Chemistry Reference
In-Depth Information
2004). These and numerous other organometallic coordination system have been used
in SPs (Rodriguez et al. 2002). The reversible kinetics of interest are those of ligand
exchange. In Pd(II) and Pt(II) complexes, ligand exchange occurs through a sterically
congested associative mechanism. Added bulk in the N-alkyl substituents R therefore
slows the exchange while exerting a lesser effect on the relative energy of the roughly
isosteric end points. For example, the association constants for 1a
.
2b and 1b
.
2b in
dimethylsulfoxide (DMSO) are (1.6 and 1.3)
10
3
M
21
, respectively, but the rates of
ligand exchange differ by nearly 2 orders of magnitude (70-100 s
21
for 1a
.
2b and
1.0 s
21
for 1b
.
2b; Yount et al. 2003).
The pincer motif can be incorporated into SPs such as linear SPs 3
.
4. A 1:1
mixture of 3:4a (4.6 wt%) forms linear SPs, and there is a concomitant increase in
solution viscosity. That the increased viscosity of the SP solution is not influenced
by the reversibility of the metal -ligand bond is demonstrated by direct comparison:
a similar solution of 3
.
4b has the same viscosity, and so the equilibrium structure of
the SP determines the viscosity of the SP solutions; the transience of the main chain
does not contribute (Yount et al. 2003).
The situation becomes more complex when considering nonlinear responses such
as shear thinning. Under high steady shear, the viscosity of polymer solutions often
drops as a result of chain alignment in the flow field. The same behavior has been
observed in SPs, but now there are two possible mechanisms for viscosity loss: the
alignment of effectively intact SPs or the scission of the relatively weak SP bonds
to decrease the average SP molecular weight. The ambiguity is complicated by the
typically unknown contributions of an applied mechanical force to the dissociation
rate of the supramolecular interaction. For example, Paulusse and Sijbesma (2004)
showed that ultrasound-induced flow can rupture coordinative bonds in main-chain
SPs, and Kersey et al. (2006) reported the force dependence of ligand exchange reac-
tions in pincer coordination motifs such as 3
.
4. A discussion of force-dependent
supramolecular dynamics is presented in Section 3.5. The elucidation of shear thin-
ning, shear thickening, and other nonlinear viscoelastic responses of SPs under large,
rapid deformations constitute one rich area of relatively undeveloped activity in
the field.
As solutions become increasingly concentrated and enter the solid state, intact
chain slippage mechanisms slow dramatically. In these regimes, the dissociation
rates of the SP interaction might change as well, but in general the change will be
greatest in the dynamics of the intact SP. The expectation is therefore that at some
point a transition will occur, after which SP dissociation provides the principal relax-
ation mechanism. Recent work on UPy-terminated polycaprolactones in the melt
state, for example, reveals ideal Maxwell viscoelastic behavior in the solid state
that is described by a single relaxation element with a lifetime of 1.6 ms (van Beek
et al. 2007). This lifetime corresponds well to that of the UPy dimer; thus, the
mechanism of relaxation appears to involve dissociation of the dimers, subsequent
(relatively) rapid rearrangement of the SP subunits, and reassociation of dimers.
An understanding of the full structure-activity space (density of SP unit, temperature
dependence, yield under high strain rates, etc.) relevant to a given application, and
especially under which conditions the SP dissociation is the limiting dynamic
