Synthesis, thermal, rheological characteristics, and enzymatic degradation of aliphatic polyesters with lignin-based aromatic pendant groups

Article abstract of DOI:10.1002/pola.29534

This research aims to produce lignin-based biodegradable polyesters with improved thermal quality. A series of aliphatic polyesters with lignin-based aromatic side groups were synthesized by conventional melt-polycondensation. Decent molecular weight (21–

Full text of DOI:10.1002/pola.29534

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POLYMER SCIENCE
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ORIGINAL ARTICLE
Synthesis, thermal, rheological characteristics, and enzymatic degradation
of aliphatic polyesters with lignin-based aromatic pendant groups
1
1
2
1
Carlos R. Arza, Ping Wang, Javier Linares-Pastén, Baozhong Zhang
1
Centre of Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, SE-22100, Lund, Sweden
Division of Biotechnology, Department of Chemistry, Lund University, P.O.Box 124, 22100, Lund, Sweden
2
Correspondence to: B. Zhang (E-mail: baozhong.zhang@chem.lu.se)
Received 19 July 2019; Revised 8 October 2019; accepted 12 October 2019
DOI: 10.1002/pola.29534
ABSTRACT: This research aims to produce lignin-based biode-
gradable polyesters with improved thermal quality. A series of
aliphatic polyesters with lignin-based aromatic side groups were
synthesized by conventional melt-polycondensation. Decent
TGA results, the thermal decomposition temperatures of the
ꢀ
polyesters are all above 390 C, which make them superior com-
pared with commercial biodegradable polyesters like polylactic
acid or polyhydroxyalkanoates. Finally, rheological characteris-
tics and enzymatic degradation of the obtained polyesters were
also measured. © 2019 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2019
−
1
molecular weight (21–64 kg mol ) was achieved for the poly-
merizations. The molecular structures and thermal and mechan-
ical properties of the obtained polyesters were characterized. As
a result, the obtained polyesters are all amorphous, and their
glass-transition temperature (T
dant aromatic group (31–51 C). Furthermore, according to the
g
) depends on the size of the pen-
KEYWORDS: amorphous; biodegradable; bioplastics; enzymatic
ꢀ
degradation; lignin; polyesters; thermal stability; thermal properties
INTRODUCTION Bio-based polymers have received growing
attention from society, because of their potential to solve the
challenges caused by the production of conventional poly-
mers, such as fossil resource depletion and greenhouse gas
(terephthalate) in the backbone, which greatly enhanced its
thermal stability and glass-transition temperature (T ). Ali-
g
phatic polyesters without aromatic units (e.g., PLA and PHAs)
ꢀ
usually have lower T
g
values, such as T
g
~50 C for PLA and
1
–9
ꢀ
emissions.
However, the growing littering issue with plastic
4 C for polyhydroxybutyrate (PHB), respectively, and thermal
stability. In order to improve the thermal properties of the
polyesters while keeping the aliphatic esters so that they can
remain biodegradable, rigid aromatic units can be introduced
as side groups on aliphatic polyester backbones. It has been
reported that PLA with phenyl side groups showed signifi-
wastes cannot be solved only by the transition to bio-based
polymers, because most bio-based polymers on the market
today are nonbiodegradable (e.g., partially bio-based bio-PET,
®
10,11
PlantBottle ).
They will stay persistently in nature and
deteriorate the plastic pollution, once they enter the environ-
ment through unavoidable leakage of the plastic recycling sys-
ꢀ
33–36
cantly increased T
PHAs with pendant aromatic groups were also reported,
although the impact on their T values was not significant
because of the flexible polymer backbones and linkers
value (e.g., 100 C for polymandelide).
g
1
2
tem.
Therefore, bio-based biodegradable polymers are
1
3–16
expected to become ideal materials for the future.
g
Today, most important bio-based biodegradable polymers are
aliphatic polyesters (i.e., polymers with ester bonds derived
from aliphatic carboxylic acids and alcohols), such as poly-
37,38
between the phenyl groups and the backbones.
Other
rigid cyclic units were also introduced as side groups on ali-
39–41
phatic polyesters to increase the T value.
g
1
7–26
lactic acid (PLA) and polyhydroxyalkanoates (PHAs).
Their production capacities have been growing fast over the
past years, but their market share compared with that of non-
biodegradable bio-based polymers (e.g., bio-based PET)
remains small. A particular challenge for the application of ali-
phatic polyesters (e.g., PLA and PHAs) is their inferior thermal
quality compared with that of PET, the current market leader
In order to develop bio-based aliphatic polyesters with aromatic
pendant groups, aliphatic dicarboxylic acids (or dicarboxylates)
with pendent aromatic groups have great potential. These mole-
cules can be used as monomers in conventional melt-
polycondensation with bio-based aliphatic diols (e.g., ethylene
glycol, 1,3-propanediol, and 1,4-butanediol), yielding aliphatic
polyesters with tuneable thermal and mechanical properties.
Today, unsubstituted benzene (or phenyl group) was used as
2
7–32
for polyesters.
PET has been widely used in textile and
packaging industries because of its rigid aromatic units
Additional supporting information may be found in the online version of this article.
2019 Wiley Periodicals, Inc.
©
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the pendant group for the synthesis of aliphatic dicarboxylic
Ethanol (99.7%) was purchased from Solveco. All chemicals
were used as received without purification.
4
2,43
acid (or dicarboxylate) monomers and polyesters.
However,
unsubstituted phenyl group is not widely existing in nature, nei-
ther is it straightforward to be prepared from biomass
resources. Therefore, naturally existing aromatic units will have
great potential for the development of bio-based monomers and
polyesters with desirable physical properties.
Synthesis of monomers and polymers
3
-(3,4-Dimethoxyphenyl)pentanedioic acid (3b)
Monomers 3-(4-methoxyphenyl)pentanedioic acid (3a) and
3-(3,4,5-trimethoxyphenyl)pentanedioic acid (3c) were pre-
pared according to a previously reported procedure. The
synthesis of 3b is described as a general method. 3a and 3c
were synthesized similarly. To a well-stirred solution of
5
7
The most abundant aromatic biomass is lignin, which is a phe-
nolic biopolymer that accounts for ~15–30% of lignocellulosic
4
4,45
biomass in plants.
Currently, ~70 million tons of lignin is
3
,4-dimethoxybenzaldehyde (7.16 g, 43.1 mmol, 1.00 eq.) in
methyl acetoacetate (10.00 g, 86.15 mmol, 2.00 eq.) in a
0 mL vial placed in an ice-bath was added dropwise piperi-
dine (0.45 mL, 0.43 mmol, 0.010 eq.). After being stirred for
2 h at room temperature, the reaction mixture solidified,
which was washed with ethanol (100 mL) and Et O (100 mL),
isolated from the pulp and paper industry annually, and most
4
6
of them were subjected to incineration. Valorization of lignin
has therefore attracted increasing attention since the past
5
4
7,48
decade.
Particularly, lignin contains abundant aromatic
7
sub-structures (e.g., vanillin, ferulic acid, vanillic acid,
syringaldehyde), which can be valuable for the production of
2
and dried over vacuum for 3 h. The resulting solid was dis-
solved in a KOH solution (184 g, 3.29 mol KOH in 95 mL etha-
nol and 140 mL water), and heated at reflux for
4
9–52
various aromatic molecules.
bisphenyl monomers were reported by enzymatic dimeriza-
For instance, symmetrical
5
3
tion of vanillin, among which the dicarboxylate and diol
1
h. Afterward, the solution was concentrated in vacuo until
monomers were used to prepare polyesters with tuneable T
g
approximately ~30% initial volume remained, and then it was
dissolved in 450 mL of deionized water and washed with
ꢀ
54
values (−5 to 139 C). In another approach, acetylferulic
acid- or vanillic acid-based AB monomers (hydroxyl-carboxylic
acids) were synthesized and used to produce polyesters with
Et
2
O (3 × 50 mL). Finally, the remaining aqueous phase was
ꢀ
acidified at 0 C with conc. HCl until pH ~ 1, and the formed
precipitate was filtrated and washed with cold water. Recrys-
tallization from oDCB (×3) yielded 3b as beige needle-like
crystals (4.5 g, yield: 39%).
5
5
comparable thermal properties as PET’s. Vanillic acid-based
dicarboxylic acid monomers were also reported, which were
polymerized with aliphatic diols to yield polyesters with desir-
ꢀ
56
able thermal stability and T values (5–67 C). To the best
g
ꢀ
1
of our knowledge, lignin-based aromatics have not been used
as side groups for aliphatic dicarboxylic acids or dic-
arboxylates for the synthesis of biodegradable polyesters with
improved thermal properties.
m.p 191 C (DSC); H NMR (400.13 MHz, DMSO-d , δ): 12.06
6
(br, 2H, (Hh)), 6.85 (ds, 1H, (Hg)), 6.83 (d, 1H, (Hf)), 6.75 (dd,
1H, (He)), 3.73 (s, 3H, (Hd)), 3.70 (s, 3H, (Hc)), 3.36 (q, 1H,
1
3
(Hb)), and 2.54 (dd, 4H, (Ha)); C NMR (100.61 MHz, DMSO-
d6, δ): 172.97, 148.49, 147.39, 135.96, 119.26, 11.67, 111.42,
Herein, we report on a one-step synthesis of three aliphatic
dicarboxylic acids with lignin-based aromatic pendent groups,
starting from various lignin-based molecules (4-hydrox-
ybenzaldehyde, vanillin, and syringaldehyde) with 1-3 met-
hoxy substituents. Afterward, the obtained dicarboxylic acids
were polymerized with ethylene glycol to yield three linear
aliphatic polyesters with pendent lignin-based aromatic side
groups. The molecular structures and the thermal, mechanical,
and rheological properties of the obtained polyesters were
characterized. The biodegradation of the obtained polyesters
with bacterial ester-active enzymeswas also evaluated.
5
1
9
5.50, 55.47, 40.37, and 37.63; IR (ATR): ν = 3153, 1720,
703, 1513, 1288, 1220, 1201, 1181, 1143, 1115, 1020,
−1
20, 867, 822, 747, 561, and 548 cm
.
3
-(4-Methoxyphenyl)pentanedioic acid (3a)
Beige needle-like crystals (yield: 41%).
ꢀ
1
m.p 166 C (DSC); H NMR (400.13 MHz, DMSO-d , δ): 12.02
6
(
3
br, 2H, (Hf)), 7.17 (d, 2H, (He)), 6.82 (d, 2H, (Hd)), 3.71 (s,
13
H, (Hc)), 3.36 (q, 1H, (Hb)), and 2.54 (dd, 4H, (Ha));
C
NMR (100.61 MHz, DMSO-d , δ): 172.87, 157.78, 135.29,
6
1
1
7
28.42, 113.58, 54.95, 40.42, and 37.21; IR (ATR): ν = 3232,
723, 1684, 1520, 1256, 1232, 1213, 1141, 1018, 883, 810,
-1
66, 639, and 591 cm .
EXPERIMENTAL
Materials
3
-(3,4,5-Trimethoxyphenyl)pentanedioic acid (3c)
3
3
,4-Dimethoxybenzaldehyde (99%), p-anisaldehyde (98%),
,4,5-trimethoxybenzaldehyde (98%), methyl acetoacetate
Beige needle-like crystals (yield: 46%).
ꢀ
1
(>99%), piperidine (99%), potassium hydroxide (KOH)
(90%), 1,2-dichlorobenzene (oDCB) (99%), hydrochloric acid
(HCl) (37%), ethylene glycol (EG) (>99%), and titanium
(IV) isopropoxide (TTIP) (97%) were purchased from Sigma-
m.p 198 C (DSC); H NMR (400.13 MHz, DMSO-d , δ): 12.10
6
(
br, 2H, (Hf)), 6.56 (s, 2H, (He)), 3.74 (s, 6H, (Hd)), 3.61 (s,
1
3
3
H, (Hc)), 3.37 (q, 1H, (Hb), and 2.57 (dd, 4H, (Ha)). C NMR
, δ): 172.96, 152.59, 139.25, 136.17,
104.68, 59.89, 55.83, 40.10, and 38.24; IR (ATR): ν = 3072,
(
100.61 MHz, DMSO-d
6
Aldrich. Ethyl acetate (EtOAc), diethyl ether (Et
2
O), xylenes
(
Analytical grade, ACS), and tetrahydrofuran (THF) (HPLC
1
7
720, 1693, 1594, 1460, 1426, 1255, 1183, 1123, 999, 837,
82, 654, and 595 cm−
1
.
grade, without stabilizer) were purchased from Scharlau.
2
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Synthesis of polyester 5c (General polymerization protocol)
The synthesis of 5c is described as a general polymerization
method. 5a and 5b were synthesized similarly. EG (1.52 g,
Viscotek 305 TDA, which included a guard column and two
Malvern Panalytical general purpose mixed bed columns with
an exclusion limit of 20 × 10 Da for polystyrene. Detection
6
2
4.5 mmol, 8.00 eq.) and 3c (0.91 g, 3.06 mmol, 1 eq.) were
consisted of a conventional dual cell refractive index detector,
a four-capillary bridge viscometer, and a light scattering
detector operating at 3 mW, at a wavelength of 670 nm, and
measurements angles of 90 and 7 . The three detectors were
calibrated with a polystyrene standard (96 kDA) from Poly-
mers Laboratories. Molecular weights were determined by
the triple detection method using the OmiSEC 5.12 software
(Malvern). Differential scanning calorimetry (DSC) measure-
ments were performed using TA Instruments DSC Q2000.
added to a two-necked round-bottomed flask equipped with a
mechanical stirrer, After being stirred for 30 min at 180 C
under N
xylene solution, 0.5 mol % with respect to 3c) was added via
a syringe, and the temperature was raised to 200 C. After the
esterification of 3c was completed (~3 h, monitored by H-
NMR), the reaction mixture was stirred overnight with an
increased flow of N
ture was raised to 220 C for 5 h. Afterward, the reaction mix-
ture was cooled to room temperature, dissolved in THF
ꢀ
flow, TTIP (4.35 mg, 0.38 mL of a 11.40 mg mL−
1
ꢀ
ꢀ
2
ꢀ
1
ꢀ
2
at 200 C. Finally, the reaction tempera-
ꢀ
ꢀ
−
−1
The samples were studied with a heating rate of 10 min
1
under nitrogen with a purge rate of 50 mL min . The
ꢀ
(
7 mL), and precipitated in 500 mL of deionized water. The
sequence consisted of a heating ramp from 40 to 200 C,
ꢀ
beige small fibers formed were collected by gravity filtration,
washed with water (3 × 200 mL), and dried at 50 C under
followed by a cooling ramp to −50 C, and finally a heating
ꢀ
ꢀ
ramp to 220 C, which was employed to determine the T
g
.
vacuum overnight to yield 0.95 g (97%) of beige fiber-like
Thermogravimetric analysis (TGA) was performed with a
thermogravimetric analyzer TA Instruments Q500 at
1
powder 5c. H NMR (400.13 MHz, DMSO-d
6
, δ): 6.51 (br, 2H,
a
ꢀ
−1
(
(
(
He)), 4.05 (br, 4H, (Hh)), 3.70 (br, 6H, (Hd)), 3.57 (br, 3H,
Hc)), 3.37 (q, 1H, (Hb), and 2.64 (dd, 4H, (Ha)); C NMR
heating rate of 10 C min , under nitrogen with a purge rate
13
−1
of 50 mL min . Dynamic mechanical analysis (DMA) was
100.61 MHz, DMSO-d , δ): 171.14, 152.66, 138.38, 136.06,
performed on a TA Instruments Q800 dynamic mechanical
analyzer in oscillatory bending mode at a frequency of 1 Hz,
an oscillatory amplitude of 15 μm, and with a temperature
6
1
04.52, 61.86, 59.83, 55.75, 39.78, and 38.12; IR (ATR):
ν = 2938, 1728, 1589, 1509, 1458, 1422, 1237, 1149,
−
1
ꢀ
1
119, 1041, 1003, 966, 907, and 829 cm
.
ramp of 3 C. Optically clear free-standing rectangular speci-
mens (l: 8 mm, w: 5 mm, t: 1 mm) were prepared by com-
ꢀ
pression molding at 150
C for 10 min (Supporting
Polyester 5a
Information Fig. S3). Rheological analysis was performed on
an Advance Rheometer AR2000 ETC from TA Instruments.
The characterization was carried out under N2 atmosphere,
using a disposable 20 mm diameter aluminum parallel-plate
geometry with a gap of ca. 1 mm. Dynamic strain sweep mea-
surements were carried out at 1 Hz from 0.1 to 100% oscilla-
tory strain in order to determine the viscoelastic linear
region. Dynamic frequency sweep experiments were per-
formed in the linear viscoelastic region at an oscillatory strain
of 2% and a frequency range of 0.1–500 rad s− . Reversed
phase liquid chromatography–mass spectrometry (LC-MS)
was performed on a XEVO-G2 ESI-QToF mass spectrometer,
and Acquity UPLC equipped with a Acquity CSH C18 column
Beige fiber-like powder (93%).
1
H NMR (400.13 MHz, DMSO-d , δ): 7.12 (d, 2H, (He)), 6.77
6
(
d, 2H, (Hd)), 3.97 (br, 4H, (Hh)), 3.65 (br, 3H, (Hc)), 3.34 (q,
1
3
1H, (Hb), and 2.60 (dd, 4H, (Ha)); C NMR (100.61 MHz,
DMSO-d , δ): 171.01, 157.88, 134.31, 128.30, 113.57, 61.77,
6
54.84, 39.89, and 37.14; IR (ATR): ν = 2961, 1727, 1611,
−
1
1513, 1245, 1136, 1112, 1030, 829, and 548 cm
.
1
Polyester 5b
Beige fiber-like powder (92%).
1
H NMR (400.13 MHz, DMSO-d , δ): 6.82 (br, 1H, (Hg)), 6.74
6
(
br, 1H, (Hf)), 6.74 (br, 1H, (He)), 4.00 (br, 4H, (Hh)), 3.69 (br,
(
1.7 μm, 2.1 × 100 mm), all from Waters. The mobile phases
3
H, (Hd)), )), 3.65 (br, 3H, (Hc)), 3.37 (q, 1H, (Hb), and 2.62
contained 0.1% formic acid in water (A) and 0.1% formic
acid in acetonitrile (B), and the gradient profile was
1
3
(
dd, 4H, (Ha)); C NMR (100.61 MHz, DMSO-d , δ): 171.08,
6
1
5
1
7
48.48, 147.46, 134.92, 119.18, 111.43, 111.07, 61.78, 55.38,
5.27, 39.61, and 37.51; IR (ATR): ν = 2924, 1727, 1516,
0
.0–0.7 min 5% B, 0.7–8.0 min 5–99% B, followed by 99% B
ꢀ
for 3 min. The column was kept at 60 C and the flow rate
was 0.5 mL min . Diode-array detection was performed
451, 1421, 1256, 1235, 1136, 1060, 1023, 967, 809, and
−1
65 cm−
1
.
between 190 and 300 nm and the mass spectra between m/z
50 and 1200 were generated in positive electrospray mode
Analytical methods
using a capillary voltage of 3 kV, cone voltage of 40 V, source
ꢀ ꢀ
temperature 120 C, desolvation temperature 500 C, cone gas
Nuclear magnetic resonance (NMR) measurements were car-
ried out on a Bruker DRX400 spectrometer at the proton fre-
quency of 400.13MHz and carbon frequency of 100.61 MHz.
Fourier transform infrared (FTIR) spectra were measured
with an attenuated total reflection (ATR) setup using a
Bruker Alpha FT-IR spectrometer. Twenty four scans were
co-added using a resolution of 4 cm . Size exclusion chroma-
tography (SEC) was performed in chloroform at 35 C with a
flow rate of 1.0 mL min . The SEC equipment used was a
of 50 L h− and desolvation gas 800 L h (both N
1
−1
). Lock
2
mass correction was performed using leucine enkephalin
(according to Waters standard recommendations). Extracted
ion chromatograms were plotted for m/z 283.118 (C14
for polymer 5a, m/z 313.129 (C15 ) for polymer 5b and
m/z 349.139 (C16 ) for polymer 5c. The peaks moni-
H O )
18 6
H O
20 7
−
1
22 8
H O
ꢀ
tored at these signal were the dominating in the total ion
chromatograms.
−
1
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ꢀ
Enzymes production and purification
37 C for 72 h. For Tth, the polymers were soaked in a mix-
ture of phosphate buffer (1.6 mL, PH 8.5), DMSO (0.2 mL) and
Tth (0.2 mL, 0.79 mg mL− ). The reaction mixture was incu-
58
PTEase, without signal peptide, from Ideonella sakaiensis and
lipase Tth from Thermoanaerobacter thermohydrosulfuricus SOL1
1
59
ꢀ
(
Tth) were produced in recombinant strains of Escherichia coli.
The genes encoding the enzymes were chemically synthesized
GenScript USA Inc. Piscataway, N.J.) with optimized codons for
bated with shaking at 200 rpm at 50 C for 72 h. After incuba-
tion, the samples were centrifuged at 13,000 g for 10 min,
and the supernatant was analyzed with LC-MS.
(
E. coli. PETase gene was inserted in the vector pET28b (restric-
tion sites NdeI and XhoI), in frame with the sequence encoding a
histidine tag in the N-terminal; while, Tth gene was inserted in
pET21b (restriction sites NdeI and XhoI) in frame with the
sequence encoding a histidine tag in the C-terminal. Chemically
competent cells E. coli BL21(DE3) were transformed, by thermic
shock, with the mentioned plasmids. The recombinant strains
were grown in 300 mL Luria Bertani (LB) medium, sup-
plemented with 100 μM ampicillin for the strain harboring
pET21b:Tth and with kanamycin 34 μM for the strain harboring
pET28b:PETase. The growth phase for both recombinant strains
RESULTS AND DISCUSSION
Synthesis of monomers
Monomers 3a-c were synthesized using a green chemistry
protocol by a solvent-free reaction of an aromatic aldehyde
(
1a-c) and methyl acetoacetate (2) with piperidine catalyst,
5
7
followed by a hydrolysis with aqueous KOH (Scheme 1).
Three potentially bio-based aromatic aldehydes (1a-c) were
used as the starting materials, which contained 1–3 methoxy
groups on the aromatic ring. It was reported that these aro-
matic aldehydes can be conveniently prepared by methylation
of lignin-based aromatic aldehydes (4-hydroxybenzaldehyde,
ꢀ
was incubated at 37 C until an optical density of 0.6 at 600 nm
of wavelength. Thereafter, the production of recombinant
6
0–62
enzymes was induced with 1 mM isopropyl β-D-
vanillin, and syringaldehyde, respectively)
using dimethyl
ꢀ
63
1
-thiogalactopyranoside. PETase was produced at 25 C for 12 h;
carbonate (a green reagent). The ketone-ester (2) for the
reaction was an inexpensive chemical, of which three syn-
thetic routes from bio-sourced chemicals are available. First, it
could be prepared using Claisen condensation of methyl ace-
tate, which was an ester of bio-sourced chemicals methanol
and acetic acid. Second, it could be synthesized by oxidation
ꢀ
while, Tth at 37 C for 6 h.
Both enzymes were purified by immobilized metal ion affinity
chromatography (IMAC). First, cell pellets were collected by
centrifugation, washed, and resuspended in binding buffer
(
Tris, NaCl 500 mM, pH 7.4), followed by disintegration by
64
of methyl crotonate, which could be prepared by a depoly-
sonication. The cell extract containing Tth was subjected to an
65
merization of bio-based polymer PHB. Finally, it could also
be prepared by a methoxycarbonilation of acetone with
ꢀ
59
additional heat treatment at 60 C for 30 min. After that, the
cell extracts were centrifuged at 23,000 g, 20 min, and the
resulting supernatants were directly loaded onto a chromato-
graphic FF-column. Bound proteins were eluted by elution
buffer (Tris, NaCl 500mM, imidazole 500 mM, pH 7.4). Puri-
fied enzymes were analyzed by SDS-PAGE and concentrations
were estimated spectrophotometrically at 280 nm.
dimethyl carbonate, both of which were potentially bio-based
66,67
chemicals.
The possible synthetic routes to prepare 1a-c
and 2 from bio-sourced raw materials were summarized in
the Supporting Information Fig. S1.
The obtained monomers 3a-c were polymerized with ethylene
glycol 4 (EG, a bio-based diol used for the production of bio-
based PET) to produce the corresponding polyesters 5a-c. A
conventional two-step melt-polycondensation protocol (esteri-
fication and polycondensation) was employed using TTIP
(0.5 mol %) catalyst. The first step (esterification between
Enzymatic reactions
To analyze the degradation of 5a-c, two enzymes (PETase and
Tth) were used. For PETase, the polymer samples were
soaked in a mixture of phosphate buffer (1.6 mL, PH 7.0),
DMSO (0.2 mL) and PETase (0.2 mL and 1.58 mg mL− ). The
reaction mixture was incubated with shaking at 200 rpm at
1
carboxylic acid and ethylene glycol) was completed after 3 h
using an excess (8 eq.) of EG at 200 C under continuous N
ꢀ
2
SCHEME 1 Synthesis of monomers 3a-c, and their polymerization with ethylene glycol (4) to produce aliphatic polyesters with
aromatic pendant groups (5a-c).
4
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TABLE 1 Characterization of Polyesters 5a-c
ꢀ
T
g
( C)
a
b
DSC
DMA
(kg mol−
1
)
PDI
T
10 ( C)
c
ꢀ
T
d
c
( C)
ꢀ
C
d
1
C
d
2
(K)
C1
g
C2 (K)
g
Polyester
M
n
(kg/mol)
M
w
5
5
5
a
b
c
21.3
41.2
63.5
68.5
83.6
3.30
1.94
2.16
31
45
51
27
47
51
363
360
359
416
397
392
2.82
2.46
3.36
138.8
103.1
127.5
9.45
95.2
41.2
95.5
12.71
11.47
136.0
a
, 10 ꢀC min−1. T
c
, 10 ꢀC min−1
Based on the reference temperature T
DSC: second heat ramp; N
point of step transition.
2
g
determined as inflection
TGA: N
2
.
d
ꢀ
r
= 140 C.
b
DMA: bending mode, 3 min−1, 1 Hz. T
mum in E’’ curves.
ꢀ
g
determined as the peak maxi-
flow. The second step (polycondensation) was performed
for 5a-b, see Fig. 1) or right (higher retention volume, as
for 5c) of the corresponding peak retention volume. Normaliz-
ing the three SEC curves with respect to their retention times
provided a clearer view on the shapes of the curves
(Supporting Information Fig. S5). The presence of the shoul-
ders on the SEC chromatograms was consistent with the melt
rheological behavior of the polyesters (see later section). The
observed higher PDI value of 5a compared with 5b-c might
be due to some unidentified side reactions during the poly-
merization of 3a.
ꢀ
overnight at 200 C with an increased N
by an additional 5 h stirring at 220 C. It was observed that
2
flow rate, followed
ꢀ
the additional 5 h with a stronger N
2
flow at higher tempera-
ꢀ
ture (220 C) facilitated the removal of EG and increased the
viscosity of the polymer melts. After the polymerization, the
reaction mixture was cooled to room temperature, forming an
optically clear, cognac-brown material as the crude polymer
products. A straightforward precipitation of the crude poly-
mer solution (in THF) into water yielded polymers 5a-c as
white or off-white powders (or fibrous powders). The
obtained polyesters 5a-c showed decent molecular weights
The chemical structures of monomers 3a-c and polyesters 5a-
1
c were confirmed by H-NMR spectra (Fig. 2). First, all the sig-
according to SEC measurements (M = 21.3, 41.2, and 63.5 kg
n
−
1
nals of monomers 3a-c were unambiguously assigned [Fig. 2
mol , and PDI = 3.30, 1.94, and 2.16, respectively, see
Table 1). Interestingly, the achieved molecular weight was
higher when the number of methoxy groups in the monomer
increased, despite the fact that the increased number of sub-
stituents could lead to steric hindrance during polymerization.
Furthermore, it was observed that the SEC chromatograms of
(
A,C,E)]. The resonance of the carboxylic acid protons was
observed at 12.02 (f), 12.06 (h), and 12.10 (f) ppm, respec-
tively. The aromatic protons were observed at 6.54–7.20 ppm.
Specifically, 3a [Fig. 2(A)] showed two aromatic doublets at
6
.82 and 7.17 ppm (d and e, respectively). 3b [Fig. 2(C)] dis-
played a doublet of doublets at 6.75 ppm (e), a doublet at
.83 ppm (f), and a doublet of singlet at 6.85 ppm (g). For
monomer 3c [Fig. 2(E)], only an aromatic singlet at 6.56 ppm
e) was observed. Furthermore, the methoxy groups of 3a-c
were also observed at 3.71 ppm [c, Fig. 2(A)], 3.70 and
.73 ppm [c and d, Fig. 2(C)], and 3.61 and 3.74 ppm [c and d,
5
a-c showed shoulders to the left (lower retention volume, as
6
(
3
Fig. 2(E)]. The chemical shifts of the tertiary backbone CH
proton were observed at 3.36 ppm as a quintet for 3b [b,
Fig. 2(C)]. However, the corresponding signal was not discern-
able for 3a and 3c, because of the severe overlap with the
water signal at ~3.3 ppm. Finally, the resonances of the CH
2
group (a) were observed at 2.55 ppm (doublet of doublets),
which partially overlapped with the solvent peak (DMSO).
1
After the polymerization, all the H-NMR spectra of the
corresponding polyesters [Fig. 2(B,D,F)] were broadened, indi-
cating the formation of polymers. The disappearance of the
carboxylic acid proton signals at ~11.7 ppm indicated the
complete esterification (Step 1). Furthermore, the incorpora-
tion of ethylene “bridge” in the polymer backbones was con-
firmed by the appearance of the new ester CH signal at 3.97,
2
FIGURE
polyesters 5a-c. The retention volume of a polystyrene standard
PS, ~96 KDa) was indicated as the dashed line. The
shoulders on the curves were indicated with arrows. [Color
figure can be viewed at wileyonlinelibrary.com]
1
SEC chromatograms of the refractive index of
4.00, and 4.05 ppm (h, for 5a-c, respectively).
(
M
n
Furthermore, the chemical structures of the obtained polyes-
ters were characterized by FTIR spectroscopy. As shown in
Figure 3, a broad carboxylic acid O─H stretching band
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1
FIGURE 2 H-NMR spectra of monomers 3a-c (A, C, and E), and polyesters 5a-c (B, D, and F). [Color figure can be viewed at
wileyonlinelibrary.com]
1
(
(
3300–2500 cm− ) and two carbonyl C═O stretching bands
stretching band in the monomers was not observed in the
spectra of the polyesters 5a-c, which confirmed the depletion
of monomers. In the meantime, the two carbonyl C═O
stretching bands of monomers changed into one single broad
C═O stretching band centered at 1726 cm− , which showed
red shift compared to the corresponding bands for the mono-
mers. Because the existence of the two C═O stretching bands
in the spectra of monomers was attributed to the hydrogen
bonding interactions between carbonyl C═O and O─H groups
in the crystalline structure of the monomers, the observation
that there was only one C═O stretching band in the spectra of
all polyesters confirmed the consumption of COOH groups
−
1
1800–1625 cm
)
of monomers 3a-c were clearly
6
8
observed. After the polymerization, the characteristic O─H
1
6
9
and the formation of non-hydrogen bonded ester groups.
The observed red shift of the C═O stretching band after the
polymerization was also consistent with the formation of
7
0
esters for other biodegradable aliphatic polyesters.
The thermal transition of 5a-c (fiber-like powders) was stud-
ied using differential scanning calorimetry (DSC) and dynamic
mechanical analysis (DMA). As shown in Figure 4(A), the T
for all the three polyester powders were shown in the second
DSC heating cycles, and the T values increased as a function
g
s
FIGURE 3 FTIR spectra of monomers 3a-c and polyesters 5a-c in
the longer wave length (left) and short wavelength (right)
regions. [Color figure can be viewed at wileyonlinelibrary.com]
g
of increased number of OMe substituents (T ~31, 45, and
g
ꢀ
51 C, for 5a-c, respectively). No melting endotherm was
6
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FIGURE 4 DSC (A) and DMA (B) curves of polyesters 5a-c. [Color
figure can be viewed at wileyonlinelibrary.com]
FIGURE
5
TGA weight loss (A) and derivative weight
(B) curves of polyesters 5a-c. [Color figure can be viewed at
wileyonlinelibrary.com]
observed for all the three polymers, which suggested an amor-
phous morphology for all polyesters. The T
polyesters were further confirmed by DMA measurements.
The T values were obtained as the peak maximum in the E’’
g
values of the
The flow characteristics of 5a-c were evaluated by rheological
analysis. The specimens for rheology measurements were pre-
ꢀ
pared by compression molding at 150 C for 10 min, of which
g
DMA curves, and these values were in agreement with those
Tg values obtained from DSC.
5a and 5b displayed yellow coloration to some extent
(Supporting Information Fig. S3). 5c did not show significant
coloration after compression molding (as beige films). Time–
temperature superposition (TTS) was applied to the data
obtained from the rheology measurements between 500 and
The thermal stability of the polyesters was studied using ther-
mogravimetric analysis (TGA). As shown in Figure 5, the TGA
thermograms of 5a-c displayed a single-step weight loss pro-
−1
ꢀ
ꢀ
0
.1 rad s with at 5 C intervals (from 80 to 140 C for 5a,
file, with the 10% weight loss temperature (T10%) at ~359,
ꢀ
and 100 to 150 C for 5b-c). Figure 6(A) showed the storage
G’ and loss G’’ moduli master curves of 5a-c, obtained by
applying the TTS principle.
ꢀ
3
60, and 363 C, respectively. The decreased thermal stability
of polyesters 5a to 5c was attributed to the increased number
of electron-donating methoxy groups per backbone repeating
units, which is consistent with the observations for other
All the polyesters displayed a single relaxation found at the
crossover frequency (where G’ equals to G’’), whose inverse was
identified as the terminal relaxation time. This is the characteris-
tic reptation time (τrep), which confirmed that the polyesters
possessed sufficient molecular weight for entanglements. Gener-
ally, for nonaggregating polymers, power-law behavior in the
terminal region at low frequencies is characterized by slopes
1 and 2 for G’’ and G ’, respectively. However, slopes of 0.85 and
1.03 for G’ and G ’, as well as a broader transition from the termi-
nal to the rubbery plateau was observed for the obtained
7
1,72
methoxy-containing polymers.
Compared with commer-
cially available aliphatic polyesters PLA and PHA
ꢀ
ꢀ
(
T
10%~291–334 C and 261–270 C, respectively), the new
polyesters 5a-c exhibited superior thermal stability, which
4
0,73–77
may greatly facilitate their processing and applications.
In addition, polyesters 5a-c also showed higher thermal sta-
bility compared with other reported aliphatic polyester with
ꢀ
rigid structures such as Coates’ (T10%~315 C) and Baker’s
(
ꢀ
78,79
onset weight loss at 330 C) aliphatic polyesters.
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8
5
curve fitting.
Figure 7(A) showed the shift factors of
the polyesters and their WLF fit curves, which corroborated
the validity of the WLF behavior of the polyesters. Parameters
g
1
C
C
1
g
2
and C
2
depend on T
r
, so they were transformed into C and
85
values using Tg, which could be compared with the
g
g
known values in the literature. C and C values were calcu-
1
2
8
5
lated according to the following equation
:
C1C2
g
1
C =
À
Á
ð2Þ
ð3Þ
C2 + Tg −Tr
À
Á
g
2
C = C2 + Tg −Tr
g
1
g
2
where C and C are the T -dependent parameters. The values
g
g
1
g
2
for C and C were listed in Table 1, which correlated well
with reported values for other nonaggregating polymers,
including commercial polyesters PET and PLA [Fig. 7(B)].
8
5–89
Finally, a preliminary biodegradation investigation of the
obtained polyesters 5a-c was performed by treating the poly-
mer powders with two different enzymes, PETase from
5
8
Ideonella sakaiensis
and thermostable lipase (Tth) from
5
9
Thermoanaerobacter thermohydrosulfuricus, for 3 days. The
aqueous phase was subjected to LC-MS analysis. As shown by
the extracted ion chromatograms (Supporting Informa-
tion Figs. S6–S8), monomer and dimer signals (confirmed by
high-resolution mass spectrometry, Supporting Informa-
tion Figs. S9 and S10) as the degraded products were clearly
observed at ~3 min and 4 min after the treatment with PETase
(
Supporting Information Figs. S6–S8A). The corresponding sig-
nals were very small or not observed when there was no PET-
ase (negative control, Supporting Information Figs. S6–S8B). In
case of the other enzyme, Tth, the monomer and dimer signals
were observed (Supporting Information Figs. S6–S8C), but
there was no significant difference compared with the negative
control (Supporting Information Figs. S6–S8D). This indicated
that enzyme Tth was not effective to degrade these new polyes-
ters in 3 days.
FIGURE 6 Storage G’ and loss G’’ moduli (A), and complex
ꢀ
viscosity η* (B) master curves of polyesters at T
r
= 140 C. For
clarity, 5b-c master curves in (A) have been shifted vertically by
factors of 10 and 100, respectively. [Color figure can be viewed
at wileyonlinelibrary.com]
polyesters 5a-c. This behavior is typical for polymers with non-
80–84
uniform molecular weight distributions,
which was consis-
CONCLUSIONS
tent with the observed shoulders on the SEC curves (Fig. 1).
Furthermore, melt rheology also provided suitable curve overlap
generating complex viscosity η* master curves over a frequency
range of six orders of magnitude [Fig. 6(B)]. All the polyesters
Three lignin-based aromatic aldehydes (4-hydroxybenzaldehyde,
vanillin, and syringaldehyde) were used to synthesize a series of
aliphatic dicarboxylic acids with pendent aromatic groups. These
dicarboxylic acids were successfully polymerized with ethylene
glycol to produce a series of polyesters with varied size of the
0
showed zero-shear viscosity (η ) in the low-frequency limit.
Especially, 5b and 5c exhibited η above 1000 Paꢁs, which con-
0
firmed their high-molecular weight according to the SEC data.
At higher frequencies, shear thinning was observed for all the
polyesters after a broad transition caused by the nonuniform
molecular weight distribution (Fig. 1).
g
pendant groups. The T values of the resulting polyesters were in
ꢀ
the range of 31–51 C, which were lower than that of PET
ꢀ
TM
(T
g
~65 C), but were comparable or higher than that of PTT
ꢀ
(Sorona ; T ~35 C). Furthermore, all these polymers showed
g
good thermal stability with initial weight loss (10%) above
Polyesters 5a-c obeyed the Williams–Landel–Ferry (WLF)
equation as follows:
ꢀ
350 C. According to the rheological analysis, the flow character-
istics of the new polyesters displayed characteristic reptation
behavior, which suggested that these polyesters possessed suffi-
cient molecular weight for entanglements. Enzymatic degradation
tests indicated that these polymers could be degraded into mono-
mer or dimer species within 3 days by PETase. This work pro-
vided the first insight into the design of new bio-based aliphatic
polyesters with desirable thermal properties toward various
−
C1ðT −TrÞ
logðaTÞ =
ð1Þ
C2 + ðT−TrÞ
where a is the temperature shift factor to generate the mas-
T
ter curves, T is the temperature, T is a reference temperature
r
ꢀ
(
140 C), and C
1
and C
2
are empirical constants from the
8
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This work was financially supported by the Åforsk Foundation
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No. 2016/1489), the Crafoord Foundation (Nos. 20160774
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én acknowledges J. Gust. Richert Memorial Foundation for
financial support. The authors also thank Mats Wallin from
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