Biomass-Derived Poly(ether-amide)s Incorporating Hydroxycinnamates

Article abstract of DOI:10.1021/acs.biomac.9b00044

Lignin-derived chemicals have great potential as feedstock to produce polymeric materials, due to the low cost and high abundance of lignin biomass. Lignin is one of the few nonpetroleum sources of aromatic carbon, a desirable moiety in high-performance polymers. Herein we describe the synthesis and characterization of a series of 21 poly(ether-amide)s that incorporate hydroxycinnamates derived from lignin. Three different hydroxycinnamates (coumaric acid, ferulic acid, sinapinic acid) were incorporated into dimers, and then copolymerized with a series of seven aliphatic and aromatic diamines via interfacial polymerization. The resultant polymers exhibited poor solubility in standard organic solvents (excluding DMF), but exhibited moderate glass transition temperatures and moderate thermal stabilities. Additionally, the polymers exhibit excellent resistance to hydrolysis. The modularity of this synthetic approach could be used to rapidly generate diverse polymers with a broad range of well-tuned properties.

Full text of DOI:10.1021/acs.biomac.9b00044

Article
pubs.acs.org/Biomac
Cite This: Biomacromolecules XXXX, XXX, XXX−XXX
Biomass-Derived Poly(ether-amide)s Incorporating
Hydroxycinnamates
Brianna M. Upton† and Andrea M. Kasko*,‡
†Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States
‡Department of Bioengineering, University of California, Los Angeles, 410 Westwood Plaza, Los Angeles, California 90095, United
States
S
* Supporting Information
ABSTRACT: Lignin-derived chemicals have great potential as feedstock to produce polymeric
materials, due to the low cost and high abundance of lignin biomass. Lignin is one of the few
nonpetroleum sources of aromatic carbon, a desirable moiety in high-performance polymers.
Herein we describe the synthesis and characterization of a series of 21 poly(ether-amide)s that
incorporate hydroxycinnamates derived from lignin. Three different hydroxycinnamates
(coumaric acid, ferulic acid, sinapinic acid) were incorporated into dimers, and then
copolymerized with a series of seven aliphatic and aromatic diamines via interfacial
polymerization. The resultant polymers exhibited poor solubility in standard organic solvents
(excluding DMF), but exhibited moderate glass transition temperatures and moderate thermal
stabilities. Additionally, the polymers exhibit excellent resistance to hydrolysis. The modularity
of this synthetic approach could be used to rapidly generate diverse polymers with a broad
range of well-tuned properties.
few biobased equivalents to commodity, aromatic polymers5,6
INTRODUCTION
■
have been produced, in stark contrast to the vast number of
In the United States, plastics are typically made from chemicals
derived from hydrocarbon gas liquids (HGLs), 86% of which
biobased aliphatic polymeric materials available.7−9 The
difficulty in the production of these biobased materials results
from a lack of sources for biologically derived aromatic
monomers.
comes from natural gas processing and 14% of which comes
from crude oil refineries.1 In 2017, 13% of total petroleum
products and 3% of total natural gas products were consumed
Lignin is the only naturally occurring, aromatic polymer and
is one of three components of lignocellulosic biomass. Unlike
cellulose and hemicellulose, lignin is regularly isolated and
discarded as a byproduct of several industrial processes. p-
Hydroxycinnamyl alcohol monomers are polymerized in plant
cell walls by an oxidative radical process to produce lignin
which exhibits both ether moieties and carbon−carbon
linkages. Due to the uncontrolled polymerization process and
the presence of functional groups capable of radical chain
transfer, the resulting polymeric structure is poorly defined and
highly heterogeneous. In turn, the heterogeneous polymeric
structure makes utilizing lignin in polymers or composites
difficult. However, as an aromatic polymer, lignin exhibits
desirable properties, such as high strength and good thermal
stability.10−12 While lignin itself is poorly defined, its
monomers (monolignols) are well-defined, aromatic small
molecules, making them ideal starting materials for polymer
synthesis. Lignin is a significant source of potential aromatic
building blocks,13 and identifying and optimizing polymer-
in chemical production and other noncombustion uses.
Although the majority of both gas and oil resources are
currently used for energy and heat (93%), not petrochemical
production, the relative fraction of these fossil fuels consumed
for noncombustion applications is expected to shift in the next
decades due to the rise in electric vehicles and increase in
efficiency of combustion engines.2 This expected shift, along
with expected increased worldwide demand for fossil fuels and
price fluctuations in oil and natural gas drive the demand for
alternative feedstocks for chemical production. Perhaps more
importantly, environmental issues surrounding the isolation of
fossil fuels and the generation of petrochemicals, and the
environmental impact of accumulation of (nondegradable)
plastic waste is also driving increased interest in alternative
feedstocks from sustainable sources and biobased plastics.3
The synthesis of biobased polymers centers on the direct
production of biopolymers or biomonomers from biological
sources.4 Commercial products produced from biobased
polymers are available, but generally exhibit weak thermo-
mechanical properties in comparison to other commodity
materials. Some biobased feedstocks have been identified and
utilized to replace materials that were previously made from
petroleum-derived feedstocks (i.e., poly(lactic acid) clamshell
food containers). However, analogues of specialty or high
performance polymers have yet to be developed. Specifically,
Special Issue: The Rational Design of Multifunctional Renewable-
Resourced Materials
Received: January 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.biomac.9b00044
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1H, 13C, and FTIR Spectroscopy. 1H NMR spectra were
recorded on Bruker AV-300 or Bruker DRX-500 spectrometers at
room temperature in d6-DMSO, unless otherwise noted. Chemical
shifts are reported with respect to internal solvent, 2.50 ppm (d6-
ization methods for these monomers is crucial to developing
high performance biobased aromatic polymers.
Modified monolignol-precursors have been used to synthe-
size polyesters,5,6,14 poly(ester-urethane)s,15,16 and poly-
(anhydride-ester)s.17,18 Before incorporation, monolignols are
commonly reduced to remove the unsaturation in their
aliphatic side chain. In a notable exception Nguyen et al.
compared unsaturated polyesters with their saturated ana-
logues.19 Unsaturated polymers exhibited higher glass
transition temperatures (Tg), presumably due to the limited
rotational freedom about the sp2 carbon−carbon bonds.
Our group recently reported the synthesis and character-
ization of a series of hydroxycinnamate-based poly(ester-
amide)s as biobased, biodegradable analogues of commodity
aramids (Figure 1).20 These aromatic-aliphatic poly(ester-
1
DMSO), 7.16 (C6D6), or 7.26 (CDCl3) for H NMR spectra. 13C
NMR spectra were recorded on a Bruker AV-500 spectrometer with a
dual cryoprobe (13C, 1H). Chemical shifts are reported with respect to
internal solvent, 39.52 ppm (d6-DMSO), 128.06 ppm (C6D6), or
77.16 ppm (CDCl3) for 13C NMR spectra. Infrared absorption
spectra were collected using a Jasco 4210-FT/IR spectrometer from
4000 to 400 cm −1 from KBr pellets. All samples were dried at 100 °C
in a vacuum oven overnight (at least 12 h) prior to characterization.
Molecular Weight Characterization. Molecular weight (Mn and
Mw) and dispersity (Đ = Mw/Mn) were determined using gel
permeation chromatography (GPC). Samples were dissolved in 0.01
M LiBr in N,N-dimethylformamide (DMF) at a concentration of 2−5
mg/mL and were passed through a 0.20 μm PTFE filter before
injection. GPC for all polymers was conducted on a Jasco system
equipped with a refractive index detector, a UV detector, a Waters
Styragel guard column, and four Waters HR Styragel 5 μm columns
(100−5 K, 500−30 K, 50−100 K, 5−600 K) using 0.01 M LiBr in
N,N-dimethylformamide (DMF) at 40 °C and a flow rate of 1.0 mL/
min. Calibration was performed using near-monodisperse polystyrene
standards (Mn = 1250 to 549 000 Da) from Jordi Laboratories, and
chromatograms were analyzed using ChromNAV chromatography
software.
Figure 1. Previously synthesized and characterized monolignol-based
Thermal Characterization. All samples were dried at 100 °C in a
vacuum oven overnight (at least 12 h) prior to all thermal
characterization. Thermogravimetric analysis (TGA) was conducted
on a PerkinElmer Pyris Diamond TG/DTA Thermogravimetric/
Differential Thermal Analyzer. The TGA instrument was operated
under an argon atmosphere, using platinum crucibles. Samples (6−12
mg) were heated from 25 to 800 °C at a rate of 10 °C min/mL. Pyris
Manager was used to analyze the data. Decomposition temperatures
Td5, Td10, and Td25 were measured at 5, 10, and 25% mass loss,
respectively. Differential scanning calorimetry (DSC) was performed
on a PerkinElmer DSC 8000 to determine glass transition
temperature (Tg). Samples (5−8 mg) were heated from −30 to 200
°C at a rate of 10 °C/min and cooled to −30 °C at a rate of 10 °C/
min. A minimum of two heating and cooling cycles were performed
and Tg was measured from the second heating cycle. Pyris Manager
was used to analyze the data.
poly(ester-amide)s.
amide)s were insoluble in common organic solvents, had
moderate thermal stability, and were generally amorphous.
While these polymers had useful physical properties, they were
subject to hydrolytic degradation over an extended period of
time due to the ester linkages. We therefore designed biobased
poly(ether-amide)s by replacing the ester linkages in the
polymer backbone with ether linkages to prevent hydrolytic
degradation, improve the thermal properties, and improve
polymer solubility. In this report, we describe the synthesis and
characterization of a series of 21 poly(ether-amide)s from three
hydroxycinnamate-based ether dimers and a series of seven
aliphatic or aromatic diamines, including their resulting
thermal properties and hydrolytic stability.
Tert-Butyl Ether Dimer Synthesis. p-Coumaroyl tert-Butyl
Ether Dimer (3a). p-Coumaroyl tert-butyl ester (4.04 g, 18.4 mmol,
2.2 equiv) was weighed and added to a two-necked round-bottom
flask along with 30 mL of N,N-dimethylformamide (DMF). The
reaction mixture was cooled to 0 °C and sodium hydride (0.44 g, 18.4
mmol, 2.2 equiv) was added portionwise. After 30 min, a solution of
1,4-dibromobutane (1.0 mL, 8.3 mmol, 1 equiv)) in 5 mL DMF was
added dropwise over 30 min via an addition funnel. The reaction was
allowed to slowly warm to room temperature and stir overnight. After
12 h at room temperature, the reaction was then heated to reflux for 6
h. The reaction was then cooled to room temperature, diluted with
ethyl acetate (70 mL), washed with deionized water (2 × 70 mL) and
brine (1 × 50 mL). The organic layer was separated, dried over
MgSO4, and volatiles were removed via rotary evaporation to yield the
crude product as an off-white crystalline solid. The crude product was
purified on silica gel via flash column chromatography using a gradient
of 7:3 to 3:2 hexanes:ethyl acetate as the eluent. Yield: 2.08 g, 50.4%.
1H NMR (300 MHz, C6D6): δH 7.88 (d, 1H, CCHCHCOOtBu),
EXPERIMENTAL SECTION
■
Materials. 4-Hydroxybenzaldehyde (98%, TCI America), vanillin
(99%, Alfa Aesar), 3,5-dimethoxy-4-hydroxybenzaldehyde (98%,
Acros), malonic acid (99%, Alfa Aesar), tert-butanol (99%, Alfa
Aesar), acetic anhydride (99.5%, Fisher), acetone (ACS grade,
Fisher), sulfuric acid (conc., ACS grade, Fisher), piperidine (99%,
Spectrum), trifluoroacetic acid (99.5%, Fisher), 1,4-diaminobutane
(98+%, Alfa Aesar), 1,8-diaminooctane (98%, Acros), 1,10-
diaminodecane (97%, Acros), p-phenylenediamine (99+%, Acros),
m-phenylenediamine (99+%, Acros), sodium hydroxide (ACS grade,
Fisher), and N,N-dimethylformamide (HPLC grade, Alfa Aesar) were
purchased and used as received. Pyridine (99%, Fisher) was distilled
over KOH prior to use. Dichloromethane (ACS grade, Fisher) was
distilled from CaH2 prior to use. Hexamethylene diamine (99.5%,
Acros) was sublimed under reduced pressure prior to use.
Ethylenediamine (98%, Acros) was distilled prior to use to remove
discoloration. Meldrum’s acid,21 tert-butyl malonate,22 and 1,4-
dibromobutane23 were synthesized according to published proce-
dures. The synthesis of the p-coumaroyl tert-butyl ester (2a), feruloyl
tert-butyl ester (2b) and sinapoyl tert-butyl ester (2c) were previously
reported.20 Silica gel 60 (230−400 mesh, Fisher) was used for column
chromatography. Thin layer chromatography (TLC) was conducted
with silica gel 60-F245 plates and visualized with a hand-held UV
lamp. NMR solvents d6-DMSO, CDCl3, and C6D6 were obtained
from Cambridge Isotope Laboratories and used as received.
7.15 (d, 2H, ArH), 6.63 (d, 2H, ArH), 6.45 (d, 1H, CCH
CHCOOtBu), 3.49 (br s, 2H, −OCH2CH2), 1.60 (br s, 2H,
−OCH2CH2), 1.51 (s, 9H, C(CH3)3). 13C NMR (500 MHz, C6D6):
δC 166.72 (COOC(CH3)3), 161.18 (COCH2CH2), 143.82 (CH
CHCOOtBu), 130.19 (Ar), 128.39 (CCHCHCOOtBu, under
C6D6), 118.75 (CHCHCOOtBu), 115.32 (Ar), 80.04 (C(CH3)3),
67.66 (COCH2CH2), 28.66 (C(CH3)3), 26.34 (COCH2CH2).
Feruloyl tert-Butyl Ether Dimer (3b). Yield: 2.10 g, 52.5%. 1H
NMR (300 MHz, C6D6): δH 8.04 (d, 1H, CCHCHCOOtBu), 6.97
(d, 1H, ArH), 6.94 (s, 1H, ArH), 6.63 (d+d, 2H, ArH and
B
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Scheme 1. Synthesis of Hydroxycinnamate-Based Ether Dimers from Precursor Aldehydes 1a, 1b, and 1c
CCHCHCOOtBu), 3.70 (br s, 2H, OCH2CH2), 3.38 (s, 3H, Ar
OCH3), 1.84 (br s, 2H, OCH2CH2), 1.64 (s, 9H, C(CH3)3). 13C
NMR (500 MHz, C6D6): δC 166.75 (COOtBu), 151.56 (C
OCH3), 150.58 (COCH2CH2), 144.43 (CCHCHCOOtBu),
128.37 (CCHCHCOOtBu), 122.69 (Ar), 118.61 (Ar), 113.10
(CCHCHCOOtBu), 110.81 (Ar), 80.08 (OC(CH3)3), 68.65
(COCH2CH2), 55.56 (COCH3), 28.67 (OC(CH3)3), 26.60
(COCH2CH2).
Thionyl chloride (20 equiv) was added to the flask along with a drop
of DMF, and the reaction was heated to reflux for 6 h. Volatiles were
removed under vacuum to yield the diacid chloride dimer. The
desired diamine (3 equiv) was weighed and added to a 100 mL beaker
along with 15 mL of 0.5 M NaOH and a stir bar. The isolated diacid
chloride dimer was dissolved in 15 mL of distilled dichloromethane
and added quickly to the rapidly stirring diamine solution. A solid
precipitate formed immediately and the resulting slurry was allowed
to stir for 2 min. The slurry was transferred to an Eppendorf tube and
centrifuged at 4.4 × 103 rpm for 20 min. The biphasic solution was
decanted from the Eppendorf tube and the residual solid was washed
with water (2 × 15 mL). After each washing, the slurry (water and
polymer) was centrifuged at 4.4 × 103 rpm for 10 min. The resulting
solid was dried under reduced pressure to yield the corresponding
polymer as a yellow solid. Characterization data for each poly(ether-
amide) can be found in Supporting Information.
1
Sinapoyl tert-Butyl Ether Dimer (3c). Yield: 1.98 g, 48.6%. H
NMR (300 MHz, C6D6): δH 7.88 (d, 1H, CCHCHCOOtBu), 6.54
(d+s, 3H, CCHCHCOOtBu and ArH), 4.15 (br m, 2H,
OCH2CH2), 3.27 (s, 6H, ArOCH3), 2.09 (br m, 2H, OCH2CH2),
1.52 (s, 9H, C(CH3)3). 13C NMR (500 MHz, C6D6): δC 166.26
(COOtBu), 154.43 (COCH3), 144.34 (CCHCHCOOtBu),
140.56 (COCH2CH2), 130.14 (CCHCHCOOtBu), 119.52
(CCHCHCOOtBu), 105.82 (Ar), 79.97 (OC(CH3)3), 73.14
(COCH2CH2), 55.68 (COCH3), 28.32 (OC(CH3)3), 27.35
(COCH2CH2).
Diacid Ether Dimer Synthesis. p-Coumaryl Diacid Ether Dimer
(4a). p-Coumaryl tert-butyl ether dimer (2.08 g, 4.2 mmol, 1 equiv)
was weighed and added to a round-bottom flask along with
trifluoroacetic acid (12.9 mL, 168.3 mmol, 40 equiv). Reaction was
stirred at room temperature for 4 h. Volatiles were removed under
vacuum to yield the product as a white solid. Yield: 1.52 g, 94.4%. 1H
NMR (300 MHz, d6-DMSO, sparingly soluble): δH 7.64 (d, 2H,
ArH), 7.56 (d, 1H, CHCHCOOH), 6.98 (d, 2H, ArH), 6.39
(d, 2H, CHCHCOOH), 4.09 (br s, 2H, OCH2CH2), 1.88 (br s,
2H, OCH2CH2). 13C NMR (500 MHz, d6-DMSO, sparingly soluble):
δC 167.84 (COOH), 160.33 (COCH2CH2), 143.78 (CH
CHCOOH), 129.96 (Ar), 126.76 (CCHCHCOOH), 116.44
(CHCHCOOH), 114.83 (Ar), 67.30 (OCH2CH2), 25.29
(OCH2CH2).
RESULTS AND DISCUSSION
■
Dimer Synthesis. On the basis of our previous work
synthesizing a series of hydroxycinnamate-based poly(ester-
amide)s, we designed a set of hydroxycinnamate-based
poly(ether-amide)s to be resistant to hydrolysis, to exhibit
increased solubility in common organic solvents, and to exhibit
lower melting and glass transition temperatures while retaining
thermal stability. A series of three hydroxycinnamate-based
ether dimers were synthesized in three steps from the
precursor aldehydes (1a, 1b, and 1c). As shown in Scheme
1, the tert-butyl protected esters (2a, 2b, and 2c) were
synthesized in one step via a Doebner-modified Knoevenagel
condensation with tert-butyl malonate and 4-hydroxybenzalde-
hyde (1a), vanillin (1b), or 3,5-dimethoxy-4-hydroxybenzalde-
hyde (1c), respectively. The yields for this condensation
reaction were lower than expected, but the yield increased with
increasing methoxy substitution. While the electronic effects of
aldehyde substitution are rarely discussed for Knoevenagel
condensation reactions, the inclusion of electron donating
groups clearly affected the yield of the isolated enolates. The
sodium salt of the protected ester monomers was synthesized
with sodium hydride and then dimerized with 1,4-dibromo-
butane in N,N-dimethylformamide (DMF) via Williamson
etherification. The dimerization reaction did not proceed to
completion at room temperature after 18 h, so the reaction was
subsequently heated to reflux, in an attempt to increase
conversion. After heating to reflux for 6 h, the reaction still did
not reach full conversion. Furthermore, increasing the reaction
time led to the formation of undesired side products.
Alternative methods (such as using a different base to
deprotonate the protected ester monomers) were also
attempted. These modifications increased the yield of side
products (dimers with the addition of multiple ether linkers)
Ferulyl Diacid Ether Dimer (4b). Yield: 1.5433 g, 92.4%. 1H NMR
(300 MHz, d6-DMSO): δH 7.55 (d, 1H, CHCHCOOH), 7.31 (s,
1H, ArH), 7.20 (d, 1H, ArH), 6.99 (d, 1H, ArH), 6.46 (d, 1H,
CHCHCOOH), 4.08 (br s, 2H, OCH2CH2), 3.81 (s, 3H, Ar
OCH3), 1.89 (br s, OCH2CH2). 13C NMR (500 MHz, d6-DMSO,
sparingly soluble): δC 167.90 (COOH), 150.12 (COCH3), 149.09
(COCH2CH2), 144.19 (CCHCHCOOH), 127.02 (CCH
CHCOOH), 122.64 (Ar), 116.65 (Ar), 112.48 (CCHCHCOOH),
110.49 (Ar), 67.92 (OCH2CH2), 55.67 (COCH3), 25.44
(OCH2CH2).
1
Sinapyl Diacid Ether Dimer (4c). Yield: 1.59 g, 97.8%. H NMR
(300 MHz, d6-DMSO): δH 7.56 (d, 1H, CHCHCOOH), 7.02 (s,
2H, ArH), 6.56 (d, 1H, CHCHCOOH), 3.93 (br s, 2H,
OCH2CH2), 3.79 (s, 6H, ArOCH3), 1.78 (br s, 2H, OCH2CH2).
13C NMR (500 MHz, d6-DMSO): δC 167.76 (COOH), 153.27
(COCH3), 144.25 (CCHCHCOOH), 138.43 (COCH2CH2),
129.67 (CCHCHCOOH), 118.37 (CCHCHCOOH), 105.78
(Ar), 72.16 (COCH2CH2), 56.01 (COCH3), 26.18 (COCH2CH2).
Poly(ether-amide) Synthesis. General Polymerization Proce-
dure. All polymers were synthesized according to a previously
developed protocol in our laboratory.20 The desired diacid ether
dimer (1 equiv) was weighed and added to a round-bottom flask.
C
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Scheme 2. Synthesis of Poly(ether-amides) from Monolignol-Based Ether Dimers and Diamine Linkers via Interfacial
Polymerization
rather than the desired ether dimer (Scheme S1). Despite the
lack of full conversion, the desired ether dimers (3a, 3b, and
3c) were achieved in moderate yield (48.6% to 52.5%, 1H and
13C NMR Figures S1−S6) after purification via column
chromatography. Additionally, the unreacted tert-butyl ester
monomers were also recovered via column chromatography,
allowing their use in later reactions. The pure ether dimers
were then deprotected using trifluororoacetic acid (TFA), to
yield 4a, 4b, and 4c, in nearly quantitative yield (1H and 13C
NMR Figures S7−S12). Similar to the previously synthesized
ester analogues, a key structural property of these ether dimers
is the retention of the degree of unsaturation in their aliphatic
side chain. The retention of this functionality should increase
the thermal stability and glass transition temperature of the
resulting poly(ether-amide)s.19 We hypothesize that the
balance between the rigidity associated with carbon−carbon
double bond and the flexibility associated with ether linkages
should generate a series of materials with tunable physical
properties.
Poly(ether-amide) Synthesis. In the literature, poly-
(ether-amide)s are commonly aliphatic block copolymers
synthesized by condensation reactions of aliphatic polyether
and polyamide segments. While aliphatic materials are useful,
they typically feature low thermal stability and low Tg values,
due to high chain flexibility and minimal interchain
interactions. In contrast to these materials, we were interested
in isolating aromatic-aliphatic poly(ether-amide)s, as biobased
mimics to aramids, such as Kevlar or Nomex. We hypothesized
utilizing hydroxycinnamate-based dimers would yield polymers
with physical strength similar to aramids, while utilizing
aliphatic diamine linkers would increase the solubility and
processability of the isolated polymers. Commercial aramids
are typically produced via solution-phase methods, however we
were unable to identify a common solvent for the dimer and
the diamine. We therefore utilized interfacial polymerization
for poly(ether-amide) synthesis. Interfacial polymerization is
an ideal polymerization technique as it does not require strict
stoichiometric conditions, is less sensitive to impurities,
requires short reaction times, and is commonly performed
under mild reaction conditions.24 Two immiscible monomer
solutions, typically one aqueous and one nonaqueous, are
prepared and carefully combined for polycondensation either
by drawing a fiber from the interface,25 or as a precipitate from
a vigorously stirring solution. Acid chloride hydrolysis is the
largest limiting factor for this polymerization method, however
due to the hydrophobic nature of the aromatic carboxylic acid
ether dimer used, hydrolysis should be minimal. We initially
attempted to draw fibers from the interface between the two
solutions, but the poly(ether-amide)s produced were too
brittle for isolation by this method. Vigorous stirring was
required during polycondensation to form high molecular
weight poly(ether-amide)s,26 similar to what was required for
synthesis of the poly(ester-amide)s reported previously.20
Three acid chloride ether dimers were generated in situ by
refluxing the carboxylic acid ether dimer in thionyl chloride for
6 h. After the removal of excess thionyl chloride, the acid
chloride dimers were then dissolved in distilled dichloro-
methane and added to a dilute solution of sodium hydroxide
containing the desired diamine (Scheme 2). The condensation
polymerizations yielded twenty-one different aromatic-aliphatic
poly(ether-amide)s. The resulting polymers were insoluble in
most common organic solvents, but were sparingly soluble in
DMF at room temperature. Poly(ether-amide)s from p-
coumaryl acid and aliphatic diamines (P1−P5) featured
exceedingly limited solubility in DMF, due to the lack of
methoxy substitution. Even after Soxhlet extraction in DMF for
24 h, P1−P5 remained largely intractable. Due to poly(ether-
amide) insolubility, polymerization was confirmed (P1−P21)
using FT-IR spectroscopy. As shown in Figure 2, the CO
stretch in the ferulic acid carboxylic acid ether monomer
shifted from ∼1786 cm−1 to 1650−1660 cm−1 in P8−P14,
confirming formation of the amide linkage. Additionally, N
H bending was seen from 1530 to 1560 cm−1 and CO
stretching was seen from 1033 to 968 cm−1, confirming
formation of amide bonds as well as the retention of the ether
dimer throughout the polymerization reaction. Similar
stretching frequencies can be seen in both the sinapyl and p-
coumaryl-based polymers (Figures S13 and S14), confirming
formation of the series of poly(ether-amide)s. The solubility of
the poly(ether amides) varied with hydroxycinnamate
substitution (increasing substitution on the aromatic ring
increased solubilty, measured as the weight fraction of the
D
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crude product that is soluble (yields listed in Supporting
Information). One of the goals of our design was to improve
the solubility of these biobased polymers compared to the
previsouly synthesized poly(ester-amide)s. While the coumar-
yl-based poly(ether-amide)s were generally less soluble than
their poly(ester-amide) analogues, both the ferulyl and sinapyl
poly(ether-amide)s were more soluble than their poly(ester-
amide) analogues (Table S1, Supporting Information).
Molecular Weight Characterization. The number and
weight-average molecular weights (Mn and Mw, respectively) of
P1−P21 were determined using gel permeation chromatog-
raphy (GPC) (Table 1, Figures S15−S17). The isolated
poly(ether-amide)s were sparingly soluble in DMF, with a
large fraction remaining intractable (20−82 wt % insoluble,
Table S1). This insolubility suggests that the molecular weights
of the poly(ether-amide)s are higher than those determined by
GPC, as GPC only characterizes the soluble fraction. This is
clearly demonstrated in polymers P1−P5, which exhibited
extremely low molecular weights by GPC (Mn: 3 to 6 kDa)
when the polymers were added to DMF at room temperature.
Kwolek and Morgan observed a similar trend in the synthesis
and characterization of aromatic-aliphatic polyamides.27 In
their work, polyamides containing aliphatic linkers less than
nine carbons in length or aromatic units without functional
groups were insoluble in all common organic solvents, even
DMF. We attempted to dissolve suspensions of the aromatic-
aliphatic p-coumaryl-based polymers using several approaches.
Boiling DMF was added to each polymer sample and stirred
until the suspension had cooled to room temperature. After
filtration, GPC traces showed that the addition of heated DMF
Figure 2. IR fingerprint region of P8−P14 and the corresponding
ferulyl dimer starting material showing the formation of the amide
linkage and presence of the ether linkage in the dimer.
Table 1. Molecular Weight Characterization of Poly(ether-amide)s by Gel Permeation Chromatography
a
a
b
Polymer
P1
Dimer
Diamine
Mn
Mw
Đ
p-Coumaryl
Ethylene diamine
3,700
6,740
3,950
5,620
6,330
6,280
6,410
8,110
3,650
4,060
5,060
11,020
5,680
15,780
9,240
16,500
9,130
1.37
1.64
1.44
2.80
1.46
2.62
1.42
4.73
1.20
1.26
1.43
1.23
1.94
2.03
1.98
1.58
2.08
1.55
1.34
1.79
1.81
1.71
2.16
1.40
2.38
2.29
c
c
c
c
c
c
c
c
c
c
c
c
c
c
P2
P3
P4
P5
1,4-Diaminobutane
Hexamethylenediamine
1,8-Diaminooctane
1,10-Diaminodecane
38,360
4,380
c
5,130
P6
P7
P8
P9
m-Phenylenediamine
p-Phenylenediamine
Ethylene diamine
1,4-Diaminobutane
Hexamethylenediamine
1,8-Diaminooctane
1,10-Diaminodecane
m-Phenylenediamine
p-Phenylenediamine
Ethylene diamine
1,4-Diaminobutane
Hexamethylenediamine
1,8-Diaminooctane
1,10-Diaminodecane
m-Phenylenediamine
p-Phenylenediamine
15,210
7,690
21,800
9,460
Ferulyl
Sinapyl
12,780
18,490
19,420
13,870
12,800
12,920
12,220
26,640
28,270
28,630
12,160
7,550
24,830
37,470
38,510
21,900
26,630
20,080
16,430
47,560
51,060
49,050
26,300
10,600
34,730
19,290
P10
P11
P12
P13
P14
P15
P16
P17
P18
P19
P20
P21
14,580
8,430
a
b
c
Determined by GPC, 1.0 mL min−1, 0.01 M LiBr in DMF. Đ = Mw/Mn. Molecular weight determined after Soxhlet extraction, 24 h, with 0.01 M
LiBr in DMF.
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resulted in a slight increase in the solubility of higher molecular
weight fractions (Figure 3). To determine if additional heating
extremely low Tgs. In contrast to this generalization, the
poly(ether-amide)s synthesized featured moderately high Tg
values, as shown in Table 2. The glass transition temperature
Table 2. Thermal Characterization of Poly(ether-amide)s by
DSC and TGA
T
Td5
bc
T
T
d2b5 e
g a
d1b0 d
,
,
,
Polymer
(°C)
(°C)
(°C)
(°C)
p-Coumaryl
P1
P2
P3
P4
P5
P6
P7
P8
90.8
91.8
84.3
80.6
66.2
243.6
213.3
238.3
254.4
121.6
150.3
139.1
143.9
123.5
170.9
194.2
123.3
216.5
233.7
226.5
133.5
106.2
121.6
177.8
170.8
204.8
290.3
280.9
294.0
291.7
142.4
175.9
185.1
276.7
251.9
241.6
264.8
157.6
317.9
310.9
271.4
225.5
166.7
154.5
254.8
192.8
247.4
335.8
327.0
340.4
328.1
301.0
410.8
368.9
344.5
353.9
317.9
310.8
307.5
404.3
386.5
317.7
350.3
289.4
328.3
309.0
248.8
333.7
79.1
Figure 3. GPC traces of P1 using different extraction techniques to
improve high molecular weight polymer solubility.
112.3
103.1
92.4
98.1
110.1
82.3
108.3
124.7
106.9
102.3
85.2
Ferulyl
P9
improved solubility, a 24 h extraction with a Soxhlet extractor
was performed. Samples prepared via this method exhibited a
significant increase in the observed molecular weight of P1−
P4, as shown in Table 1. However, even after utilizing Soxhlet
extraction, molecular weights observed for P5 were still low.
Limited molecular weights were also observed for ferulyl and
sinapyl-based polymers synthesized with 1,10-diaminodecane,
P12 and P19 respectively. Because P19 was the most soluble of
the series, its overall molecular weight (soluble + insoluble)
was likely lower than P5 or P12, which had larger insoluble
fractions. One explanation for these lower molecular weights is
the reduced solubility of 1,10-diaminodecane in the aqueous
solution, as only lower molecular weight polymers were
isolated for P5, P12, and P19. Similar results were seen in our
previously studied poly(ester-amide) system.20
P10
P11
P12
P13
P14
P15
P16
P17
P18
P19
P20
P21
Sinapyl
84.9
77.9
84.8
81.1
a
b
c
Determined by DSC. Determined by TGA. Td5 = temperature at
d
e
5% mass loss. Td10 = temperature at 10% mass loss. Td25
temperature at 25% mass loss.
=
Generally, the poly(ether-amide)s exhibited weight average
molecular weights (Mw) between 20 and 40 kDa and
dispersities between 1.5 and 2.2. The highest molecular weight
polymers were produced from sinapyl acid-based ether dimers,
likely due to increased solubility with multiple aromatic
methoxy substituents. The broad series of polymers obtained
show the effectiveness of this polymerization method for the
rapid generation of a series of polymers with tunable
properties. Based on the polymerization method used, the
dispersity of the resulting materials is not well controlled.
However, values obtained are within acceptable values for
commercial applications, suggesting that this polymerization
method could be appropriate for use in an industrial setting.
Thermal Properties. The thermal behavior of polymers
P1−P21 were studied using both differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA)
(Figures S18−S23). Generally, aromatic polymers exhibit
higher glass transition temperatures (Tg) than aliphatic
polymers due to a lack of rotation about bonds between
aromatic monomers, in comparison to aliphatic sp3 bonds.
This lack of rotational freedom causes reduced chain mobility
and backbone flexibility. Additionally, with the inclusion of
bulky side groups, Tg should also increase, as these
modifications cause decreased chain mobility by limiting
rotation about bond in the backbone. Because the polymers in
this study incorporate both aliphatic and aromatic compo-
nents, and because the degree of methoxy substitution is also
systematically varied, structural effects on Tg are complex.
All of the poly(ether-amide)s were amorphous by DSC, as
only a single glass transition temperature (Tg) was observed.
Due to high backbone flexibility, polyethers typically exhibit
(Tg) should decrease as the length of the aliphatic spacer
increases due to increased chain mobility and backbone
flexibility, while the incorporation of aromatic diamines as
linkers should increase the observed Tg. While these trends are
generally seen throughout the series of isolated poly(ether-
amide)s, there are also several significant exceptions.
As shown in Figure 4, both the length of the aliphatic linker,
as well as the identity of the hydroxycinnamate dimer, have
large effects on the observed Tg. The incorporation of
substituted aromatic dimers (sinapyl-based dimers instead of
p-coumaryl-based dimers), increases the observed Tg by
limiting the rotational flexibility of the polymer chains. As
expected, Tg generally increased with increasing degrees of
Figure 4. Glass transition temperature as a function of aliphatic linker
length.
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Figure 5. Hydrolytic stability of P15 (left) and P17 (right) in 1 M aqueous HCl; GPC traces as a function of time (0−21 days).
methoxy substitution, as seen in the overall increase in Tg
between P1−P7 and P15−P21. Consistent with our previous
study, poly(ether-amide)s with long aliphatic linkers (such as
hexamethylenediamine, 1,8-diaminooctane, 1,10-diaminode-
cane) exhibited Tg higher than expected, as seen in P10 and
P11. These deviations in glass transition temperature may be
due increased chain interactions, which prevent polymer chain
movement. Wang et al. saw a similar effect in the synthesis of
poly(ether-amide)s with polyethylene glycol spacers.28 The
observed Tg increased as the length of the polyethylene glycol
spacer increased, possibly due to increased interchain
interactions. However, it is difficult to ascribe a definitive
cause to this increased Tg, as the samples also likely differ in
molecular weight (as discussed below). In addition to effects
seen by the incorporation of longer aliphatic spacers, the
substitution pattern of aromatic diamines also significantly
affected the observed glass transition temperature. The use of
m-phenylenediamine instead of p-phenylenediamine showed a
decrease in the observed Tg,, seen in a comparisons between
P13 and P14. The meta-substitution pattern causes a nonlinear
structure, which will minimize π−π interactions and prevent
the formation of strong hydrogen bonds between chains. In
turn, the reduced interchain interactions cause a significant
reduction in the observed Tg.
Molecular weight also played a significant role in the
observed Tg of the poly(ether-amide)s. Polymer chain ends
have more rotational freedom than interchain units, exhibiting
greater free volume. A larger concentration of chain ends in a
polymer sample (by the presence of smaller molecular weight
polymers or a greater fraction of smaller molecular weight
polymers) will lower the observed Tg, as free volume is
inversely proportional to glass transition temperature. With
this in mind, a large fraction of smaller molecular weight
polymers in a nonmonodisperse sample can have effects similar
to that observed with the addition of plasticizers. As
exemplified by their higher dispersity, P9 and P12 feature an
increased fraction of smaller molecular weight polymers, which
yields lower glass transition temperatures than would expected
based on the length of their linker and Tg observed for other
ferulyl-based polymers. As seen in Table 2, P11 exhibited a
higher Tg than P18. While both polymers are synthesized with
1,8-diaminooctane, P11 is synthesized with the ferulyl-based
dimer (single methoxy substitution) and should feature a lower
glass transition temperature than P18. However, P11 exhibits a
weight-average molecular weight more than 5 kDa greater than
P18. This difference in molecular weight, and thus decreased
number of chain ends present, may be responsible for the
observed increase in Tg. Additionally, P18 is more polydisperse
with a dispersity of 2.16, in comparison to P11 which features a
dispersity of 1.58.
The thermal stability of the poly(ether-amides) was studied
using thermogravimetric analysis (TGA). Polymer samples
were heated from 30 to 800 °C under a nitrogen atmosphere
(Figures S18−S20). Most of the polymers in this series
featured initial mass loss (Td5) between 120 and 210 °C, lower
temperatures than would be typically expected for these
materials (Table 2). Despite efforts to remove water prior to
the thermal decomposition studies, water loss was likely the
cause of low initial decomposition temperatures. As these
materials contain amide linkages, they are more hygroscopic
that pure hydrocarbons and the retention of water is not
surprising. The hypothesis that the presence of water in the
polymer samples results in a low (Td5), as a much higher
temperature is required to reach 25% mass loss (Td25). For
example, P6 exhibits an initial decomposition temperature
(Td5) of 150.3 °C but requires the polymer sample to be
heated to 410.8 °C to cause 25% mass loss. The length of the
aliphatic diamine linker appeared to have no significant effect
on the thermal stability of the poly(ether-amide)s. The
incorporation of aromatic diamine linkers also did not have a
substantial effect on thermal stability, regardless of the pattern
of substitution. Overall, the polymers have mild thermal
stability and potential for use in melt processing.29 Prior to
large scale thermal processing, these poly(ether-amide)s
should undergo more rigorous purification to remove residual
water. After removal of the residual water, these materials will
likely exhibit increased thermal stability in contrast to the data
discussed here.
Hydrolytic Stability. While ester linkages are susceptible
to hydrolysis over extended time scales, the degradation of
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amide bonds is slow enough that they are typically considered
nondegradable in aqueous conditions. Additionally, the
incorporation of an ether linkage into the hydroxycinnamate
dimer eliminates hydrolysis of the polymer backbone as a
degradation mechanism, as ethers are only cleavable under
harsh chemical conditions. In our previous report, the
hydroxycinnamate-based poly(ester-amide)s underwent hydro-
lytic degradation under both neutral and mildly acidic pH (pH
7 and 5, respectively). Chemical structure affected the rate of
hydrolysis; for example, polymers synthesized with aliphatic
diamine linkers degraded more rapidly than those with
aromatic diamine linkers. To test the hydrolytic stability of
the poly(ether-amide)s, two representative samples (P15 and
P17) were suspended in 1 M HCl and monitored by GPC.
Over the course of 3 weeks, no significant changes were
observed in GPC, indicating that the samples are hydrolytically
stable. Notably, these conditions were much harsher than
those applied to the previously reported poly(ester-amide)s,
which underwent degradation even at neutral pH (Figure 5).
The purpose of this paper was to demonstrate hydrolytically
stable polymeric materials containing aromatic groups from
starting materials derived from biomass, and to improve their
solubility compared to our previous report. While we did
achieve increased solubility for the polymers incorporating
ferulic acid and sinapinic acid, on average roughly 35% of the
sample was still insoluble. In order for these polymers and/or
our previously reported poly(ester amide)s incorporating
hydroxycinnamates to be commercially appealing, several
advances must be considered. First, while pathways exist for
producing the phenolic aldehydes used as starting material in
this work, the reported methods are not yet sufficiently
efficient to ensure both low cost and low environmental
impact. Nonetheless, this is an active research area and likely to
advance in the coming decade. Second, the synthesis of the
monomers, dimers and polymers followed traditional organic
chemistry approaches, and will need to be modified and
optimized using green solvents and reagents. Third, the
intractability of the products limited their characterization.
The limited solubility prevented processing of the polymers
from powders into fibers or films, and thus the physicome-
chanical properties remain unknown. Chemical modifications
to the polymer chains to enhance their solubility are necessary,
and may be achieved using the unsaturated bond in the
hydroxycinnamates.
has provided a unique set of functional materials, with the
prospect of their use in the development of other novel
materials.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.bio-
mac.9b00044.
■
S
1H and 13C NMR spectra, polymerization yields and
solubility, polymer infrared spectroscopy, GPC traces,
DSC traces, TGA traces, decomposition study procedure
(PDF)
AUTHOR INFORMATION
Corresponding Author
*Andrea M. Kasko. E-mail: akasko@ucla.edu.
■
ORCID
Brianna M. Upton: 0000-0002-9408-5265
Andrea M. Kasko: 0000-0003-2355-6258
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.M.U. acknowledges the UCLA Department of Chemistry for
a Departmental Graduate Research Fellowship and UCLA
Graduate Division for a Dissertation Year Fellowship.
ABBREVIATIONS
Đ = Mw/Mn, dispersity
■
Mn, Number-average molecular weight
Mw, Weight-average molecular weight
Tg, glass transition temperature
DMF, N,N-dimethylformamide
DMSO, dimethyl sulfoxide
DSC, Differential scanning calorimetry
GPC, gel permeation chromatography
kDa, kilodalton
NMR, nuclear magnetic resonance spectroscopy
PTFE, poly(tetrafluoroethylene)
TFA, trifluororoacetic acid
TGA, Thermogravimetric analysis
TLC, thin layer chromatography
UV, ultraviolet
CONCLUSIONS
■
We have described the synthesis of a series of aromatic-
aliphatic poly(ether-amide)s from hydroxycinnamate-based
ether dimers and various aliphatic and aromatic diamines.
Aliphatic diamines were chosen as comonomers to improve the
solubility and processability of hydroxycinnamate-based
polymers. Additionally, the five aliphatic diamines used in
this study are all available from biological sources. While the
isolated poly(ether-amide)s were insoluble in standard organic
solvents (excluding DMF), the moderate glass transition
temperatures and thermal stability observed offer promise for
these materials in melt processing applications. Additionally,
the polymers exhibit excellent resistance to hydrolysis. The
modularity of this synthetic approach could be used to rapidly
generate diverse polymers with a broad range of well-tuned
properties. The versatility of this method increases the
potential applications for this polymer system industrially.
This targeted upgrading of underutilized biobased materials
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