Synthesis of biorenewable and water-degradable polylactam esters from itaconic acid

Article abstract of DOI:10.1039/c6gc01081d

Itaconic acid, a naturally occurring compound mass-produced via fermentation of glucose, was reacted with ethanolamine or ethylene diamine to afford hydroxy-acid or diacid monomers containing the 2-pyrrolidone lactam. Homopolymerization or copolymerization with diols, respectively, yielded polylactam esters with higher glass transition temperatures and faster hydrolytic degradation compared to the commercial polyester polylactic acid (PLA).

Full text of DOI:10.1039/c6gc01081d

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Synthesis of biorenewable and water-degradable
polylactam esters from itaconic acid†
Cite this: DOI: 10.1039/c6gc01081d
Pengxu Qi,a Hsiao-Li Chen,a,b Ha Thi Hoang Nguyen,a Chu-Chieh Linb and
Received 15th April 2016,
Accepted 16th June 2016
Stephen A. Miller*a
DOI: 10.1039/c6gc01081d
www.rsc.org/greenchem
Itaconic acid, a naturally occurring compound mass-produced via is derived from bio-ethylene glycol and petrochemical ter-
fermentation of glucose, was reacted with ethanolamine or ethylene ephthalic acid.13 Perhaps the most successful, fully biorenew-
diamine to afford hydroxy-acid or diacid monomers containing the able polyester is polylactic acid (PLA), made from corn starch
2-pyrrolidone lactam. Homopolymerization or copolymerization or other carbohydrates.14 While PLA is 100% biobased, its
with diols, respectively, yielded polylactam esters with higher glass useful temperature range is limited by its low glass transition
transition temperatures and faster hydrolytic degradation compared temperature (Tg) of 50 °C15 and it composts very poorly in the
to the commercial polyester polylactic acid (PLA).
environment,16 thus requiring industrial composting con-
ditions.17 PET exhibits a notably higher Tg of 67 °C,18 making it
suitable for a wider range of applications. The superior thermal
properties seem to originate with the aromatic/aliphatic motif
present in the polymer backbone (Fig. 1). Recreating this hard/-
soft motif with bioaromatics from lignin (vanillin) or ligno-
cellulose (ferulic acid, coumaric acid), our research group19–22
has reported several new biogenic polyesters and copolyesters
with Tg values ranging from 78 °C to 153 °C,22 surpassing the
important benchmark of 100 °C for polystyrene (PS).23
As the commercial plastics industry was born and thrived in
the last century, a great portion of finite resources was dedi-
cated to polymer production. A handful of commercial poly-
mers—poly(ethylene terephthalate) (PET), polyethylene (PE),
polyvinylchloride (PVC), polypropylene (PP), polystyrene (PS),
and polycarbonate (PC)—have dominated the plastics market
for many decades because of their low cost and unique
thermal and mechanical properties.1 However, the overutiliza-
tion of these materials has revealed dire environmental issues.
The depletion of fossil fuel resources, along with persistence
in the environment—especially the oceans—have stimulated
academic and industrial research programs that seek degrad-
able polymers derived from biorenewable resources.2–4 Such
alternatives to incumbent polymers could appreciably improve
the sustainability of the short-term packaging industry.5
In our continued pursuit of high Tg thermoplastics, we
reasoned that certain biobased heterocycles might replace our
Among a wide variety of linear polymers, polyesters have
great potential because (1) structural diversity allows tailoring
of their properties, (2) the main-chain ester functionality is an
obvious point of water-degradation or bio-degradation, and (3)
many are accessible from natural building blocks.6–9 Thus,
considerable research has focused on partially or completely
biobased polyesters.4,10–12 The fastest growing, partially biore-
newable polyester is 30% biobased PET (PlantBottle®), which
aThe 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
bDepartment of Chemistry, National Chung Hsing University, Taichung City, 402,
Taiwan. E-mail: cchlin@dragon.nchu.edu.tw; Tel: +886-0422-840411
†Electronic supplementary information (ESI) available: Synthetic details, com-
plete polymer characterization data, and polymer degradation data. See DOI:
10.1039/c6gc01081d
Fig. 1 The glass transition temperature (Tg) of polylactic acid (PLA) can
be excelled by incorporating aromatic (PET), bioaromatic (PHFA), or bio-
heterocyclic (PEPC) motifs into the polyester main-chain.
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chosen bioaromatics and confer the polymer chain rigidity for this reaction, but water proved to give the highest yield
and/or chain-chain interactions that are essential for high (71% after recrystallization). No catalyst was necessary since
glass transition temperature materials.24 Herein, we have ethanolamine was suitably active. Note that ethanolamine can
focused on the 2-pyrrolidone ring system to function as the be made in three steps from bioethanol or biosynthetically
hard component of a repeating hard/soft motif. This lactam from serine.44
was selected for several reasons. It can be prepared by reaction
The HEPC monomer was subjected to a solvent-free, two-
of a primary amine with the inexpensive and naturally-occur- stage melt-polymerization protocol with a variety of catalysts.
ring molecule, itaconic acid (IA). The α,β-unsaturated carbonyl A pre-polymerization stage under nitrogen at 180 °C commenced
functional group of IA reacts efficiently with a variety of oligomerization and thus minimized loss of monomer to evapor-
primary amine nucleophiles in environmentally acceptable sol- ation during the second stage, which ramped to 240 °C with
vents with simple workup procedures.25–28 The five-carbon dynamic vacuum. Removal of the water byproduct ensured the
diacid IA has been promulgated as one of the top twelve renew- condensation polyesterification reaction (Scheme 1).
able chemicals available from biomass by the U.S. Department
Table 1 shows the polymerization results from a survey of
of Energy National Renewable Energy Laboratory because of its eight different catalysts, including Brønsted acids (entries
scalability, sustainability, and nontoxicity.29 It is presently 1–3), Brønsted bases (entries 4–5), and Lewis acids (entries
mass-produced by glucose fermentation with Aspergillus 6–11). Considering the polymer yield and molecular weight
terreus.30 This biosynthetic method yields 40 to 80 million kg achieved with 1.0 mol% catalyst loading, the Brønsted bases
per year31 with a cost near $1.5 USD per kg.32 Previous excelled the Brønsted acids. But, the overall winner was Lewis
polymerization research with IA has generally followed two acid Sb2O3 (entry 8) with 78% yield, Mn = 15 100, and PDI =
approaches:33 polycondensation of itaconic acid with diols to 2.7. The optimal loading of this catalyst was 2.0 mol%, which
yield polyesters;34–37 and radical homopolymerization or co- gave 84% yield, Mn = 20 800, and PDI = 2.1 (entry 9). Other
polymerization of dialkyl itaconate with other unsaturated reaction conditions were varied, but did not improve the
monomers to prepare acrylic polymers.38–41 While polyamides polymerization results. For example, a longer or hotter pre-
from itaconic acid containing the pyrrolidone ring system have polymerization stage did not increase molecular weights. And
been reported,42 it appears that polyesters containing this higher ultimate polymerization temperatures (>240 °C) gener-
lactam are novel. We predicted that such polylactam esters ally resulted in lower molecular weights and broader PDI
might be more degradable than the reported polylactam values, possibly a consequence of decomposition.
amides because of the greater hydrolytic sensitivity of esters
Scheme
2 describes another strategy for synthesizing
versus amides, generally.43
polylactam esters. In this case, two equivalents of itaconic acid
Polyethylene 2-pyrrolidone-4-carboxylate (PEPC, Fig. 1) was (IA) are reacted with ethylene diamine in water to afford a
selected as the paradigm polylactam ester because of its struc- monomer bearing two lactam rings, ethylene bis(pyrrolidone
tural similarity to PET and polydihydroferulic acid (PHFA).19 carboxylic acid) (EBPC, 78% yield). This diacid monomer was
For these three polymers, each repeat unit contains a rigid copolymerized with several α,ω-alkanediols, HO(CH2)nOH,
ring, two main-chain sp3 hybridized carbons, and ester con- where n = 2, 3, 4, 5, or 6 to give a homologous series of
nectivity (Fig. 1). Scheme 1 describes the aza-Michael addition polyalkylene ethylene bis(pyrrolidone carboxylate) (PAEBPC)
of ethanolamine to itaconic acid (IA), affording an intermedi- polymers. Table 2 summarizes the polymerization results and
ate amino diacid. Intramolecular cyclization generates water compares the formed polymers as a function of the number of
and the γ-lactam ring, which is part of the targeted monomer
N-(2-hydroxyethyl)-2-pyrrolidone-4-carboxylic acid (HEPC),
formed as the racemate. Several polar solvents were explored
Table 1 Conditions for polymerizing HEPC to PEPC (see Scheme 1)a
Catalyst
(mol%)
Yieldb
(%)
Mn
b
Entry
Catalyst
(g mol−1
)
PDIb
1
2
3
4
5
6
7
8
p-TSA
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
0.5
56
42
24
63
80
60
81
78
84
73
67
9800
7200
5600
3.2
2.9
6.4
3.7
2.7
3.4
2.6
2.7
2.1
2.3
2.9
Amberlyst®c
H2SO4
K2CO3
Na2HPO4
MgO
11 400
13 800
10 200
5200
15 100
20 800
18 200
6000
Sn(Oct)2
Sb2O3
Sb2O3
Sb2O3
Sb2O3
9
10
11
a 180 °C under nitrogen for 4 hours, followed by a temperature ramp
over 8 hours to 240 °C with dynamic vacuum. b GPC in hexafluoroiso-
propanol (HFIP) at 40 °C vs. polymethyl methacrylate standards.
c Amberlyst® 15 hydrogen form (dry).
Scheme 1 The aza-Michael addition of ethanolamine to itaconic acid
(green) followed by intramolecular cyclization affords racemic HEPC, a
lactam/hydroxy-acid suitable for polymerization to the polylactam ester,
PEPC, via Fischer esterification.
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methylene spacers, n, present in the diol. These polymeriz-
ations employed the optimized conditions of Table 1: no
solvent and 2.0 mol% Sb2O3 catalyst with a temperature ramp
from 180 to 240 °C.
Table 2, entry 2 shows the result when the diacid : diol ratio
is 1.0 : 1.0. In this case, the yield (63%) and molecular weight
(Mn = 10 000) are relatively low, and the PDI is somewhat broad
(PDI = 4.1). Entry 3 shows that these numbers can be improved
(yield = 88%; Mn = 23 200; PDI = 2.3) when a slight excess of
diol is employed. Quite plausibly, the diacid : diol ratio of
1.0 : 1.2 ensures the incorporation of ethylene glycol into the
initially formed oligomers, avoiding sub-stoichiometric diol
during the vacuum stage, which would lead to low molecular
weight polymer. With the capability of Sb2O3 to catalyze both
direct esterification and transesterification, any excess, volatile
diol can be removed during the vacuum stage of the polymeriz-
ation. The excess diol strategy also worked for the longer diols
(n = 3–6) of Table 2, entries 4–7, affording yields from 86 to
91% and good Mn values from 19 500 to 24 900.
Scheme 2 Two equivalents of itaconic acid (green) react with ethylene
diamine to afford the monomer ethylene bis(pyrrolidone carboxylic acid)
(EBPC). Polyesterification at 180–240 °C with 2.0 mol% Sb2O3 catalyst
with alkanediols (n = 2–6) yields polyalkylene ethylene bis(pyrrolidone
carboxylate) (PAEBPC) polymers with properties dependent on n.
Table 2 Molecular weight and thermal data for PEPC (entry 1), PAEBPC polymers made from diols of varying length (entries 2–7), and commercial
samples of PLA (entry 8) and PET (entry 9)a
Monomer ratio
(diacid : diol)
Entry
1
Polymers
Yield (%)
84
Mn b (Da)
Mw b (Da)
PDIb
2.1
Tg c (°C)
T50 d (°C)
20 800
44 700
60
387
2
3
4
5
6
7
1.0 : 1.0
1.0 : 1.2
1.0 : 1.1
1.0 : 1.1
1.0 : 1.05
1.0 : 1.05
63
88
90
88
91
86
10 000
23 200
19 500
24 900
21 200
23 400
40 800
52 500
50 300
59 800
47 300
78 200
4.1
2.3
2.6
2.4
2.2
3.3
56
62
50
39
29
24
359
374
368
373
373
376
8
9
Commercial samplee
Commercial samplef
91 200
34 100
201 800
75 500
2.2
2.2
52
83
343
414
a With 2.0 mol% Sb2O3, 180 °C under nitrogen for 4 hours, followed by a temperature ramp over 8 hours to 240 °C with dynamic vacuum. b GPC
in hexafluoroisopropanol (HFIP) at 40 °C vs. polymethyl methacrylate standards. c Determined by DSC. d TGA temperature at which 50% mass
loss was observed under nitrogen. e Ingeo® PLA cup sample from NatureWorks, LLC. f Bottle grade PET copolymer from Trader Joe’s® Natural
Mountain Spring Water.
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Table 2 summarizes the polylactam ester thermal pro-
Fig. 2 shows the time-dependent molecular weight analysis
perties, as measured by differential scanning calorimetry for the polylactam ester of Table 2, entry 4 (from HOCH2CH2-
(DSC) and thermogravimetric analysis (TGA). The highest glass CH2OH) over 360 days. Exposed only to air, the polymer Mn
transition temperatures (Tg) of the series belong to PEPC decreased 10% from 19 500 to 17 600 during this time, but for
(entry 1, 60 °C) and the PAEBPC from ethylene glycol (entry 3, the submerged sample, the polymer Mn decreased steadily and
n = 2, 62 °C). Note that the lower molecular weight of entry 2 completely from 19 500 to 570 (below the detection limit for
negatively impacts its Tg value compared to the same polymer polymer with this instrument). Most likely, the polymer dis-
of entry 3 (56 vs. 62 °C). For the series of PAEBPC polymers solution enhanced the hydrolysis kinetics. The facile dis-
with n = 2 to 6, it is evident that lactam ring dilution by solution of these polymers can be appreciated because of the
increasing alkylene connector length proportionally decreases hydrophilicity of structurally-related lactams and polylactams:
the glass transition temperature. The Tg ranged from 62 °C N-methylpyrrolidone (NMP), which is miscible with water;45
(n = 2) to 24 °C (n = 6); so each additional methylene group and polyvinyl pyrrolidone (PVP), which is soluble in water.46 In
dropped the Tg by about 10 °C. According to DSC, these poly- contrast, the water-insolubility of PLA47 renders it much slower
mers are not crystalline; no melting temperatures are observed. to hydrolyze, despite its similar sterics (alpha substitution)
This is somewhat expected since the polymers are likely atactic near the ester functional group.
with insufficient long-range conformational order. Because
Furthermore, 1H NMR analysis of the immersed polymers
lactam ring formation is stereorandom, the HEPC monomer is after one year shows regeneration of the corresponding diol
obtained as the racemate (R/S) and the EBPC monomer is and the EBPC di-carboxylic acid monomer. Fig. 3 shows the 1H
obtained as a nearly equal ratio of diastereomers (55 : 45, see NMR spectra for the polyethylene [ethylene bis(pyrrolidone
the ESI†). Finally, Table 2 shows the T50 values for these poly- carboxylate)] polymer (PEEBPC, Table 2, entry 3), the degra-
lactam esters, the temperature at which 50% mass loss occurs dation products after one year of agitation in water, and auth-
according to thermogravimetric analysis (TGA) under nitrogen. entic EBPC monomer. By integration, about 94% of the ester
The T50 decomposition temperatures (359–387 °C) excel that of functional groups have hydrolyzed to afford ethylene glycol.
commercial PLA (343 °C, entry 8) and approach that of com- The matching of the degradation spectrum to that of the orig-
mercial PET (414 °C, entry 9).
inal EBPC monomer confirms that hydrolysis has occurred at
Since all polyesters potentially undergo hydrolysis, a pre- the ester group, but not at the amide functional group—
liminary degradation study was conducted under environmen- leaving the lactam ring intact.
tally relevant conditions of (1) air or (2) neutral water. Six
This degradation pathway is also confirmed by MALDI-TOF
different polylactam esters (Table 2, entries 1 and 3–7) were and ESI-TOF analysis, which clearly shows PEPC (Table 1,
molded into approximately 5 mm flattened pellets. These were entry 1) yielding the monomer HEPC (173 Da) + sodium
simply exposed to air, or agitated continuously in deionized (23 Da) = 196 Da and higher oligomers with repeat units of
water with an orbital shaker. Interestingly, all polymers in 155 Da, upon acid-catalyzed hydrolysis (see the ESI†). Simi-
water began to dissolve and their dissolution was complete larly, PPEBPC (Table 1, entry 4) yields its monomer PEBPC
after 30 days. After one year, the polymers exposed only to air (342 Da) + sodium (23 Da) = 365 Da and higher oligomers with
exhibited just a slight decrease in molecular weight (10% or repeat units of 324 Da. Thus, it is clear that the main-chain
less) according to Gel Permeation Chromatography (GPC),
whereas the polymers submerged in water were completely
hydrolyzed to monomer (Mn ∼ 500).
Fig. 2 According to molecular weight analysis, complete hydrolytic
degradation of polypropylene/[ethylene bis(pyrrolidone carboxylate)]
(Table 2, entry 4) was observed in neutral water over 360 days, whereas
minimal degradation is observed in air.
Fig. 3 1H NMR spectra of polymer PEEBPC (top, from Table 2 entry 3)
and ester hydrolysis degradation products EBPC and ethylene glycol
(middle) obtained after agitation in water for one year. Authentic
monomer EBPC (bottom) is shown for comparison.
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ester group is the key feature that allows for polymer degra-
dation. Compare this hydrolysis behavior to that of polylactam
Acknowledgements
amides reported by Kaneko et al.,42 which resemble PEPC This research was supported by the National Science
(Fig. 1) except they are constructed with alkylene diamines and Foundation (CHE-0848236, CHE-1305794, and CRIF-MU
thus bear no ester functionality. In this case, strongly alkaline CHE-0541761). Hsiao-Li Chen was supported by the Graduate
solutions (pH > 10), one year’s time in soil, or UV-irradiation Student Study Abroad Program (GSSAP) administered by the
(250–450 nm) in water effected pyrrolidone ring opening National Chung Hsing University of Taiwan.
(hydrolysis) and polymer solubilization, but not chain scission.
Apparently the remaining amide main-chain functionality is
robust and survives these various conditions.
Notes and references
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Conclusions
The pyrrolidone lactam ring has been incorporated into an ali-
phatic polyester and this hard/soft motif of the polymer repeat
unit was targeted to mimic that of aromatic/aliphatic polymers
such as polyethylene terephthalate (PET). Importantly, the
hydroxy-acid monomer (HEPC, with one lactam ring) and the
diacid monomer (EBPC, with two lactam rings) are synthesized
from biogenic itaconic acid, an inexpensive, scalable platform
chemical increasingly available via glucose fermentation. The
solvent-free, melt homopolymerization of HEPC or copolymeri-
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2.0 mol% Sb2O3 afforded the best yields (84–91%) and the
highest molecular weights (Mn = 19 500–24 900). For these
polylactam esters, the lactam : methylene (–CH2–) ratio scaled
proportionally to the glass transition temperature (Tg). For the
highest ratio of 1 : 2, the Tg is 60 to 62 °C (Table 2, entries 1
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