Thiourea-tagged poly(octadecyl acrylate) gels as fluoride and acetate responsive polymer gels through selective complexation

Article abstract of DOI:10.1039/c0cc03256e

Fluoride and acetate-responsive polymer gels were prepared by incorporation of p-nitrophenylthiourea into poly(octadecyl acrylate) gels, and the colours and volumes were changed selectively in the presence of these ions by complexation in THF.

Full text of DOI:10.1039/c0cc03256e

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Thiourea-tagged poly(octadecyl acrylate) gels as fluoride and acetate
responsive polymer gels through selective complexationw
Janakiraman Krishnamurthi,a Toshikazu Ono,a Shogo Amemori,a Hiromu Komatsu,b
Seiji Shinkaia and Kazuki Sada*ab
Received 14th August 2010, Accepted 9th November 2010
DOI: 10.1039/c0cc03256e
Fluoride and acetate-responsive polymer gels were prepared by
incorporation of p-nitrophenylthiourea into poly(octadecyl
acrylate) gels, and the colours and volumes were changed
selectively in the presence of these ions by complexation in THF.
Stimuli-responsive polymer gels that can change their volumes
drastically in response to various external physical and
chemical stimuli have been of much interest due to various
applications for actuators, drug delivery and microfluidic
Fig. 1 Schematic illustration of an anion recognition gel.
devices.1–3 Among them, the stimuli-responsive polymer gels
to specific chemical substances have been much studied due to
application of molecular recognitions for material sciences.4–10
However, they are mostly constructed by conjugation
of receptors or host molecules with thermally-responsive
hydrogels that undergo volume phase transitions in aqueous
phase such as polyelectrolytes, proteins, or poly(NIPAM).11–15
Thus, the guest molecules as the external stimuli have been still
limited owing to difficulty in designing host–guest systems with
high affinity in water.
Recently, we demonstrated that lipophilic polyelectrolyte
gels bearing tetraalkylammonium tetraphenylborate had
Fig. 2 Molecular structures of 1, G1 and G2.
extraordinary high swelling abilities in non-polar organic
solvents such as dichloromethane and THF.16,17 They also
and swelling behaviours in the presence of tetra(n-butyl)-
exhibited discontinuous and drastic volume changes in
response to changes of solvent polarities.18 These results
revealed that electrostatic repulsion and osmotic pressure
among the dissociated ion pairs are readily useful in nonpolar
media as driving force for volumetric changes of the polymer
networks. This prompted us to design novel anion-responsive
polymer gels peformed in nonpolar media by incorporation
of anion receptors into the lipophilic polymer chain as
schematically shown in Fig. 1. Complexation of specific anions
with the receptors should enforce free counter cations to
entrap in the inside of the polymer gels, which induces osmotic
pressure and electrostatic repulsion to expand the gel. Non-polar
media should allow us to use hydrogen bonds between the
anion receptors and the target anions, which might increase
binding affinity compared to that in water. Here, we demonstrate
preparation of thiourea-tagged poly(octadecyl acrylate) gels
ammonium salts with various anions shown in Fig. 2. A
thiourea moiety was selected as the receptor site, because of
remarkable binding abilities and colour changes to the specific
anions such as fluoride and carboxylate by hydrogen
bonds.19–24
Cross-linked poly(octadecyl acrylate) with a small amount
of thiourea groups was prepared by radical copolymerization
gelation from octadecyl acrylate (ODA), thiourea monomer 1
and ethylene glycol dimethacrylate (EGDMA) as a cross-linker
in the presence of azobisisobutyronitrile (AIBN) as the initiator
in the benzene–methanol mixture. The feed ratio was fixed at
95/5/1 for ODA/1/EGDMA. The formed gel (G1) was washed
with methanol for 48 h, then air-dried at room temperature.
The sample was cut into cylinders of about 1.0 mm in length
and dried in vacuo at 40 1C. As a reference, cross-linked
poly(octadecyl acrylate) without the thiourea moiety (G2)
was prepared by the same procedure in the absence of 1.
The dried and cut gels were transferred into THF solutions of
tetra(n-butyl)ammonium salt (TBA) with various concentrations
and kept for 48 hours at room temperature for equilibrium
swelling. Then, swollen gels were taken out from the solution
and then weighted. Swelling degrees (Q) were calculated as the
following equation:
a Department of Chemistry and Biochemistry,
Graduate School of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
b Department of Chemistry, Faculty of Science, Hokkaido University,
Sapporo 060-0810, Japan. E-mail: sadatcm@mail.sci.hokudai.ac.jp;
Fax: +81 11-706-3473; Tel: +81 11-706-3473
w Electronic supplementary information (ESI) available: Synthetic
procedure for G1 and G2 and titration of 1 with TBAF and TBAAcO.
See DOI: 10.1039/c0cc03256e
Q = (Wwet ꢀ Wdry)/Wdry (wt/wt)
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Chem. Commun., 2011, 47, 1571–1573 1571

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Fig. 3 Images of G1: (a) a dried gel, (b) a swollen gel in THF,
(c) a swollen gel in THF in the presence of TBA fluoride (25 mM) and
(d) TBA acetate (25 mM).
where Wwet and Wdry are the weights of the dried gel and the
wet gel, respectively.
A slightly yellow dried gel (G1) turned to be red and
yellow-green by immersion in the THF solutions of the
fluoride ion and acetate ion, respectively, as shown in Fig. 3.
The colour changes occurred only in the solutions of these two
anions, and other salts as well as salt-free THF remained
colourless after swelling. This specific colour changes were
attributed to complexation of p-nitrophenylthiourea moieties
in the polymer chains with fluoride or acetate ions as the
external stimuli. UV-vis titration experiments between the
monomer 1 and acetate or fluoride in acetonitrile indicated
that their binding constants (K) at 25 1C were 3.7 ꢁ 103 Mꢀ1
and 9.2 ꢁ 103 Mꢀ1 for fluoride and acetate, respectively. These
values were similar in magnitude to those of the reported
thiourea derivatives.19,21 Then, we investigated the swelling
behaviours of G1 in the presence of various tetra(n-butyl)-
ammonium salts at 25 mM (Fig. 4a).
Fig. 4 (a) Swelling degrees (Q) of G1 (grey bars) and G2 (open bars)
in the presence of TBA salts (25 mM) in THF. (b) The log plot of salt
concentrations to relative swelling degrees (Q0) of G1 in the presence of
various TBA salts in THF; (K) acetate, (E) fluoride, (’) chloride, (m)
hexafluorophosphate anion.
polymer gels, and the swelling abilities should be further
enhanced. However, in the presence of excess amount of
the salts, the polarity of the media should increase with the
increase in the salt concentration, and compatibility of the
poly(octadecyl acrylate) to the solution decreases. Therefore,
the swelling degrees decreased gradually. These effects were
observed for concentration dependence of the swelling degrees
in the presence of the ions that have no abilities to form
complexes with the thiourea groups. In order to clarify the role
of complexation, we tried removal of the fluoride anion from
the red swollen gels (Q = 40) by soaking in a large volume of
the THF mixture of methanol (8 : 2). After immersion for
48 hours at room temperature, the red gels collapsed and
turned to be nearly colourless. The swelling degree decreased
to 18, which was close to that of the swelling degree with
salt-free THF. A few further cycles of alternative immersion in
a 1 mM THF solution of TBA fluoride and THF–methanol
(8 : 2) provided the alternative changes of swelling degrees and
colour, as shown in Fig. 5. These results clearly indicate
enhancement of swelling degrees by specific anions that can
interact with the thiourea binding sites, and G1 should be
regarded as novel anion-responsive smart polymer gels with
selective colour changes.
In the fluoride and acetate solutions, the swelling degrees
increased to about 40, compared to those in the salt-free
solution (Qo = 20). On the other hand, in the bromide (Br)
and chloride (Cl) solutions, they were nearly equal to those
without salts. In the solution of more hydrophobic anions such
as hexafluorophosphate (PF6) and tetraphenylborate (BPh4),
they decreased to less than 20. Then, concentration
dependence of the swelling degrees was investigated, and
enhancement of the swelling degrees in the presence of TBA
salts were estimated by relative swelling ratios; Q0 = Q/Qo,
where Qo is the swelling degree of G1 in salt-free THF, as
shown in Fig. 4b. The swelling degrees were abruptly increased
to their double on introduction of a fluoride or acetate anion.
Then, the high swelling degrees were observed in the ranges
with several ten millimolar and then they were gradually
decreased with the increase in the anion concentrations. Since
the estimated concentration of the thiourea groups in the
polymer network of G1 was 0.15 mmol gꢀ1, the binding of
the external ions resulted in expansion of G1. The osmotic
pressure and electrostatic repulsion might be generated by
free moving tetra(n-butyl)ammonium counter cations in the
In conclusion, we demonstrate the fluoride or acetate
anion-sensitive smart materials accompanied by the two
changes of appearance of the polymer gels: their size and
colour in the swelling state. Hence, they should be highly
useful in an anion detecting and removing technology.
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Notes and references
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Although the swelling behaviours of ion-sensitive hydrogels
have been well documented in aqueous solutions,9,10 our
system lays on the reliable molecular recognition of thiourea
for specific anions with the aid of hydrogen-bonded complexation
in nonpolar media. Our result suggests that drastic volumetric
change of polymer gels should be acquired by increase of
osmotic pressure by entrapping external guest molecules,
which should be a powerful tool for designing guest-responsive
smart materials. Moreover, utility of the non-polar media
enables us to use
a library of well-defined hydrogen-
bonded host–guest systems in supramolecular chemistry for
their designs.25–27 Preparation and swelling abilities of
various guest-responsive polymer gels are under current
investigation.
The financial support for this research was provided by the
Grant-in-Aid (B) No. 20350056, Nanotechnology Network
Project (Kyushu-area Nanotechnology Network) and the
Grant-in-Aid for the Global COE Program, ‘‘Science for
Future Molecular Systems’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT).
T.O. acknowledges the JSPS research fellowship for young
scientists.
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1571–1573 1573