Chemistry Reference
In-Depth Information
Figure 7.13 Schematic representation of the structure of a cross-linked MSP gel prepared by
Loveless and coworkers (Yount et al. 2005; Loveless et al. 2007) from poly(vinyl pyridine)
(PVP) cross-linked with bis-Pd
II
-orPt
II
-pincer compounds (18 and 19).
more thermodynamically and kinetically stable Pt
II
analogues demonstrate that it is
the off-rate of the metal binding motif (and not the binding constant) that is respon-
sible for the observed change in viscosity. This highlights the importance of kinetics
in dictating the properties of MSPs and suggests that “slow” means “strong.”
With the knowledge of how these systems behave, Loveless and coworkers (2007)
set out to gain control of the sol -gel and gel -gel transitions of these materials. The
reversible nature of both the pyridine/Pd
II
and pyridine/Pt
II
interactions was demon-
strated by use of competitive interacting additives. For example, either acids (such as
HCl and H
2
SO
4
), which can protonate the PVP, or competitive coordinating ligands
[such as dimethylaminopyridine (DMAP) or Cl
2
ions], which can displace the
pyridine in the complexes, reduce the cross-linking and weaken the gels. When
acids were used, addition of a weak base reversed the effect and resulted in the refor-
mation of the gel. The stimuli-responsive behavior of these gelswasmost profound close
to the percolation threshold of the cross-linker. For example, for the 10 wt% PVP in
DMSO system with the cross-linker 19a at the percolation threshold (0.8 wt%), the
