme 3(a), in which the reaction between epoxides and active esters led to an epoxy
network. In this mechanism, DMAP attacked the epoxide and formed an alkoxy anion in step 1. The
alkoxy anion attacked the ester group and released a phenoxy anion in step 2. The phenoxy anion
attacked the carbon of the C-N bond in step 3, releasing DMAP that will catalyze another epoxide
moiety. The alkoxy anion formed in step 1 further attacked the other ester group and gradually led to
a network (steps 4 and 5, and further reactions). For property comparison, we also cured (1-4) by
DDM, forming the (x)/DDM thermosets in Scheme 3(b). The reaction between epoxides and amines
form hydroxyl groups and led to the formation of an epoxy network. The IR spectra of (x)/DDM
were shown in Fig. 8. An obvious signal of a hydroxyl group was observed at 3385 cm-1 for
(x)/DDM. The signal of a carbonyl group was presented at the same position as of (x), indicating the
active esters do not participate in the crosslinking reaction. Table 1 lists the gel content of SC(x). The
gel content is larger than 98% for each sample. Also, the chloroform solution remained transparent
after the test. The two observations indicated a network was formed in each SC(x), supporting the
self-curing reaction of (1-4) to epoxy networks.
20

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Green Chemistry
View Article Online
DOI: 10.1039/C9GC01184F
Fig. 8. FTIR spectra of (x), SC(x) and (x)/DDM. x=1-4.
N
O
Z
O
O
O
Z
Step 3
O
O
+
O
O
(a) Self-cured
O
O
O
N
Step 2
O
O
N
N
N
Z
O
N
N
O
O
O
O
+
O
O
N
O
Step 5
N
O
Z
Z
O
Step 1
O
O
N
O
Step 4
O
O
Z
Z
SC(x)
O
Z
(1)
SC(1)
(1)/DDM
(2)
SC(2)
(2)/DDM
(b) Cured with DDM
O
O
O
O
O
O
O
OH
H2N
Z
O
Z
O
NH
OH
Z
O
N
(3)
SC(3)
(3)/DDM
Step 2
(4)
SC(4)
(4)/DDM
Step 1
O
O
O
O
OH
(x)/DDM
(I)
Scheme 3. Parts of the structures of (a) SC(x) and (b) (x)/DDM. x=1-4.
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Thermal properties
View Article Online
DOI: 10.1039/C9GC01184F
Fig. 9(a) plotted the DSC thermograms of the self-curing epoxy thermosets, SC(x). The Tg
o
values are 114, 88, 73, and 147 C, respectively. No exothermic peaks are observed in the figure,
indicating they were fully cured. DMA thermograms of SC(x) are displayed in Fig. 9(b). There are
modulus plateaus for SC(1)-SC(4) after glass relaxation, indicating they belong to thermosets. The Tg
values, defined by the peak temperature of the tanδ curve, are 131, 107, 92 and 161 oC, respectively.
For SC(1)-SC(3), the Tg values decreased with the increasing chain length of diacid. The SC(4), due
do the higher rigidity of furan than alkylene, exhibited the highest Tg. Fig. 10 displays the TGA
thermograms of SC(x). Depending on the structure, the Td5 (5 wt% degradation temperatures) are in
the range of 350-392 oC (Table 1). The char yields (CY) at 800 oC are in the range of 17-27 %. As to
the mechanical properties, we have tried to measure the mechanical properties of thermosets. Fig. S2
shows a typical stress-strain curve of SC(3) and (3)/DDM. Unfortunately, we observed that the test
stopped after the force reached the maximum loading force of the instrument (100 Nt). Therefore, no
exact data on tensile strength and elongation at break were reported here. However, we have
observed that the initial modulus of SC(3) is 1.86 GPa, which is much larger than that (0.78 GPa) of
(3)/DDM. The result demonstrates the advantage of the self-curing thermoset in rigidity.
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Green Chemistry
View Article Online
DOI: 10.1039/C9GC01184F
Fig. 9. (a) DSC and (b) DMA thermograms of SC(x). x=1-4.
110
100
90
80
70
60
50
40
30
20
10
0
SC(1)
SC(2)
SC(3)
SC(4)
100
200
300
400
500
600
700
800
Temperature (oC)
Fig. 10. TGA thermograms of SC(x). x=1-4.
DSC, DMA, and TGA thermograms of (x)/DDM were shown in Fig. S3 and S4 for property
o
comparison. The thermal properties of all thermosets are listed in Table 1. (x)/DDM shows 8-20 C
higher Tg than SC(x), probably due to the rigid diphenylmethane structure. However, (x)/DDM
shows much lower rubbery modulus than SC(x) for x=1-3, demonstrating the higher rigidity of SC(x)
after glass transition. For example, the rubbery modulus is 15 MPa for (2)/DDM and 52 MPa for
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SC(2). The lower rigidity of (x)/DDM at rubbery state can also be supported by the higher height of
View Article Online
DOI: 10.1039/C9GC01184F
the tanδ curve, as listed in Table 1. This indicates that the packing of SC(x) is more compact than
that of (x)/DDM for x=1-3. The interaction between secondary alcohol and furan might explain the
slightly higher rubbery modulus of (4)/DDM than that of SC(4). The hydrogen bonding between
furan and secondary alcohol groups reinforces the thermomechanical properties of (4)/DDM. The
signal at 1102 cm-1 (C-O-C of furan) up-shifted to 1098 cm-1 in the (4)/DDM (Fig. S5). The up-shifts
in wavenumber indicates an interaction between the furan and secondary alcohol groups, which is
supported by the Palmese et al.49 TGA data show that (x)/DDM displays lower 5 wt% decomposition
temperature than SC(x). For example, 5 wt% decomposition temperature is 350 oC for (2)/DDM and
is 395 oC for SC(2), demonstrating the advantage of self-curing strategy on thermal stability.
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Table 1. Thermal and dielectric properties of SC(x) and (x)/DDM
View Article Online
DOI: 10.1039/C9GC01184F
Tg (oC) Tg (oC) Rubbery Height Td5% CY
(DSC)b (DMA)c E’ (MPa)d of tanδ (oC)e (%)f
g
h
Sample
GC (%)a
Dk
Df
SC(1)
(1)/DDM
SC(2)
>98
>98
>98
>98
>98
>98
>99
>99
114
118
88
131
139
107
120
92
42
18
52
15
29
21
32
36
0.60 370
0.98 364
0.66 395
1.03 350
0.70 392
1.03 368
0.61 350
0.74 341
17
24
27
20
17
25
24
28
3.10 0.007
3.03 0.019
3.08 0.008
3.05 0.022
3.01 0.008
3.08 0.017
3.15 0.010
3.10 0.020
(2)/DDM
SC(3)
99
73
(3)/DDM
SC(4)
91
112
161
181
147
(4)/DDM
169
a The gel content tested in chloroform.
b The Tg was determined at the half height of the step in a DSC thermogram.
c The peak temperature of a tanδ curve in a DMA thermogram
d The storage modulus at rubbery state was determined at Tg+30 oC.
e The 5% decomposition temperature in a TGA thermogram.
f The char yield at 800 oC in a TGA thermogram (wt%).
g The dielectric constant measured at 1 GHz at room temperature.
h The dissipation factor measured at 1 GHz at room temperature.
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Dielectric property
View Article Online
DOI: 10.1039/C9GC01184F
Table 1 also lists the dielectric properties of all thermosets. The dielectric constants of SC(x) are
around 3.0-3.1, which are approximately the same as those of (x)/DDM. However, the dissipation
factors of SC(x) are only 0.007-0.010, which are 50-63 % lower than those of (x)/DDM. The
highly-polar secondary alcohol, as evidenced by the hydroxyl absorption at 3385 cm-1 in Fig. 8, is
responsible for the high dissipation factors of (x)/DDM. In contrast, no secondary alcohol was
formed for SC(x), according to the curing chemistry and the lack of hydroxyl absorption at 3385 cm-1
in Fig. 8. In a circuit, the signal propagation loss is proportional to frequency, dissipation factor, and
the square root of the dielectric constant.50, 51 Therefore, a dielectric material with a low dielectric
constant and dissipation factor, especially with a low dissipation factor, is desired. This result
demonstrates the advantage of the self-cured epoxy thermosets in signal propagation loss in a circuit.
Conclusions
We have successfully prepared four fully bio-based epoxy compounds (1-4) from eugenol
through the esterification of the phenolic group, followed by the oxidation of the allylic group.
Through this method, the active ester and epoxide moieties were simultaneously integrated into each
compound. According to the NMR analysis (Fig. 5) of a model reaction of glycidyl phenyl ether and
phenyl furan-2-carboxylate, we confirmed that the active ester and epoxide could react with each
other. According to the obvious exotherms in the DSC thermograms (Fig. 7) and the shift of carbonyl
absorption in IR spectra (Fig. 8), we reported the DMAP-catalyzed self-curing reaction of (1-4).
Fully bio-based epoxy thermosets were achieved according to this chemistry. The self-cured epoxy
thermosets, SC(x), show better properties than the DDM-cured thermosets, (x)/DDM, except for a
26

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Green Chemistry
slight reduction in Tg values. Especially, due to the lack of secondary alcohols, the dissipation factors
View Article Online
DOI: 10.1039/C9GC01184F
of SC(x) were 50-63 % lower than (x)/DDM, demonstrating the advantage of SC(x) in signal
propagation loss in a circuit. In short, a strategy for preparing fully bio-based epoxy thermosets with
low dissipation factors are revealed in this work.
Acknowledgments
This work was financially supported by the “Advanced Research Center For Green Materials Science
and Technology” from The Featured Area Research Center Program within the framework of the
Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of
Science and Technology in Taiwan (MOST 107-3017-F-002-001 and 107-2221-E-005-025-MY3 )
Notes
The authors declare no competing financial interest.
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DOI: 10.1039/C9GC01184F
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