Thermoset elastomers derived from carvomenthide

Article abstract of DOI:10.1021/bm501450c

Renewable thermoset elastomers were prepared using the plant-based monomer carvomenthide. Controlled ring-opening transesteri fication polymerization of carvomenthide using diethylene glycol as an initiator gave α,ω-dihydroxyl poly(carvomenthide) (HO-PCM-OH), which was subsequently converted to carboxy-telechelic poly-(carvomenthide) (HOOC-PCM-COOH) by esterification with excess succinic anhydride through a one-pot, two-step process, leading to no crystallinity, high viscosity, strong thermal resistance, and low glass transition temperature of the resulting functionalized polyester. Thermal curing processes of the resulting 3, 6, and 12 kg mol^-1 prepolymers were achieved with trifunctional aziridine to give cross-linked PCM elastomers. The thermal properties, mechanical behavior, and biocompatibility of the rubbery thermoset products were investigated by differential scanning calorimetry, thermal gravimetric analysis, dynamic mechanical analysis, tensile tests under static and cyclic loads, and cell adherence. These new materials are useful candidates to satisfy the design objective for the engineering of a variety of soft tissues. (Figure Presented).

Full text of DOI:10.1021/bm501450c

Article
pubs.acs.org/Biomac
Thermoset Elastomers Derived from Carvomenthide
Jinyoung Yang,†,∥,# Sangjun Lee,†,§,# Woo Jin Choi,‡ Howon Seo,⊥ Pilhan Kim,⊥ Geon-Joong Kim,∥
Young-Wun Kim,*,†,§ and Jihoon Shin*,†
†Center for Biobased Chemistry, ‡Chemical Materials Solutions Center, Korea Research Institute of Chemical Technology (KRICT),
141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Korea
§Department of Green Chemistry and Environmental Biotechnology, University of Science & Technology (UST), 113 Gwahak-ro,
Yuseong-gu, Daejeon 305-350, Korea
∥Department of Chemical Engineering, Inha University, 100 Inharo, Nam-gu, Incheon 402-751, Korea
⊥Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro,
Yuseong-gu, Daejeon 305-701, Korea
S
* Supporting Information
ABSTRACT: Renewable thermoset elastomers were prepared
using the plant-based monomer carvomenthide. Controlled
ring-opening transesterification polymerization of carvomen-
thide using diethylene glycol as an initiator gave α,ω-
dihydroxyl poly(carvomenthide) (HO-PCM-OH), which was
subsequently converted to carboxy-telechelic poly-
(carvomenthide) (HOOC-PCM-COOH) by esterification
with excess succinic anhydride through a one-pot, two-step
process, leading to no crystallinity, high viscosity, strong
thermal resistance, and low glass transition temperature of the
resulting functionalized polyester. Thermal curing processes of the resulting 3, 6, and 12 kg mol−1 prepolymers were achieved
with trifunctional aziridine to give cross-linked PCM elastomers. The thermal properties, mechanical behavior, and
biocompatibility of the rubbery thermoset products were investigated by differential scanning calorimetry, thermal gravimetric
analysis, dynamic mechanical analysis, tensile tests under static and cyclic loads, and cell adherence. These new materials are
useful candidates to satisfy the design objective for the engineering of a variety of soft tissues.
Telechelic polymer chains having reactive functional groups
such as acrylate,9 epoxide,10 isocyanate,11 hydroxyl,12,13 and
amine14 are basic building blocks for various polymeric
materials. In particular, carboxy-telechelic polymers with low
molar mass have been highly favored owing to their ability to
serve as starting materials for the preparation of polymers
having ester and amide linkages.
Direct polymerization can enable a straightforward synthetic
route to carboxy-ended polymers. Poly((R,S)-3-hydroxybuty-
rate) containing dicarboxylic acid as chain-end functionality was
obtained by anionic polymerization of β-butyrolactone initiated
by succinic acid disodium salt, which was followed by
protonation.15 Telechelic poly(butyl acrylate) and poly-
(styrene) with a carboxy group were produced in a single-
step procedure with 4,4′-azobis(4-cyanovaleric acid) as an
initiator via reversible addition−fragmentation chain transfer
(RAFT) polymerization using dicarboxy trithiocarbonate as a
chain transfer agent (CTA) and dead-end polymerization,
respectively.16,17 The use of unprotected maleic acid as a CTA
led to efficient incorporation of carboxy end groups in
INTRODUCTION
■
Current movement in the chemical industry toward replacing
petroleum-based feedstock with renewable alternatives is being
driven by (a) concerns about diminishing fossil resources or
global warming, (b) present knowledge and technology
allowing the processing of one type of biomass, such as lactic
acid, furfural, or vegetable oil, with only small capital
investment, (c) general demand for renewable or green
products from consumers, and (d) concerns on the political
side to reduce the dependence on fossil feedstock and to
decrease the environment impact.1 However, renewable
feedstock can also provide an interesting sustainable platform
through the intended design of biobased polymers having
distinct or superior physichemical properties.2−4
One biorenewable resource is carvone, which is a component
of the oils extracted from caraway seed Carum carvi and
spearmint Mentha spicata.5 Thousands of tons of carvone are
harvested annually and used in the pharmaceutical, flavor,
fragrance, insect control, and food industries.6 Carvone is
converted to carvomenthone via hydrogenation, which is
subsequently followed by a Baeyer−Villiger oxidation to yield
the seven-membered lactone carvomenthide, which is suitable
for use as a polymerizable monomer.7,8
Received: September 29, 2014
Revised: November 26, 2014
Published: December 16, 2014
© 2014 American Chemical Society
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poly(olefin) during ring-opening metathesis polymerization
(ROMP) of the cyclic olefin, cis-cyclooctene (COE).18
sustaining deformation, owing to its intrinsic elasticity, and of
working in a mechanically dynamic environment such as that of
cardiac tissue,33 vascular tissue,34 cartilage tissue,35 retinal
tissue,36 nerve tissue,37 and tympanic membrane.38
Low Tg carboxy-telechelic poly(olefin)s were synthesized via
ROMP of 3-hexyl-cis-cyclooctene (3-Hex-COE) and COE with
maleic acid as a CTA, followed by subsequent hydrogenation.
The prepolymers were chosen for the synthesis of cured
elastomers in the presence of polyfunctional aziridines, showing
Tg values similar to those of the starting polyolefins and
elongations at break up to 230% with tensile strength in the
range of 1.05 to 1.74 MPa.19 Supramolecular polymer gels
formed with carboxy-terminated telechelic poly(butadiene)
(PBD(COOH)2) and a linear poly(amidine) (PAmd) having
an N,N′-disubstituted acetamidine group through use of a
carboxylate salt bridge were studied. The gel exhibited a high G′
value of about 1 MPa at ambient temperature and Tgel of 37
°C.20 Viscoelastic properties for supramolecular soft materials
formed by ionic hydrogen bonds between carboxy-terminated
telechelic poly(ethyl acrylate) (PEA(COOH)2) and poly-
(ethylenimine) were also investigated.21
On the other hand, the postpolymerization functionalization
of an end group can lead to carboxy-telechelic macromolecules.
End-carboxylated telechelic poly(ε-caprolactone) (PCL) (F =
1.4−2.0) was cross-linked thermally with poly(styrene)-
epoxided poly(butadiene)-poly(styrene) triblock copolymer to
investigate the curing behavior and the mechanical properties of
the thermoset elastomers. The resulting cured polymers, having
high temperature elasticity above the Tg of poly(styrene),
showed stress−strain curves that depended on the blend ratio
and molar mass of the carboxy prepolymer.22 Mono-, bi-, and
trifunctional carboxy-telechelic microspheres were prepared via
suspension polymerization of carboxy-terminated oligocapro-
lactones and divinylbenzene. Their ion-exchange capacities
corresponded to the total carboxy amount.23 The direct
polyesterification of hydroxyl-telechelic poly(ethylene tereph-
thalate) (PET) with carboxy-terminated PCL (F = 1.98) or
carboxy-ended poly(propylene oxide) (PPO), in which the
number of functionality (F) was not determined, was used to
synthesize a multiblock poly(ester-ether) exhibiting thermo-
plastic elastomer properties.24,25 After the primary hydroxyl end
group in poly(tetramethylene oxide) (PTMO) was converted
to carboxylic acid with F ∼ 2.0, as estimated by the chemical
titration method, the carboxy-ended PTMO was cured with an
epoxy resin and pyromellitic dianhydride (PMDA); the
resulting material, however, showed no shape memory
properties.26
Poly(glycerol sebacate) (PGS), commercially available as the
brand Regenerez, is a tough polyester prepared via poly-
condensation of glycerol and sebacic acid for soft tissue
engineering.27 PGS exhibits biocompatibility, biodegradability,
and bioresorbability as highly relevant properties in biomedical
applications; in addition, it shows thermoset elastomeric
properties.28−30 The soft elastomeric nature of PGS, shown
in its stress−strain curve, is similar to that of vulcanized rubber
due to the three-dimensional network of random coils caused
by both the covalent cross-linking and the hydrogen-bonding
interactions among hydroxyl groups attached to the polymer
backbone.31 PGS materials have a tensile Young’s modulus of
0.025 to 1.2 MPa, an ultimate tensile strength over 0.5 MPa,
and strain to failure of greater than 330%.27 The Young’s
modulus of PGS is between that of ligaments and human
myocardium (0.02−0.5 MPa).32 The maximum strain at break
of PGS is similar to that of arteries and vein (<260%). The
resulting superior mechanical properties make PGS capable of
Herein, using the ring-opening transesterification polymer-
ization (ROTEP) of carvomenthide (CM) followed by
postpolymerization functionalization via a one-pot, two-step
process, we report the preparation and characterization of a
new thermoset elastomer system from renewable carboxy-
telechelic poly(carvomenthide) (PCM). We demonstrated the
tin-catalyzed polymerization of CM to give α,ω-dihydroxyl
PCM (HO-PCM-OH) with various molar masses (Mn = 3, 5,
and 12 kg mol−1), precisely controlled low dispersities (Đ =
1.05−1.09) that lead to a higher cross-linking state in the
designed thermoset polymers, and low glass transition
temperatures of about −36 °C without any crystallinity. We
also prepared α,ω-dicarboxy-functionalized PCM (HOOC-
PCM-COOH) prepolymer, through esterification of the polyol
with succinic anhydride (SA), which was followed by curing
with aziridine cross-linker to yield elastomeric materials. As a
result, we postulated that thermoset elastomers, having
potentially useful properties, could be generated with
advantages such as no crystallinity and a flexible backbone of
renewable polyester, whereas there was some crystallinity or
rigid backbone in thermoset elastomers produced by
petroleum-based monomers having no substitution such as
cycloalkene and caprolactone.19,22,39−41 We pursued a protocol
for the synthesis of prepolymers that would make them greener
and more amenable to large-scale preparations. Herein, we
report (a) new methods for preparing HO-PCM-OH polyols
and HOOC-PCM-COOH prepolymers with low dispersities,
(b) the use of such prepolymers to prepare thermally cross-
linked elastomers with triarmed aziridine, and (c) the
characterization of these polyols, prepolymers, and elastomers
using 1H and 13C NMR spectroscopy, size-exclusion
chromatography (SEC), differential scanning calorimetry
(DSC), thermal gravimetric analysis (TGA), dynamic mechan-
ical analysis (DMA), tensile testing, and cytocompatibility.
Depending on the application target of interest, these
renewable elastomers hold promise for use as scaffolds for
soft tissue engineering.
EXPERIMENTAL SECTION
■
Materials. All air- or moisture-sensitive compounds were handled
under an inert atmosphere in a glovebox. Diethylene glycol (Aldrich)
used for polymerizations was distilled under reduced pressure over
sodium and stored in a glovebox. Tin(II) 2-ethylhexanoate (Aldrich)
used for polymerization was also distilled and stored in a glovebox. D-
Hydrocarvone, palladium on carbon (10 wt %), succinic anhydride,
Oxone, and m-chloroperoxybenzoic acid (m-CPBA, ≤77%) were used
as received from Sigma-Aldrich without further purification.
Trimethylolpropane tris(2-methyl-1-aziridinepropionate) (TAz) was
obtained from Tokyo Chemical Industry. All other solvents and
reagents were used as received from the commercial source without
further purification.
Measurements. 1H NMR spectra were collected from CDCl3
solution on a Bruker DPX-500 spectrometer operating at 500 MHz.
13C NMR spectra were also collected at 125 MHz. Proton and carbon
chemical shifts were referenced to CDCl3 (7.26 and 77.23 ppm,
respectively). Polymer samples were prepared in CDCl3 (Euriso-Top)
at a concentration of approximately 10 mg mL−1 for 1H NMR and 100
mg mL−1 for 13C NMR. Molar masses (Mn and Mw) were determined
by size-exclusion chromatography (SEC) in chloroform as the mobile
phase at 40 °C versus poly(styrene) standards (Polymer Laboratories)
under a constant volumetric flow rate (1 mL min−1) using an Agilent
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1260 LC system equipped with a Styragel guard column (30 × 4.6
mm) and three successive Styragel columns (300 × 7.8 mm) (HT4,
HT3, and HT2) packed with rigid 10 μm styrene divinylbenzene
particles. GC-MS experiments were conducted on an Agilent
Technologies 6890 GC system equipped with a HP-1 capillary
column (30 m × 0.25 mm × 0.1 μm) and 5973N MSD using electron
ionization. Differential scanning calorimetry (DSC) measurements
were performed using a TA Q-1000 DSC under a nitrogen flow of 50
mL min−1. The polymer samples (7−10 mg) were heated at 150 °C,
held for 5 min to erase the influence of thermal history, cooled to −80
°C, and then reheated at 200 °C. The rates of heating and cooling
were 10 °C min−1. The values reported were obtained from the second
heating cycle. An indium standard was used for calibration. Total acid
number (TAN) of the carboxy telechelic HOOC-PCM-COOH
samples was determined using titration technique (ASTM D664)
with a Metrohm 888 Titrando titrator. Thermal gravimetric analysis
(TGA) was performed on a TA Q-500 TGA instrument under a
nitrogen flow of 100 mL min−1 at a heating rate of 10 °C min−1 in a
temperature range from 25 to 600 °C. Tensile testing for the cured
elastomers was carried out at room temperature on a QRS-S11H
universal testing machine (Quro) for normal strain−stress properties.
Elastomeric recovery properties of the elastomers were also
determined using an Instron 5567 universal testing machine at room
temperature. Tensile properties of the ASTM D1708 microtensile
bars, which had a uniform sheet thickness (∼0.55 mm), were tested at
strain rate of 130 mm min−1; all values are reported as the average and
standard deviation of at least four samples. Dynamic mechanical
analysis (DMA) was carried out using an ARES-G2 rotational
rheometer (Waters) under nitrogen. The shear modulus and tan δ
with temperature from −50 to 170 °C was measured in a rectangular
torsion fixture at ω = 1.0 rad s−1 and γ = 1.0%.
(m, 1H major, 1H minor), 1.75−1.38 (m, 3H major, 5H minor),
1.35−1.25 (m, 1H major), 1.09 (d, J = 7.0 Hz, 3H minor), 1.01 (d, J =
6.5 Hz, 3H major), 0.89 (m, 6H major, 6H minor). 13C NMR (125
MHz, CDCl3): δ 212.8, 46.2, 44.9, 44.4, 34.7, 32.3, 28.5, 19.2, 18.9,
13.9 (major isomer); 214.2, 44.3, 43.8, 42.6, 30.9, 30.1, 24.6, 19.6, 19.5,
15.4 (minor isomer). GC-MS data (m/z): 41, 55, 69, 83, 97, 110, 126.
A round-bottomed flask (1 L) was loaded with carvomethone (50.0
g; 324 mmol), chloroform (300 mL), and m-chloroperbenzoic acid
(94.4 g; 421 mmol) at 0 °C with stirring. The reaction flask was
capped, and the mixture was allowed to stir for approximately 48 h.
The reaction mixture was filtered under reduced pressure to remove
solids. The filtrate was washed with saturated sodium bisulfite solution
(3 × 300 mL), saturated sodium bicarbonate solution (3 × 300 mL),
deionized water (1 × 300 mL), and brine (2 × 300 mL), successively.
The organic layer was dried over anhydrous magnesium sulfate and
vacuum filtered through Celite 545. The filtrate was concentrated via
rotary evaporation under reduced pressure to remove the solvent and
to give a slightly yellow oil; there was a conversion of ∼99%, as
1
determined by H NMR analysis. The product was purified by three
times vacuum distillation (90−100 °C, 120 mTorr) to yield CM,
which is a colorless and transparent oil (36.0 g; 212 mmol and overall
1
yield of 65.4%). H NMR (500 MHz, CDCl3): δ 4.42 (qdd, J = 9.6,
6.4, and 9.4 Hz, 1H major, 1H minor), 2.80 (dd, J = 5.7 and 14.1 Hz,
1H minor), 2.75 (dd, J = 3.7 and 14.1 Hz, 1H minor), 2.47 (m, 2H
major), 1.91 (td, J = 3.8 and 15.2 Hz, 1H major, 1H minor), 1.81 (d, J
= 13.3 Hz, 1H major, 1H minor), 1.73−1.40 (m, 4H major, 4H
minor), 1.34 (dd, J = 6.4 and 1.6 Hz, 3H major, 3H minor), 1.03−0.88
(dd, J = 6.4 and 60.4 Hz, 6H minor), 0.91−0.85 (dd, J = 6.9 and 11.5
Hz, 6H major). 13C NMR (125 MHz, CDCl3): δ 174.7, 75.7, 39.7,
37.3, 35.2, 32.8, 30.5, 21.9, 18.1, 17.8 (major isomer); 173.3, 75.5, 38.1,
37.7, 31.5, 29.3, 28.3, 21.6, 20.0, 19.4 (minor isomer).
In Vitro Biocompatibility. Glass Petri dishes (DURAN, 40 mm
diameter) were coated with a chloroform solution (5%) of the
aziridine and carboxy-telechelic prepolymers such as HOOC-PCM-
COOH(3), HOOC-PCM-COOH(6), and HOOC-PCM-COOH(12).
The coated dishes were transferred into a vacuum oven at 50 °C after
evaporation of the solvent in air. The prepolymer was cross-linked into
the elastomer after 24 h at 180 °C. A control dish was coated with
CHCl3 solution (5%) of PLGA (50:50, relative molar mass 30−60 kg
mol−1, Aldrich), and the solvent was evaporated for 24 h in air. An
NIH 3T3 mouse embryonic fibroblast cell was grown in DMEM
(Gibco) containing 10% FBS (Gibco) and antibiotics (penicillin 100
units/mL and streptomycin 100 μg/mL, Sigma-Aldrich) and kept in a
5% CO2 incubator at 37 °C. The coated glass dishes were sterilized by
UV radiation for 30 min and filled with growth medium for 4 h to
eliminate impurities. Then, each dish was washed with sterilized water
three times and seeded with 50 000 NIH 3T3 cells in 2 mL of cell
culture media. At 72 h after plating, the morphology of the cells in
each dish was observed under a bright-field light microscope (Eclipse
Ti-U, Nikon). Cellar proliferation rate on PLGA and XL-PCM films
was measured by MTT assay at days 1, 3, and 5 to probe
mitochondrial activity. To obtain live/dead cell images, cells were
observed with custom-built confocal microscope after staining with
carboxyfluorescein succinimidyl ester (CFSE) and propidium iodide
(PI).
The following procedure was developed for the preparation of CM
using Oxone in an aqueous ethanol solution. A round-bottomed flask
(1 L) was charged with D-carvomenthone (6.850 g; 44.4 mmol),
sodium bicarbonate (29.9 g; 355 mmol), and ethanol absolute (200
proof, 200 mL). The reaction was stirred at room temperature as a
solution of Oxone (54.6 g; 177.6 mmol) in water (200 mL) was added
via syringe pump over 48 h. After the addition was stopped, the
reaction was allowed to stir for an additional 8 h. The reaction mixture
was vacuum filtered through Celite 545 to remove the solids. The
filtrate was concentrated to approximately 10 mL via rotary
evaporation under reduced pressure, affording conversion of ∼99%,
as determined by 1H NMR analysis. The product was purified by three
time vacuum distillation (90−100 °C, 120 mTorr) to give CM, which
is a colorless and transparent oil (4.19 g; 24.6 mmol and overall yield
of 55.9%).
Synthesis of Hydroxyl-Telechelic Poly(carvomenthide) (HO-
PCM-OH). The general procedure to prepare the hydroxyl-telechelic
PCM using bulk ROTEP of CM was as follows:41 In a nitrogen-filled
glovebox, CM (19, 32, or 74 equiv) was transferred to a pressure vessel
(350 mL) equipped with a stir bar. Diethylene glycol (1 equiv based
on two end-hydroxyl groups) was injected into the reaction vessel
followed by the addition of tin(II) 2-ethylhexanoate (0.2 or 0.5 equiv).
The reaction vessel was sealed, taken out of the glovebox, and placed
in a thermostatic oil bath at 100 °C for 22 h (94−97% conversion of
monomer). The reaction was quenched by exposure to the air. The
crude product was diluted with chloroform (ca. 30 mL). After pouring
methanol (250 mL) into the crude product solution, the polymer
solution was precipitated in order to recover the hydroxyl PCM and to
remove the remaining monomer. The removal process was repeated
twice. Sample was dried at 90 °C in a vacuum oven (60−70% isolated
Synthesis of Carvomenthide (CM). CM was synthesized from D-
dihydrocarvone using literature procedures with some modifica-
tions.7,41 A high-pressure reactor (Parr Instrument Company) was
charged with D-dihydrocarvone (100.0 g; 657 mmol), palladium on
carbon (10 wt % and 1 g; 0.94 mmol), and n-hexane (300 mL). The
reactor was sealed and charged three times with 15 bar of nitrogen to
remove much of the air. The solution was then charged with 15 bar of
H2 and placed at room temperature while stirring rapidly. The reactor
was recharged once when the pressure subsided to about 5 bar. The
reaction was allowed to proceed like this for 12 h before disassembling
the reactor. The reaction mixture was then vacuum filtered through
Celite 545, washed with n-hexane, and dried under vacuum to give
carvomenthone as colorless and transparent oil (100.3 g; 650 mmol
1
yield based on product weight calculated by conversion). H NMR
(500 MHz, CDCl3): δ 4.85 (m, 1H, Hd from the repeating unit of
hydroxyl PCM), 4.21 (m, 2H, Hb from incorporated initiator), 3.76
(m, 1H, Ha from the end unit of hydroxyl PCM), 3.67 (t, J = 4.9 Hz,
2H, Hc from incorporated initiator), 2.23 (dd, J = 5.4 and 15.1 Hz, 1H
from the repeating unit of hydroxyl PCM), 2.08 (dd, J = 7.2 and 15.1
Hz, 1H from the repeating unit of hydroxyl PCM), 1.72 (m, 2H from
the repeating unit of hydroxyl PCM), 1.56 (m, 1H from the repeating
unit of hydroxyl PCM), 1.45 (m, 1H from the repeating unit of
1
and overall yield of 98.9%). H NMR (500 MHz, CDCl3): δ 2.48−
2.27 (m, 2H major, 3H minor), 2.12−2.02 (m, 2H major), 1.91−1.82
248
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Scheme 1
hydroxyl PCM), 1.31 (m, 1H from the repeating unit of hydroxyl
PCM), 1.23 (m, 1H from the repeating unit of hydroxyl PCM), 1.18
(d, J = 6.1 Hz, 3H from the repeating unit of hydroxyl PCM), 0.83
(dd, J = 6.7 and 12.9 Hz, 6H from the repeating unit of hydroxyl
PCM). 13C NMR (125 MHz, CDCl3) in Figure S2d: 173.7, 173.4
(C1), 173.3, 70.9 (C2), 69.0 (C3), 68.3 (C4), 67.8 (C4), 63.2 (C5),
40.5 (C6), 36.3 (C7), 33.4 (C8), 29.7 (C9), 26.6 (C10), 19.9, 19.1,
18.6.
Hz, 1H from the repeating unit of carboxy PCM), 1.69 (m, 2H from
the repeating unit of carboxy PCM), 1.54 (m, 1H from the repeating
unit of carboxy PCM), 1.43 (m, 1H from the repeating unit of carboxy
PCM), 1.27 (m, 1H from the repeating unit of carboxy PCM), 1.20
(m, 1H from the repeating unit of carboxy PCM), 1.16 (d, J = 6.2 Hz,
3H from the repeating unit of carboxy PCM), 0.81 (dd, J = 6.8 and
15.3 Hz, 6H from the repeating unit of carboxy PCM). 13C NMR (125
MHz, CDCl3): 176.3 (C1), 173.5, 173.3 (C2), 173.2, 171.6 (C3), 71.4
(C4), 70.7 (C5), 68.9 (C6), 63.1 (C7), 40.4, 36.2, 33.3, 29.6, 29.1,
28.8 (C8), 26.4, 19.8, 19.0, 18.5.
One-Pot, Two-Step Process for Synthesis of Carboxy-
Telechelic Poly(carvomenthide) (HOOC-PCM-COOH). In a nitro-
gen-filled glovebox, CM (19, 32, or 64 equiv) was transferred to a
pressure vessel (350 mL) equipped with a stir bar. Diethylene glycol
(1 equiv based on two end-hydroxyl groups) and tin(II) 2-
ethylhexanoate (0.2 or 0.5 equiv) were injected into the reaction
vessel.. The reaction vessel was sealed, taken out of the glovebox, and
placed in a thermostatic oil bath at 100 °C for 22 h (94−97%
conversion of monomer). After cooling the vessel to room
temperature, the vessel was charged with 1,4-dioxane, and the mixture
was stirred to completely dissolve the polymer. An excess of SA (8 or
12 equiv for one hydroxyl group (1 equiv) in HO-PCM-OH) and
DMAP (0.01 equiv) as a catalyst were added to the vessel. The vessel
was sealed and returned to 90 °C for 50 h (>99% conversion from
hydroxyl group to carboxy group). The reaction was quenched by
cooling and then adding chloroform, suspended to remove the
remaining SA, and precipitated into methanol to remove unreacted
CM. The recovered carboxy PCM was dried at 90 °C in a vacuum
oven (61−95% isolated yield based on product weight calculated by
conversion).
Synthesis of Carboxy-Telechelic Poly(carvomenthide)
(HOOC-PCM-COOH). The general procedure to prepare the
carboxy-telechelic PCM via postpolymerization functionalization was
as follows:25 The hydroxyl-telechelic PCM (1 equiv based on two end-
hydroxyl groups) was introduced into a pressure vessel (350 mL) with
a magnetic stir bar. 1,4-Dioxane was added to the vessel, and the
mixture was stirred to completely dissolve the polymer at room
temperature for 30 min. An excess of succinic anhydride (SA) (8 or 12
equiv for one hydroxyl group (1 equiv) in HO-PCM-OH) and
dimethylaminopyridine (DMAP) (0.01 equiv) as a catalyst were added
to the vessel. The reaction vessel was sealed and placed in a
thermostatic oil bath at 90 °C for 50 h. After cooling the vessel, the
reaction was quenched by cooling and then adding chloroform (300
mL) to precipitate most of the unreacted SA. The solution was
vacuum filtered through Celite 545 to remove the solids. The filtrate
was centrifuged at 3000 rpm for 10 min to remove small amounts of
reprecipitated SA, and then the supernatant was completely
concentrated via rotary evaporation under reduced pressure. The
concentrated polymer solution was suspended with n-hexane (800
mL). Small quantities of chloroform (10−20 mL) were added to
dissolve the polymer in the suspension and to keep the tiny amounts
of SA undissolved. The polymer solution was vacuum filtered through
Celite 545 to remove the solids. The filtrate was completely
concentrated via rotary evaporation under reduced pressure. It was
dried at 90 °C in a vacuum oven (60−70% isolated yield based on
product weight calculated by conversion). 1H NMR (500 MHz,
CDCl3): δ 4.83 (m, 1H, Hd from the repeating unit of carboxy PCM),
4.19 (m, 2H, Hb from incorporated initiator), 3.65 (t, J = 4.8 Hz, 2H,
Hc from incorporated initiator), 2.59 (dd, J = 6.0 and 24.2 Hz, 4H, He
from the end unit of carboxy PCM), 2.21 (dd, J = 6.3 and 15.1 Hz, 1H
from the repeating unit of carboxy PCM), 2.08 (dd, J = 7.3 and 15.1
Thermal Cross-linking of Carboxy-Telechelic Poly-
(carvomenthide). The general procedure to prepare the cross-linked
poly(carvomenthide) elastomers via thermal curing was as follows with
some modifications:19 Carboxy-telechelic PCM and trimethylolpro-
pane tri(2-methyl-1-aziridine propionate) (TAz) in 3:2.1 molar ratio
(acid/aziridine = 1:1.06) were charged into a round-bottomed flask
with a magnetic stir bar. Chloroform was added to the flask, and the
mixture was stirred to completely dissolve the prepolymer and TAz at
room temperature for 10 min. The mixture was then slowly cast into a
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Figure 1. Expanded region of the 1H NMR spectra (500 MHz, CDCl3) of (a) HO-PCM-OH(3) before purification, (b) after performing one-pot,
two-step process and before purification of HOOC-PCM-COOH(3), (c) purified HOOC-PCM-COOH(3) after precipitation, and (d) three
enlarged regions of 13C{1H} NMR spectrum (125 MHz, CDCl3) of HOOC-PCM-COOH(3) after precipitation (* = PCM homopolymer,42 ** =
unreacted CM, *** = remained SA, and **** = 1,4-dioxane).
PFA Petri dish to make a film. The cast solution was placed under a
flow of air at room temperature for 24 h to evaporate the solvent,
which was subsequently dried at 50 °C in a vacuum oven for 12 h to
completely remove chloroform. The dish was placed in a preheated
oven at 180 °C, and the material was cured for 10 min. A slightly
yellow transparent thermoset elastomer was obtained. Densities of
eco-friendly alternative (>99% conversion) to the above-
mentioned method of using petroleum-based chemicals
(Scheme 1B), which is followed by several distillations for
purification (Figure S1).41 Isolated yield of obtained CM was
over 56%. Detailed characterization data for carvomenthone
and CM are provided (see Experimental Section).
1.0357
0.002, 1.0205
0.001, and 1.0067
0.006 g/cm3 were
Preparation of Hydroxyl-Telechelic Poly(carvomenthide).
As can be seen in Scheme 1, α,ω-hydroxyl-functionalized
poly(carvomenthide) (HO-PCM-OH) was prepared by ring-
opening polymerization transesterification polymerization
(ROTEP) of CM. The polymerization was carried out in
bulk using tin(II) ethylhexanoate (Sn(Oct)2) as a catalyst and
diethylene glycol (DEG) as a bifunctional initiator at 100 °C
under an inert atmosphere for 22 h on a 30 g scale (see
Experimental Section for more details). Initial carvomenthide/
DEG ratios of 19:1, 32:1, and 74:1 were selected to target
molar masses of 3.0, 6.0, and 12.0 kg mol−1, designated as HO-
PCM-OH(3), HO-PCM-OH(6), and HO-PCM-OH(12),
obtained for XL-PCM(3), XL-PCM(6), and XL-PCM(12), measured
with density determination kit in MeOH at 25 °C, respectively.
RESULTS AND DISCUSSION
■
Synthesis of Prepolymers. Synthesis of Carvomenthide.
Dihydrocarvone was hydrogenated using palladium on carbon
(Pd/C) to prepare carvomenthone, which was subsequently
oxidized into carvomenthide (CM) using m-chloroperbenzoic
acid (m-CPBA) in a chlorinated solvent with a modified
literature procedure (Scheme 1A).7,41 In addition, we now
report the Baeyer−Villiger oxidation of carvomenthone using
Oxone (KHSO5·KHSO4·K2SO4), which is a green inorganic
oxidant and an ethanol/water/NaHCO3 system as a potentially
1
respectively. H NMR spectroscopy analysis (Figures 1a and
S2a) was used to determine DEG chain connectivity, monomer
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a
Table 1. Characterization Data for HO-PCM-OH and HOOC-PCM-COOH
Mn (theo.)
Mn (NMR)
Mn (SEC) (kg mol−1
)
Mn (titr.) (kg mol−1
)
Tg
(°C)
T
d
e
f
g
h
j
d k
polymer
[M]0/[I]0
(kg mol−1
)
(kg mol−1
)
(Đ)
(TAN)
F
(°C)
b
h
h
h
i
HO-PCM-OH(3)
19
3.2
5.3
3.0
5.0
5.0 (1.09)
7.8 (1.06)
17.4 (1.05)
4.4 (1.10)
7.4 (1.06)
16.5 (1.06)
2.01
2.02
1.99
2.05
1.97
1.98
−35
−38
−34
−27
−28
−27
255
301
299
293
310
317
b
HO-PCM-OH(6)
32
b
HO-PCM-OH(12)
74
11.9
3.4
11.5
3.4
l
l
c
HOOC-PCM-COOH(3)
HOOC-PCM-COOH(6)
8
2.9 (38.5)
5.6 (20.2)
c
i
8
5.5
5.6
l
c
i
HOOC-PCM-COOH(12)
12
12.0
b
12.4
13.6 (8.2)
a
c
See Experimental Section for synthetic details. M is carvomenthide (CM), and I is diethylene glycol (DEG). M is succinic anhydride (SA), and I
is dihydroxyl-terminated poly(carvomenthide) (HO-PCM-OH) macroinitiator. Calculated based on monomer conversion, as determined by H
NMR spectroscopy. Calculated from relative integrations of PCM repeating units and DEG for PCM by H NMR spectroscopy. Determined by
size-exclusion chromatography (SEC) in CHCl3 at 40 °C relative to poly(styrene) standards. Calculated based on total acid number (TAN), which
was a quantity of KOH in milligrams needed to react with the acid in an amount of the carboxy-telechelic PCM in grams. Functionalities for
d
1
e
f
1
g
h
hydroxyl group calculated from relative integration of methylene protons of DEG and a methine proton of terminal chain for dihydroxyl-terminated
i
PCM by 1H NMR spectroscopy. Functionalities for carboxylic acid group determined from relative integration of methylene protons of DEG and
j
methylene protons of terminal chain for dicarboxy-terminated PCM by 1H NMR spectroscopy. Determined by DSC (2nd heating cycle) at 10 °C
k
l
min−1. 5% weight loss determined by TGA at 20 °C min−1 in N2. Performed by one-pot, two-process reaction (sequential addition).
conversion, and Mn for PCM. Changing DEG into the
corresponding ester in the PCM induced a quantitative
downfield shift of the two methylene proton peaks (Hb and
Hc; Figures 1 and S2). The methine proton resonance adjacent
to the end hydroxyl group of a PCM chain was observed (Ha)
at δ 3.76 ppm. Comparison of the integration values between
the methine proton (Ha) at the PCM end chain and the
methylene protons (Hb and Hc) from the DEG initiator
showed a ratio of exactly 2:8, proving the presence of two
terminated hydroxyl groups per chain with minimized
adventitious initiation. On the basis of the conversion (97,
96, and 93%) and the initial monomer-to-initiator contents, the
theoretical Mn values for HO-PCM-OH(3), HO-PCM-OH(6),
and HO-PCM-OH(12) were found to be 3.2, 5.3, and 11.9 kg
mol−1, respectively (Table 1). Mn values of 3.0, 5.0, and 11.5 kg
mol−1 were also obtained by calculations of the relative
integration between the methylene protons of the incorporated
DEG at δ 4.21 and 3.67 ppm (4 protons) and the methine
protons (Hd) from PCM repeating units at δ 4.85 ppm in the
1H NMR spectra of telechelic hydroxyl PCM samples; these
results are in good agreement with the theoretical values,
indicating well-controlled molar mass. By size-exclusion
chromatography (SEC) using poly(styrene) standards, the Mn
values for the hydroxyl-terminated PCM polymers were 5.0,
7.8, and 17.4 kg mol−1, respectively; the polydispersity (Đ)
values were 1.09, 1.06, and 1.05, individually (Table 1 and
Figure S3). Finally, three PCM polyols having different molar
masses were prepared to investigate the effect of the PCM
chain length on the properties of the resulting cross-linked
elastomers. Hydroxymethylene protons at the initiating portion
of the PCM homopolymer, which resulted from small
quantities of water in CM, were observed at δ 3.98 ppm (*
in Figure 1) even if CM monomer was distilled three times with
CaH2 to completely remove the aqueous impurity.42 However,
no evidence of PCM homopolymer in SEC analysis was
apparent, and the SEC traces of the hydroxyl-telechelic PCM
samples were symmetrically unimodal with low Đ values,
demonstrating that the PCM homopolymer had a very low
molar mass, like that of an oligomer, with negligible quantities.
In short, this simple way of preparing α,ω-hydroxyl-terminated
poly(lactone) shows significant advances over previously
reported synthetic methods.43,44
converted to carboxy end groups using succinic anhydride (SA)
as a reagent and dimethylaminopyridine (DMAP) as a catalyst
via a postpolymerization functionalization, designated as
HOOC-PCM-COOH(3), HOOC-PCM-COOH(6), and
HOOC-PCM-COOH(12).24,25 Upon completion of the
reaction, the resonance of the terminal methine proton (Ha)
of the hydroxyl-terminated PCM at δ 3.76 ppm disappeared,
and a new signal for the two terminal methylene protons of the
carboxy-terminated PCM was observed at δ 2.59 ppm in the 1H
NMR spectrum (He; Figures 1b,c and S2b,c) with high levels of
coupling (>99% conversion). HOOC-PCM-COOH(3), when
compared to that of HO-PCM-OH(3) (Figure S2d), also
exhibited new resonances in the 13C NMR spectrum at δ 176.3
(C1), 171.6 (C3), 71.4 (C4), and 28.8 (C8) ppm,
corresponding to the carbonyl groups, methine carbon, and
methylene carbons of the terminal carboxy group (Figure 1d).
To simplify the carboxy-terminated PCM synthesis further,
we developed a one-pot, two-step method. As shown in
Scheme 1, CM was first polymerized using a bulk system with
DEG and Sn(Oct)2 to obtain the corresponding polyols, which
were not isolated or purified. Instead, SA, DMAP, and 1,4-
dioxane were added to the reaction mixture, and heating was
continued at 90 °C for 50 h. The conversion of CM from
secondary hydroxyl group to carboxy group at the termini of
the PCM chains reached over 99%. The residual CM was a
constant 3−7% before and after the postpolymerization
functionalization of SA, suggesting that the CM was not
involved in the carboxy-telechelic PCM (Figures 1a,b and
S2a,b). As illustrated in Figure 1c, the relative integration
between the carboxy terminal PCM methylene protons (4
protons in the half chain) and the initiated DEG methylene
protons (4 protons in the half chain) led to a ratio of exactly 1
to 1 and suggested that the conversion was almost completed
with the range of functionalities (F = 1.97−2.05) (Table
1).19,45,46 Using another relative integration between the
methylene protons of the incorporated DEG portion and the
methine protons from the PCM repeating units in the 1H NMR
spectrum (Figures 1c and S2c), Mn values of 3.4, 5.6, and 12.4
kg mol−1 were determined for HOOC-PCM-COOH(3),
HOOC-PCM-COOH(6), and HOOC-PCM-COOH(12), re-
spectively, nearly identical to those calculated for the hydroxyl-
terminated PCM samples (Table 1). The Mn values for the
carboxy-terminated PCM samples using SEC with chloroform
as an eluent and calculated with poly(styrene) standards were
Synthesis of Carboxy-Telechelic Poly(carvomenthide). The
secondary hydroxyl terminal groups of the PCM samples were
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4.4, 7.4, and 16.5 kg mol−1, individually; the respective Đ values
were 1.10, 1.06, and 1.06 (Table 1 and Figure 2). These values
amine oxidation caused by high-temperature curing (∼180 °C).
During the thermal cross-linking reaction between carboxy and
aziridine functionalities, the acid played the role of a proton
donor for N-protonation of the aziridine; the acid followed this
by attacking the nitrogen atom to open the aziridine ring and
then to generate a carbocation as an intermediate, the
counterion COO−.48,49 The cured elastomers, which had high
degrees of cross-link density, were prepared using commercially
available trimethylolpropane tri(2-methyl-1-aziridine propio-
nate) (TAz) as a trifunctional cross-linking agent. Aziridine and
the carboxy-telechelic PCM were formulated using chloroform
as a solvent. The mixture was transferred to a perfluoroalkoxy
(PFA) Petri dish. The solvent in the cast solution was
evaporated at ambient temperature in an air flow hood; then,
the cast film was dried at 50 °C under reduced pressure to
completely remove remaining chloroform. The dish with the
mixture was placed in an oven preheated at 180 °C; the dish
with mixture was subsequently cured for 15 min, leading to
slightly yellow and transparent thermoset elastomers (Figure
S4). Three sets of elastomers were prepared using a method in
which α,ω-dicarboxy-functionalized PCM samples having
different molar masses (3.4, 5.6, and 12.4 kg mol−1) were
mixed with TAz at a 3:2.1 mole ratio. It was significant that the
mole ratio of the aziridine to the acid of the sets of elastomers
must be kept constant at about 1:1. The gel fraction test was
used to demonstrate that 0−3 wt % of soluble materials in the
cross-linked elastomer remained after the cross-linking reaction
owing to the presence of a small portion of unreacted
bifunctional chains and the resulting aziridine decomposition
during the curing (Table 2).
Thermal Properties. The thermal properties of the
hydroxyl-terminated PCM (HO-PCM-OH), carboxy-function-
alized PCM (HOOC-PCM-COOH), and cross-linked PCM
(XL-PCM) samples were studied by differential scanning
calorimetry (DSC, Figure 3). We observed a slightly higher
glass transition temperature (ca. −27 °C) for the HOOC-
PCM-COOH samples compared to those of the corresponding
polyols (ca. −36 °C) (Table 1). This small increment can be
expected given that hydrogen bonding between the terminated
carboxy group and repeating ester group in HOOC-PCM-
COOH polymers is stronger than that between the end
hydroxyl group and repeating ester group in HO-PCM-OH
polymers.50 However, the effect of having the carboxyl groups
on dimer formation ought to be more important for leading to
a higher cross-linking state. A slightly higher glass transition
Figure 2. Size-exclusion chromatography data (1 mg/mL chloroform)
for (a) HOOC-PCM-COOH(3), (b)HOOC-PCM-COOH(6), and
(c) HOOC-PCM-COOH(12).
were comparable to those of the starting hydroxyl-terminated
PCM samples. In addition, after measuring total acid number
(TAN) of the carboxy-telechelic PCM samples using chemical
titration (ASTM D664), the TAN values of 38.5, 20.2, and 8.2
mg KOH g−1 were altered to molar masses of 2.9, 5.6, and 13.6
kg mol−1, respectively (Table 1).
Taken together, all data related to the molar mass obtained
by the one-pot, two-step method indicated that the
postpolymerization functionalization did not induce adverse
side reactions leading to cleavage or cross-linking of polymer
chains. In sum, this simple method for the preparation of α,ω-
carboxy-terminated poly(lactone) using succinic anhydride was
a new attempt to make a renewable secondary hydroxyl-
terminated poly(lactone), going beyond previously reported
synthetic protocols regarding petroleum-based primary hydrox-
yl group-containing polyols.23−25,47
Preparation of Thermoset Elastomers. The carboxy
groups at the termini of the PCM chains can be used as a
reactive moiety for the prepolymer for the synthesis of higher
molar mass, optically clear, flexible, and stretchable elastomers
via thermal cross-linking (Scheme 1 and Figure S4). Using
HOOC-PCM-COOH(3), HOOC-PCM-COOH(6), and
HOOC-PCM-COOH(12), we carried out successful cross-
linking reactions with trifunctional amine to form a secondary
amine and an ester in the three-dimensional-network polymers;
however, there was slight polymer yellowing due to partial
a
Table 2. Characterization Data for Cross-Linked Carboxy-Telechelic Poly(carvomenthide)
g
residual strain εR (%)
storage
Young’s
tensile
strain at
degree of
gel
Tg
T
T
modulus G′
modulus E
strength σb
break εb
cross-linking
b
c
g d
d e
f
g
g
g
h
i
i
polymer
fraction (°C) (°C) (°C)
(105 Pa)
(MPa)
(MPa)
(%)
(n)
cycle 1 cycles 2 to 20
j
j
XL-PCM(3)
XL-PCM(6)
0.983
0.999
0.968
−15
−17
−19
−10
−14
−15
298
313
321
2.2−3.4
1.4−2.5
0.8−2.0
1.13 0.13
1.04 0.09
0.52 0.04
1.14 0.09
1.16 0.13
1.24 0.19
181
8
152 18
121 12
2−3
2−3
7−8
0−1
0−1
2−4
224 18
625 61
j
XL-PCM(12)
70
5
a
b
See Experimental Section for details. Determined by shaking the samples in a large excess of methylene chloride for 96 h, changing the solvent
c
every 24 h. The samples were dried at 50 °C until constant weight was obtained. Determined by DSC (2nd heating cycle) at 10 °C min−1.
d
e
Determined by DMA with rectangular torsion fixture. Oscillation test from −50 to 170 °C at 3 °C min−1, ω = 1.00 rad s−1 and γ = 1.0%. 5% weight
f
loss determined by TGA at 20 °C min−1 in N2. Determined by DMA with an 8 mm parallel plate geometry. Oscillatory frequency sweep test at 25
g
h
°C min−1; γ = 1.0% and ω = 0.1−100 rad s−1. Mechanical properties determined on an ASTM D1708 microtensile bars. Degree of cross-linking
i
(n) was calculated with E values obtained from the tensile tests. Percent of plastic deformation measured after 1 cycle or 20 cycles from 0 to 50%
j
strain at 130 mm min−1. 8.8, 5.5, and 2.8 wt % TAz were mixed with the carboxy-telechelic PCM samples before curing for XL-PCM(3), XL-
PCM(6), and XL-PCM(12), respectively.
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When DMA experiments with temperature were performed
with a rectangular torsion fixture, tension force was applied to
prevent curvature development in the samples caused by
thermal expansion. However, since XL-PCM polymers had a
rubbery property and could be elongated by tension force
above the glass transition temperature (Tg −20 to −10 °C), the
dimensions of the samples could be changed over a range of
temperatures, whereas the dimensions of samples in the
oscillatory frequency sweep test under isothermal conditions
were constant. The G′ values at 25 °C obtained by oscillatory
frequency sweep tests were used to calculate the entanglement
molar mass, Me.19 The G′ values of the XL-PCM(3, 6, and 12)
samples over a frequency range from 0.1 to 100 rad s−1 were
0.22−0.34, 0.14−0.25, and 0.08−0.20 MPa, respectively (Table
2 and Figure 4). The entanglement molar masses Me ≈ 7.6−
Figure 3. DSC analysis for HO-PCM-OH polyols, HOOC-PCM-
COOH prepolymers, and XL-PCM cross-linked polymers.
temperature (−19 to −15 °C) for the XL-PCM samples (Table
2) was also found in comparison with those of the
corresponding carboxy prepolymers. It is assumed that the
thermal cross-linking limited the motion of the polymer chain
ends.
The thermal stability of HO-PCM-OH, COOH-PCM-
COOH, and XL-PCM polymers was assessed using thermal
gravimetric analysis (TGA; Figure S5). Taking into consid-
eration the fact that the 5% mass loss (excluding small amounts
of solvent) temperatures of the polyols were in the range of
255−299 °C under a nitrogen atmosphere, the carboxy-
telechelic polymers and the PCM samples led to improved
thermal stabilities (293−317 and 298−321 °C, respectively)
(Tables 1 and 2). The TGA of the cured polymers in particular
showed a slightly higher decomposition temperature (5%
weight loss) than that of the carboxy prepolymers.
Dynamic Mechanical Properties. Dynamic mechanical
analysis (DMA) experiments of XL-PCM samples demon-
strated the effect of temperature and frequency on dynamic
elastic properties. The storage moduli (G′) of the cured
elastomers as a function of temperature and frequency are
shown in Figure S6 and 4.
The low-temperature (<−25 °C) shear modulus was
constant at about 1 × 109 Pa. Pronounced drops in G′ (Figure
S6a) and sharp tan δ peaks (Figure S6b) occurred around the
Tg values (−15 to −10 °C) of the XL-PCM samples; these
peaks and drops were observed by DSC (−19 to −15 °C)
(Figure 3). Therefore, the Tg values obtained from DSC and
DMA were in good agreement (Table 2). In addition, the Tg
values of the XL-PCM materials obtained by DSC and DMA
increased with decreasing PCM chain length, indicating a
higher density of cross-linking (Figure S6a).51 In particular, the
tan δ (= G″/G′) values decreased as the molar mass of the
prepolymer dropped, suggesting a higher cross-linking density
and greater hardness in the XL-PCM elastomers (Figure
S6b).52 The stable plateau in the storage modulus for the XL-
PCM(3, 6, and 12) samples started at ca. −5 to −10 °C and
extended to 170 °C. The G′ values of the XL-PCM(3, 6, and
12) samples decreased as the chain length between coupling
junctions of the XL-PCM materials increased and the
corresponding cross-linking decreased (Figure 6a).
Figure 4. Viscoelastic properties (storage modulus, G′) versus
frequency at 25 °C for the cured XL-PCM polymers.
11.7, 10.1−18.1, and 12.5−31.1 kg mol−1 for the XL-PCM(3, 6,
and 12), respectively, were approximated by the following
equation
Me = ρRT/G
where ρ is the density, R is the universal gas constant, T is the
temperature, and G is the storage shear modulus. The Me values
between coupling junctions in the cross-linked polymer is
inversely proportional to the density of the coupling
junctions.53 Although the calculated Me values are higher
than those of the carboxy-telechelic PCM samples, the Me
values increased as the chain length of the carboxy-telechelic
PCM samples increased (3.4, 5.6, and 12.4 kg mol−1,
respectively). However, further studies are needed to explain
the cause of difference between calculated Me values and the
molar masses of the PCM samples.
Tensile Properties. The mechanical properties of three
cross-linked elastomers (ASTM D1708 microtensile bars)
having different molar masses of HOOC-PCM-COOH (3.4,
5.6, and 12.4 kg mol−1) were investigated (Table 2 and Figure
5). All values in the analysis were obtained from at least four
XL-PCM samples. Linear responses at low strains (<20%) were
observed in the stress−strain (S−S) curves for all of the
thermally cured polymers. Young’s moduli (E) (0.52−1.13
MPa) for the XL-PCM samples increased significantly with an
increase of the trifunctional cross-linker (TAz) content (2.8,
5.5, and 8.8 wt %, respectively), indicating higher cross-linking
density. The cross-linking densities of the XL-PCM(3, 6, and
12) elastomers were expressed by n (moles of active network
chains per unit volume); these n values were 152 18, 121
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10, and 20 were compared (Figure 6). The reciprocating tensile
test showed small unrecoverable plastic deformation (ca. εR =
2−8%) in the first cycle, which might be caused by the initial
alignment of the microstructure (Table 2);58−60 however, the
XL-PCM(3 and 6) samples having a higher cross-link density
exhibited negligible values of residual strain (εR = 0−1%)
(Table 2 and Figure 6a,b) from cycles 2 to 20 when compared
to those of XL-PCM(12) with reduced connection points
(Table 2 and Figure 6c); the XL-PCM(3 and 6) samples also
maintained mechanical integrity, which is highly desirable for
elastomeric material. The results confirm that these thermoset
elastomers showed excellent levels of recovery with a maximum
strain of 50%.
In Vitro Cytocompatibility. The cured PCM elastomers
appeared to be biocompatible in vitro. In order to investigate
cell adherence and proliferation on the elastomer surface, NIH
3T3 mouse fibroblast cells were seeded homogeneously on
glass Petri dishes coated with three kinds of XL-PCM films as
well as with the control PLGA films. At day 1 after seeding, no
significant difference was found between XL-PCM samples and
PLGA (P < 0.05). At day 5, each XL-PCM material showed a
significantly higher growth rate than that on the control PLGA
(P < 0.05), showing no tendency toward molar mass among the
three XL-PCM groups, which was related to the elastic property
of the rubbers (Figure 7). The cellular morphology on the
Figure 5. Representative stress−strain curves for (a) XL-PCM(3), (b)
XL-PCM(6), and (c) XL-PCM(12) of cross-linked polymers.
12, and 70
5 mol m−3, respectively, calculated using the
following equation54
n = E/3RT
where E is Young’s modulus, R is the universal gas constant,
and T is the temperature. The n values also can be calculated
using the relation in which the Young’s modulus E is three
times the shear modulus G when Poisson’s ratio is 0.5 for
rubber-like materials.55 The n values from the G′ values for the
XL-PCM(3, 6, and 12) were 89−137, 57−101, and 32−81 mol
m−3, respectively. Therefore, the n values obtained from
oscillatory frequency sweep tests were comparable to those
obtained from tensile tests. We also observed that, when
compared to ultimate elongation of the XL-PCM(3 and 6) (ca.
181 and 224%), the elongation (εb) of XL-PCM(12) (ca.
625%) significantly improved with the increased length of the
carboxy-terminated prepolymer to 12.4 kg mol−1 and the
decreased cross-linking density. In contrast, the tensile
strengths (σb) of all XL-PCM samples were similar (1.14−
1.24 MPa) regardless of the increased chain extender content
or the PCM chain length.19 The mechanical strengths are
comparable or superior to those of previous reported
elastomers.56,57
Figure 7. Comparison of growth rate of NIH 3T3 mouse fibroblast
cells cultured on glass dishes coated with XL-PCM(3, 6, and 12) or
PLGA (control). MTT absorption measured at 570 nm.
To further understand the elastic recovery and level of plastic
deformation of the cured elastomers, a cyclic tensile test to
determine the true elasticity of the XL-PCM(3, 6, and 12)
samples was carried out, subjecting the tensile bars prepared by
ASTM D1708 to 20 loading/unloading cycles at a strain of 50%
with a constant speed of displacement (130 mm min−1). The
hysteresis loops of the cross-linked polymers for cycles 1, 2, 3,
control PLGA surface and on the XL-PCM surfaces at days 1
and/or 3 after seeding was visualized with a fluorescence
microscope for live/dead staining (Figure 8) as well as with a
bright-field light microscope (Figure S7). The cells on the
cured PCM coatings showed cellular morphologies and growth
Figure 6. Tensile recovery properties of (a) XL-PCM(3), (b) XL-PCM(6), and (c) XL-PCM(12) from 0 to 50% strain at 130 mm min−1 for 20
■
◆
●
cycles: five loadings (filled symbols) and unloadings (open symbols) were performed for cycles 1 ( ), 2 ( ), 3 ( ), 10 (▶), and 20 (★).
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Article
Figure 8. Live/dead cell staining images of NIH 3T3 mouse fibroblast cells. Carboxyfluorescein succinimidyl ester (CFSE, green) stains all cells, and
propidium iodide (PI, red) stains only dead cells. (a) All dead cells damaged by a 30 min incubation with 70% methanol were stained by PI (positive
control), whereas no cells were stained by PI after a 30 min incubation with a phosphate buffer solution (PBS, negative control). (b, c) No dead cell
staining was observed for NIH 3T3 mouse fibroblast cells cultured on glass dishes coated with PLGA and XL-PCM(12) at days 1 and 3.
rates superior to those on the control PLGA, suggesting that
COOH, and XL-PCM samples for sequential additions, SEC
traces for HO-PCM-OH polyols, optical images for the
thermoset PCM elastomer films, and TGA for all polymers.
This material is available free of charge via the Internet at
http://pubs.acs.org.
PCM is at least as biocompatible as that of PLGA in vitro.61
CONCLUSIONS
■
A thermoset elastomer system using renewable PCM derived
from plant-based materials and having superb mechanical
properties was designed. The hydroxyl terminated PCM
polyols having controlled molar masses were synthesized
through ROTEP of CM with Sn(Oct)2. Subsequently,
dicarboxy end-functionalized PCM prepolymers to produce
cured elastomers with no crystallinity, high viscosity, strong
thermal resistance, and low glass transition temperature were
prepared through esterification using SA. NMR spectroscopy
confirmed well-defined polymer structures, and SEC showed
narrow molar mass distributions. We also reported a one-pot,
two-step method for the synthesis of carboxy-telechelic
polyester. Using trifunctional aziridine, three carboxy-telechelic
PCM prepolymers having varied molar masses were thermally
cross-linked to give three-dimensional network polyester
elastomers. All polymers including polyols, prepolymers, and
elastomers exhibited high thermal resistance. Dynamic
mechanical analysis (DMA) of XL-PCM samples demonstrated
the viscoelastic properties of the XL-PCM biomaterials. Tensile
experiments demonstrated rubbery behaviors with exceptional
elongation at break, tensile strength, and recovery under static
or cyclic load. These results offer useful approaches for the
molecular design and synthesis of engineering thermoset
elastomers and related materials including tissue engineering
scaffolds.
AUTHOR INFORMATION
Corresponding Authors
*(Y.-W.K.) E-mail: ywkim@krict.re.kr.
*(J.S.) E-mail: jshin@krict.re.kr.
■
Author Contributions
#J.Y. and S.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.S. thanks the Korea Research Institute of Chemical
Technology (KK1404-A0) and the Ministry of Trade, Industry,
and Energy Republic of Korea (10048633) for financial support
of this work. The authors greatly appreciate Jeong-Mae Son and
Dong O Shin for experimental support.
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* Supporting Information
1H and 13C NMR spectra of carvomenthone, carvomenthide
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