Doubly degradable dynamers: Dynamic covalent polymers based on reversible imine connections and biodegradable polyester units

Article abstract of DOI:10.1039/c2gc35875a

We report the preparation and features of environmentally friendly Green Dynamers (GDs), dynamic polymers presenting sequential double-degradation features, chemical and biological. They were obtained by connecting biodegradable oligomers (such as small units of polybutylene adipate, PBA, and polybutylene succinate, PBS) through chemically degradable reversible imine bonds. The GDs possess water disintegratability, mendability and biodegradability. Their degradation rate may be modulated through and increases with the number of imine groups in the polymeric chain. The GDs were thermodynamically stable and maintained their molecular weight and their mechanical strength in air. However, when they were contacted with water, the imine bonds easily hydrolyzed, breaking up the polymer chains into oligomers. After such water disintegration, the residual oligomers were biodegraded into CO₂ and water. In addition, the imine bonds were restored by evaporation of the water, leading to recovery of the molecular weight and of the mechanical properties of the GDs. The present GDs may be considered to represent a class of doubly-degradable polymers, combining chemical and biological degradability features, that operate in mild environmental conditions. This concept may contribute to solve the problem of waste elimination associated with the use of non-degradable polymeric materials.

Full text of DOI:10.1039/c2gc35875a

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PAPER
Doubly degradable dynamers: dynamic covalent polymers based on
reversible imine connections and biodegradable polyester units
Kazuyuki Fukuda,a,b Manabu Shimoda,b Makoto Sukegawa,b Tadahito Nobori*a,b and Jean-Marie Lehn*a
Received 8th June 2012, Accepted 6th August 2012
DOI: 10.1039/c2gc35875a
We report the preparation and features of environmentally friendly “Green Dynamers” (GDs), dynamic
polymers presenting sequential double-degradation features, chemical and biological. They were obtained
by connecting biodegradable oligomers (such as small units of polybutylene adipate, PBA, and
polybutylene succinate, PBS) through chemically degradable reversible imine bonds. The GDs possess
water disintegratability, mendability and biodegradability. Their degradation rate may be modulated
through and increases with the number of imine groups in the polymeric chain. The GDs were
thermodynamically stable and maintained their molecular weight and their mechanical strength in air.
However, when they were contacted with water, the imine bonds easily hydrolyzed, breaking up the
polymer chains into oligomers. After such water disintegration, the residual oligomers were biodegraded
into CO2 and water. In addition, the imine bonds were restored by evaporation of the water, leading to
recovery of the molecular weight and of the mechanical properties of the GDs. The present GDs may be
considered to represent a class of doubly-degradable polymers, combining chemical and biological
degradability features, that operate in mild environmental conditions. This concept may contribute to
solve the problem of waste elimination associated with the use of non-degradable polymeric materials.
very serious waste problems.20 In addition, biodegradability is
also required for therapeutic and medical applications. It presents
Introduction
Dynamic polymers (dynamers) are reversible polymeric entities
of either a molecular/covalent or supramolecular/non-covalent
nature.1,2 Covalent dynamers result from the polycondensation
of monomers via reversible chemical reactions generating labile
chemical bonds whose formation and dissociation can be con-
trolled through the physical and chemical conditions of their
environment. Therefore, such dynamers have an adaptive ability
under the effect of external physical stimuli or chemical
effectors.3–10 They provide the possibility of developing new
functional materials with water disintegrative,11 self-healing12–15
and drug-release16–19 properties.
Currently, thermoplastic polymers are widely used as raw
materials in various industrial fields, largely because of their
moldability and good mechanical properties.20 If the distinctive
abilities of dynamers, such as disintegration or self-repairing,
were introduced to thermoplastic polymers by linking their oli-
gomers through reversible bonds, it could open the door to new
applications in flushable, construction, agricultural and medical
materials. Biodegradability is also a highly desirable property for
such applications, because the persistent, non-biodegradable
polymeric materials present high environmental load and cause
great merits such as easing the damage to tissues and organs
from surgical processes, whilst also reducing costs. Drug deli-
very and implant systems from dynamers would offer the possi-
bility of controlling and prolonging the effective period of
therapeutic agents, whereas non-biodegradable ones require a
removal process from the body at the end of their activity
period.16,21
Herein we report on the development of such degradable
dynamic polymers, “green dynamers”, based on biodegradable
polyesters and polyethers, presenting the desirable properties
mentioned above and eliminating many of the problems asso-
ciated with the use of the usual “static”, persistent polymers.
Scheme 1 presents the concept of green dynamers (GDs). They
can be obtained by connecting biodegradable oligomers, such as
small units of poly(butylene adipate) (PBA), poly(butylene suc-
cinate) (PBS), poly(lactic acid) (PLA), polyethylene glycol,
PEG, and other biodegradable polymers, through dynamic,
reversible imine bonds. The imine bonds are thermodynamically
stable and maintain the molecular weight and the mechanical
strength of ordinary polymers. However, they readily undergo
hydrolysis22 and cleave to aldehydes and amines even at room
temperature. One may thus expect that, after disintegration by
water, the residual oligomers are gradually biodegraded to CO2
and water. The rate of water disintegration is much faster than
that of biodegradation. In addition, GDs possess self-healing
properties because the imine bonds can be restored by evapo-
ration of the water.
aISIS, Université de Strasbourg, 8 allée Gaspard Monge, 67000
Strasbourg, France. E-mail: lehn @unistra.fr;
Fax: (+33)-3–6885-5115; Tel: (+33)-3-6885-5145
bR&D Center, Mitsui Chemicals Inc., 580-32 Nagaura, Sodegaura,
Chiba 299-0265, Japan
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of N,N′-(1,4-phenylenedimethlyidene)-bis(2-(2-aminoethoxy)-
ethanol) as a pale yellow solid. mp. 44–46 °C; 1H NMR
(400 MHz, DMSO): δ = 3.44 (m, 8H), 3.68 (m, 8H), 4.58 (t,
2H), 7.79 (s, 4H), 8.37 (s, 2H); elemental analysis calcd (%) for
C16H24N2O4: C 62.3, H 7.8, N 9.1; found: C 62.3, H 8.1, N 9.1.
Preparation of biodegradable oligomers
Poly(butylene succinate) (PBS). To a 1 L separable flask
equipped with a water quantification receiver with a condenser, a
thermometer, a bent tube and an agitation blade, were added
320.40 g (2.70 mol) of succinic acid (Wako Pure Chemical
Industries) and 303.5 g (3.30 mol) of 1,4-butanediol (Wako Pure
Chemical Industries) and the mixture was heated progressively
under a nitrogen atmosphere to 180 °C. When water formation at
180 °C was hardly observed, 1.32 g of a 1% tin(II) 2-ethylhexyl-
hexanoate toluene solution (Wako Pure Chemical Industries) was
added to the reaction mixture and the reaction was continued
until Mn measured by titration increased to 1950. The PBS with
different Mn and PBAwere also synthesized by similar methods.
Scheme 1 Schematic representation of water-disintegration, self-
healing and biodegradation processes of a green dynamer.
Experimental
Materials
All reagents and solvents were purchased at the highest commer-
cial quality and were used without further purification.
Preparation of green dynamers. To a 200 ml separable flask
equipped with a condenser, a thermometer and an agitation
blade, were added 100.00 g (hydroxyl group 0.103 mol) of PBS
(Mn 1950) and 15.80 g (hydroxyl group 0.103 mol) of bis-imine
compound. After the mixture was melted at 130 °C under nitro-
gen atmosphere, 0.89 g of 1% dibutyltin(IV) dilaurate toluene
solution (Wako Pure Chemical Industries) was added. Thereafter,
16.45 g (isocyanate group: 0.195 mol) of hexamethylenediiso-
cyanate (HDI) (Mitsui Chemicals) was added dropwise over
10 min and then HDI that remained in the dropping funnel was
rinsed with 1.30 g of toluene. After continuing the reaction at
130 °C under nitrogen for 3 h, the corresponding polymeric
material, of green dynamer type, was obtained. The green dyna-
mers based on other oligomers such as PBA and PEG were also
synthesized by similar methods.
Measurements
1H NMR spectra were recorded on Bruker Avance 400 spec-
trometers. The spectra were internally referenced to the residual
proton solvent signal. The molecular weight of polymers was
determined by gel permeation chromatography (GPC). Poly-
styrene was used as a standard substance. A water solubility test
of the film was carried out by weighing the film immersed in dis-
tilled water at 35 °C for 24 h and dried. Water disintegratability
of a film was tested in distilled water (around pH 7). The tensile
strength and tensile modulus were measured by a tensile tester
(Instron1123) at a pull rate of 50 mm min−1. The differential
scanning calorimetry measurement were carried out using DSC
system (SII DSC7020). The glass transition temperature (Tg) was
obtained from the second heating at a heating rate of 10 °C min−1
.
Calculation of the number average molecular weight, Mn,
of GDs in water disintegration
Thermogravimetric analysis (TGA) was performed on a thermal
analyzer system (Shimadzu TA-60) at a heating rate of 10 °C
min−1 under nitrogen atmosphere. A biodegradability test of a
film was carried out according to ISO14855.
The number average molecular weight, Mn, of GDs in the course
of disintegration in water was calculated from NMR data using
the following eqn (1).
Mn ¼ Mn;initial=ð1 þ n ꢀ ꢀ ꢀ α=100Þ
where Mn,initial is the initial and theoretical Mn calculated as:
Mn;initial ¼ Mn;HDI ꢀ ꢀ ꢀ n þ Mn;OH ꢀ ꢀ ꢀ ðn þ 1Þ
in which Mn,HDI is the molecular weight of HDI, Mn,OH is the
average of molecular weight of bis-imine component and bio-
degradable oligomer, n is the number of repeating unit [n =
R/(1 − R)] which is determined by the ratio between the number
of isocyanate groups and hydroxy groups of the starting
materials (R = NCO/OH), and α is the mol% of aldehyde (new
end groups formed by water disintegration) which is calculated
from the NMR data as:
ð1Þ
Synthesis of the bis-imine component
N,N′-(1,4-Phenylenedimethlyidene)-bis(2-(2-aminoethoxy)
ethanol). To a 300 ml separable flask equipped with a Dean-
Stark trap with a condenser, a thermometer, a glass agitation
blade and a dropping funnel, were added 13.40 g (0.10 mol) of
terephthalaldehyde (Sigma-Aldrich) and 120 ml of toluene. The
mixture was stirred and chilled below 5 °C in an ice water bath.
Then, 21.46 g (0.20 mol) of 2-(2-aminoethoxy)ethanol (Tokyo
Chemical Industry: TCI) was added dropwise to the flask over
30 min. Thereafter, the mixture was refluxed until water for-
mation stopped. The mixture was cooled to room temperature
and then filtered to collect the precipitate which was washed
with 120 ml of toluene. The crystalline solid obtained was dried
under a nitrogen atmosphere at 50 °C overnight to yield 28.3 g
ð2Þ
α ¼ ð2IA þ IBÞ ꢀ ꢀ ꢀ 100=ð2IA þ 2IB þ 2ICÞ
ð3Þ
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where IA, IB, and IC stand for the integral of the aromatic proton
of units A, B, and, C in Fig. 3, respectively.
Results and discussions
Preparation, mechanical properties, and thermal properties of
green dynamers
Fig. 1 Pictures of a roll film (20 cm × 60 m × 30 μm) and of a non-
woven fabric made of GD based on PBS (Mn = 2500).
Scheme 2 lines out the preparation of the green dynamers. They
were obtained by connecting a PBS oligomer presenting two
hydroxyl groups and a diol compound incorporating imine
bonds through hexamethylene diisocyanate (HDI). GDs based
on other oligomers were also prepared using similar methods,
only exchanging PBS for the desired oligomers. Films were
obtained by heat pressing the material at 130 °C for 5 min and
the mechanical properties of these films were evaluated.
Table 1 shows the mechanical properties of GDs films with
respect to low density polyethylene (LDPE) films. They had
excellent tensile strengths comparable to those of LDPE. Fur-
thermore, the mechanical properties of the GDs were controlla-
ble by simply changing the type of oligomers, as these
determine the mechanical properties. GDs based on PBS
(75 wt%) and PEG (25 wt%) oligomers had almost half the
tensile modulus compared to those made of only PBS. The intro-
duction of PEG into the GD based on PBS decreased the tensile
modulus. This preparation method gives easy control of the
mechanical properties, a notable subject in material science, as
they play a crucial role in applications.3b These GDs also
showed thermoplasticity, a feature of much importance for
making roll films, laminated films, or nonwoven fabrics (Fig. 1).
Thermal characterization of the GD based on PBS was accom-
plished by DSC and TGA. The Tg of GD based on PBS was
−36 °C, which is almost the same as the Tg of PBS. The weight
losses (under nitrogen atmosphere) were 1, 5, and 50% at
Fig. 2 Pictures of the water disintegration tests at 23 °C after 80 h for
PBS film (a) and green dynamer films (thickness 15 mm) based on PBS
of Mn = 6300 (b) and Mn = 2500 (c).
temperatures of 278, 319, 384 °C, respectively. The GD also
possessed a good thermal stability.
Water disintegratability and self-healing properties of green
dynamers
First, a water solubility test was performed with a film (thickness
100 μm) of GD (Mn = 18 000 by GPC) based on PBS (Mn =
2500). When the film was immersed in distilled water at 35 °C
for 24 h and dried, the weight retention rate of the film was
98.8 wt%, which confirms that this film was not water soluble.
Water disintegratability of films (size: 11 cm2; thickness:
15 μm) having a different number of imine bonds was evaluated
in distilled water (around pH 7). Fig. 2 shows the photos of the
water disintegration test at 23 °C after 80 h for (a) a PBS film
without imine bonds and GD films based on PBS of (b) Mn =
6300 and (c) Mn = 2500. The conventional PBS film (a) did not
change shape even over 7 d. On the other hand, marked changes
in shape were observed in the GD films (b) and (c). The film (c)
based on PBS (Mn = 2500) disintegrated from an area of 11 cm2
into 1–2 cm2 fragments and the Mn measured by GPC decreased
from 18 000 to 8500. The film (b) (PBS, Mn = 6300) had a small
crack after 80 h. It disintegrated to small pieces like film (c) after
185 h. The slower rate of water disintegration of film (b) com-
pared to (c) may be attributed to a lower number of imine bonds.
Thus, the disintegration rate depends on and may be modulated
by the number of imine bonds in the dynamer, the rate increasing
with the number of imine bonds.
Scheme 2 Preparation of green dynamers by connecting a PBS oligo-
mer with a diol compound, containing imine bonds, through conden-
sation with hexamethylene-1,6-bisisocyanate.
Table 1 Mechanical properties of green dynamers and LDPE
“Green” dynamers
NMR studies were carried out to elucidate the mechanism of
disintegration by water. Fig. 3 shows the NMR spectra of GD
films (thickness 15 μm) before (a) and after immersing in dis-
tilled water at 50 °C for 24 (b) and 240 h (c). Before immersion,
the NMR spectrum showed very small peaks of aldehyde
derived from unit B (one imine and one aldehyde group). These
aldehydes were formed in the film preparation process at high
PBS (75 wt%)
PEG (25 wt%)
Based polymer
PBS
PBA
LDPE
Melting point (°C)
Tensile strength (MPa)
Tensile modulus (MPa)
105
24
490
107
27
600
105
20
210
110
20
150
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Fig. 4 Changes in mean molecular weight, Mn, of green dynamers
soaked in water at different temperatures of 23, 35, and 50 °C.
Fig. 3 NMR spectra of green dynamer films before (a) and after
soaking in distilled water at 50 °C for 24 (b) and 240 h (c).
automatic potentiometric titrator. The degree of biodegradation
of the films was calculated from the amounts of CO2 and the
theoretical amount of CO2 of the samples. It was found to be
16.0 and 66.2% after 10 and 38 d, respectively. Thus GDs
indeed possess biodegradation properties, which reduce the
environmental load.
temperature. As the immersion time in water became longer, the
peaks of unit B and unit C increased with respect to those of unit
A, containing the fully formed two imine groups. This means
that the imine bonds dissociate to aldehyde and amine through
hydrolysis, thus causing the water disintegration of the GD film.
The proton NMR data also indicate that the deformation of the
shape of the film did not need a large amount of dissociation of
imine bonds. The amount of aldehyde calculated from NMR was
only 19% after 240 h which is enough for the deformation of the
shape of GD films, as illustrated in Fig. 2(c).
Fig. 4 shows the changes of Mn calculated from the NMR
spectra of GD films immersed in water at different temperatures.
As may be expected, disintegration was accelerated at higher
temperatures. Thus, GDs are stable for use, without decompo-
sition on contact with moisture in the air or a small amount of
water, due to their insolubility, but disintegrate on contact with a
large amount of water. In addition, the time it takes for disinte-
gration can be controlled from 1 h to a few days simply by chan-
ging the number of imine bonds (with respect to the length of
the polymeric chain) in the polymer molecule.
The self-healing properties of dynamers are also of much
interest for numerous applications.12–15 To investigate the self-
healing properties of GDs, NMR measurements were carried out
on films as just prepared, after immersing in water at 50 °C for
14 h, and after subsequent drying at 80 °C for 240 h. After
immersing in water, the Mn calculated from NMR data decreased
from 13 000 to 7000. When the film was dried at 80 °C for
240 h, the Mn of the film recovered to 13 000, which is compar-
able to the Mn before immersion in water, thus demonstrating
that the GDs possess self-healing properties.
Conclusions
In conclusion, green dynamers presenting moldability and good
mechanical properties, also demonstrate excellent water disinte-
gratability, mendability and biodegradability. The films do not
deteriorate in shape or disintegrate with moisture in the air
because the GDs investigated here do not have water solubility.
They lose their shape only when they come into contact with a
large amount of water. Therefore, the GD films can be stored in
a normal atmosphere, but the imine bonds tend to dissociate at
higher temperatures in the presence of a small amount of water.
Products made of green dynamers can be discharged into toilet
bowls or kitchen sinks without any clogging of pipes because of
their water disintegratability.11 After being discharged into the
environment, they are biodegraded by microorganisms, in a
sewage disposal treatment center or in the countryside, thus pre-
serving the natural environment from contamination.20 GDs can
be used in domestic and personal care, in hygienic products, in
agricultural and horticultural materials, in civil engineering, in
construction materials, as well as in pharmaceutical preparations
and medical devices.16,21 A vast range of specialized as well as
consumer products can take advantage of the remarkable proper-
ties displayed by dynamers1,2 in general, and green dynamers in
particular.
Acknowledgements
We thank Mitsui Chemicals Inc. for financial support of this
work.
Biodegradatability of green dynamers
A biodegradability test was performed using films of GDs based
on PBS (Mn = 1950). The film with a 30 μm thickness was well
mixed in activated compost23 and transferred to a reaction
vessel. The vessel was kept at 58 2 °C and the amounts of
CO2 produced were measured by periodical titration using an
Notes and references
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C/N ratio and pH of compost were 12 and 8.7, respectively.
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