Cross-Linking via Complexation with Cucurbit[8]uril
A R T I C L E S
ultrafast network recovery after destruction of the existing
network Via shear stress, etc. Furthermore, it is worth noting
that while dissociation rate constants have been measured and
5
4-58
57,59
reported for CB[6]
di)cationic guests, no report on the binding dynamics of a CB[8]
ternary complex has appeared in the literature to date. In both
CB[6] and CB[7] complexes, the reported k and k values are
and CB[7]
1:1 complexes with
(
a
d
much lower and can be measured by NMR, while in our system
we are only measuring the rate constants for second guest
binding which is largely driven by hydrophobic interactions and
3
5
desolvation of the second guest.
Microstructure of Supramolecular Hydrogels. The micro-
structure of these supramolecular hydrogels (2b/7b/CB[8]) were
investigated using scanning electron micrography (SEM) after
cryo-drying. The supramolecular networked systems retain the
characteristic bright red color of the ternary complex charge-
transfer band, identifying that the supramolecular network is
still intact while in the solid state. Figure 10 shows SEM images
of the 1:1 mixture of 1b and 2b at 5 wt % in water with
increasing cross-link densities demonstrating that each of the
hydrogels has a very uniform, highly ordered microstructure.
As the cross-link density increases, the pore size within the
polymeric matrix decreases from approximately 12 to 3 µm
between 2.5% and 10% cross-linking, corroborating the rheo-
logical observations. The decrease in pore size with increasing
cross-link density is consistent with typical covalent cross-linked
hydrogels.
Figure 9. Network relaxation rate ꢀ (the G′ and G′′ crossover frequency
in hertz) as a function of the % cross-linking in the supramolecular hydrogel
formed from the 1:1 mixture of 2b and 7b with CB[8].
further confirm that the observed increase in viscosity is due to
cross-linking through 1:1:1 heteroternary complexation.
Looking further into the oscillatory rheology of the system,
ꢀ
changes as a function of cross-link density according to an
-1
inverse power law with a dependence of (% cross-link) (Figure
), consistent with theoretical models for supramolecular
9
51
transient networks. According to the seminal studies by Craig
et al.
52,53
the kinetics of molecular cross-links in supramolecular
systems are of greater importance than binding thermodynamics
in determining the bulk viscoelastic properties of the material.
It has been demonstrated that extrapolation of the relaxation
rate plot back to the cross-link density at the gel point yielded
the ‘apparent’ intrinsic relaxation rate (ꢀint) of the network (i.e.,
the dissociation rate of supramolecular cross-links), found to
The hydrogel microstructure was also used as a platform to
probe the hydrogel response to thermal stimuli. A sample of
2b/7b/CB[8]2.5% cross-linked with 0.25 mol equiv of CB[8] gave
the hydrogel depicted on the left of Figure 11. SEM micrographs
clearly demonstrate that the microstructure consists of large
pores approximately equal in size to those described previously
d
be equivalent to the k determined from corollary NMR studies
5
3
of the small-molecule supramolecular motif. Applying the
same method by using the cross-link density at the gel point
determined from viscosity titrations (1.25%), extrapolation of
the plot in Figure 9 yields a dissociation rate constant of the
(
1
12 µm) for the 2.5% cross-linked sample depicted in Figure
0d. Qualitative evidence for the thermal reversibility of this
-
1
hydrogel is demonstrated with an inverted vial test. Upon
heating, the hydrogel undergoes a gel-to-sol conversion and
readily flows as the second guest K value decreases at increased
a
ternary complex k
This k value is remarkably high when considering the
intrinsic motion limitations of the ternary complex within the
D cross-linked network. The implication of such a high k when
considering the solution K value for second guest binding from
ITC measurements (Table 3, entry 6) is an association rate
d
) 1200 s .
d
temperatures. This conversion is completely reversible, and the
hydrogel reforms upon cooling (Figure 11a). If more CB[8] is
then added to the vial (up to a total of 1 mol equiv), heating of
the gel and subsequent shaking allow for dissolution of the
additional CB[8], yielding a hydrogel with 10% cross-linking
upon cooling. This hydrogel, now formed with a higher cross-
link density, remained fully thermally reversible. An immediate
observation is that the red color of the hydrogel is much more
intense on account of a higher concentration of ternary
complexes within the network structure. This is consistent with
observations made previously in CB[8] ternary complex sys-
3
d
a
7
-1 -1
constant (k
a
) 9.6 × 10 M s ) which is approaching the
diffusion limit (Figure 2b). Studies are currently underway to
further analyze binding dynamics for CB[8] ternary complex-
ation both with small molecules and with polymeric materials.
While the association rate constant obtained using the method
developed by Craig et al. is high, it is a reasonable value as k
d
is expected to be greater than 200 s- (limit of the NMR) due
1
to the appearance of singular, broad peaks observed in previ-
37
1
7
tems. Additionally, SEM micrographs identify a drastic change
in the hydrogel microstructure as much smaller pore sizes are
now measured which are consistent with those previously
measured for a directly prepared sample of 10% cross-linking
ously published NMR studies. These studies clearly demon-
strate fast exchange of the second guest for MV/Np/CB[8]
ternary complexes in both small molecule and polymeric
systems. These observations offer important insight into the
binding mechanism of CB[8] ternary complexation as the
primary driving force for complexation is an extremely high
association rate (as opposed to a very low dissociation rate),
yielding dynamically cross-linked materials with potentially
(
54) Mukhopadhyay, P.; Zavalij, P. Y.; Isaacs, L. J. Am. Chem. Soc. 2006,
128, 14093–14102.
(
(
(
55) Mock, W. L.; Shih, N. Y. J. Org. Chem. 1986, 51, 4440–4446.
56) Mock, W. L.; Shih, N. Y. J. Am. Chem. Soc. 1989, 111, 2697–2699.
57) Marquez, C.; Nau, W. Angew. Chem., Int. Ed. 2001, 40, 3155–3160.
(
(
(
51) Jongschaap, R.; Wientjes, R.; Duits, M.; Mellema, J. Macromolecules
001, 34, 1031–1038.
52) Yount, W.; Loveless, D.; Craig, S. Angew. Chem., Int. Ed. 2005, 44,
746–2748.
53) Yount, W.; Loveless, D.; Craig, S. J. Am. Chem. Soc. 2005, 127,
4488–14496.
(58) Marquez, C.; Hudgins, R. R.; Nau, W. M. J. Am. Chem. Soc. 2004,
126, 5808–5816.
2
(59) Jeon, W.; Moon, K.; Park, S.; Chun, H.; Ko, Y.; Lee, J.; Lee, E.;
Samal, S.; Selvapalam, N.; Rekharsky, M.; Sindelar, V.; Sobransingh,
D.; Inoue, Y.; Kaifer, A.; Kim, K. J. Am. Chem. Soc. 2005, 127,
12984–12989.
2
1
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