Supramolecular hydrogels formed from poly(viologen) cross-linked with cyclodextrin dimers and their physical properties

Article abstract of DOI:10.3762/bjoc.8.182

Supramolecular materials with noncovalent bonds have attracted much attention due to their exclusive properties differentiating them from materials formed solely by covalent bonds. Especially interesting are rotor molecules of topological complexes that s

Full text of DOI:10.3762/bjoc.8.182

Supramolecular hydrogels formed from
poly(viologen) cross-linked with cyclodextrin dimers
and their physical properties
Yoshinori Takashima, Yang Yuting, Miyuki Otsubo, Hiroyasu Yamaguchi
and Akira Harada*§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1594–1600.
Department of Macromolecular Science, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan
doi:10.3762/bjoc.8.182
Received: 28 May 2012
Email:
Accepted: 17 August 2012
Published: 20 September 2012
Akira Harada* - harada@chem.sci.osaka-u.ac.jp
*
Corresponding author
This article is part of the Thematic Series "Superstructures with
cyclodextrins: Chemistry and applications".
§
Telephone +81-6-6850-5445; Fax +81-6-6850-5445
Keywords:
Guest Editor: H. Ritter
cyclodextrins; poly(viologen); supramolecular hydrogel
©
2012 Takashima et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Supramolecular materials with noncovalent bonds have attracted much attention due to their exclusive properties differentiating
them from materials formed solely by covalent bonds. Especially interesting are rotor molecules of topological complexes that
shuttle along a polymer chain. The shuttling of these molecules should greatly improve the tension strength. Our research employs
cyclodextrin (CD) as a host molecule, because CD effectively forms polyrotaxanes with polymers. Herein we report the formation
of supramolecular hydrogels with an α-CD dimer (α,α-CD dimer) as a topological linker molecule, and a viologen polymer (VP) as
the polymer chain. The supramolecular hydrogel of α,α-CD dimer/VP forms a self-standing gel, which does not relax (G' > G'') in
the frequency range 0.01–10 rad·s−1. On the other hand, the supramolecular hydrogel decomposes upon addition of bispyridyl
decamethylene (PyC10Py) as a competitive guest. Moreover, the β-CD dimer (β,β-CD dimer) with VP does not form a supra-
molecular hydrogel, indicating that complexation between the C10 unit of VP and the α-CD unit of the α,α-CD dimer plays an
important role in the formation of supramolecular hydrogels.
Introduction
Development of functional soft materials has attracted much which act as cross-linkers, slide on the axial polymer chain. In
attention due to the numerous practical applications [1-3]. Typi- contrast, chemical gels do not exhibit cross-linker slippage.
cally, soft materials fall into one of two types of gels: physical
gels and chemical gels [4-9]. Recently, topological cross-linked Previously, there have been some reports of supramolecular
polyrotaxanes have been identified as tertiary gels, which complexes with cyclodextrin (CD) dimers. A supramolecular
should create a new paradigm in materials science [10]. Poly- hydrogel, which was constructed by the formation of an inclu-
rotaxanes form topological gels, because the rotor molecules, sion complex between the copolymer with an adamantyl group
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Results and Discussion
Preparation of CD dimers and viologen
and CD dimer, showed a lower critical solution temperature
LCST) [11]. Another report indicated that adding selenium or
(
platinum complexes yields supramolecular assemblies of derivatives
bis(molecular tube)s cross-linked with the β-CD dimer, which Figure 1 depicts the chemical structures of the cyclodextrin
form nanofibers [12-15]. Moreover, mechanically linked poly- dimers (α,α-CD dimer, α,β-CD dimer, and β,β-CD dimer) and
rotaxane with the α-CD and poly(ethylene glycol) (PEG) pyridyl derivatives (PyC10Py and viologen polymer (VP)). The
produces a hydrogel material, which exhibits unique physical α,α-CD and β,β-CD dimers are prepared by reacting the corres-
properties [10].
ponding 6-amino-CDs and terephthalic acid using 4-(4,6-
dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride
Previously, we have prepared a polyrotaxane using α-CD and n-hydrate (DMT-MM) as a condensing reagent in DMF. The
PEG [16,17]. The α-CD/PEG polyrotaxane forms a hydrogel α,β-CD dimer is prepared by reacting 6-amino-α-CD and 6-O-
material in high concentrations [18]. Using polyelectrolytes as (4-carboxylphenylamide)-β-CD using DMT-MM in DMF.
threading molecules results in complexation between the poly- These CD dimers are purified by preparative reversed-phase
electrolyte and α-CD within the range of the 1H NMR time chromatography using DIAION HP-20 beads. As described in
scale due to the slow equilibrium [19,20]. Cationic groups, such the experimental section, the reaction of 1,10-dibromodecane
as pyridinium and pyridylpyridinium terminal groups, inhibit with 4,4’-bipyridyl in DMF gives VP, where the number of VP
the decomposition of polyrotaxane and stabilize the complexes units is 20 and was determined by the ratio of integral values of
between α-CD and cationic alkanediyl compounds [21-23]. the end group and the main chain unit in the 1H NMR spectrum.
Herein, to study the formation of supramolecular hydrogels
Hydrogelation between the CD dimer and
with the α,α-CD dimer, we chose the viologen polymer (VP),
which possesses multiple cations, as the axis molecules. viologen polymer
Decamethylene units function as recognition sites of α-CD, and Mixing the α,α-CD dimer and VP in aqueous solutions at room
bipyridyls work as electric barriers.
temperature slightly increases the viscosity of the α,α-CD
Figure 1: Chemical structures of the CD dimer (α,α-CD dimer, α,β-CD dimer, and β,β-CD dimer) and guest derivatives [PyC10Py and viologen
polymer (VP)]. Bromonium anions are omitted in guest derivatives.
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Beilstein J. Org. Chem. 2012, 8, 1594–1600.
dimer/VP, but hydrogels are not formed. On the other hand, To confirm complementarity between the CD dimer and VP, we
after heating at 100 °C for 7 h, an aqueous solution of the then investigated the formation of supramolecular hydrogels of
α,α-CD dimer/VP forms a supramolecular hydrogel containing VP with the α,β-CD dimer and the β,β-CD dimer. For each
over 30 mM (VP unit/CD unit 4:1) (Figure 2). The bipyridyl sample, the CD concentration was adjusted to 120 mM (VP
group of VP functions as an electric barrier, which prevents unit/CD unit 4:1). Even with a dimer/VP concentration greater
threading and dethreading of the α-CD unit in the α,α-CD than 60 mM, the α,β-CD dimer/VP and β,β-CD dimer/VP do
dimer onto the decamethylene unit of VP at 30 °C; this observa- not form supramolecular hydrogels. These results indicate that
tion suggests that the α-CD unit of the α,α-CD dimer cannot complexation between the C10 unit of VP and the α-CD unit of
exceed the electric barrier at 30 °C, whereas after heating the α,α-CD dimer is important for the formation of cross-links
at 100 °C, the α-CD unit exceeds the barrier to form poly- between VPs (Figure 3). The cavity size of β-CD is too large to
rotaxanes.
allow formation of a stable cross-linked polyrotaxane complex.
Figure 2: Photographs of hydrogelation with various concentrations of α,α-CD dimer/VP in water. Aqueous solution of α,α-CD dimer/VP forms the
hydrogel at concentrations above 30 mM.
Figure 3: Hydrogelation of VP with various CD derivatives (VP unit/CD 4:1) at 25 °C. (a) Concentrations of CDs are 120 mM. (b) Proposed structure
of the α,α-CD dimer/VP supramolecular hydrogel.
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Beilstein J. Org. Chem. 2012, 8, 1594–1600.
The α,β-CD dimer and β,β-CD dimer do not function as indicating the formation of a polyrotaxane VP/α-CD complex.
crosslinking molecules between VP and β-CD. Actually, the The association–dissociation equilibrium between VP and α-CD
association constant of α-CD with decamethylene is much is slow on the NMR time scale. On the other hand, upon add-
higher than that of β-CD with decamethylene [24]. Conse- ition of PyC10Py to the VP/α-CD complex (VP unit/α-CD/
quently, the association constant plays an important role in gel PyC10Py 1:2:8), the splitting peaks of the VP/α-CD complex
formation.
disappear, and then signals of PyC10Py split due to complexa-
tion between PyC10Py and α-CD (Figure 4c). These results indi-
To confirm a supramolecular hydrogel formed by crosslinking cate that the excess PyC10Py disturbs complexation of α-CD
VP with the α,α-CD dimer, we added PyC10Py as a competitive and VP. The sol state of the α,α-CD dimer/VP in the presence
guest to the supramolecular hydrogel of the α,α-CD dimer/VP. of PyC10Py is attributed to the dissociation of VP and the α,α-
After adding PyC10Py (PyC10Py/VP unit/α,α-CD dimer 8:8:1) CD dimer, suggesting that complexation between VP and the
and heating at 100 °C, the supramolecular hydrogel of the α,α- α-CD units is necessary to form the gel.
CD dimer/VP changes to the sol even at a high concentration
Viscoelastic property of the α,α-CD dimer/VP
(
[α,α-CD dimer] = 60 mM), because the α-CD unit of the α,α-
CD dimer forms an inclusion complex with PyC10Py. This hydrogel
inclusion-complex formation causes the cross-links between the Figure 5 shows the storage elastic modulus (G') and loss elastic
α,α-CD dimer and VP to decompose.
modulus (G'') for an α,α-CD dimer/VP hydrogel (60 mM) at
0 °C. The master curve of the hydrogel is similar to the Voigt
2
1
H NMR study of complexation of the α,α-CD
Model. G'' relaxes as the frequency increases. However, the
hydrogel does not relax (G' > G'') in the frequency range
dimer/VP
To observe the competitive effect of PyC10Py, we conducted 0.01–10 rad·s−1, indicating a self-standing gel. This behavior
H NMR studies on complexation between α-CD/VP and a are similar to chemically cross-linked gels even though the α,α-
competitive experiment using PyC10Py. Figure 4 shows CD dimer/VP hydrogel is topologically cross-linked between
H NMR spectra of VP/PyC10Py, VP/α-CD, and VP/α-CD/ VPs with the α,α-CD dimer. This result confirms that com-
1
1
PyC10Py. Addition of α-CD causes peak splitting of the plexation of VP and the α,α-CD dimer is stable and responsible
decamethylene and pyridyl protons of VP (VP unit/α-CD 1:2), for the stability of the hydrogel.
Figure 4: 500 MHz 1H NMR spectra of VP (VP unit 2 mM) with α-CD and PyC10Py (VP unit/CD/PyC10Py 1:2:8) in D2O at 30 oC: (a) VP and PyC10Py,
(b) VP and α-CD, and (c) VP, PyC10Py, and α-CD.
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Beilstein J. Org. Chem. 2012, 8, 1594–1600.
for 4 days. After evaporation of the solvent, the residue was
dissolved in water (10 mL) and poured into acetone (100 mL).
The product was collected and purified by reversed-phase chro-
matography (elution: water–acetonitrile) to give α,α-CD dimer
as a white solid in 22% yield. 1H NMR (DMSO-d6, 500 MHz) δ
8
.36 (t, 2H, -NH), 7.88 (s, 4H, Ph), 5.59–5.40 (m, 24H,
O(2,3)H of α-CD), 4.97–4.78 (m, 12H, C(1)H of α-CD),
.55–4.41 (m, 10H, O(6)H of α-CD), 3.84–3.48 (m,
4
C(3,6,5,3,4)H of α-CD); MALDI–TOF m/z: 2095 [M + Na]+.
Preparation of α,β-CD dimer
a) Methyl terephthalate-β-CD
Figure 5: G' and G'' of the α,α-CD dimer/VP hydrogel as a function of
frequency (ω). Applied shear strain amplitude is 1%.
Conclusion
Mixing VP and the α,α-CD dimer creates a hydrogel, which is
expected to realize supramolecular materials with a high tensile To a solution of 6-NH2-β-CD (566 mg, 0.50 mmol) in dried
strength and self-healing abilities. The complementarity DMF (7 mL) was added methyl terephthalate succinimidyl ester
between α-CD and the decamethylene units plays an important (137.6 mg, 0.50 mmol). After stirring for 2 days at rt, the solu-
role in the formation of supramolecular hydrogels composed of tion was poured into acetone (100 mL) to give methyl tereph-
α,α-CD dimer/VP. VP has an electric barrier between the thalate-β-CD as a yellow solid in 43% yield. 1H NMR (DMSO-
decamethylene units, which is a unique feature of this supra- d6, 500 MHz) δ 8.46 (t, 1H, -NH), 8.00 (d, 2H, Ph), 7.95 (s, 3H,
molecular hydrogel. The electric barrier prevents dethreading of -CH3), 7.94 (d, 2H, Ph), 5.83–5.59 (m, 14H, O(2,3)H of β-CD),
α-CD from VP, yielding a self-standing supramolecular 4.95–4.79 (m, 7H, C(1)H of β-CD), 4.45–4.32 (m, 6H, O(6)H
hydrogel. We will electrochemically control the elasticity of the of β-CD), 3.74–3.51 (m, C(3,6,5,3,4)H of α-CD); TLC: Rf 0.22
α,α-CD dimer/VP hydrogel.
(n-butanol/ethanol/water 5:4:3).
Experimental
b) Terephthalic acid-β-CD
Preparation of α,α-CD dimer
6
0
-NH2-α-CD (120 mg, 0.12 mmol) and terephthalic acid (8 mg, To a solution of methyl terephthalate-β-CD (605 mg,
.50 mmol) were dissolved in dried DMF (20 mL). DMT-MM 0.47 mmol) in water (120 mL) was added NaOH (0.1 M, 7 mL).
(
34 mg, 0.12 mmol) was added and the mixture was stirred at rt After stirring for 12 hours at rt, the solution was concentrated
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Beilstein J. Org. Chem. 2012, 8, 1594–1600.
and purified by DIAION HP-20 column. The column was methylene in decamethylene), 2.21 (m, 96H, β methylene in
flushed with water (500 mL) and then eluted with water/ decamethylene ), 1.72–1.30 (m, 288H, χ, δ, ε methylene in
methanol 80:20 (v/v). The fraction was concentrated to give decamethylene).
terephthalic acid-β-CD as a yellow solid in 70% yield. 1H NMR
Preparation of [Py-(CH2)10-Py]2+·2Br−
(
DMSO-d6, 500 MHz) δ 8.39 (t, 1H, -NH), 7.98 (d, 2H, Ph),
7
4
.90 (d, 2H, Ph), 5.83–5.59 (m, 14H, O(2,3)H of β-CD), (PyC10Py)
.95–4.79 (m, 7H, C(1)H of β-CD), 4.45–4.32 (m, 6H, O(6)H Pyridine (158 mg, 2.0 mmol) and 1,10-dibromodecane (315 mg,
of β-CD), 3.74–3.51 (m, C(3,6,5,3,4)H of α-CD); TLC: Rf 0.32 0.80 mmol) were dissolved in acetone and heated under reflux
(
n-butanol/ethanol/water 5:4:3).
for 3 d. After evaporation of the solvent, the residue was
dissolved in methanol (20 mL) and poured into diethyl ether
(200 mL). The product was collected by centrifugation to give
PyC10Py in 91% yield as a brown solid. 1H NMR (D2O,
c) α,β-CD dimer
5
8
7
00 MHz) δ 8.90 (d, J = 6.6 Hz, 4H, 2-position of pyridine),
.62 (t, J = 8.2 Hz, 2H, 4-positon of pyridine), 8.14 (t, J =
.7 Hz, 4H, 3-positon of pyridine), 4.67 (t, J = 7.3 Hz, 4H, α
methylene in decamethylene), 2.08 (m, 4H, β methylene in
decamethylene), 1.42–1.30 (m, 12H, χ, δ, ε methylene in
decamethylene).
Rheological measurements
Dynamic viscoelasticity were measured by using an Anton Paar
MCR301 rheometer at a strain of 0.1%. The storage elastic
modulus (G') and loss elastic modulus (G'') were measured at
The synthetic procedure was the same as α,α-CD dimer, using 20 °C. The sample concentration was adjusted to 1.0 wt %.
terephthalic acid-β-CD (65 mg, 50 μmol), 6-NH2-α-CD (59 mg,
6
0 μmol), DMT-MM (17 mg, 60 μmol), dried DMF (8 mL) to
give α,β-CD dimer in 36% yield as a white solid. 1H NMR
DMSO-d6, 500 MHz) δ 8.32, 8.27 (m, 2H, -NH), 7.89 (s, 4H,
Supporting Information
(
Supporting Information File 1
Additional information and 1H NMR spectra of all new
compounds.
Ph), 5.80–5.44 (m, 26H, O(2,3)H of CDs), 4.97–4.78 (m, 13H,
C(1)H of CDs), 4.53–4.35 (m, 11H, O(6)H of CDs), 3.86–3.37
(
m, C(3,6,5,3,4)H of α-CD); TLC: Rf 0.04 (n-butanol/ethanol/
[http://www.beilstein-journals.org/bjoc/content/
water 5:4:3); MALDI–TOF m/z: 2259 [M + Na]+.
supplementary/1860-5397-8-182-S1.pdf]
Preparation of β,β-dimer
β,β-Dimer was prepared according to our previous report [25].
Acknowledgements
This work was supported by the “Core Research for Evolu-
Preparation of viologen polymer (VP)
tional Science and Technology” program of the Japan Science
1
,10-Dibromodecane (7.3 g, 24 mmol) was added to a solution and Technology Agency.
of 4,4’-bipyridyl (4 g, 24 mmol) in DMSO (40 mL). After being
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