Renewable Thermoplastics Based on Lignin-Derived Polyphenols

Article abstract of DOI:10.1021/acs.macromol.7b00064

A series of renewable triphenylmethane-type polyphenols (TPs) were synthesized from lignin-derived guaiacols (methylguaiacol and propylguaiacol) and aldehydes (4-hydroxybenzaldehyde, vanillin, and syringaldehyde). By converting guaiacols to catechols through ortho-demethylation, the newly formed phenolic para site remarkably improved the reactivity as reflected by conversion of TPs. Optimized reagent molar ratios were aldehyde/catechol (1:4) and aldehyde/H₂SO₄ (1:3). A typical TP (VAN-M-CAT) was converted to glycidyl ether (GE-VAN-M-CAT) to examine its feasibility as precursor to epoxy thermosets. The resulting network exhibited excellent glassy modulus (12.3 GPa), glass transition temperature (167 °C), and thermal stability, which were attributed to the rigid triphenylmethane framework, high functionality (n = 5), and high cross-link density. A fully biobased epoxy comonomer (VAN-LIN-EPO), which was prepared by esterification of VAN-M-CAT with linoleic acid followed by epoxidation, could tune the material properties. This study widens the synthesis route of fully biobased polyphenols, which can yield polymers with excellent properties.

Full text of DOI:10.1021/acs.macromol.7b00064

Article
pubs.acs.org/Macromolecules
Renewable Thermoplastics Based on Lignin-Derived Polyphenols
†
,†,‡
Shou Zhao and Mahdi M. Abu-Omar*
†
‡
Department of Chemistry & Biochemistry and Department of Chemical Engineering, University of California, Santa Barbara, Santa
Barbara, California 93106, United States
*
S Supporting Information
ABSTRACT: A series of renewable triphenylmethane-type
polyphenols (TPs) were synthesized from lignin-derived
guaiacols (methylguaiacol and propylguaiacol) and aldehydes
(
4-hydroxybenzaldehyde, vanillin, and syringaldehyde). By
converting guaiacols to catechols through ortho-demethylation,
the newly formed phenolic para site remarkably improved the
reactivity as reflected by conversion of TPs. Optimized reagent
molar ratios were aldehyde/catechol (1:4) and aldehyde/
H SO (1:3). A typical TP (VAN-M-CAT) was converted to
2
4
glycidyl ether (GE-VAN-M-CAT) to examine its feasibility as
precursor to epoxy thermosets. The resulting network
exhibited excellent glassy modulus (12.3 GPa), glass transition
temperature (167 °C), and thermal stability, which were
attributed to the rigid triphenylmethane framework, high functionality (n = 5), and high cross-link density. A fully biobased epoxy
comonomer (VAN-LIN-EPO), which was prepared by esterification of VAN-M-CAT with linoleic acid followed by epoxidation,
could tune the material properties. This study widens the synthesis route of fully biobased polyphenols, which can yield polymers
with excellent properties.
INTRODUCTION
can be realized through ortho-demethylation of ortho methoxy,
■
1
4,15
a characteristic group of lignin-derived phenols.
As
Polyphenols are important precursors to polymers like epoxy
thermosets. As development of renewable materials has become
increasingly important, partially or fully biobased polyphenols
have been prepared with the purpose to replace or supplement
petroleum-based bisphenol A (BPA).
generally produced by condensation of phenol with ketone or
aldehyde in the presence of an acid. Using this approach,
several biobased polyphenols have recently been reported: (1)
diphenolic acid based on cellulose-derived levulinic acid, (2)
bisphenol obtained from cellulose-based 2,3-pentanedione, (3)
bisphenol based on lignin-derived creosol, and (4) triphenol
prepared from lignin-based vanillin and guaiacol.
For phenol−aldehyde condensation, the phenolic para
position is the preferred coupling site due to its higher
reactivity over the ortho site.
phenols are often characterized as para site occupied by alkyl
illustrated in Scheme 1, ortho-demethylation of typical lignin-
based guaiacols leads to new para and ortho sites. These newly
formed sites, especially the para site, could enhance reactivity.
Meanwhile, demethylation also increases the number of
functional OH groups. TPs with higher OH content have the
potential to further improve the cross-linking density and
1
−3
Polyphenols are
4
13,16,17
mechanical properties of subsequently formed polymers.
5
Recent development in catalytic lignin depolymerization
techniques is capable of converting bulk lignin into various
smaller aromatic molecules including aldehydes, e.g., 4-
hydroxybenzaldehyde (HBA), vanillin (VAN), and syringalde-
6
7
8
1
8,19
hyde (SYA)
and guaiacols such as 4-methylguaiacol (M-
2
0,21
GUA) and 4-propylguaiacol (P-GUA).
Condensation
9
,10
between aromatic aldehyde and phenol yields triphenyl-
methane-type phenol (TP). TP-based materials are supposed
to be highly rigid due to the triphenylmethane framework and
highly functional (n = 5). To tune the rigidity, vegetable oils
with flexible carbon chains may represent a renewable
However, lignin-derived
groups, while partial ortho sites are substituted by methoxy
1
1,12
groups,
which makes them especially difficult to couple to
aldehydes. Because of this obstruction, either para-available
22,23
8
modifier.
Especially, unsaturated fatty acids (e.g., linoleic
phenols like guaiacol or ortho-specified coupling catalysts like
7
acid, LA) are often used as plasticizer or copolymer adjusting
zinc acetate have been used as alternative methodologies, even
24,25
the rigidity of thermoplastics.
The integration of lignin-
though these approaches are frequently associated with
limitations such as insufficient phenol candidates and limited
orientation and stretch of functional hydroxyls obtained via
based TPs and unsaturated fatty acids would yield renewable
polymers with tunable mechanical properties.
1
3
ortho coupling.
To increase the applications of lignin-based phenols, creating
a para site on para-substituted phenol could be a more
universal and straightforward methodology. This improvement
Received: January 10, 2017
Revised: March 6, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.7b00064
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Scheme 1. Synthesis Route of Fully Renewable Triphenylmethane-Type Polyphenols from Lignin-Derived Aldehydes and Para-
a
Substituted Guaiacols
a
Condensation reactions between aldehydes and guaiacols give conversions below 5%. Ortho-demethylation of guaiacols to corresponding catechols
significantly improve the reactivity. The conversions of methylcatechol (M-CAT) based TPs are more than 90%, while propylcatechol (P-CAT)-
based TPs are more than 45%. Reaction conditions: molar ratio of aldehyde:phenol = 1:4, aldehyde:H SO = 1:3, room temperature for 2 days.
2
4
Absolute ethanol is used as solvent. Conversions and isolated yields of different lignin-based TPs are listed in Table 2.
In this study, we first describe the effective synthesis of fully
renewable polyphenols from the above-mentioned aldehydes
and para-substituted guaiacols. By creating a para site on para-
substituted guaiacols through ortho-demethylation, reactivity of
the afforded catechols significantly improves. The proposed
TPs have unique molecular architecture: three aromatic rings
and five functional OH groups. To explore their potential as
precursors to epoxy resin, a typical TP (VAN-M-CAT,
prepared from condensation of vanillin and methylcatechol)
is then reacted with epichlorohydrin to produce glycidyl ether
prepolymer (GE-VAN-M-CAT). VAN-M-CAT was selected
because it was easily separated from the reaction mixture, giving
the highest yield (88%) among all TPs. Meanwhile, VAN-M-
CAT is reacted with chlorinated linoleic acid via esterification
followed by epoxidation to make an epoxy comonomer (VAN-
LIN-EPO), which acts as a plasticizer that tunes the rigidity of
the resulting thermosets. The mixture of GE-VAN-M-CAT and
VAN-LIN-EPO with different mass ratios is cured with a
hardener (diethylenetriamine, DETA) to make epoxy networks.
Thermal and mechanical performances of biobased thermosets
are characterized to evaluate their potential for replacing BPA-
based counterparts.
Ortho-Demethylation of 4-Methylguaiacol and 4-Propyl-
guaiacol. Lignin-based phenols were ortho-demethylated as pre-
1
4
viously reported. Briefly, 4-propylguaiacol (P-GUA, 16.6 g, 0.1 mol)
was added to 83 g of 48% aqueous hydrobromic acid. The reaction
mixture was magnetically stirred at 120 °C for 20 h, cooled to ambient
temperature, saturated with NaCl, and extracted three times with
diethyl ether. The organic layer was dried over MgSO4 and
concentrated using rotary evaporation to yield P-GUA-derived
catechol (P-CAT) as a yellow oil (94% yield). M-GUA demethylated
product (M-CAT) was prepared using the same method with 92%
yield. Proton NMR spectra of M-GUA, M-CAT, P-GUA, and P-CAT
are shown in Figures S1 and S2.
Synthesis of Lignin-Based Triphenylmethane-Type Phenols
TP). M-CAT-Based TPs. Vanillin (VAN, 1.05 g, 7 mmol) and M-CAT
(
(3.47 g, 28 mmol) were dissolved in 3 mL of absolute ethanol. To this
solution, 2.0 g of concentrated sulfuric acid, dissolved in 1.5 mL of
absolute ethanol, was slowly added while stirred. An ice bath was used
to control the temperature below 10 °C. After the addition of sulfuric
acid, the temperature was increased to room temperature. The system
was gently stirred for 2 days under room temperature. Then, 100 mL
of H O and 15 mL of diethyl ether were successively added to
2
precipitate flakes. The precipitate was then collected through filtration,
washed with water three times, and dried at 65 °C under vacuum for 2
days to obtain the TP product (VAN-M-CAT) as a white powder
1
(2.35 g, 88% isolated yield). H NMR (acetone-d , 400 MHz) δ: 7.64
6
(
s, 2H, Ar−OH), 7.39 (s, 2H, Ar−OH), 6.74 (d, J = 8.1, 1H, Ar−H),
6
6
.69 (s, 1H, Ar−H), 6.65 (s, 2H, Ar−H), 6.42 (d, J = 7.8, 1H, Ar−H),
EXPERIMENTAL SECTION
■
.23 (s, 2H, Ar−H), 5.39 (s, 1H, Ar −CH), 3.69 (s, 3H, −OCH ),
3
3
13
General. 4-Hydroxybenzaldehyde, vanillin, syringaldehyde, 4-
methylguaiacol, 4-propylguaiacol, 4-methylcatechol, 48% aqueous
hydrobromic acid, epichlorohydrin, tetrabutylammonium bromide,
diethylenetriamine (DETA), peracetic acid (32 wt % in dilute acetic
acid), linoleic acid, and oxalyl chloride were purchased from Aldrich
Chemical Co. Ethanol (200 proof) was purchased from Decon
Laboratories, Inc. Sulfuric acid (98%) was obtained from Fisher
Scientific. All chemicals were used as received without further
purification.
2.00 (s, 6H, −CH ). C NMR (acetone-d , 400 MHz) δ: 148.09,
3
6
145.54, 143.71, 143.08, 136.21, 135.08, 128.24, 122.76, 118.18, 117.24,
115.31, 113.98, 56.13 (−OCH ), 49.70 (Ar −CH), 18.80 (−CH ).
3
3
3
+
[C H O − H ]: 381.4.
22
22
6
SYA-M-CAT was obtained from M-CAT and syringaldehyde (SYA)
using the above-mentioned method (2.05 g, 71% isolated yield). H
1
NMR (acetone-d , 400 MHz) δ: 7.65 (s, 2H, Ar−OH), 7.40 (s, 2H,
6
Ar−OH), 7.08 (s, 1H, Ar−OH), 6.63 (s, 2H, Ar−H), 6.33 (s, 2H, Ar−
H), 6.23 (s, 2H, Ar−H), 5.37 (s, 1H, Ar −CH), 3.67 (s, 6H, −
3
B
DOI: 10.1021/acs.macromol.7b00064
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
a
Scheme 2. Glycidylation of VAN-M-CAT with Epichlorohydrin
a
Three major products with mono-epoxy-substituted (GE-VAN-M-CAT-1), tri-epoxy-substituted (GEVAN-M-CAT-3), and penta-epoxy-
substituted (GEVAN-M-CAT-5) were isolated using a preparative HPLC, and their molar ratio was measured to be 14:55:31 using analytical
HPLC spectra.
1
3
OCH ), 2.00 (s, 6H, −CH ). C NMR (acetone-d , 400 MHz) δ:
Preparation of Glycidylated Ether of VAN-M-CAT (GE-VAN-
3
3
6
1
48.45, 143.75, 143.09, 135.30, 135.06, 134.93, 128.25, 118.15, 117.16,
08.10, 56.54 (−OCH ), 50.03 (Ar −CH), 18.76 (−CH ). [C H O
M-CAT). GE-VAN-M-CAT was prepared by reaction of VAN-M-CAT
(
2 g, 5.2 mmol) and epichlorohydrin (30 g, 320 mmol).
1
−
3
3
3
23 24
7
+
Tetrabutylammonium bromide (0.84 g, 2.6 mmol) was used as a
phase transfer catalyst. The mixture was heated at 95 °C for 1 h and
followed by a dropwise addition of 10 g of 20% w/w NaOH solution.
The reaction was kept for another 3 h, and the mixture was washed
with water, extracted with ethyl acetate, and concentrated with a rotary
evaporator to yield GE-VAN-M-CAT as a viscous oil (2.8 g). The
catechol-like structure of VAN-M-CAT is likely to produce
benzodioxane derivatives during glycidylation (Scheme 2). Using a
preparative scale HPLC, three major glycidylated products, i.e., mono-
epoxy-substituted (GE-VAN-M-CAT-1), tri-epoxy-substituted (GE-
VAN-M-CAT-3) and penta-epoxy-substituted GE-VAN-M-CAT-5)
were isolated. Structures of the major glycidylated products were
measured by NMR and mass spectra. An analytical HPLC was used to
detect peaks of each epoxidized product and determine their molar
ratio in the mixture to be GE-VAN-M-CAT-1:GE-VAN-M-CAT-
3:GE-VAN-M-CAT-5 = 14:55:31. Epoxy equivalent value of GE-VAN-
M-CAT mixture was determined to be 495 mmol epoxy/100 g by
HCl/acetone chemical titration method. This is in accordance with the
calculated value (531 mmol epoxy/100 g) using the above ratio.
H ]: 411.4.
For HBA-M-CAT that is derived from 4-hydroxybenzaldehyde
HBA) and M-CAT, only the isolation method is different from the
above. In detail, after the reaction was complete, 100 mL of H O was
poured into the mixture prior to the addition of 20 mL of diethyl ether
to extract the product. The ethereal extract was dried with MgSO , and
the solvent was allowed to evaporate slowly to yield colorless crystals,
which were subsequently washed with cold ether and dried at 65 °C
under vacuum for 2 days to yield a white powder (2.07 g, 84% isolated
(
2
4
1
yield). H NMR (acetone-d , 400 MHz) δ: 7.58 (s, 5H, Ar−OH), 6.84
6
(
d, J = 8.4, 2H, Ar−H), 6.75 (d, J = 8.4, 2H, Ar−H), 6.64 (s, 2H, Ar−
H), 6.20 (s, 2H, Ar−H), 5.38 (s, 1H, Ar −CH), 1.99 (s, 6H, −CH ).
3
3
1
3
C NMR (acetone-d , 400 MHz) δ: 156.32, 143.68, 143.10, 135.34,
6
1
1
35.53, 135.15, 131.27, 128.17, 120.93, 118.18, 117.27, 116.85, 115.89,
15.72, 49.24 (Ar −CH), 18.74 (−CH ). [C H O − H ]: 351.4.
P-CAT-Based TPs. P-CAT-based TPs (HBA-P-CAT, VAN-P-CAT,
and SYA-P-CAT) were prepared using the same reaction conditions as
VAN-M-CAT. The desired TPs were separated from unreacted
phenols and aldehydes using silica gel chromatography (hexane/ethyl
acetate, 3:1 to 1:1) to give:
+
3
3
21 20
5
GE-VAN-M-CAT-1, yellow oil, 14 mol % in epoxidized product
1
mixture. H NMR (CDCl , 400 MHz) δ: 6.27−6.80 (7H, Ar−H), 5.37
3
1
HBA-P-CAT, orange solid, 1.20 g, 42% isolated yield. H NMR
(1H, f), 4.13−4.28 (5H, c′, g′, h), 3.94−4.10 (3H, c, g), 3.82−3.87
(
acetone-d , 400 MHz) δ: 7.53 (s, 4H, Ar−OH), 6.81 (d, J = 8.2, 2H,
(4H, i, i′), 3.76 (3H, d), 3.37 (1H, b), 2.89 (1H, a), 2.73 (1H, a′), 2.03
6
(6H, e). 13C NMR (CDCl
, 400 MHz) δ: 113.58−140.51 (Ar−C),
Ar−H), 6.73 (d, J = 8.1, 2H, Ar−H), 6.66 (s, 2H, Ar−H), 6.22 (s, 2H,
3
Ar−H), 5.57 (s, 1H, Ar −CH), 2.35 (t, J = 7.2, 4H, −CH −), 1.47 (dt,
73.31 (h), 70.07 (c), 65.07 (g), 61.74 (i), 55.89 (d), 50.14 (f), 48.99
3
2
13
+
J = 15.0, 7.4, 4H, −CH −), 0.85 (t, J = 7.2, 6H, −CH ). C NMR
(b), 44.99 (a), 18.66 (e). [C31
H O + Na ]: 573.
34 9
2
3
GE-VAN-M-CAT-3, brown oil, 55 mol % in epoxidized product
(
acetone-d , 400 MHz) δ: 156.27, 143.82, 143.04, 136.82, 134.81,
6
1
mixture. H NMR (CDCl , 400 MHz) δ: 6.23−6.80 (7H, Ar−H), 5.37
1
(
32.66, 131.07, 117.79, 117.34, 115.68, 47.99 (Ar −CH), 34.85
3
3
+
(1H, f), 4.18−4.25 (5H, c′, g′, h), 3.92−4.07 (4H, c, g), 3.78−3.82
−CH −), 24.84 (−CH −), 14.45 (−CH ). [C H O − H ]: 407.4.
2
2
3
25 28
5
1
(2H, i, i′), 3.74 (3H, d), 3.22−3.37 (3H, b), 2.61−2.88 (6H, a, a′),
VAN-P-CAT, orange solid, 1.07 g, 35% isolated yield. H NMR
13
1
(
(
.99−2.03 (6H, e). C NMR (CDCl , 400 MHz) δ: 113.40−149.39
(
acetone-d , 400 MHz) δ: 7.66 (s, 2H, Ar−OH), 7.39 (s, 1H, Ar−
3
6
Ar−C), 73.29 (h), 70.08 (c), 65.18 (g), 61.74 (i), 55.84 (d), 50.12
OH), 7.37 (s, 2H, Ar−OH), 6.72 (d, J = 8.0, 1H, Ar−H), 6.66 (s, 2H,
Ar−H), 6.63 (s, 1H, Ar−H), 6.41 (d, J = 7.2, 1H, Ar−H), 6.24 (s, 2H,
Ar−H), 5.58 (s, 1H, Ar −CH), 3.68 (s, 3H, −OCH ), 2.36 (t, J = 7.2,
+
f), 49.09 (b), 44.66 (a), 18.93 (e). [C H O + Na ]: 629.
34
38 10
GE-VAN-M-CAT-5, brown oil, 31 mol % in epoxidized product
3
3
1
mixture. H NMR (CDCl , 400 MHz) δ: 6.32−6.80 (7H, Ar−H), 5.40
3
4
6
1
1
2
H, −CH −), 1.48 (dt, J = 15.0, 7.4, 4H, −CH −), 0.85 (t, J = 7.2,
2
2
13
(1H, f), 4.09−4.25 (5H, c′), 3.88−4.99 (5H, c), 3.74 (3H, d), 3.22−
H, − CH ). C NMR (acetone-d , 400 MHz) δ: 148.06, 145.54,
3
6
13
3
4
4
.36 (5H, b), 2.60−2.87 (10H, a, a′), 2.03 (6H, e). C NMR (CDCl ,
3
43.85, 143.02, 137.48, 134.75, 132.71, 122.78, 117.52, 117.27, 115.33,
00 MHz) δ: 113.29−146.78 (Ar−C), 70.28 (c), 55.77 (d), 50.23 (b),
13.85, 107.64, 56.16 (−OCH ), 48.44 (Ar −CH), 34.85 (−CH −),
+
3
3
2
9.20 (f), 45.26 (a), 18.89 (e). [C H O + Na ]: 686.
+
37 42 11
4.78 (−CH −), 14.47 (−CH ). [C H O − H ]: 437.4.
2
3
26 30
6
Esterification of VAN-M-CAT with Linoleic Acid and
1
SYA-P-CAT, orange solid, 1.08 g, 33% isolated yield. H NMR
Epoxidation. Linoleic acid was first converted to linoleoyl chloride
(
acetone-d , 400 MHz) δ: 7.68 (s, 2H, Ar−OH), 7.36 (s, 2H, Ar−
26
6
(LC) to increase its reactivity. To a solution of linoleic acid (4.2 g,
5 mmol) dissolved in 35 mL of dry dichloromethane was added
OH), 7.05 (s, 1H, Ar−OH), 6.66 (d, J = 1.8, 2H, Ar−H), 6.28 (dd, J =
1
1
2
0
1
1
5.7, 1.8, 4H, Ar−H), 5.57 (s, 1H, Ar −CH), 3.66 (s, 6H, −OCH ),
3
3
slowly 4.23 g (33.3 mmol) of oxalyl chloride at 0 °C. The temperature
was then raised to room temperature and stirred for 4 h. The reaction
mixture was concentrated with rotary evaporator to yield LC as a
yellowish oil (4.13 g, 92% isolated yield).
Esterification between VAN-M-CAT and LC was then performed
by a solvent-free and catalyst-free condition as established by a
.36 (t, J = 7.2, 4H, −CH −), 1.45 (dt, J = 15.0, 7.4, 4H, −CH −),
2
2
13
.85 (t, J = 7.2, 6H, −CH ). C NMR (acetone-d , 400 MHz) δ:
3
6
48.45, 143.89, 143.01, 136.58, 135.11, 134.64, 132.76, 117.68, 117.48,
08.11, 56.58 (−OCH ), 48.81 (Ar −CH), 34.85 (−CH −), 24.75
3
3
2
+
(
−CH −), 14.48 (−CH ). [C H O − H ]: 467.6.
2
3
27 32
7
C
DOI: 10.1021/acs.macromol.7b00064
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
2
7
previous study. In detail, VAN-M-CAT (1 g, 2.6 mmol) and LC
8.97 g, 30 mmol) were introduced in a 50 mL reactor with a
RESULTS AND DISCUSSION
Synthesis of Renewable TPs under Optimized
Conditions. Molar ratios of aldehyde/phenol and aldehyde/
■
(
nitrogen−gas bubbling system and an outlet connected to a wash
bottle holding a NaOH solution. The mixture was then stirred and
heated at 130 °C for 15 h under a continuous nitrogen stream. The
obtained esterified product, VAN-M-CAT-LIN, was washed using cold
ethanol to remove the excess acid chloride and further purified by a
silica column using an eluent of hexane/ethyl acetate (10:1) to yield
H SO are capable of affecting the yield of TPs. To obtain
2
4
optimal conditions, reaction between vanillin and methylca-
techol was studied as an example due to the facile isolation and
purification of the product VAN-M-CAT. Isolated yields of
VAN-M-CAT under various synthesis conditions are listed in
Table 1. Effect of vanillin/methylcatechol molar ratio was
2
.79 g, 63% isolated yield.
VAN-M-CAT-LIN was then epoxidized using peracetic acid to
make epoxy comonomer. 1.5 g (0.88 mmol, 8.8 mmol of double bond)
of VAN-M-CAT-LIN was dissolved in 25 mL of dichloromethane in a
Table 1. Effect of Stoichiometric Ratio of Reactants and
Amount of Catalyst and Solvent on the Isolated Yield of
VAN-M-CAT
5
0 mL round-bottomed flask. To this flask was added dropwise
peracetic acid (4.2 g, 17.6 mmol). The reaction mixture was stirred at
room temperature overnight, washed 3 times with brine, and extracted
with ethyl acetate. The ethyl acetate was removed using a rotary
evaporator to yield epoxidized VAN-M-CAT-LIN (VAN-M-CAT-
LIN-EPO) as a yellowish oil (1.4 g, 86% isolated yield).
Formation of Biobased Epoxy Networks. Three epoxy
monomer mixtures with weight ratio GE-VAN-M-CAT to VAN-M-
CAT-LIN-EPO of 100:0, 75:25, and 50:50 are respectively mixed with
diethylenetriamine (DETA) with stoichiometric ratio of epoxy vs
vanillin/
vanillin/
2
vanillin/
b
isolated yield
(%)
a
a
entry
M-CAT
H SO4
EtOH
1
2
3
4
5
6
1:2
1:3
1:4
1:4
1:4
1:3
1:3
1:3
1:1
1:5
1:3
1:3
1:3
1:3
1:3
1:3
1:6
72
83
88
75
89
83
1:4
−
NH for curing. The mixtures were stirred for 10 min, degassed under
a
b
vacuum to remove entrapped air, and poured into molds for curing.
Given GE-VAN-M-CAT was highly reactive with DETA, the mixtures
were cured at room temperature for 8 h, followed by 60 °C for 4 h and
Molar ratio. Weight ratio. Condensation was conducted at room
temperature for 2 days.
8
0 °C for 4 h. The obtained cured epoxy thermosets, denoted as
studied first. For entry 1, condensation of 1 equiv of vanillin
with 2 equiv of methylcatechol gave a yield of 72%. Increasing
methylcatechol/vanillin ratio to 3:1 (entry 2) readily improved
isolated yield to 83%. As the ratio raised to 4:1, the yield
increased further to 88% (entry 3). The effect of catalyst
amount was then investigated (entries 3−5). The amount of
VAN100LIN , VAN LIN , and VAN LIN , were subjected to
mechanical and thermal analyses.
0
75
25
50
50
Analysis Methods. The chemical structure of TPs was followed
using NMR and Fourier transform infrared (FTIR) spectroscopy. The
NMR spectra were performed on a Bruker Avance ARX-400
spectrometer while a Bruker Avance-III-800 was used for taking 2D
HMQC spectra. Deuterated acetone or chloroform was used as
solvent. FTIR analyses were conducted using a Thermo-Nicolet Nexus
H SO was insufficient when equal moles of H SO and vanillin
2
4
2
4
were used (entry 4, 75% isolated yield). After increasing the
H SO /vanillin ratio to 3:1 (entry 3), the yield increased to
4
70 FTIR spectrometer equipped with an ultrahigh-performance,
2
4
8
3
8%. It is noteworthy that the yields of entry 5 (89%) and entry
are similar, even though the H SO /vanillin ratio is increased
versatile attenuated total reflectance (ATR) sampling accessory. The
spectra were scanned over a wavenumber range of 400−4000 cm
with a resolution of 4 cm .
−1
2
4
−1
to 5:1. Thus, H SO /vanillin ratio of 3:1 is effective at
2 4
Conversions of phenol−aldehyde condensation reaction were
measured by high-performance liquid chromatography (HPLC,
Agilent 1260 Infinity Quaternary), with a Zorbax Eclipse XDB-C18
Column (250 × 74.6 mm). ESI−MS analyses in negative mode were
performed using a 7 T Thermo Scientific LQIT/FT−ICR mass
spectrometer. A Waters Delta Prep 4000 HPLC was used to separate
and collect each of the glycidylated products of VAN-M-CAT in 50
mg scale.
Crystal of VAN-M-CAT was obtained from slow evaporation of an
ether solution at room temperature. Single crystals were mounted on
Mitegen microloop mounts using a trace of mineral oil and cooled in
situ to 100(2) K for data collection on a Nonius KappaCCD
diffractometer equipped with a graphite crystal, incident beam
monochromator using Mo Kα radiation (λ = 0.710 73 Å). Data
were collected using the Nonius Collect software and processed using
HKL3000 and corrected for absorption and scaled using Scalepack.
Dynamic mechanical properties were characterized using a DMA
catalyzing the coupling reaction and considered optimal.
The amount of solvent ethanol was also found to impact
product yields (compare entries 3 and 6). In entry 3, to fully
dissolve 1 equiv of vanillin and 4 equiv of methylcatechol, the
weight ratio of ethanol/vanillin had to be at least 3. For
production of VAN-M-CAT, using the least amount of solvent
is advantageous since it minimizes the product dissolved in
solvent and facilitates isolation of product. This was confirmed
when weight ratio of ethanol/vanillin increased to 6 in entry 6.
Even though increasing solvent could facilitate mixing of
reactants, the yield in entry 6 (83%) was still slightly lower than
that in entry 3. Therefore, overall evaluation of the above-
mentioned conditions revealed entry 3 as the optimal synthesis
condition (molar ratio of vanillin/M-CAT = 1:4, vanillin/
H SO4 = 1:3), which produced high isolated yield while
2
consuming relatively less H SO and ethanol.
2
4
The optimized condition was subsequently used for
condensation of other lignin-based aldehydes and phenols.
Initially, direct condensation of aldehydes (HBA, VAN, or
SYA) with para-substituted guaiacols (M-GUA or P-GUA) was
studied using HPLC to measure the conversion. However, it
turned out that conversions of all reactions were negligible
(<5%). In an effort to increase the conversion, reaction time
was increased up to 7 days, but with no enhancement in
conversion. As ortho site of M-GUA and P-GUA is the only
available position for condensation, low reactivity due to steric
2
980 (TA Instruments). Rectangular specimens with dimensions of 30
mm length, 10 mm width, and 2.5 mm thickness were measured in a
single-cantilever mode. The measurements were conducted from 25 to
2
00 °C at a heating rate of 3.00 °C/min and a frequency of 1 Hz. The
temperature at the maximum in the tan δ curve was taken as Tα
(
related to glass transition).
Thermal stability studies were carried out on a TGA Q500 (TA
Instruments) under a nitrogen flow of 40 mL/min. Samples (15−20
mg) were placed in a platinum pan and scanned from 30 to 600 °C at
a ramp rate of 20 °C/min.
D
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effect of the ortho sites significantly decreased the reactivity of
guaiacols. To increase the reactivity, M-GUA and P-GUA were
demethylated to the corresponding catechols (M-CAT and P-
CAT).
VAN > SYA. As the number of electron-donating (methoxy)
groups increases from HBA to SYA, the electrophilicity
decreases, which reduces their reactivity in electrophilic
substitution reactions.
Compared to the negligible conversions of para-substituted
guaiacols, Table 2 shows significant increase in conversion and
Structure of TPs. Proton and carbon NMR spectra of
VAN-M-CAT are shown in Figure 1. The proton peak at 5.39
ppm corresponds to the triphenyl methyl group, which
indicates successful coupling of vanillin with methylcatechol.
Aromatic protons are found at 6.74, 6.69, 6.65, 6.42, and 6.23
ppm. The methoxy group is observed at 3.69 ppm while the
methyl peak at 2.00 ppm. As for the carbon NMR, the methoxy,
triphenylmethyl, and methyl groups are observed at 56.13,
Table 2. Conversions and Isolated Yields of TPs Derived
a
from Lignin-Based Aldehydes and Catechols
conv
(%)
isolated yield
(%)
entry aldehyde catechol
polyphenol
1
2
3
4
5
6
HBA
HBA
VAN
VAN
SYA
M-CAT
P-CAT
M-CAT
P-CAT
M-CAT
P-CAT
HBA-M-CAT
HBA-P-CAT
VAN-M-CAT
VAN-P-CAT
SYA-M-CAT
SYA-P-CAT
97
50
95
48
90
45
84
42
88
35
71
33
4
9.70, and 18.80 ppm, respectively. The characteristic triphenyl
methyl peak is also observed for other lignin-based TPs (as
depicted in Figures S3−S7), which confirms the formation of
triphenylmethane framework. The structure of M-CAT and P-
CAT based TPs are also confirmed by IR and mass spectra in
Figures S8−S14.
X-ray structure of VAN-M-CAT is also measured to confirm
the structure and determine the coupling site. It is observed in
Figure 2 that vanillin couples exclusively at the para sites of
both methylcatechol molecules. This can be explained by
higher reactivity of the para site. Meanwhile, the para site has
less steric hindrance compared to ortho sites when subjected to
condensation. The high reactivity of phenolic para position
highlights the role of demethylation, which could be an effective
way of modifying lignin-derived phenols, especially for those
with para substituted and ortho occupied by methoxy group.
Besides, the stretched orientation of functional hydroxyls of
SYA
a
Reaction conditions: molar ratio of aldehyde:phenol = 1:4,
aldehyde:H SO = 1:3, room temperature for 2 days. Absolute ethanol
2
4
is used as solvent.
yield when the corresponding catechols are used. For example,
TPs based on M-CAT have conversions in the range of 90−
9
7%, with isolated yields of 71−88%. By comparison, P-CAT-
based TPs have lower conversion (45−50%) and isolated yields
(
33−42%), which might be attributed to the greater steric effect
of the propyl group. Structure of aldehydes also has impact on
conversion. Generally, conversion follows the order: HBA >
Figure 1. Proton (A) and carbon (B) NMR spectra of VAN-M-CAT. Solvent: acetone-d6.
E
DOI: 10.1021/acs.macromol.7b00064
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benzodioxane can also catalyze the reaction between monomer
30,31
and cross-linker.
Introduction of flexible components is another way of
adjusting rigidity. By esterifying VAN-M-CAT and linoleic acid,
followed by epoxidation, a fully biobased epoxy monomer with
rigid core and flexible branches was synthesized (VAN-LIN-
EPO) as shown in Figure 4. Figure 5, panel A, exhibits the
Figure 2. X-ray structure of VAN-M-CAT. Crystal was obtained from
the slow evaporation of an ether solution at room temperature.
TPs is also advantageous for making polymers with desirable
properties.
Epoxy Monomers from VAN-M-CAT and Fatty Acid.
Glycidylation of VAN-M-CAT with epichlorohydrin yields
three products with different substitution of oxirane groups
(Scheme 2). Proton, carbon, HMQC NMR spectra, and mass
spectra of each product are illustrated in Figures S15−S21.
Formation of epoxy groups is also confirmed using IR spectra.
As seen in Figure 3, panel A, after VAN-M-CAT is glycidylated1
to GE-VAN-M-CAT, its broad hydroxyl band at 3401 cm−
decreases significantly, and it is accompanied by the presence of
Figure 4. Structure of fully biobased epoxy prepolymer VAN-LIN-
EPO, which is synthesized via esterification between VAN-M-CAT
and linoleic acid, and followed by epoxidation.
−1
an epoxy ring band at 912 cm and a C−O−C ether linkage at
−1
1
027 cm . The catechol groups of VAN-M-CAT make it
inevitable to cause side reactions like intramolecular cyclization
between two adjacent oxiranes. Benzodioxane derivative can
occur in either one or both catechols to form mono-epoxy-
substituted (GE-VAN-M-CAT-1) or tri-epoxy-substituted (GE-
VAN-M-CAT-3) products. From the viewpoint of functionality,
the appearance of benzodioxane derivative is unfavorable since
it is unreactive with amine hardener/cross-linker and is likely to
create dangling chain ends that impair cross-linking in the
NMR spectra of linoleic acid. The peak at δ 5.4 corresponds to
CC double bonds, while peaks at δ 0.9−2.8 are related to the
saturated part of the carbon chain. After linoleic acid is
esterified with VAN-M-CAT to yield VAN-M-CAT-LIN
(VAN-LIN), characteristic peaks of both linoleic acid and
VAN-M-CAT (δ 6.5−6.9, aromatic H; δ 5.5, Ar −CH; δ 3.7,
3
28
resulting polymers. However, if the penta-epoxy-substituted
GE-VAN-M-CAT-5) is the only glycidylation product, the
−OCH and δ 2.0, −CH ) are evident (Figure 5, panel B). To
3
3
(
produce epoxy comonomer, VAN-LIN is epoxidized using
peracetic acid to yield VAN−LIN−EPO. As seen in Figure 5,
panel C, the double bond peak of VAN-LIN-EPO disappears
while a new epoxy peak at δ 2.9 appears. As for the FTIR
spectra of VAN-LIN in Figure 3, panel B, it reveals a CC
formed network could be highly brittle due to the high rigidity
and functionality (related to cross-link density) of VAN-M-
CAT. Therefore, from the viewpoint of processing and
application, the presence of certain amount of benzodioxane
byproducts is favorable since it helps adjust the rigidity of cured
networks. As the major product, tri-epoxy-substituted product
2
9
−1
stretching vibration band at 3008 cm and an ester bond at
−1
1764 cm . When VAN-LIN is epoxidized with peracetic acid,
−1
(
55%) could effectively reduce the rigidity while still keep cross-
the CC band is gone while a new epoxy band at 912 cm
linked within the network. Meanwhile, hydroxyl groups of
appears.
Figure 3. FTIR spectra of (A) VAN-M-CAT and its epoxidized product GE-VAN-M-CAT and (B) VAN-LIN and its epoxidized product VAN-LIN-
EPO.
F
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similar epoxy equivalent value (531 and 539 mmol epoxy/100
g, respectively), the flexible nature of VAN-LIN-EPO
remarkably decreases cross-linking density of the resulting
polymer. On one hand, the saturated component of the carbon
side chain introduces void volume and yields a network that
33
deforms more readily. On the other hand, closed looping
could be formed by the hardener and epoxies of VAN-LIN-
EPO in adjacent carbon chains, which limits the direction of the
resulting network. The plasticizer role of VAN-LIN-EPO can
also be reflected through height of tan δ, which is the ratio of
loss to storage modulus. As seen in Figure 6, height of tan δ
decreases from VAN LIN to VAN100LIN , suggesting lower
50
50
0
segmental mobility and fewer relaxing species in
34,35
VAN100LIN0.
Storage modulus (E′) values are also presented in Figure 6.
Without addition of VAN-LIN-EPO, VAN1 LIN exhibits a
00
0
high glassy E′ of 12.3 GPa, which could be attributed to the
high rigidity and functionality of GE-VAN-M-CAT mixture.
Addition of flexible VAN-LIN-EPO comonomer effectively
decreases the modulus, as E′ of VAN LIN and VAN LIN
75
25
50
50
decreases to 6.2 and 3.2 GPa, respectively. This confirms the
plasticizer nature of VAN-LIN-EPO. It is noteworthy that Tα
and E′ of BPA diglycidyl ether (DGEBA)/DETA network were
36
Figure 5. Proton NMR structure of (A) linoleic acid, (B) VAN-LIN,
product of esterification between VAN-M-CAT and linoleoyl chloride
and (C) epoxidized VAN-LIN (VAN-LIN-EPO). Solvent: CDCl3.
reported to be 137 °C and 3.6 GPa, which are lower than
those of GE-VAN-M-CAT/DETA. This finding highlights
renewable TP-based epoxy networks possess marked mechan-
ical performance to replace or supplement petroleum-based
thermosets.
Dynamic Mechanical Analysis (DMA). Cross-link density
(
ν ) is a key parameter that determines performance of epoxy
Thermogravimetric Analysis (TGA). Figure 7 exhibits a
one-step degradation profile for all epoxy networks. Thermal
e
3
2
thermosets. νe can be reflected from α-relaxation temperature
Tα, related to glass transition temperature) since increased
(
covalent cross-links restricts the mobility of polymer segments,
which leads to higher T . Inset of Figure 6 illustrates T of
α
α
VAN100LIN0 (167 °C), VAN LIN (111 °C), and
75
25
VAN LIN (82 °C), which gradually decreases as portion of
50
50
VAN-LIN-EPO increases from 0 to 50 wt %. Even though GE-
VAN-M-CAT mixture and VA-LIN-EPO are calculated to have
Figure 7. Thermogravimetric analysis thermograms of epoxy networks
with different weight ratio of GE-VAN-M-CAT and VAN-LIN-EPO.
VAN LIN , for example, represents epoxy network with 75 wt % GE-
75
25
VAN-M-CAT and 25 wt % VAN-LIN-EPO in the prepolymer mixture.
stability of cured thermosets increases with cross-linking
density. This can be reflected from the shift of onset
degradation temperature (expressed as T , temperature at
d5
5
% weight loss) from 220 °C of VAN LIN to 245 and 269
50 50
Figure 6. DMA curve of epoxy networks with different weight ratio of
GE-VAN-M-CAT and VA-LIN-EPO as a function of temperature.
VAN LIN , for example, represents epoxy network with 75 wt % GE-
°
C of VAN LIN and VAN100LIN . As cross-linking density
75 25 0
increases, polymer chains become more constrained, which
causes lower molecular mobility during thermal expansion.
Meanwhile, the tortuous pathway in highly cross-linked
network postpones mass exchange. Statistic heat-resistant
75
25
VAN-M-CAT and 25 wt % VAN-LIN-EPO in prepolymer mixture.
Temperature at the maximum in tan δ curve is taken as T (related to
α
glass transition).
G
DOI: 10.1021/acs.macromol.7b00064
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index temperature (T ), which is calculated using T and Td30
ORCID
s
d5
(
temperature at 30% weight loss) (eq 1), reflects the thermal
Mahdi M. Abu-Omar: 0000-0002-4412-1985
Notes
37
stability of cured networks. Table 3 illustrates T increases
s
The authors declare no competing financial interest.
Table 3. Thermogravimetric Data of T , T (Temperature
d5
d30
at 5% and 30% Weight Loss), T (Statistic Heat-Resistant
s
ACKNOWLEDGMENTS
■
Index Temperature), and Char600 (Char Residue at 600 °C)
of Epoxy Networks with Different Ratio of GE-VAN-M-CAT
and VAN-LIN-EPO
Financial support for this research was provided by the
University of California, Santa Barbara, and the Mellichamp
Academic Initiative in Sustainability. S.Z. thanks Yuan Jiang for
mass spectrometry measurements. Shou Zhao is the recipient of
a Mellichamp graduate fellowship.
epoxy networks
VAN LIN
Td5 (°C)
Td30 (°C)
Ts (°C)
Char600 (%)
220
245
269
317
313
318
136
140
146
14.0
14.9
17.3
50
50
25
VAN LIN
75
ABBREVIATIONS
VAN100LIN0
■
TP, triphenylmethane-type phenol; HBA, 4-hydroxybenzalde-
hyde; VAN, vanillin; SYA, syringaldehyde; M-GUA, 4-
methylguaiacol; P-GUA, 4-propylguaiacol; M-CAT, 4-methyl-
catechol; P-CAT, 4-propylcatechol; DETA, diethylenetriamine;
LA, linoleic acid; HBA-M-CAT, TP prepared from HBA and
M-CAT; VAN-M-CAT, TP prepared from VAN and M-CAT;
SYA-M-CAT, TP prepared from SYA and M-CAT; HBA-P-
CAT, TP prepared from HBA and P-CAT; VAN-P-CAT, TP
prepared from VAN and P-CAT; SYA-P-CAT, TP prepared
from SYA and P-CAT; GE-VAN-M-CAT, glycidylated ether of
VAN-M-CAT; VAN-LIN, esterification product of VAN-M-
CAT and LA; VAN-LIN-EPO, epoxidized VAN-LIN;
slightly from VAN LIN (136 °C) to VAN LIN (140 °C)
50
50
75
25
and VAN100LIN (146 °C), verifying the role of cross-linking
0
density on thermal stability. Besides, Char600 (char formed at
6
00 °C) of networks is also observed to increase from
VAN LIN to VAN100LIN0.
50
50
T = 0.49[T + 0.6(T − T )]
(1)
s
d5
d30
d5
CONCLUSIONS
■
Synthetic routes to renewable triphenylmethane-type poly-
phenols were demonstrated, widening the applications of
typical lignin-based para-substituted guaiacols. Ortho-demethy-
lation of para-substituted guaiacols to give the corresponding
catechols was key in providing new and highly reactive para
sites, which effectively improves the reactivity for aldehyde
coupling. Under optimized conditions, M-CAT and P-CAT
based TPs are obtained in high to moderate yields in the range
VAN100LIN , cured resin with weight ratio of GE-VAN-M-
0
CAT to VAN-LIN-EPO (100:0); VAN LIN , cured resin with
75
25
weight ratio of GE-VAN-M-CAT to VAN-LIN-EPO (75:25);
VAN LIN , cured resin with weight ratio of GE-VAN-M-CAT
50
50
to VAN-LIN-EPO (50:50).
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DOI: 10.1021/acs.macromol.7b00064
Macromolecules XXXX, XXX, XXX−XXX