Polyethylene ferulate (PEF) and congeners: polystyrene mimics derived from biorenewable aromatics

Article abstract of DOI:10.1039/c5gc01104c

Ferulic acid and p-coumaric acid are abundant, biorenewable precursors for the synthesis of polyethylene ferulate (PEF) and polyethylene coumarate (PEC), as well as cognate copolymers with prescribed hydrogenation of the main-chain double bond. By controlling the comonomer feed ratios, copolymers with tunable thermal properties are obtained, including the thermal range occupied by polystyrene (PS).

Full text of DOI:10.1039/c5gc01104c

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Polyethylene ferulate (PEF) and congeners:
polystyrene mimics derived from biorenewable
Cite this: DOI: 10.1039/c5gc01104c
aromatics†
Received 23rd May 2015,
Accepted 3rd July 2015
Ha Thi Hoang Nguyen, Marcus H. Reis, Pengxu Qi and Stephen A. Miller*
DOI: 10.1039/c5gc01104c
www.rsc.org/greenchem
Ferulic acid and p-coumaric acid are abundant, biorenewable pre- threaten human health after prolonged exposure. Even though
cursors for the synthesis of polyethylene ferulate (PEF) and poly- polystyrene is recycled in some regions (e.g., Europe), most is
ethylene coumarate (PEC), as well as cognate copolymers with either discarded into landfills or directly into the open
prescribed hydrogenation of the main-chain double bond. By con- environment, often concentrating in rivers and oceans—
trolling the comonomer feed ratios, copolymers with tunable especially for foamed polystyrene, which floats.
thermal properties are obtained, including the thermal range
Contrasting the positive material advantages of PS against
its negative environmental impact, we have sought to develop
new classes of environmentally compatible polystyrene
mimics. These should be sourced from plants instead of toxic,
fossil fuel feedstocks and they should degrade with greater
facility in natural environments, leaving behind benign by-
products.
occupied by polystyrene (PS).
Worldwide, polystyrene (PS) is one of the most popular com-
modity plastics, accounting for about 9% of the global plastic
market.1 This hydrocarbon polymer with aromatic side groups
is synthesized from styrene, a monomer wholly derived from
non-renewable fossil fuels (Fig. 1). Despite its broad utility as a
packaging plastic, polystyrene is neither biorenewable nor bio-
degradable and persists in the environment long after its use.
It has been realized that unreacted styrene monomer can
remain in the polymer and is continually released throughout
the PS life cycle.2 This situation is potentially calamitous
because styrene has been reported as a hazardous air pollu-
tant3 and a human carcinogen.4 For this reason, polystyrene
containers (e.g., cups and food packaging) can potentially
The optimal source for renewable aromatics is lignin,
which makes up about 30% of biomass and is the second
most abundant organic polymer on earth.5 Ferulic acid (FA)
and p-coumaric acid (CA) are the two most abundant hydroxy-
cinnamic acids in the plant kingdom6,7 and these can be
sourced from lignin and, especially, the lignocellulose of
megacrops sugarcane and corn. Not only are these aromatic
acids abundantly available from non-food agricultural waste
(bagasse, stover), they possess antioxidant properties which
protect against oxidative stress in cells and the progression of
age-related diseases,8 as well as reduce the risk of some types
of cancer.9–12 Present in virtually all plants, these hydroxy-
cinnamic acids are especially abundant in “superfoods”.
In 2010, our group reported the synthesis of polydihydro-
ferulic acid (PHFA), a polyethylene terephthalate (PET) mimic
made from either vanillin or ferulic acid building blocks
(Fig. 2).13 An important revelation was the necessity of flexible
segments along the polymer backbone. Polyferulic acid itself
(PFA) is an insoluble, intractable material with a melting temp-
erature (Tm) of 325 °C and a glass transition temperature (Tg)
of 150 °C.14 These high values predict poor processability,
especially as the Tm approaches the decomposition tempera-
ture. In the case of its hydrogenated analogue, PHFA, the two
sp3 carbons in the main chain suitably reduce the melting
temperature to 234 °C, but still allow for a Tg (73 °C) to
compete with that of PET (67 °C). While a Tg of 73 °C is a laud-
able achievement for a renewable thermoplastic polyester,
Fig. 1 The aromatic component of polystyrene (PS) confers desirable
thermal properties, but PS is wholly derived from non-renewable fossil
fuel feedstocks and has poor degradation behaviour.
The George and Josephine Butler Laboratory for Polymer Research, Department of
Chemistry, University of Florida Gainesville, Florida 32611-7200, USA.
E-mail: miller@chem.ufl.edu; Fax: +1-352-392-9741; Tel: +1-352-392-7773
†Electronic supplementary information (ESI) available: Synthetic details, com-
plete polymer characterization data, and method to determine copolymer com-
position. See DOI: 10.1039/c5gc01104c
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subjected to dynamic vacuum for 8 hours in order to eliminate
the by-product water generated from the step-growth reaction,
thus driving the polymerization. All the polymerizations were
performed in the dark by covering the apparatus with alumi-
num foil to avoid unwanted photochemical reactions.17 As
shown in Table 1, the six homopolymers were obtained in
excellent yields (85.9 to 93.1%) with appreciable molecular
weights. Mn values ranged from 1600 to 22 900 and the poly-
dispersity index (PDI) values were somewhat typical for step-
growth polymerizations, ranging from 1.6 to 3.6. The homo-
polymers showed good thermal stability, exhibiting a 50%
mass loss under nitrogen at temperatures from 394 to 536 °C.
Sasiwilaskorn et al.18 explored the synthesis of polyethylene
coumarate (PEC) homopolymer (cf., Table 1, entry 3) using N,
N-dicyclohexylcarbodiimide (DCC) as a coupling agent, but the
polymer was impure with no reported thermal properties or
molecular weight characterization. Hydroxyethylcoumaric
methylester has been investigated as a comonomer for photo-
crosslinkable polymers.19 A patent from 1998 reported poly-
propylene ferulate (cf., Table 1, entry 5) synthesis for
producing phase retarder films, but thermal properties and
molecular weights were not reported.20 Meier et al.21 discov-
ered polycondensation of hydroxyethyldihydroferulic methyl-
ester to form polyethylene dihydroferulate (cf., Table 1, entry 2)
using triazabicyclodecene (TBD) as an organocatalyst.
However, our direct polycondensation from hydroxyethyl-
dihydroferulic acid using Sb2O3 as the catalyst gave a signifi-
cantly higher molecular weight (Mn = 22 100 vs. 5440) and a
higher glass transition temperature (Tg = 32 °C vs. −27 °C).
As recorded in Table 1, the polyalkylene hydroxycinnamates
and polyalkylene dihydrohydroxycinnamates exhibited
different thermal behaviours depending on the number of
methylene spacers, the presence of aromatic methoxy substitu-
ents, and the unsaturation or saturation of the alpha and beta
carbons. For the three polyalkylene ferulate polymers, the
glass transition temperature decreased significantly with an
increase in the number of methylene groups; Tg = 113 °C for
n = 2, 82 °C for n = 3, and 51 °C for n = 6, (Table 1, entries 1, 5,
and 6, respectively). The aromatic methoxy substituent had a
minimal effect on the Tg. Polyethylene ferulate (PEF, Table 1,
entry 1), with an aromatic methoxy group, showed a glass tran-
sition temperature (113 °C) merely 4 degrees greater than that
of polyethylene coumarate (109 °C, PEC, Table 1, entry 3),
having no methoxy group. The hydrogenated analogues com-
pared similarly; polyethylene dihydroferulate (32 °C, Table 1,
entry 2) showed a glass transition temperature merely 8
degrees greater than that of polyethylene dihydrocoumarate
(24 °C, Table 1, entry 4). Comparison of polyethylene ferulate
with its hydrogenated analogue reveals a precipitous drop in
the Tg: Δ = 81 °C = 113 °C–32 °C (Table 1, entries 1 and 2).
Likewise, comparison of polyethylene coumarate with its
hydrogenated analogue reveals an abrupt drop in the Tg: Δ =
85 °C = 109 °C–24 °C (Table 1, entries 3 and 4).
Fig. 2 Polydihydroferulic acid (PHFA), with main chain saturation, is a
more tractable variant of polyferulic acid (PFA).
values matching those of PS, near 95 to 100 °C,15 would consti-
tute an additional achievement.
Herein, we reveal our subsequent pursuits of crafting
renewable polymers, with improved thermal properties, from
hydroxycinnamic acids. The hydrogenation of the double bond
in ferulic acid or p-coumaric acid is one approach for incorpo-
rating the necessary conformational flexibility into the polymer
main chain. An alternative strategy is to leave the double bond
intact and install sp3 hybridized carbons elsewhere. Thus,
monomers were synthesized by appending alkylene spacers of
various lengths to ferulic acid and p-coumaric acid by reaction
with chloroalkylene alcohols (Scheme 1) via adaptation of lit-
erature procedures.16 Additional saturation and flexibility were
introduced by subsequent hydrogenation of hydroxyethyl-
ferulic acid (n = 2) and hydroxyethylcoumaric acid (n = 2),
affording hydroxyethyldihydroferulic acid and hydroxyethyl-
dihydrocoumaric acid monomers, respectively.
Once the monomers were obtained, homopolymerizations
were conducted using 1 mol% of Sb2O3 catalyst. Following a
temperature ramp, typically from 150 to 250 °C over a period
of 4 hours under an atmosphere of argon, the reactions were
Overall, the effect of polymer structure on the glass tran-
sition temperature follows reasonable predictions. As the poly-
mers acquire more conformational flexibility with larger n or
Scheme 1 Synthetic route from coumaric acid (R = H) and ferulic acid
(R = OMe) to polyalkylene hydroxycinnamates (blue, left) and polyalky-
lene dihydrohydroxycinnamates (orange, right).
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Table 1 Homopolymerization results and characterization of polyalkylene hydroxycinnamates and polyalkylene dihydrohydroxycinnamatesa
Entry
1
Polymers
Yield (%)
89.6
Mn b (Da)
Mw b (Da)
PDIb
1.6
Tg c (°C)
Tm c (°C)
T50% d (°C)
1600
2500
113
n.o.
536
2
3
4
5
6
87.1
85.9
90.5
93.1
92.7
22 100
79 400
3.6
32
109
24
n.o.
n.o.
123
n.o.
n.o.
394
433
415
405
397
e
e
e
—
—
—
22 900
7700
63 400
13 700
26 600
2.8
1.8
2.6
82
10 000
51
a 1 mol% Sb2O3; mixtures were typically melted under argon during a 4 hour temperature ramp from 150 to 250 °C; 8 hours of dynamic vacuum
at 250 °C. b Obtained by GPC in hexafluoroisopropanol (HFIP) at 40 °C versus polymethyl methacrylate (PMMA) standards. c Determined by DSC,
n.o. = not observed. d Temperature at which 50% mass loss is observed under nitrogen. e Insolubility of the polymer prevented GPC analysis.
double bond saturation (a greater fraction of sp3 vs. sp2
carbons), lower temperatures suffice to effect long-range chain
motions. And, while the effect is smaller, a methoxy group
increases the conformational barriers for chain motion,
increasing the Tg. Incidentally, polymer solubility also tracked
with polymer flexibility. Gel permeation chromatography
(GPC) analysis of the most rigid polymers gave small (Table 1,
entries 1 and 5) or absent (Table 1, entry 3) peaks—suggesting
that insolubility may belie the real molecular weights. The
most flexible polymers (Table 1, entries 2, 4, and 6) were easily
solubilized in the GPC solvent, hexafluoroisopropanol (HFIP),
gave larger GPC signals, and showed higher molecular weights
(Mn between 10 000 and 22 900). For all six homopolymers,
only polyethylene dihydrocoumarate (Table 1, entry 4) exhibi-
Scheme 2 Synthetic route to copoly(hydroxyethyldihydroferulic acid/
hydroxyethylferulic acid) (R
=
OMe) and copoly(hydroxyethyl-
ted a melting temperature, which was measured at 123 °C. It
seems the other five polymers resist crystallite formation
under the conditions tested.
dihydrocoumaric acid/hydroxyethylcoumaric acid) (R = H).
A copolymerization strategy was implemented to tune the
degree of saturation in the polymer main chain which, in ranged from 1600 to 39 000 with half of the samples above Mn
turn, should modulate the glass transition temperature.22 = 10 900. The lower molecular weights (Table 2, entries 6–11)
As described in Scheme 2, copoly(hydroxyethyldihydroferulic correlated with polymer rigidity (greater unsaturation) and
acid/hydroxyethylferulic acid) (R = OMe) and copoly(hydroxy- may simply be a result of poor solubility in the GPC solvent
ethyldihydrocoumaric acid/hydroxyethylcoumaric acid) (R = H) (HFIP). The two polymers with the greatest hydroxyethyl-
copolymers can be synthesized with 1 mol% of Sb2O3 catalyst coumaric acid content (Table 2, entries 15 and 16) were com-
under the same conditions that functioned for the homopoly- pletely insoluble in HFIP.
A few of the samples show
merizations. As tabulated in Table 2, 1H NMR spectroscopy uncharacteristically high PDI values, but most were in the
confirmed that the incorporation fraction closely matched the typical range for a step-growth polymerization (1.5 to 3.1).
feed fraction for these copolymerizations. The copolymers
An additional investigation of these peculiar molecular
were obtained in high yields (76.4 to 94.1%) and Mn values weights was conducted by tracking the GPC behaviour and
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Table 2 Polymerization results and characterization of copolymers prepared according to Scheme 2a
Monomer feed (%)
Inc. FAb
(mol%)
Yield
(%)
Mn
Mw
Tg
Tg Fox
(°C)
T50%
(°C)
c
c
d
e
f
Entry
(Da)
(Da)
PDIc
(°C)
1
2
3
4
5
6
7
8
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
0
12
19
27
37
53
56
66
74
84
100
87.1
91.7
88.4
89.5
90.9
90.9
76.4
82.5
89.9
90.6
89.6
22 100
39 000
26 500
22 800
12 900
5700
5900
3400
3200
1900
79 400
3.6
5.9
3.1
2.7
1.9
3.0
1.9
1.9
1.9
1.5
1.6
32
37
41
46
52
58
66
77
87
98
113
32
38
45
53
60
67
75
84
93
394
388
389
385
386
392
389
396
414
441
356
228,700
81 100
60 400
25 000
16 900
11 100
6400
9
10
11
6000
2900
2500
102
113
1600
Monomer feed (%)
Inc. CAb
(mol%)
Yield
(%)
Mn
Mw
Tg
Tg Fox
(°C)
T50%
(°C)
c
c
d
e
f
Entry
(Da)
(Da)
PDIc
(°C)
12
13
14
15
16
100
90
50
10
0
0
10
50
90
100
0
10
51
92
100
90.5
92.2
94.1
87.3
85.9
22 900
29 400
10 900
63 400
134 500
40 500
2.8
4.6
3.7
24
29
56
94
109
24
31
61
98
109
415
415
418
430
433
g
g
g
—
—
—
g
g
g
—
—
—
a 1 mol% Sb2O3; mixtures were typically melted under argon during a 4 hour temperature ramp from 150 to 250 °C; 8 hours of dynamic vacuum
at 250 °C. b Incorporation of hydroxyethylferulic acid (hydroxyethyl FA) and hydroxyethylcoumaric acid (hydroxyethyl CA) in the copolymers
determined by 1H NMR. c Obtained by GPC in hexafluoroisopropanol (HFIP) at 40 °C versus polymethyl methacrylate (PMMA) standards.
d Determined by DSC. e Calculated using the Fox equation. f Temperature at which 50% mass loss is observed under nitrogen. g Insolubility of the
polymers prevented GPC analysis.
glass transition temperature as a function of the polymeri-
zation time. Thus, eight aliquots were removed during an
eleven hour copolymerization of hydroxyethyldihydroferulic
acid and hydroxyethylferulic acid (in a 30 : 70 ratio, akin to
Table 2, entry 8) with a temperature ramp from 180 °C to
240 °C (see the ESI†). After one hour, only oligomers were
observed, with Mn = 400 and Mw = 870. Over the next 10 hours,
molecular weight increased and then seemingly decreased
with a maximum Mn of 5900 for Tp = 5 hours. We attribute the
apparent molecular weight reduction to an artifact of the GPC
analysis because of insolubility in HFIP. Corroborating evi-
dence is the unwavering rise in the glass transition tempera-
ture over time, with measured Tg values progressing as
expected for a step-growth polymerization: 42, 63, 66, 69, 72,
73, 74, and 77 °C.
Fig. 3 Glass transition temperatures observed and plotted for copoly
Fig. 3 illustrates the fine control possible over the glass
(hydroxyethyldihydroferulic acid/hydroxyethylferulic acid) (R
= OMe,
transition temperature of the copolymers. As predicted by the
Fox equation,23 the glass transition temperatures followed a
proportional increase as the unsaturated monomer content
light grey diamonds) and copoly(hydroxyethyldihydrocoumaric acid/
hydroxyethylcoumaric acid) (R = H, dark grey squares).
increased, reaching
a maximum with 100% unsaturated
monomer fraction. Thus, a steady increase in the Tg is possible
by increasing the feed fraction of the unsaturated comonomer. Tg of 98 °C, which is the targeted match of the Tg for poly-
For the ferulic acid-based copolymers (R = OMe), the Tg range styrene. For the coumaric acid-based copolymers (R = H), the
is 32 °C to 113 °C and the 50 : 50 (feed) copolymer has a Tg of Tg range is slightly lower from 24 °C to 109 °C and the 50 : 50
58 °C (Table 2, entry 6). The 10 : 90 (feed) copolymer exhibits a (feed) copolymer has a Tg of 56 °C (Table 2, entry 14). The
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10 : 90 (feed) copolymer exhibits a Tg of 94 °C, which is essen-
tially the targeted match of the Tg for polystyrene. Note that
the Tg range of these two copolymers spans the Tg values of
several common commodity plastics, from polylactic acid
(PLA, 55 °C) to polyethylene terephthalate (PET, 67 °C) to poly-
vinyl chloride (PVC, 82 °C) to polystyrene (PS, 95–100 °C) to
polymethyl methacrylate (PMMA, 105 °C).
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Because chain structure and flexibility are paramount to poly-
meric thermal properties, the naturally abundant phytochemi-
cals ferulic acid and p-coumaric acid have been converted into
four targeted monomers with varying degrees of conformational
freedom: (1) hydroxyethylferulic acid, (2) hydroxyethyl-
dihydroferulic acid, (3) hydroxyethylcoumaric acid, and (4)
hydroxyethyldihydrocoumaric acid. The homopolymerizations
and copolymerizations of these monomers yield a novel family
of biorenewable thermoplastics with programmable glass tran-
sition temperatures (Tg). For example, the polyethylene ferulate
(PEF) homopolymer exhibited a Tg of 113 °C and the polyethy-
lene dihydroferulate homopolymer exhibited a Tg of 32 °C. The
corresponding copolymer series spanned the full range of
intermediate Tg values, matching those of several high-volume
commodity polymers. Importantly, the ferulic acid-based co-
polymer with a 10 : 90 feed ratio (predominantly composed of
hydroxyethylferulic acid with 10% of the hydrogenated conge-
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polystyrene (PS). Similarly, the 10 : 90 coumaric acid-based
copolymer showed a Tg of 94 °C, suggesting that it could also
serve as a PS mimic.
These polyalkylene hydroxycinnamates revealed important
structure/property relationships, including:
a
significant
decrease in the Tg with an increasing number of methylene
groups in the main chain; a slight increase in the Tg with the
inclusion of an aromatic methoxy substituent; and, a dramatic
decrease in the glass transition temperature upon saturation
of the main chain CvC double bond to a single bond. Also, as
the polymer main chain unsaturation increased, their solubi-
lity and tractability decreased, leading to a lower molecular
weight analysis, which could be real or simply a result of
diminished solubility.
Future work will test mechanical properties in addition to
the degradation behaviour of the polymers and copolymers.
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degradation to be likely. Degradation pathways via simple
hydrolytic degradation are also conceivable and yield by-pro-
ducts that are not just benign, but beneficial to human
health.24
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