One-step synthesis of thermosensitive nanogels based on highly cross-linked poly(ionic liquid)s

Article abstract of DOI:10.1002/anie.201202957

A gel for all seasons: Thermosensitive nanogels based on highly cross-linked poly(ionic liquid)s (CLPNs) were prepared in one step by the copolymerization of imidazolium-based monomers with cross-linkers in selective solvents. Reversible nanogel-macrogel transitions of CLPNs in methanol could be achieved by changing the temperature (see picture). Copyright

Full text of DOI:10.1002/anie.201202957

Angewandte
Chemie
DOI: 10.1002/anie.201202957
Ionic Gels
One-Step Synthesis of Thermosensitive Nanogels Based on Highly
Cross-Linked Poly(ionic liquid)s**
Yubing Xiong,* Jingjiang Liu, Yujiao Wang, Hong Wang, and Rongmin Wang*
Ionic liquids (ILs) have attracted considerable attention in
a wide variety of fields because of their excellent properties,
such as immeasurably low volatility, non-flammability, high
polarity, high ionic conductivity, and a wide window of
electrochemical potential.[1] In the past few years, the
utilization of ILs for the design of advanced materials and
polymer-based physical-chemical systems has been studied
widely.[2] In particular, ILs have been extensively utilized in
the preparation of a new class of solid state electrolytes with
high ionic conductivity known as ionic gels,[3] which have
found use in sensors, solar cells, capacitors, polymer light-
emitting cells, CO2 absorption, and other applications.[4]
Currently, ILs are usually either gelled with polymers,[5]
incorporated with inorganic substances (silica nanoparticles
and carbon nanotubes),[6] or mixed with other ILs to prepare
IL-based gels.[7] The design and synthesis of low molar mass
gelators are also effective methods.[8] However, these meth-
ods and techniques for the preparation of IL-based gels
present several limitations, ranging from expensive materials
to complex synthetic procedures,[9] which make them less
attractive
Recently, we have demonstrated a facile one-step syn-
thetic strategy for the preparation of IL-based cross-linked
polymeric nanogels (CLPNs) by the conventional radical
copolymerization of a phosphonium-based IL (PIL) and the
cross-linkers ethylene glycol dimethacrylate (EGDMA) and
divinylbenzene (DVB) in selective solvents.[10] Nevertheless,
when the imidazolium-based IL (ImIL), 1-vinyl-3-(ethoxy-
carbonyl)methyl imidazolium chloride was copolymerized
with a cross-linker under the same conditions, the copolymers
precipitated from the solvent, resulting in particles in the
submicrometer range.[11] Similar results were also reported by
Han et al.[12] The precipitation of ImIL-based cross-linked
copolymers from the solvent during the polymerization
process was due to the lower number of poly(ImIL) chains
in the copolymer.[11] This result was presumably caused by the
lower polymerization reactivity of ImIL monomers, as
compared with the cross-linkers.
Herein, geminal dicationic, 1,4-butanediyl-3,3’-bis-1-vinyl
imidazolium halides ([BVIm]X = 1·X; X = Cl or Br), were
developed to provide further insight into the mechanism of
the one-step synthesis of CLPNs, and to prepare ImIL-based
CLPNs through the copolymerization of 1·X with cross-
linkers in selective solvents. For comparison, 3-butyl-1-vinyl-
imidazolium bromide ([C4VIm]Br) was also prepared. The
results revealed that ImIL-based CLPNs can be easily
prepared through the copolymerization of 1·X with
EGDMA and DVB in selective solvents. Stable, blue
opalescent CLPN solutions can be prepared without any
precipitation when the cross-linking copolymerization of 1·X
is conducted in methanol or ethanol (Supporting Information,
Figure S1). However, the copolymers precipitated from the
solvent when [C4VIm]Br was copolymerized with the cross-
linkers under the same conditions. These results confirm the
assumption that the polymer particles cannot be stable
because of the low reactivity of ImIL monomers, as compared
with the cross-linkers. In contrast, the polymer particles can
be stabilized by introducing geminal ImIL monomers. More
importantly, the ImIL-based CLPN is thermosensitive, and
can reversibly transform to precipitate or macrogel in
methanol with a change in temperature (Scheme 1). Herein,
Scheme 1. One-step synthesis of ImIL-based CLPN, with photographs
showing the thermosensitive behavior of CLPN solutions. A) 1·Br/
EGDMA=5:1 (mol/mol); B) 1·Br/EGDMA=10:1 (mol/mol).
[*] Dr. Y. Xiong, J. Liu, Y. Wang, H. Wang, Prof. R. Wang
Key Laboratory of Eco-Environment-Related Polymer Materials
of Ministry of Education, College of Chemistry and Chemical
Engineering, Northwest Normal University
Lanzhou 730070 (China)
E-mail: yubing_xiong@163.com
a novel example of an ImIL-based thermosensitive nanogel
prepared by one-step cross-linking copolymerization is pre-
sented.
The sizes of the ImIL-based CLPNs were measured using
dynamic light scattering (DLS), the z-potential was also
determined. As shown in Table 1, ImIL-based CLPNs with
sizes of less than 100 nm can be prepared by the one-step
[**] This work was financially supported by the NSFC (20804031,
21164008, and 20964002). We also thank Prof. M. Jiang, Prof. P. Yao,
and Prof. D. Chen (Fudan University) for DLS, TEM, FTIR, and
rheological measurements.
Supporting information for this article, including synthetic proce-
dures and analytical data, is available on the WWW under http://dx.
doi.org/10.1002/anie.201202957.
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Table 1: Properties of CLPNs prepared in methanol with different 1·X/
cross-linker feed ratios.
Monomer
Feed ratio[a]
Dh
PDI
z-Potential
1·Br+EGDMA
1·Br+EGDMA
1·Br+EGDMA
1·Br+DVB
5:1
10:1
15:1
5:1
10:1
5:1
100
78
67
99
69
88
42
0.21
0.19
0.22
0.25
0.30
0.24
0.32
16.5
14.5
13.9
13.6
16.5
13.8
11.3
1·Br+DVB
1·Cl+EGDMA
1·Br+EGDMA[b]
5:1
[a] Molar ratio of 1·X/cross-linker. [b] Prepared in ethanol.
Dh =hydrodynamic diameter, PDI=polydispersity index of CLPN.
synthesis, and the size decreases with the increase in 1·Br in
the feed. This data is reasonable, because 1·Br acts as
a stabilizer in the process. In agreement with PIL-based
CLPNs, ImIL-based CLPNs prepared in ethanol were smaller
than those prepared in methanol, which is probably because
the solubility of ImIL is higher in ethanol than in methanol.
Also, given that all CLPNs are positively charged, they are
very stable in solution owing to mutual electrostatic repulsion.
These results demonstrate that the one-step synthesis is an
effective technique for the preparation of IL-based CLPNs.
The CLPN generated with a feed ratio of 5:1 1·Br/
EGDMA was found to precipitate from methanol when the
temperature decreased below 258C, but a stable CLPN
solution was immediately recovered at temperatures above
258C (Scheme 1A). This behavior indicates that 1·X-based
CLPN is thermosensitive. Figure 2 shows the temperature
dependence of the turbidity and diameter of CLPN (4.5 wt%)
in methanol solution. The blue opalescent solution was
translucent at temperatures above 288C, and suddenly
turned cloudy at temperatures below 258C. The discrete
transition, which occurred within a temperature difference of
38C, indicates that 1·X-based CLPNs are thermosensitive and
possess an upper critical solution temperature (UCST). The
thermosensitive behavior was also supported by DLS. The
hydrodynamic diameter (Dh) of CLPN increased rapidly from
less than 100 nm to micrometer levels in the same temper-
ature range. These results suggest that free CLPN was
suddenly desolvated to form larger aggregates, leading to
Figure 2. Temperature dependence of the transmittance at 500 nm
~
&
(c c) and the hydrodynamic diameter (Dh; c c) for
4.5 wt% CLPN (1·Br/EGDMA=5:1) in methanol.
precipitation from the solvent at temperatures below the
UCST.
However, CLPNs (1·Br/EGDMA) with feed ratios of
higher than 10:1 did not precipitate from the solvent when the
temperature was below 258C, not even at 48C. Surprisingly,
instead of precipitating, the CLPN solutions gelled when kept
at temperatures below À108C. As shown in Scheme 1B, the
reversible sol–gel transition of CLPN in methanol is driven by
temperature change. A weak gel also formed when CLPN
(1·Br/EGDMA = 5:1) completely precipitated from metha-
nol (Scheme 1A). The preliminary results showed that some
interactions between CLPN and methanol exist, which can be
enhanced by increasing the 1·Br content in the copolymers.
Dynamic rheological analysis is an effective technique for
measuring the sol–gel transition behavior. The sol–gel
transition point is indicated by the temperature at which the
storage modulus (G’) curve intersects the curve of the loss
modulus (G’’). Figure 3 shows the oscillatory temperature
sweep profiles of 4.5 wt% CLPN solutions in methanol.
When the temperature was above À78C, both CLPN
solutions exhibited a viscoelastic response, because CLPN
was in the liquid state. When the temperature was below
À78C, both G’ and G’’ increased; however, G’ increased more
rapidly than G’’. The sol–gel transition occurred when G’ was
higher than G’’, at about À78C for both CLPN solutions. The
results obtained from the temperature-dependent oscillatory
shear rheological measurements further confirmed the
sol–gel transition behavior of CLPN solution in methanol
with a 1·Br/cross-linker feed ratio above 10:1.
A variety of organo- and hydro-gelators that can immo-
bilize organic fluids and/or water have been proposed in
which various types of intermolecular interactions, such as
hydrogen-bond, host–guest, p–p, cation–p, and electrostatics,
play a significant role.[13] In an effort to obtain evidence for
the formation of a CLPN-based macrogel in methanol,
1H NMR spectroscopy, IR spectroscopy, differential scanning
calorimetry (DSC), and X-ray diffraction (XRD) were used
to elucidate the nature of the interactions between methanol
and ImIL-based CLPN.
Figure 1. Proposed H-bond network between the imidazolium ring,
BrÀ, and methanol at lower temperatures.
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analysis was conducted in deuterated DMSO. Moreover, the
addition of methanol into a 1·Br solution in DMSO resulted
in a significant decrease in the chemical shift and intensity of
the signal of the C2 proton of imidazolium (Figure S2). The
results indicate that the C2 proton of imidazolium can
exchange with the protons of polar solvents such as water
and methanol. Therefore, it is inferred that a strong H bond
can form between 1·Br and methanol.
IR spectroscopy is a powerful and effective technique for
studying the interactions between ILs and solvents, as well as
the formation/disruption of H bonds.[14] It has been reported
that the IR spectra of dried imidazolium-based ILs present
a prominent absorption band at 3058 cmÀ1, which is attributed
À
À
À
to the C H stretching vibration for C H···X . Upon uptake
À
of water, the peaks associated with the aromatic C H
À
stretching vibration and the C H stretching vibration of the
C H···X shift to higher wavenumbers.[15] According to
À
À
Pimentel and McClellan,[16] the stretching mode of an A H
À
moiety shifts to lower frequencies upon H bond formation.
Infrared spectra of ImIL-based CLPNs are shown in Fig-
ure S3. Based on the reports, the peaks at 3078 and 3137 cmÀ1
À
were attributed to the aromatic C H stretching vibration and
À
À
À
the C H stretching vibration of C H···Br , respectively. The
peaks were shifted to higher wavenumbers, as compared to
the dried samples. This illustrates the disruption/diminution
of the H bond between the imidazolium ring and BrÀ,
probably due to the presence of residual methanol or water.
To elucidate the interactions between CLPN and meth-
anol, FTIR spectra of CLPN containing methanol were
continuously collected in a temperature-controlled process
(Figures S4–S8). As shown in Figure S9a, two prominent
bands, which correspond to the vibrational region of the
~
Figure 3. Shear storage modulus (G’; ) and shear loss modulus (G’’;
!
) for CLPN at a concentration of 4.5 wt% in methanol, as a function
^
of temperature ( ). A) 1·Br/EGDMA=10:1 (mol/mol); B) 1·Br/
EGDMA=15:1 (mol/mol). Photographs show the corresponding
appearance of the sample.
À
À
aromatic C H stretching and the C H stretching of
À
À
À1
C H···Br , were observed around 3125 and 3060 cm ,
respectively. However, when methanol was added, both of
the bands shifted to significantly higher wavenumbers, 3135
and 3087 cmÀ1, respectively (Figure S9b). Compared with
these shifts, most of the other bands of CLPN remained
unchanged upon addition of methanol. The upward shift in
First, the 1H NMR spectra of 1·Br in different deuterated
solvents were obtained. As shown in Figure 4, all of the
signals confirm the desired structure of 1·Br. A prominent
characteristic found in 1·Br was the signal associated with the
C2 proton of the imidazolium ring, which was not observed
when the measurements were conducted in deuterated water
or methanol. However, the signal emerged when the NMR
À
the frequency of the C H stretching was consistent with
H bond disruption/diminution between the imidazolium ring
and BrÀ in the presence of methanol. Furthermore, the peaks
again downshifted to lower wavenumbers when methanol was
evaporated at higher temperature. This downshift is also
indicative of H bond formation between the imidazolium ring
and BrÀ upon the evaporation of methanol. At the same time,
À
the position of the O H stretching band of methanol shifted
from 3425 cmÀ1 to 3409 cmÀ1, and the intensity decreased
greatly. Therefore, the formation of a solution of CLPN in
À
À
methanol involves the partial replacement of C H···Br
bonds with H bonds between the imidazolium ring and
methanol. Considering these results, the formation of
a thermo-reversible CLPN macrogel in methanol involved
significant enhancement in the H bonds between the imida-
zolium ring, BrÀ, and methanol at lower temperatures. The
formation of a similar H-bond network was also observed in
water.[8c] Such a network could serve to enhance the enclosure
of methanol, and ultimately formed a physical macrogel
(Figure 1). When the temperature increased, the H-bond
Figure 4. 1H NMR spectra of 1·Br in various deuterated solvents.
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network was destroyed, leading to the formation of a meth-
anol solution of CLPN.
CLPNs, for example as gold nanoparticle supporters, are
currently under investigation in our laboratory.
The morphology of CLPN was examined using electronic
microscopy. Transmission electron microscopy (TEM) shows
spherical nanoparticles with sizes of less than 50 nm (Fig-
ure 5A–C). The sizes observed are smaller than those from
In summary, ImIL-based CLPNs were facilely prepared
by one-step cross-linking copolymerization of geminal dicat-
ionic monomers and cross-linkers in selective solvents. Stable
CLPN solutions were obtained and the particle sizes could be
fine-tuned by changing the 1·X/cross-linker feed ratio. There-
fore, the one-step synthesis herein presented is an efficient
method for the preparation of IL-based CLPNs. Moreover,
ImIL-based CLPNs are thermosensitive and display rever-
sible, temperature-driven nanogel–macrogel transitions in
methanol. ImIL-based CLPNs were also found to be effective
catalysts for CO2 cycloaddition reactions with epoxides, and
could be readily separated by filtration, because of their
thermosensitive properties. Such attributes make them
a robust material platform suitable for a wide range of
applications.
Received: April 18, 2012
Revised: June 13, 2012
Published online: && &&, &&&&
Keywords: carbon dioxide fixation · copolymerization ·
.
hydrogen bonds · ionic liquids · nanogel
Figure 5. Representative TEM (A, B, and C) and SEM (D, E, and F)
images of CLPN. A) 1·Br/DVB=10:1; B) CLPN prepared in ethanol;
C) 1·Br/EGDMA=10:1; D) 1·Br/DVB=10:1; E) 1·Br/EGDMA=10:1;
F) cryo-dried gel (1·Br/EGDMA=10:1).
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DLS measurements, because the particles were swollen in
solution. Figures 5D and E show SEM images of CLPN
obtained after precipitating the reaction solutions using
diethyl ether. The CLPN aggregated and formed larger
particles during the precipitation process. It can also be
observed from Figure 5E that the aggregates are crystalline
and consist of spherical CLPN particles. However, Figure 5F
shows that cryo-dried CLPN gel has a regularly arranged
structure. The crystal structure of ImIL-based CLPNs may
result from poly(1·Br) chains on the CLPN surface. The
crystal structure of ImIL-based CLPNs was also confirmed by
XRD and DSC measurements (Figures S10 and S11).
ImIL-based CLPNs demonstrated excellent activity in the
cycloaddition reaction of CO2 with epoxides. As summarized
in Tables S1 and S2, and in Figure S12, cyclic carbonates with
high yield and selectivity were obtained. Also, CLPN could be
easily separated and reused without detectable decrease in
activity. Furthermore, a binary catalyst system combining an
ImIL-based CLPN and K2CO3 was explored for the trans-
esterification of
a
cyclic carbonate with methanol
(Scheme S1). The results showed that the binary catalyst
was effective in the reaction, and dimethyl carbonate was
obtained in high yield (Table S3) and excellent selectivity.
More importantly, CLPN could be readily separated by
filtration after precipitation from methanol at temperatures
below the UCST. As expected, ImIL-based CLPNs have
much potential as both catalysts and novel catalyst supporters
that can be readily recycled owing to their thermosensitive
properties. More potential applications of thermosensitive
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Communications
Ionic Gels
A gel for all seasons: Thermosensitive
nanogels based on highly cross-linked
poly(ionic liquid)s (CLPNs) were pre-
pared in one step by the copolymerization
of imidazolium-based monomers with
cross-linkers in selective solvents. Rever-
sible nanogel–macrogel transitions of
CLPNs in methanol could be achieved by
changing the temperature (see picture).
Y. Xiong,* J. Liu, Y. Wang, H. Wang,
R. Wang*
&&&& — &&&&
One-Step Synthesis of Thermosensitive
Nanogels Based on Highly Cross-Linked
Poly(ionic liquid)s
Ionische Gele
Ein Gel fꢀr alle Fꢁlle: Temperatur-
empfindliche Nanogele auf der Grund-
lage hoch vernetzter polymerer ionischer
Flꢀssigkeiten (CLPNs) wurden in einem
Schritt durch die Copolymerisation eines
Imidazolium-Monomers und eines Netz-
bildners in selektiven Lçsungsmitteln
erhalten. Temperaturꢁnderungen bewirk-
ten reversible Nanogel-Makrogel-ꢂber-
gꢁnge der CLPNs in Methanol (siehe
Bild).
Y. Xiong,* J. Liu, Y. Wang, H. Wang,
R. Wang*
&&&& — &&&&
One-Step Synthesis of Thermosensitive
Nanogels Based on Highly Cross-Linked
Poly(ionic liquid)s
6
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Angew. Chem. Int. Ed. 2012, 51, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Ionic Gels
A gel for all seasons: Thermosensitive
nanogels based on highly cross-linked
poly(ionic liquid)s (CLPNs) were pre-
pared in one step by the copolymerization
of imidazolium-based monomers with
cross-linkers in selective solvents. Rever-
sible nanogel–macrogel transitions of
CLPNs in methanol could be achieved by
changing the temperature (see picture).
Y. Xiong,* J. Liu, Y. Wang, H. Wang,
R. Wang*
&&&& — &&&&
One-Step Synthesis of Thermosensitive
Nanogels Based on Highly Cross-Linked
Poly(ionic liquid)s
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!