Communications
Table 3: Determination of hydrodynamic diameter of the complexes of 5
with 6, 7 and 8 at 258C in water.
However, in our studies, we could show conclusively that the
fluorinated anions 7 and 8 have a high tendency to form
inclusion complexes with CD (Table 1).
As shown in Table 2, the cloud points (LCST) of the
aqueous NIPAAM-CD copolymer 5 change significantly with
ILs 6, 7, and 8 as guest molecules. The LCST values of
System
Diameter [nm]
15.1
5
5+6
11.5
9.8
Table 2: LCSTof copolymer 5 and its complexes with 6, 7, and 8 in water.
Polymer
5
5+6
5+7
5+8
LCST [8C][a]
36
43
53
34
[a] Determined by turbidity measurements.
5+7
5+8
complexed copolymers 5 increase after addition of ionic guest
molecules 6 and 7. This increased hydrophilicity of pseudo-
polyanions results from the free carboxylate (IL 6) and
sulfonate (IL 7) groups, which are preferentially located in the
aqueous phase and therefore responsible for these increased
LCST values of 438C and 538C, respectively.
Complexation of the bis(trifluoromethylsulfonyl)imide
anion 8 leads to a compound in which the hydrophobic
trifluoromethylsulfonyl group is preferentially located in the
centre of the CD-cavity, leading to a decreased LCST value.
The cloud points of the copolymer 5 decrease with increasing
hydrophobicity of the ionic liquids, as expected (Table 2).[4]
This observed influence of ILs on the LCST values is a result
of the complexation of negatively charged guests in CD
cavity. In other words, starting from a neutral polymer, we
obtained a pseudopolyanion (Scheme 3).
16.5
Scheme 4. Intramolecular interactions of guest molecules (IL 6 and IL
7) with covalently bonded CD lead to shrinkage with IL anions 6 and 7
(left), or to expansion of the polymer chains with IL 8 (right) For
details, see text.
Scheme 3. Generation of pseudopolyanions.
The hydrodynamic diameters of copolymer 5 without
guests, and copolymer 5 with charged guest molecules
included, were measured by DLS (Table 3). Surprisingly, the
results indicate that polymer 5 has a larger mean coil size than
that of its complexes with IL 6 and 7. In both cases (5 + 6 and
5 + 7), the negative charges of the anions are in contact with
the water phase, owing to the hydrophobic cavity of CD. A
possible reason for this situation is the increasing ionic
strength, which leads to some intramolecular interactions
between the CD-complexed anions and the cations in
solution, resulting in a largely reduced intra- and intermo-
lecular repulsion of the chains. These attractive forces
obviously lead to a decrease of the hydrodynamic diameter
of the complexes 5 + 6 and 5 + 7 (Scheme 4). We can thus
conclude that electrostatics override entropy effects.[9]
(Scheme 4), owing to incorporation of the anion group of
guest monomer 8 into the CD ring. Furthermore, as shown in
Table 1, the entropy of the complex-formation process of IL 8
with CD is very negative, which indicates that a noncoordi-
nating character of the covered anion dominates. Thus, the
cation–cation repulsion plays a major role and determines the
extended polymer coil structure.
The light-scattering results are consistent with the out-
comes of the turbidity measurements. The complexes 5 + 6
and 5 + 7 are more hydrophilic than the pure copolymer 5,
resulting in an increase of the LCST. A decrease in hydro-
dynamic diameter is due to counterion effects. In case of IL 8,
a lower cloud point was obtained, because the ions are
covered by the CD ring and therefore spatially separated
from the cation. A higher hydrodynamic diameter is a result
of electrostatic repulsion between the cations. These results
are due to the delocalized negative charge of the anion of 8.
In contrast, the hydrodynamic diameter of the complex
5 + 8 slightly increases in comparison to the pure copolymer 5
3436
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 3435 –3437
Amajjahe, Sadik
