Poly(methyl methacrylate)s with pendant calixpyrroles and crown ethers: Polymeric extractants for potassium halides

Article abstract of DOI:10.1002/anie.200803970

(Figure Presented) Fishing for KF: Potassium fluoride and chloride salts can be extracted from aqueous media using poly(methyl methacrylate) s containing pendant calix[4]pyrrole and crown ether groups (see picture). These polymers are able to extract thes

Full text of DOI:10.1002/anie.200803970

Communications
DOI: 10.1002/anie.200803970
Polymeric Extractants
Poly(methyl methacrylate)s with Pendant Calixpyrroles
and Crown Ethers: Polymeric Extractants for Potassium
Halides**
Abdullah Aydogan, Daniel J. Coady, Sung Kuk Kim, Ahmet Akar,*
Christopher W. Bielawski,* Manuel Marquez, and Jonathan L. Sessler*
Angewandte
Chemie
9648
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The selective separation of alkaline
salts from aqueous media is of
fundamental importance in chemis-
try. It is, for instance, critical to the
production of commodity materials
(e.g., bromine and potassium) from
high-salt sources, such as the Dead
Sea and the Great Salt Lake[1] and,
on a very different scale, to the
regulation of taste[2] and the main-
tenance of osmotic balance in cells.[3]
Materials that could allow for such
separations are thus of potential
interest in a wide range of applica-
tions.[4] Polymeric systems are par-
ticularly attractive in this regard
because they are generally easy to
isolate from solutions or mixtures.
Recently, we reported that
copolymers of
a
polymerizable
derivative 2 (Scheme 1) of octame-
thylcalix[4]pyrrole (1)[5] and methyl
methacrylate (MMA) are effective
at extracting tetrabutylammonium
Scheme 1. Structures of compounds examined for their abilities to extract KF from water.
chloride or fluoride from aqueous media.[6] Unfortunately,
these polymeric materials displayed relatively low affinities
for the corresponding salts containing cations less soluble in
organic media (e.g., Na and K). We envisioned that by
appending recognition groups capable of binding such ions to
modified calixpyrrole-containing polymers, their affinities
toward common salts would be improved. For example, crown
ethers (e.g., 3) are well-known for their ability to complex
alkali cations, particularly potassium.[7] This has led to their
use in phase-transfer catalysis[8] and as extractants for picrate
anion salts under organic–aqueous interfacial conditions.[9] In
fact, polymeric systems containing crown ethers have been
used to extract the potassium salts of relatively hydrophobic
anions.[9] However, neither these latter systems nor any of
which we are aware possess the capability of extracting
“hard” potassium salts, such as KF or KCl, from aqueous
media.
Herein, we report the synthesis, characterization, and
extraction properties of mixed MMA copolymers containing
pendant calix[4]pyrrole subunits known to bind halide anions
in a 1:1 ratio in organic media[5] and benzo-[15]crown-5
subunits capable of forming 2:1 sandwich complexes with
potassium cations.[7] It was thus expected that strong,
potentially mutually enhancing, interactions would enable
these polymeric materials to extract potassium halide salts,
such as KCl and KF, from aqueous solutions.
Copolymers 4–6 (Scheme 1) were prepared from MMA,
calix[4]pyrrole 2, and the benzo-[15]crown-5 derivative 7[9]
using conventional free-radical polymerization techniques.[10]
In general, azoisobutyronitrile (1 mol%) was added to THF
solutions of these monomers in various ratios. After heating
at 708C for 17 h, the solutions were independently poured
into excess methanol, which caused the polymer to precip-
itate. After collection by filtration, the resulting materials
were characterized by NMR spectroscopy (CD2Cl2) and gel
permeation chromatography.[6] The molecular weights (33–
90 kDa) and polydispersities (PDI = 2.1–2.5) of these copoly-
mers were typical of those obtained from free-radical
polymerizations.
Initial qualitative evidence that copolymer 4, which
contains both calix[4]pyrrole and crown ether subunits,
could extract chloride salts into organic media came from a
visual test involving 9, a water-soluble dye that contains a
chloride counteranion. Treatment of an aqueous solution of 9
(25.5 mm) with a CH2Cl2 solution of copolymer 4 (effective
concentration of the calix[4]pyrrole and crown ether repeat
units was 1.56 and 1.22 mm, respectively) resulted in a colored
organic phase (Figure 1). As controls, solutions of the dye
were also exposed to CH2Cl2 solutions of 1 (1.56 mm), 3
(1.22 mm), or a mixture of 1 and 3 (1.56 and 1.22 mm,
[*] A. Aydogan, Prof. A. Akar
Istanbul Technical University
Faculty of Science & Letters, Department of Chemistry
Maslak 34469 Istanbul (Turkey)
Fax: (+90)212-2856386
E-mail: akara@itu.edu.tr
D. J. Coady, S. K. Kim, Prof. C. W. Bielawski, Prof. J. L. Sessler
Department of Chemistry and Biochemistry
The University of Texas at Austin, Austin, TX 78712 (USA)
Fax: (+1)512-4717550
E-mail: bielawski@cm.utexas.edu
sessler@mail.utexas.edu
Dr. M. Marquez
Department of Bioengineering
Arizona State University, Tempe, AZ 85287 (USA)
[**] This work was supported by the National Science Foundation (CHE-
0645563), the National Institutes of Health (GM 58907), and the
INEST Group.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803970.
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Communications
salt consisting of two hard ions, namely potassium fluoride. In
parallel, the extraction properties of 5 and 6 were examined.
These systems contain either calixpyrrole or crown ether
recognition subunits, respectively, and were designed to assess
the relative importance of each individual ion recognition unit
on the overall extraction properties of 4. As shown in
Figure 3, addition of a 3.4m D2O solution of KF to a CD2Cl2
solution of 4 (effective concentration of the calix[4]pyrrole
and crown ether repeat units 6.25 and 4.86 mm, respectively)
resulted in the appearance of a signal at d = À121.7 ppm in
the 19F NMR spectrum of the organic phase. A similar signal,
but of reduced intensity, was seen in the case of 5, whereas
very little signal was observed in the case of 6.
Figure 1. Aqueous solutions of 9 (top layers): a) After treatment with
CH2Cl2 (bottom layer). b) After treatment with a CH2Cl2 solution of 1
(bottom layer). c) After treatment with a CH2Cl2 solution of 3 (bottom
layer). d) After treatment with a CH2Cl2 solution of 1 and 3 (bottom
layer). e) After treatment with a CH2Cl2 solution of polymer 4 (bottom
layer). See text for details.
respectively); however, no transfer of color was observed.
These results were quantified using UV/Vis spectroscopy. As
shown in Figure 2, analysis of the aqueous phases of these
extraction experiments confirmed that 4 was able to extract 9
into the organic phase more effectively (greater than 54%)
than 1, 3, or their mixture. Similar qualitative and quantitative
results were observed for aqueous solutions of 10, a water-
soluble dye that contains a potassium countercation. In this
case, copolymer 4 proved more effective as an extractant
(greater than 30%) relative to either 1, 3, or the same mixture
of 1 and 3 used above (see the Supporting Information).
Encouraged by these initial results, we next sought to
address the question of whether copolymer 4 could extract a
Figure 3. 19F NMR spectra of CD2Cl2 solutions of copolymers a) 4
(effective [calix[4]pyrrole]=6.25 mm), b) 5 ([calix[4]pyrrole]=6.50 mm),
and c) 6 (no calix[4]pyrrole) after adding D2O solutions of KF (3.4m),
shaking the tubes vigorously, and then separating the phases with the
aid of centrifugation (10 min). * denotes KF.
To quantify the amount of fluorine present in the organic
phases of the aforementioned extraction experiments, fluo-
robenzene (final concentration: 14.21 mm) was added to each
sample as an internal standard (d = À114.3 ppm). On the
basis of comparative integrations (i.e., comparing total
fluoride content in the CD2Cl2 layer relative to this standard),
copolymer 4 was found to be capable of extracting KF more
efficiently (7.55 Æ 0.04 mm) then polymer 5 (5.71 Æ 0.03 mm)
under conditions where the effective concentration of the
calix[4]pyrrole repeat units in both polymers were essentially
the same (6.25 mm versus 6.50 mm for 4 and 5, respectively).
In addition, both of these polymers were found to extract
significantly more fluoride into the organic phase than 6
([F] = 0.34 Æ 0.03 mm in the CD2Cl2 layer), a copolymer that
does not contain any calix[4]pyrrole subunits, as noted above.
As control experiments, extractions were also performed in
an analogous manner using 1, 3, MMA homopolymer,[6] an
equimolar mixture of calixpyrrole 1 and crown ether 3, and
the calixpyrrole crown ether pseudo dimer 8, which was
envisioned as a small-molecule analogue of 4.[11] No quantifi-
able fluorine signal was observed in the organic phase when
any of these control systems were used as extractants.[12]
Flame emission spectroscopy (FES) was used to confirm
the coextraction of potassium in the above experiments.[13]
The organic phase obtained after extracting KF with polymer
4 afforded an emission intensity (EI) of 0.401 (at 766.5 nm,
i.e., the emission wavelength of the excited potassium ion
Figure 2. UV/Vis spectra of aqueous solutions of 9 (initial concentra-
tion=25.5 mm) after exposure to an equal volume of a CH2Cl2 solution
of polymer 4 (effective concentration of the calix[4]pyrrole and crown
ether repeat units was 1.56 and 1.22 mm, respectively), 1 (1.56 mm), 3
(1.22 mm), or a mixture of 1 and 3 (1.56 and 1.22 mm, respectively).
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produced by the flame source) after dilution with a known
amount of ethyl acetate. By way of comparison, the organic
phases produced EI values of 0.277 and 0.038, respectively,
when polymers 5 and 6 were used as extractants under
otherwise identical conditions. Based on quantification with a
series of standards,[14] extracted potassium concentrations of
6.84, 4.73, and 0.65 Æ 0.05 mm were calculated for CD2Cl2
solutions of polymers 4, 5, and 6, respectively (at effective
crown ether concentrations of 5.60, 0.00, and 5.00 mm,
respectively). These values are in good agreement with
those obtained from the 19F NMR spectroscopy data. A
summary of the KF extraction data is presented in Table 1.
it was concluded that polymer 4 extracts potassium chloride
much more effectively than it does sodium chloride.[18] This
finding, which is in accord with the relative hydration energies
of K+ and Na+ (DGh = À295 kJmolÀ1 for K+ and
À365 kJmolÀ1 for Na+),[15] suggests that these materials may
ultimately enable the selective separation of potassium halide
salts from complex aqueous mixtures. This could be especially
valuable in specialty medical applications, such as the control
of hyperkalemia, where potassium ion exchange resins (e.g.,
sodium polystyrene sulfonate; kayexalate) have seen wide-
spread use, despite being subject to inherent chemical and
clinical limitations.[19]
In conclusion, we have prepared the first well-defined and
homogenous polymeric systems capable of extracting potas-
sium fluoride and chloride salts from aqueous media. These
polymers contain pendant calixpyrrole and crown ether
subunits, key features that permit the concurrent complex-
ation of both halide and potassium ions. This, in turn, allows
the system as a whole to overcome the relatively high
hydration energies of KF and KCl and enables their
extraction from aqueous media with efficiencies that exceed
those expected on the basis of the effective concentration of
the individual receptors (crown ether and calixpyrrole). To
our knowledge this has not hitherto proved possible with any
other simple polymeric material. Ongoing efforts are focused
on fine-tuning the choice of receptors and investigating the
effect of polymer molecular weight and microstructure on the
overall extraction performance of these materials.
Table 1: Summary of KF extraction efficiencies.[a]
Cmpd[b] calix/
crown[c] (total)[d,e]
eff. [%]
eff. [%]
eff. [%]
eff. [%]
(calix)[d,f]
(total)[e,g]
(crown)[g,h]
4
5
6
8
1.0:0.8
1.0:0.0
0.0:1.0
1.0:1.0
67
88
6
121
88
61
73
12
0
137
n.d.[i]
12
n.d.[i]
0
0
0
[a] Extraction efficiencies (eff.) are reported as the percent (%) of
extractant populated with KF upon exposure to a saturated aqueous
solution of KF. See text for additional details. [b] See Scheme 1 for the
structures of the compounds studied. [c] Relative molar ratios of
calixpyrrole (calix) to crown ether (crown) units in the extractant.
[d] Calculated from total fluoride extracted. [e] Based on the total number
of ion receptors (calixpyrrole plus crown ether) in the extractant.
[f] Based on the total number of calixpyrrole units in the extractant.
[g] Calculated from total potassium extracted. [h] Based on the total
number of crown ether units in the extractant. [i] n.d.=not determined.
Received: August 11, 2008
Published online: October 16, 2008
Next, the ability of polymers 4, 5, and 6 to extract KCl
from aqueous media was evaluated using conditions analo-
gous to those employed for the KF studies described above. In
this case, after exposing the polymers to 3.4m solutions of KCl
in D2O, FES was again used to determine the relative
amounts of potassium extracted. Polymer 4 proved to be the
most effective extractant, displaying an EI value of 0.761,
which corresponded to a potassium concentration of 12.97 Æ
0.08 mm in the organic phase, with the exact quantification
being based on a series of standards.[14] Polymers 5 and 6
displayed lower EI values, namely 0.507 and 0.081, respec-
tively, which corresponded to potassium concentrations of
8.64 and 1.38 Æ 0.08 mm. The higher overall extraction values
for KCl compared to KF is consistent with the relative
aqueous solvation energies (DGh) of chloride and fluoride
anions (DGh = À340 kJmolÀ1 for ClÀ vs. À465 kJmolÀ1 for
FÀ).[15] Specifically, the more hydrophobic anion (ClÀ) was
extracted more effectively than its more hydrophilic analogue
(FÀ).[16,17]
Finally, an effort was made to determine if potassium salts
could be selectively extracted in the presence of their sodium
analogues. Towards this end, a 0.5 mL H2O solution of KCl
(134.1 mm) and NaNO3 (1.47m) was treated with polymer 4
(in 0.75 mL CH2Cl2) and analyzed using FES. (The two
aforementiond inorganic salts were combined deliberately to
form NaCl in situ.) The EI of the signal corresponding to
potassium (0.734) was over an order of magnitude greater
than the signal corresponding to sodium (0.043). On this basis
Keywords: calixpyrroles · crown compounds · extractants ·
polymers
.
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Communications
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[9] This particular crown ether was chosen because it was found to
be more effective in potassium cation extraction than the
corresponding crown-6 system when incorporated in a polymer;
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[10] A copolymer of 2 and 7 (i.e., no MMA) was also prepared but
was found to afford an insoluble white preciptate upon exposure
to aqueous solutions of KF or KCl. This result is thought to
reflect the formation of strong complexes within the polymer
matrix involving either the chloride or fluoride anion and the
calix[4]pyrrole subunit, or the potassium cation and the crown
ether subunit, perhaps in a cooperative fashion. However, the
lack of solubility precluded further analysis of this material.
Empirically, the presence of methacrylate units in the backbones
of these polymeric extractants (i.e., 4–6) appears necessary to
retain solubility in dichloromethane.
[11] Several other mixed calixpyrrole crown ether pseudo dimers,
containing different covalent linkages, different size crowns, etc.,
were also prepared; none of these proved effective as KF
extractants under the conditions described above (see the
Supporting Information).
[15] Hydration energies were taken from Table 1 of Y. Marcus, J.
Chem. Soc. Faraday Trans. 1991, 87, 2995.
[16] Further support for this conclusion was obtained from 1H NMR
spectroscopic analysis of the pyrrolic NH signal as a function of
KF and KCl concentration under the extraction conditions
described in the text. As detailed in the Supporting Information,
the NH signals shifted downfield and decreased in intensity with
increasing concentrations of the potassium salts, KCl and KF. In
accord with expectations, KCl gave rise to greater downfield
shifts in the NH peak shift (d NH = 7.39 ppm for [KCl] ꢀ
1.04 mm) than KF (d = 7.35 ppm for [KF] ꢀ 1.27 mm).
[17] The temperature of ultimate mass loss exhibited by polymers 4–6
was seen to increase (to over 4008C by thermogravimetric
analysis) after extraction with KCl or KF (see Supporting
Information). The increased thermal stabilites are consistent
with the notion that these polymers contained ionic species.
[18] NaCl and NaF extractions were also carried out using the same
conditions employed for the corresponding potassium salts.
However, no evidence of efficient extraction was observed in the
case of any of the polymeric or control systems considered
herein.
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