Responsive supramolecular gels constructed by crown ether based molecular recognition

Article abstract of DOI:10.1002/anie.200805712

Responsive supramolecular gels were constructed from crown ether terminated four-arm star poly(ε-caprolactone) (PCL-DB24C8) and dibenzylammonium- terminated two-arm PCL-DBAS (see scheme), exploiting the formation of pseudorotaxane linkages between crown e

Full text of DOI:10.1002/anie.200805712

Communications
DOI: 10.1002/anie.200805712
Supramolecular Gels
Responsive Supramolecular Gels Constructed by Crown Ether Based
Molecular Recognition**
Zhishen Ge, Jinming Hu, Feihe Huang,* and Shiyong Liu*
Supramolecular gels can be constructed from small molecules,
as well as oligomeric and polymeric species.[1] These building
blocks are typically brought together by noncovalent inter-
actions, such as hydrophobic,[2] p–p stacking,[3] multiple
hydrogen bonding,[4] and metal–ligand interactions,[5] as well
as host–guest recognition.[6] Since supramolecular gels can
respond to external stimuli, including temperature, pH,
solvent composition, and electric/magnetic fields, owing to
the relatively low activation energy required for breaking
weak bonds, they can serve as drug-delivery carriers, func-
tional membranes, and smart devices.[7] As the first artificially
synthesized host molecules, crown ethers can bind metal and
organic ammonium cations, if these guest molecules fit into
the macrocyclic cavities.[8] Previously, crown ether-based
molecular recognition has facilitated the synthesis of a variety
of supramolecular polymers with chain topologies varying
Scheme 1. Schematic representation for the fabrication of responsive
supramolecular networks from four-arm star PCL–DB24C8 and two-
arm PCL–DBAS as a result of molecular recognition between
dibenzo[24]crown-8 (DB24C8) and dibenzylammonium salt (DBAS)
from linear (AB, ABA, alternating),[9] star,[10] cross-linked,[11]
hyperbranched,[12] and dendronized polymers,[13] as well as
dendrimers.[14] Notably, reports concerning crown ether-based
supramolecular gels are relatively rare,[15] partially owing to
moieties.
synthetic difficulties encountered in the preparation of well-
defined polymer precursors embedded with multiple crown
ether hosts at predetermined positions.
ring-opening polymerization (ROP) and click reaction. At
relatively high concentrations in organic solvents such as
CHCl3, the mixture of four-arm star PCL–DB24C8 and two-
arm PCL–DBAS spontaneously form supramolecular gels,
exhibiting fully reversible thermo- and pH-induced gel–sol
transitions. 1H NMR spectroscopy, viscometry, and differ-
ential scanning calorimetry (DSC) were employed to charac-
terize this novel type of crown ether-based supramolecular
network.
Herein we report the fabrication of responsive supra-
molecular gels on the basis of reversible molecular recog-
nition between dibenzo[24]crown-8 (DB24C8) and dibenzy-
lammonium salt (DBAS) moieties, using biodegradable
poly(e-caprolactone) (PCL) segments as the scaffold
(Scheme 1). DB24C8-terminated four-arm star PCL (four-
arm star PCL–DB24C8) and DBAS-terminated two-arm PCL
(two-arm PCL–DBAS) were synthesized by a combination of
Synthetic routes for the preparation of four-arm star
PCL–DB24C8 and two-arm PCL–DBAS are shown in
Scheme 2. Firstly, to incorporate sites for molecular recog-
nition onto polymer precursors by click chemistry, alkynyl-
containing DB24C8 and dibenzylamine (DBA) moieties were
synthesized (see the Supporting Information, Scheme S1).
Alkynyl–DB24C8 was obtained in approximately 71% yield
from a macrocyclization reaction between propargyl 3,4-
dihydroxybenzoate and 1,2-phenylenebis(oxyethylenoxye-
thylenoxyethylene) ditosylate, catalyzed by LiBr under
semi-high dilution conditions (see the Supporting Informa-
tion, Figure S1). Alkynyl–DBAwas synthesized by reacting 4-
formylphenyl propargyl ether with benzylamine followed by
reduction with NaBH4 (see the Supporting Information,
Figure S2). Pentaerythritol and ethylene glycol were used as
initiators for the ROP of e-caprolactone (CL) in the presence
of Sn(Oct)2 catalyst, yielding four-arm star PCL and a,w-
dihydroxy-terminated PCL, respectively. Terminal hydroxy
groups were then converted into tosylate groups by reacting
[*] Z. Ge, J. Hu, Prof. Dr. S. Liu
Hefei National Laboratory for Physical Sciences at the Microscale
Key Laboratory of Soft Matter Chemistry
Department of Polymer Science and Engineering
University of Science and Technology of China
Hefei, Anhui 230026 (P.R. China)
Fax: (+86)551-3607348
E-mail: sliu@ustc.edu.cn
Prof. Dr. F. Huang
Department of Chemistry, Zhejiang University
Hangzhou, Zhejiang 310027 (P.R. China)
Fax: (+86)571-87953189
E-mail: fhuang@zju.edu.cn
[**] This work was financially supported by an Outstanding Youth Fund
(50425310) and research grants (20534020, 20674079, and
20874092) from the National Natural Scientific Foundation of China
(NNSFC).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805712.
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Chemie
association
constant
(Ka
CDCl3,
ꢀ 2.7 ꢀ 104 mÀ1
,
258C).[16] The supramolecular
self-assembly between equiva-
lent amounts (1:1 molar ratio,
crown ether/ammonium ion) of
four-arm star PCL20–DB24C8
and two-arm PCL18–DBAS was
initially examined by NMR
spectroscopy. The 1H NMR
spectrum (Figure 1c) was
recorded 2 h after mixing four-
arm star PCL20–DB24C8 and
two-arm PCL18–DBAS in
CDCl3 at a total concentration
of 20 glÀ1. Compared to that of
two-arm PCL18–DBAS (Fig-
ure 1a), the NMR spectrum of
the mixture revealed that char-
acteristic methylene proton sig-
nals (Ha and Hb) of DBAS,
which initially overlapped with
signals corresponding to the
PCL main chains, shifted
downfield to d = 4.5–4.7 ppm
upon
complexation
(Fig-
ure 1c). Moreover, signals
characteristic of the crown
ether (Hb and Hg) at d = 3.72
and 3.82 ppm in four-arm
Scheme 2. Synthetic routes for the preparation of a) dibenzo[24]crown-8-terminated PCL (four-arm star
PCL–DB24C8) and b) dibenzylammonium-terminated difunctional PCL (two-arm PCL–DBAS) by a combi-
nation of ring-opening polymerization and click reactions. TEA=triethylamine.
PCL20–DB24C8
(Figure 1b)
shifted upfield to the range of
d = 3.4–3.8 ppm (Figure 1c).
with tosyl chloride. Quantitative end-group transformation
was confirmed by the disappearance of the resonance signal at
d = 3.86 ppm, which is characteristic of methylene protons
adjacent to terminal hydroxy groups, in the 1H NMR spectra
of four-arm PCL–OTs and two-arm PCL–OTs (see the
Supporting Information, Figures S3 and S4).
These NMR spectroscopic signal shifts indicated that diben-
zylammonium ions at the chain terminals of two-arm PCL18–
DBAS were encircled by the macrocyclic cavities of the crown
ether, forming pseudorotaxane linkages. The percentage of
complexation between DB24C8 and DBAS moieties was
calculated to be 79% by comparing the integrals of NMR
signals characteristic of uncomplexed and complexed crown
ether moieties. Molecular recognition between terminal
crown ether and dibenzylammonium moieties, from four-
arm star PCL20–DB24C8 and two-arm PCL18–DBAS, respec-
tively, will lead to the formation of supramolecular networks
(Scheme 1).
The reversible formation of noncovalent bonds between
four-arm star PCL20–DB24C8 and two-arm PCL18–DBAS was
further investigated by NMR spectroscopy. Treatment by
heating or introduction of organic base (triethylamine, TEA)
would be expected to disrupt pseudorotaxane formation.
Figure 1 also shows 1H NMR spectra for the mixture of four-
arm star PCL20–DB24C8 and two-arm PCL18–DBAS upon
addition of TEA and upon heating to 608C. In the presence of
TEA, the characteristic resonance signals resulting from the
pseudorotaxane formation (d = 4.5–4.7 and 3.4–3.8 ppm,
Figure 1c) completely disappeared (Figure 1d). The addition
of organic base led to deprotonation of the dibenzylammo-
nium ions and the break-up of the complexation between
four-arm star PCL20–DB24C8 and two-arm PCL18–DBAS.
Next, four-arm star PCL–N3 and two-arm PCL–N3 were
obtained by azidation with NaN3. The end group functionality
was determined to be quantitative, as evidenced by the
complete disappearance of characteristic NMR signals of
tosylate and the emergence of a new signal at d = 3.28 ppm
(methylene protons adjacent to azide; see the Supporting
Information, Figures S3 and S4). Finally, four-arm star PCL–
DB24C8 and two-arm PCL–DBAS were successfully
obtained by click reactions of four-arm star PCL–N3 and
two-arm PCL–N3 with alkynyl–DB24C8 and alkynyl–DBA,
respectively. In both click reactions, alkynyl-containing spe-
cies were in excess relative to those of azido moieties to
ensure quantitative “click” end-group functionalization of
1
polymer precursors, which was again confirmed by H NMR
and FT-IR spectroscopy (see the Supporting Information,
Figures S3–S6).
Four-arm star PCL–DB24C8 and two-arm PCL–DBAS
incorporate complementary molecular recognition moieties.
Complexation between DB24C8 and DBAS led to the
formation of a pseudorotaxane, with a desirably high
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Figure 2. Variation of reduced viscosities (CHCl3, 208C) as a function
of total polymer concentrations obtained for molecular recognition
complexes of four-arm star PCL20–DB24C8 and two-arm PCL18–DBAS
(1:1 molar ratio, crown ether/ammonium ion), and the control mixture
of four-arm star PCL20–OH and two-arm PCL18–OH at the same molar
ratio.
concentrations. A comparison of reduced viscosity/concen-
tration profiles obtained for the two types of solution
mixtures tells us that the introduction of complementary
host–guest-interaction moieties at the chain terminals of non-
interacting polymer precursors can dramatically modify the
blend solution rheology.
Figure 1. Partial 1H NMR (300 MHz, CDCl3) spectra for a) two-arm
PCL18–DBAS; b) four-arm star PCL20–DB24C8; c) molecular recognition
complexes of four-arm PCL20–DB24C8 and two-arm PCL18–DBAS (1:1
molar ratio of crown ether/ammonium ion; 20 glÀ1, 208C); d) com-
plexes after treating with TEA (208C); e) complexes at 608C (C and U
denote complexed and uncomplexed moieties, respectively).
At even higher polymer concentrations, the formation of
supramolecular gels and reversible thermo- and pH-induced
gel–sol transitions can be visualized macroscopically
(Figure 3). After weighing proper amounts of four-arm star
Conversely, at elevated temperatures (608C), it was also
evident that intensities of signals at d = 4.5–4.7 and 3.4–
3.8 ppm reduced considerably, again confirming the disrup-
tion of molecular recognition (Figure 1e). In the latter case,
the percentage of remaining complexation was determined to
be roughly 10% in CDCl3 at 608C.
Pseudorotaxane formation between terminal crown ether
and dibenzylammonium moieties of four-arm star PCL20–
DB24C8 and two-arm PCL18–DBAS will lead to supramolec-
ular networks and a size increase of polymer chains.
Viscometry is a convenient and simple technique to charac-
terize the growth of polymer networks with increasing
polymer concentrations.[4a,f] The variation of reduced viscosity
as a function of polymer concentration for the solution
mixture of four-arm star PCL20–DB24C8 and two-arm PCL18–
DBAS (1:1 molar ratio, crown ether/ammonium ion) is shown
in Figure 2. For comparison, variation of reduced viscosity of
the solution mixture of four-arm PCL20–OH and two-arm
PCL18–OH was also plotted. In the concentration range 1–
30 glÀ1, reduced viscosity varied linearly with total polymer
concentrations, indicating that no significant physical entan-
glements or noncovalent interactions occurred between four-
arm PCL20–OH and two-arm PCL18–OH in CHCl3. Con-
versely, for the solution mixture of four-arm star PCL20–
DB24C8 and two-arm PCL18–DBAS, the reduced viscosity
varied exponentially with total polymer concentrations,
indicating the formation of supramolecular networks. The
dramatic increase of reduced viscosity also indicates the
growth of supramolecular networks with increasing polymer
Figure 3. a) Supramolecular gel formed from the mixture of four-arm
star PCL20–DB24C8 and two-arm PCL18–DBAS (1:1 molar ratio, crown
ether/ammonium ion) in CHCl3 (150 glÀ1, 208C); b) the same mixture
after heating to 608C (150 glÀ1); c) the same mixture after treating
with 1.1 equivalents of TEA relative to DBAS moieties (150 glÀ1
208C).
,
PCL20–DB24C8 and two-arm PCL18–DBAS into a vial, the
addition of CHCl3 and subsequent heating to 608C led to a
transparent solution. Upon cooling to 208C, nearly trans-
parent supramolecular gels formed. Heating the resultant
supramolecular gels resulted in recovery of the fluid solution.
The formation of gels at lower temperatures can be
ascribed to the restoration of the complexation between
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0.8 mmol) in DMF (5 mL) was degassed by three freeze–thaw cycles
crown ether moieties and dibenzylammonium ions, which was
disrupted at elevated temperatures (Figure 1). Moreover,
supramolecular gels formed at 208C were also converted into
fluid solution upon addition of 1.1 equivalents (per crown
ether) of TEA, as a result of the deprotonation of dibenzy-
lammonium ions, which again disrupted the supramolecular
network. Interestingly, the introduction of roughly 1.2 equiv-
alents (per crown ether) of trifluoroacetic acid (TFA) led to
the reformation of supramolecular gels. It should be noted
that these thermo- and pH-induced gel–sol transitions are
completely reversible, reflecting the dynamic nature of typical
supramolecular systems (Scheme 1).
Differential scanning calorimetry (DSC) was further
employed to characterize the pseudorotaxane linkage forma-
tion in the mixture of four-arm star PCL20–DB24C8 and two-
arm PCL18–DBAS (see the Supporting Information, Fig-
ure S7). Compared to those of the two separate components,
the DSC thermogram of the mixture (1:1 molar ratio of crown
ether/ammonium ion) revealed a broad endothermic peak in
the temperature range 55–808C, which can be attributed to
disruption of the host–guest interactions between DB24C8
and DBAS moieties. This result was in agreement with that
obtained by 1H NMR spectroscopic studies.
and sealed under vacuum. After stirring the mixture at 608C for 12 h,
it was diluted with THF (30 mL) and passed through a basic alumina
column. After removal of the solvents under reduced pressure, the
residues were dissolved in THF and purified by precipitation (three
times) in excess diethyl ether, affording two-arm PCL–DBA as a pale
yellow powder (0.88 g, yield: 93%; Mn,GPC = 5.3 kDa, Mw/Mn = 1.14).
The resultant two-arm PCL–DBA (0.472 g, 0.1 mmol) was dissolved
in THF (20 mL). After cooling to 08C, HPF6 (60 wt% in water,
0.486 g, 2.0 mmol) was slowly added. After the mixture was stirred for
30 min at room temperature, the reaction was quenched with water.
The dispersion was extracted with CH2Cl2 (3 ꢀ 40 mL). The combined
organic fractions were dried (MgSO4) and the solution was concen-
trated under reduced pressure. The remaining portion was added to
excess diethyl ether and the resultant precipitate was removed by
filtration and dried in a vacuum oven overnight at room temperature,
affording two-arm PCL–DBAS as a pale yellow solid (0.44 g, 88%;
Mn,GPC = 5.1 kDa, Mw/Mn = 1.15).
Received: November 22, 2008
Published online: January 28, 2009
Keywords: crown compounds · gels · molecular recognition ·
.
ring-opening polymerization · supramolecular chemistry
In summary, DB24C8-terminated four-arm star poly(e-
caprolactone) (PCL) and dibenzylammonium-terminated
two-arm PCL were synthesized by a combination of ring-
opening polymerization and click reaction. Based on these
well-defined precursors, responsive supramolecular gels were
constructed, taking advantage of the formation of pseudo-
rotaxane linkages between terminal crown ether and ammo-
nium moieties. The resultant supramolecular gels underwent
thermo- and pH-induced reversible gel–sol transitions. More-
over, the multiresponsive supramolecular networks contain
cavities with sizes which are expected to be facilely adjusted
by changes in the arm lengths of four-arm star PCL–DB24C8
and two-arm PCL–DBAS, which augurs well for their
application as smart nanocarriers for guest molecules and
complicated molecular devices. Further work towards this
aspect is currently underway.
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Experimental Section
Experimental details, including procedures for the synthesis of
alkynyl–DB24C8, alkynyl–DBA, four-arm star PCL–N3, and two-
arm PCL–N3, characterization methods, and data interpretation are
available in the Supporting Information.
Typical procedure for the preparation of four-arm star PCL–
DB24C8: A mixture of four-arm star PCL20–N3 (1.87 g, 0.2 mmol),
alkynyl–DB24C8 (0.531 g, 1.0 mmol), PMDETA (0.139 g, 0.8 mmol),
and CuBr (0.115 g, 0.8 mmol) in DMF (10 mL) was degassed by three
freeze–thaw cycles and sealed under vacuum. After stirring the
mixture at 808C for 8 h, it was diluted with THF (30 mL) and passed
through a basic alumina column. After removal of the solvents under
reduced pressure, the residues were dissolved in THF and purified by
precipitation (three times) in excess diethyl ether antisolvent. After
drying in a vacuum oven overnight at room temperature, four-arm
star PCL20--DB24C8 was obtained as a pale yellow solid (2.0 g, 87%,
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