Network reorganization of dynamic covalent polymer gels with exchangeable diarylbibenzofuranone at ambient temperature

Article abstract of DOI:10.1021/ja5065075

Reversible bonds and interactions have been utilized to build stimuli-responsive and reorganizable polymer networks that show recyclability, plasticity, and self-healing. In addition, reorganization of polymer gels at ambient temperature, such as room or

Full text of DOI:10.1021/ja5065075

Article
pubs.acs.org/JACS
Network Reorganization of Dynamic Covalent Polymer Gels with
Exchangeable Diarylbibenzofuranone at Ambient Temperature
Keiichi Imato,†,‡ Tomoyuki Ohishi,‡ Masamichi Nishihara,§ Atsushi Takahara,*,†,§
and Hideyuki Otsuka*,‡
†Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
‡Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550,
Japan
§Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
S
* Supporting Information
ABSTRACT: Reversible bonds and interactions have been
utilized to build stimuli-responsive and reorganizable polymer
networks that show recyclability, plasticity, and self-healing. In
addition, reorganization of polymer gels at ambient temperature,
such as room or body temperature, is expected to lead to several
biomedical applications. Although these stimuli-responsive proper-
ties originate from the reorganization of the polymer networks, not
such microscopic structural changes but instead only macroscopic
properties have been the focus of previous work. In the present
work, the reorganization of gel networks with diarylbibenzofur-
anone (DABBF)-based dynamic covalent linkages in response to
the ambient temperature was systematically investigated from the
perspective of both macroscopic and microscopic changes. The
gels continued to swell in suitable solvents above room temperature but attained equilibrium swelling in nonsolvents or below
room temperature because of the equilibrium of DABBF linkages, as supported by electron paramagnetic resonance
measurements. Small-angle X-ray scattering measurements revealed the mesh sizes of the gels to be expanded and the network
structures reorganized under control at ambient temperature.
covalent linkages by swelling measurements, small-angle X-ray
scattering (SAXS) measurements, and mechanical tests.
INTRODUCTION
■
Cross-linked polymers have a wide range of applications, from
hard structural materials to soft rubbers and gels, because of
their three-dimensional network structures. The polymer
networks are formed by physical associations between polymer
chains and/or covalent cross-linking. Reversible non-covalent
cross-links afford stimuli-responsive and reorganizable net-
works, which show recyclability, plasticity, and self-healing,
while robust covalent cross-links result in unchangeable
networks.1−4
Similarly, cross-linked polymers with reversible covalent
bonds based on dynamic covalent chemistry5 can change their
structures in response to external stimuli (temperature, pH,
light, catalyst, and so on),6,7 making them processable, self-
healable, and environmentally adaptive.4,8−14 Although these
stimuli-responsive properties originate from the rearrangement
or reorganization of the polymer networks, researchers in the
field of dynamic covalent chemistry have focused only on
macroscopic properties such as self-healing, and microscopic
structural changes have not been investigated. Several studies
have demonstrated reorganization of networks incorporating
photo-,15,16 additive-,17 or catalyst-responsive18,19 dynamic
To the best of our knowledge, no studies have focused on
both macroscopic and microscopic autonomous structural
changes of chemically cross-linked polymers at ambient
temperature, such as room temperature or body temperature,
although these could be suitable for many relevant applications.
Soft tissues in the human body, such as tendons, ligaments,
cartilage, and muscle are all cross-linked polymers containing
water (hydrogels); recently, artificial biocompatible gels with
excellent mechanical performance like these soft tissues have
been achieved.20 Therefore, reorganization of polymer gels at
ambient temperature has great potential to be applied to
biomaterials, actuators, and controlled drug delivery systems.
In the present study, we examined the network reorganiza-
tion at ambient temperature of polymer gels cross-linked by
autonomously exchangeable dynamic covalent linkages, specif-
ically, diarylbibenzofuranone (DABBF) units. DABBF exists in
an equilibrium between its dissociated and associated states at
room temperature (Figure 1), although usual dynamic covalent
Received: June 27, 2014
Published: August 6, 2014
© 2014 American Chemical Society
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The fluidity of the reaction mixture slowly decreased during the
course of the reaction. The reaction mixture completely lost
fluidity after 48 h, indicating that polymerization was successful.
After polymerization, the crude gel was immersed in 1,4-
dioxane to remove the catalyst and unreacted monomers and
subsequently freeze-dried. A typical cross-linked polymer was
prepared and used as a control sample. The control polymer
shared the same structure of the cross-linked polymer with
DABBF but had no DABBF linkage. It was prepared from PEG,
HDI, and tetrahydric 2,2-bis(4-hydroxyphenyl)propane (bi-
sphenol A) in a manner outlined for the polymer cross-linked
by DABBF (Figure 1).
De-Cross-Linking of Cross-Linked Polymers. To
determine whether DABBF linkages in the cross-linked
polymer were in a state of equilibrium and able exchange
their bonds with one another, a de-cross-linking reaction was
performed in tetrahydrofuran (THF) in air at room temper-
ature by addition of an excess of another DABBF derivative, as
published elsewhere.24,25 After the de-cross-linking reaction (42
h), a THF-soluble high-molecular-weight component (Mn =
6600) was detected by gel-permeation chromatography (GPC)
measurements (Figure S1 in the Supporting Information). This
was expected to be a linear polymer and/or a cross-linked
oligomer. Although we tried to perform a detailed analysis by
fractionation of this component, cross-linking occurred again
1
after fractionation. However, the H NMR spectrum of the
solution without fractionation after de-cross-linking clearly
indicated the presence of soluble PEG and HDI, which
constitute the polymer network (Figure S2 in the Supporting
Information). Consequently, we confirmed that the DABBF
linkages in the polymer network were in an equilibrium state in
air at room temperature and that de-cross-linking of the cross-
linked polymer occurred through bond exchange of the DABBF
linkages.
Effect of Temperature on Reorganization of Polymer
Networks. The reorganization of the polymer networks was
evaluated by monitoring the swelling behavior in several
solvents at ambient temperatures (0, 15, 25, 35, and 45 °C).
The swelling degree (Q) is defined as the weight ratio of the
amount of absorbed solvent to that of the dry polymer, as
follows (eq 1):
Figure 1. Chemical structures of diarylbibenzofuranone (DABBF) and
the cross-linked polymers.
linkages need external stimuli to induce their reversibility.21,22
A
carbon-centered radical formed from cleaved DABBF shows
greatly attenuated reactivity toward oxygen compared with
other carbon-centered radicals.23 Therefore, DABBF is a rare
dynamic covalent linkage that functions under mild conditions,
such as in air at room temperature and physiological
temperature. We have previously reported that gels cross-
linked by DABBF can macroscopically self-heal and that
micelles with DABBF in their inner core are characterized by
reversible cross-linking and de-cross-linking in air at room
temperature.24,25 However, structural changes of polymer
networks with DABBF have not been evaluated. Thus, in this
study, we investigated the reorganization of the network
structures in a DABBF-containing cross-linked polymer and the
effect of temperature on the structural changes. These were
systematically evaluated using swelling, electron paramagnetic
resonance (EPR), and SAXS measurements.
Wwet − W
dry
Q =
× 100%
W
dry
(1)
where Wdry and Wwet are the weights of the dried and wet gels,
respectively.26
Figure 2 shows the swelling behavior of the cross-linked
polymers containing DABBF and bisphenol A in different
solvents at 0, 15, 25, 35, and 45 °C. The cross-linked polymer
with DABBF was swollen with THF, and the obtained gel
attained equilibrium swelling within 24 h at low temperatures
such as 0, 15, and 25 °C (Figure 2a). At relatively higher
temperatures such as 35 and 45 °C, however, the gel did not
reach equilibrium swelling but continued to swell. The swelling
degree in THF after 120 h increased with increasing
temperature. In contrast, the bisphenol A-containing control
polymer was swollen with THF, and the gel attained
equilibrium swelling within 24 h at all temperatures (Figure
2e). The swelling degree showed almost the same value at all
temperatures, which was close to that of the DABBF-containing
gel at 0 °C. This is a significant difference between dynamic
covalent and covalent systems. Thus, our data clearly indicated
RESULTS AND DISCUSSION
■
Preparation of Cross-Linked Polymers. A polymer
cross-linked by DABBF was prepared by polyaddition of
poly(ethylene glycol) (PEG) (Mn = 1000), hexamethylene
diisocyanate (HDI), and a tetrahydric DABBF cross-linker in
the presence of di-n-butyltin dilaurate (catalyst) in 1,4-dioxane
in a manner similar to that previously reported (Figure 1).24
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Figure 2. Swelling behavior of the cross-linked polymers at ambient temperatures (0, 15, 25, 35, and 45 °C). Shown are plots of swelling degree (Q)
vs time for the DABBF-containing cross-linked polymer in (a) THF, (b) DMF, (c) 1,4-dioxane, and (d) water and for the control cross-linked
polymer in (e) THF and (f) DMF.
that the network structure incorporating DABBF reorganized
and expanded at relatively higher temperatures as a result of the
bond exchange of the DABBF linkages, with this unique
behavior originating from the equilibrium.
This suggests that the swelling is caused by the imbalance
between the repulsive forces among polymer chains and the
contractile stretching forces of the elastically active networked
structures. The polymer chains showed less compatibility with
water because water is a nonsolvent for the polymer. Therefore,
the repulsive forces were not generated, and the network
structure was not affected by temperature. The swelling
behavior of the control cross-linked polymer in DMF was
similar to that in THF and unaffected by temperature (Figure
2f). The swelling behavior and the swelling degrees of the
DABBF-containing cross-linked polymer in other solvents at
room temperature are shown in Figure S3 and Table S1 in the
Supporting Information.
As shown in Figure 2b,c, the swelling behavior and the
swelling degrees of the DABBF-containing cross-linked
polymer in N,N-dimethylformamide (DMF) and in 1,4-dioxane
were similar to those in THF, and the polymer finally dissolved
in THF, DMF, and 1,4-dioxane at 50 °C within 1 day, while the
cross-linked polymer was hardly swollen with water and the
swelling degrees were unaffected by temperature (Figure 2d).
In neutral polymer gels, the increase in entropy associated with
polymer solvation leads to stretching of the polymer chains
between cross-linking points, despite their restorative forces.27
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To investigate the equilibrium state of the DABBF linkages
in the gel network, variable-temperature EPR measurements
were performed. Figure 3a shows EPR spectra of the DABBF-
properties. The amount of the dissociated radical at 50 °C was
certainly larger than that at 0 °C, but the change was very small
in the material, from 0.001% to 0.01%.
Reorganization of Networks with Reversible Temper-
ature Changes. To provide a deeper understanding of the
effect of temperature on the network reorganization, the
swelling behavior with reversible temperature changes was
investigated. As shown in Figure 4, the polymer cross-linked by
Figure 4. Swelling behavior of the cross-linked polymers with
reversible temperature changes (0 and 40 °C): (top) swelling degrees
of the DABBF-containing cross-linked polymer (green) and the
control cross-linked polymer (black) in THF and (bottom) photo-
graphs of the swelling behavior of the DABBF-containing cross-linked
polymer.
Figure 3. (a) EPR spectra of the DABBF-containing cross-linked
polymer swollen with THF and (b) percentage of dissociated DABBF
linkages at different temperatures.
containing cross-linked polymer swollen with THF measured
over the temperature range from −100 to 50 °C. The g values
of these spectra were determined to be 2.0040, suggesting the
presence of oxygen radicals and carbon radicals. Therefore, the
detected spectra were derived from these radicals, which were
formed from cleaved DABBF units. The chemical structures of
plausible generated radicals are shown in Figure S4 in the
Supporting Information. The intensity of the peaks increased
with increasing temperature, indicating that the equilibrium of
DABBF shifted to the dissociated side. We assumed that the
equilibrium simply involves the associated DABBF and the
dissociated radicals without irreversible side reactions; under
this assumption, we calculated the ratio of dissociated DABBF
from the peak intensity by using 4-hydroxy-2,2,6,6-tetrame-
thylpiperidin-1-oxyl (TEMPOL) as a standard (Figure 3b).
Although the amount of dissociated radicals was extremely low
below 0 °C (less than 0.001%), the amount of dissociated
radicals exponentially increased above 0 °C. The steep increase
in the ratio of the dissociated DABBF linkages reduced the
contractile restorative forces generated by swelling, resulting in
continuous swelling. This indicated that the above-mentioned
unique and temperature-dependent reorganization is caused by
the extreme sensitivity of the equilibrium to the temperature.
Notably, the amount of dissociated radical was found to be
less than 0.01% even at 50 °C. The DABBF-containing gels
attained equilibrium swelling at low temperature because
almost all of the DABBF linkages were connected. The slight
increase in the percentage of the dissociated DABBF upon
heating, however, led to macroscopic changes in the material
DABBF and the control polymer were swollen with THF at 0
°C (light-blue area) and 40 °C (pink area). As expected, the
swelling degrees of the control polymer did not change with
temperature. While little change was also observed in the
swelling degree of the DABBF-containing gel at 0 °C, the gel
was drastically swollen at 40 °C, and the swelling degree
significantly increased. The temperature-dependent swelling
was confirmed by the volume changes shown in the
photographs in Figure 4. The changes were irreversible, and
the gel attained new equilibrium states. This may be due to the
compatibility of the polymer chains with THF, which leads to
high repulsive forces, and to the network, which reorganized
upon swelling at 40 °C. Reversible volume changes of some
hydrogels with physical cross-links in response to external
stimuli have been reported.28−31 The networks had stable
covalent cross-links, and the number of cross-links drastically
changed in response to external stimuli by dissociation and
association of physical cross-links, resulting in reversible
changes with upper volume limits. This may be due to the
DABBF-containing gel, which shows no stable cross-links, and
to the fact that the number of cross-links hardly changed when
the external stimulus was applied. Similar behavior was also
observed in the DABBF-containing cross-linked polymer
swollen with DMF, although the response was slightly worse
than that in THF (Figure S6 in the Supporting Information).
These results indicated that the gels cross-linked by DABBF
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swelled and reorganized in response to the ambient temper-
ature and that the network structures can be easily controlled
with the ambient temperature.
the experimental results. Figure 5b shows the time dependence
of the correlation length obtained from the fitting curves. The
correlation length increased with increasing swelling time, from
1.1 to 2.4 nm after 4 days. Similar behavior was observed in the
swelling of the DABBF-containing gel in DMF (Figure S7 in
the Supporting Information). These results support the concept
that the networks reorganized and the mesh sizes expanded
through the bond exchange of the DABBF linkages in response
to the ambient temperature. Therefore, the polymer networks
and the mesh sizes can be controlled using the swelling time
and the ambient temperature.
A plausible mechanism for the network reorganization in
response to the ambient temperature is shown in Figure 6.
Below room temperature, the gel attains the equilibrium
swelling because the repulsive forces among the polymer chains
and the contractile forces due to stretching of elastically active
networked structures become equal in magnitude. The ratio of
dissociated DABBF increases while the contractile restorative
forces decrease with increasing temperature. These changes
lead to an imbalance between the repulsive forces and the
contractile forces, resulting in expansion of the network
(continuous swelling). After the reorganization, the gel attains
a new state of equilibrium at low temperatures. In the extreme
case of increased intramolecular cross-linking due to the
reorganization, the cross-linked polymers can completely
dissolve in some solvents at ambient temperature.
If the network structure reorganizes, its mesh size is expected
to change. Therefore, the variations of the network structures in
the DABBF-containing gels were investigated by SAXS, and the
correlation lengths (ξ) of the gels were evaluated. The latter
represents the mesh size consisting of polymer chains in a
semidilute solution, which was determined by fitting eq 2 to
SAXS profiles.32,33 Equation 2 consists of two components,
namely, randomly distributed noninteracting domains in the
networks (stretched exponential formalism) and the correlation
between the polymer chains (Ornstein−Zernike formalism), as
follows:
I
soln(0)
I(q) = Iex(0) exp[−(qΞ)α] +
1 + ξ2q2
(2)
where Iex(0) and Isoln(0) are constant values, α is also a positive
constant in the range of 0.7 to 2, Ξ is the mean size of static
nonuniformities or the mean size of static concentration
fluctuations, and q is the scattering vector. Figure 5a shows
SAXS profiles, fitting curves described by eq 2, and the
correlation lengths of the gel swollen with THF for 0 to 4 days
at 40 °C. In all cases, the fitting curves successfully represented
CONCLUSIONS
■
We have shown the network reorganization of gels covalently
cross-linked by DABBF linkages at ambient temperature. The
gels were successfully prepared by polyaddition of PEG, HDI,
and a DABBF cross-linker. De-cross-linking occurred at room
temperature upon the addition of an excess of another DABBF
into the gels, as the DABBF linkages in the gels were in an
equilibrium state and could exchange their bonds. The gels
continued to swell at ambient temperatures above room
temperature but attained equilibrium swelling at temperatures
lower than room temperature. EPR measurements revealed that
the temperature-dependent swelling and the network reorgan-
ization are due to the sensitivity of the equilibrium of the
DABBF linkages to temperature. The amount of dissociated
DABBF in the gels increased from 0.001% at 0 °C to 0.01% at
50 °C. The increase in the ratio reduced the contractile
restorative forces generated by swelling, resulting in continuous
swelling. The slight increase led thus to macroscopic changes in
the swelling behavior. The gel networks irreversibly reorganized
with reversible temperature changes. SAXS measurements
showed that the mesh sizes of the gels expand through bond
exchanges of the DABBF linkages and that the network
structures can be easily controlled via the swelling time and the
ambient temperature. Evaluation of the change in network
structures is crucial, since exchange reactions of reversible
covalent bonds and resulting network reorganization in
polymer gels and cross-linked polymers have been widely
applied in the past decade to the production of smart materials.
In addition, we believe that the network reorganization of the
DABBF-containing gel in response to the ambient temperature
can find applications in a diverse range of fields in which smart
materials are used, such as biomaterials, controlled release, and
drug delivery systems.
Figure 5. (a) SAXS profiles of the DABBF-containing cross-linked
polymer swollen with THF at 40 °C for different reaction times, their
fitting curves described by eq 2, and the values of the correlation
length (ξ). (b) Values of ξ associated with the DABBF-containing
cross-linked polymer swollen with THF at 40 °C for different swelling
times. The scattering vector is defined as q = 4π sin(θ/λ).
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Figure 6. Proposed mechanism for the network reorganization of the DABBF-containing gel in response to the ambient temperature: (a) below
room temperature; (b) upon heating; (c) upon cooling to temperatures lower than room temperature.
mT/s. The concentration of the radicals formed from cleaved DABBF
was determined by comparing the area of the observed integral
MATERIALS AND METHODS
■
Synthesis of Cross-Linked Polymers. In a sample tube, a
solution of tetrahydric DABBF (1.00 g, 1.22 mmol) and PEG (Mn =
1000) (2.43 g, 2.43 mmol) in 1,4-dioxane (4.11 mL) was prepared.
HDI (0.78 mL, 4.86 mmol) and a 50 vol % solution of di-n-butyltin
spectrum with that of TEMPOL in the same solvent under the same
experimental conditions; the Mn2+ signal was used as an auxiliary
standard. The concentration of DABBF linkages was calculated by
using the gel volume after swelling for 5 days at room temperature
dilaurate in THF (3 drops) were added to the solution under a
with the assumptions that the density of the cross-linked polymer in
the dry state was 1 g/cm3 and that the composition of the polymer was
nitrogen atmosphere at room temperature. After 48 h, the obtained gel
was purified by immersing it in water for 24 h and in 1,4-dioxane for
several days and then freeze-drying it to afford a yellow solid (4.23 g,
quantitative yield).
The control polymer was synthesized in a similar manner. In a
sample tube, a solution of tetrahydric bisphenol A (185 mg, 0.49
mmol) and PEG (Mn = 1000) (984 mg, 0.98 mmol) in 1,4-dioxane
similar to that of the reaction mixture because the polymer was
obtained in quantitative yield, as shown in Figure S5 in the Supporting
Information. The g value was calculated according to the following
equation:
g = hν/βH
(1.45 mL) was prepared. HDI (0.32 mL, 1.97 mmol) and di-n-butyltin
where h is the Planck constant, ν is the microwave frequency, β is the
dilaurate (1 drop) were added to the solution under a nitrogen
Bohr magneton, and H is the magnetic field.
atmosphere at room temperature. After 48 h, the obtained gel was
SAXS Measurements. SAXS measurements were carried out on
purified by immersing it in water for 72 h and in 1,4-dioxane for
several days and then freeze-drying it to afford a white solid (1.42 g,
94% yield).
De-Cross-Linking Reaction. The cross-linked polymer with
DABBF (20 mg) and a THF solution (2.23 mL) of dihydric
beamline BL40B2 at SPring-8 using incident X-rays with wavelength of
λ = 0.1 nm. Scattered X-rays were detected using a 300 mm × 300 mm
imaging plate with a resolution of 0.1 mm/pixel and a 2158 mm
sample-to-detector distance calibrated by the average of three peaks of
silver behenate. The measured gels were swollen in THF and DMF
DABBF (20 equiv of DABBF linkages in the cross-linked polymer)
and contained in 2 mm quartz capillaries. The scattering intensity of
were placed in a sample tube. The mixture was then stirred under air at
room temperature. After 42 h, the reaction mixture was evaluated by
GPC analysis at 40 °C using the following experimental setup: a guard
column (Tosoh TSK guard column Super H-L), three columns
the polymer [ΔI(q)] was calculated by subtracting the scattering
intensity of the solvent [Isolv(q)] from that of the solution [Isoln(q)]
after the intensities were adjusted using the transmittances (Tsolv and
Tsoln, respectively), according to the equation ΔI(q) = Isoln(q)/Tsoln
Isolv(q)/Tsolv. The scattering vector was defined as q = 4π sin(θ/λ).
−
(Tosoh TSK gel SuperH 6000, 4000, and 2500), a differential
refractive index detector, and a UV−vis detector. THF was used as the
eluent at a flow rate of 0.6 mL/min. Polystyrene (PS) standards (Mn =
4920−3000000; Mw/Mn = 1.02−1.03) were used to calibrate the GPC
system. 1H NMR (400 MHz) spectroscopic measurements were
carried out to evaluate the mixture at 25 °C using a 400 MHz Bruker
spectrometer with tetramethylsilane as an internal standard in
chloroform-d (CDCl3).
Measurement of Swelling Degrees. Sliced cross-linked
polymers were placed in various organic solvents at several
temperatures, and the Q values of the cross-linked polymers were
calculated from the weights of the cross-linked polymers in the dry and
gel states. Temperature was controlled using an oil bath and an
ethanol bath. The weights of the cross-linked polymers in gel states
were measured by taking the gel out of the solution, removing excess
solution from the gel surface, and weighing. Gels were kept in darkness
during the entire process.
ASSOCIATED CONTENT
* Supporting Information
■
S
De-cross-linking reaction, swelling behavior of the DABBF-
containing gels in other solvents at room temperature, swelling
behavior with reversible temperature changes in DMF, and
SAXS profiles of the DABBF-containing gels in DMF after
reaction at 40 °C. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
takahara@cstf.kyushu-u.ac.jp
otsuka@polymer.titech.ac.jp
EPR Measurements. Variable-temperature EPR measurements
were carried out on a JEOL JES-X320 X-band EPR spectrometer
equipped with a JEOL DVT temperature controller. The measured
gels swollen with THF were contained in 3 mm glass capillaries, and
the capillaries were sealed after being degassed. Spectra were measured
using a microwave power of 0.5 mW and a field modulation of 0.01 or
0.1 mT with a time constant of 0.03 s and a sweep rate of 0.05 or 0.25
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The synchrotron radiation experiment was performed on
beamline BL40B2 at SPring-8 with the approval of the Japan
■
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(31) Nakahata, M.; Takashima, Y.; Hashidzume, A.; Harada, A.
Angew. Chem., Int. Ed. 2013, 52, 5731.
(32) Yeh, F.; Sokolov, E. L.; Walter, T.; Chu, B. Langmuir 1998, 14,
Synchrotron Radiation Research Institute (JASRI) (Proposal
2012A1003). The authors thank Dr. Noboru Ohta for his
assistance in the SAXS measurements. H.O. gratefully acknowl-
edges financial support of the Funding Program for Next
Generation World-Leading Researchers (Green Innovation
GR077) from the Cabinet Office, Government of Japan. This
research was also supported by a Grant-in-Aid for Scientific
Research (26288057) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT). K.I. acknowledges
the financial support through JSPS Research Fellowships for
Young Scientists.
4350.
(33) Shibayama, M. Macromol. Chem. Phys. 1998, 199, 1.
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