Chemistry Reference
In-Depth Information
event, creates a rational avenue for SP material design. The regions of structure-
activity space that are close to transitions between “supramolecular” and “covalent”
behavior are especially rich for applications, because we can potentially engineer
materials that with modest manipulation could provide the best of both sets of
materials in a stimulus-responsive manner.
3.3. MECHANICAL PROPERTIES OF SP NETWORKS
The UPy-terminated polycaprolactone described above constitutes what is effectively
a transient polymer network in which individual chains are held in place by entangle-
ments that are rendered transient through supramolecular interaction. Formal SP
networks, such as that shown schematically in Figure 3.6, can be created by multiple
branching (cross-linking) points from a common linear polymer. Linear polymers
aggregate through a supramolecular interaction that constitutes a chemical, rather
than topological, entanglement of two chains. With multiple branching sites per
chain, aggregation-induced molecular weights increase more rapidly with the
addition of the cross-linkers than in the linear case. At early stages of aggregation,
the same competition between relaxation mechanisms holds; the aggregates can
diffuse or slip past each other as intact structures, or pieces of the aggregated
SP can relax by dissociating and reassociating in the applied stress field. At appropri-
ate concentrations of SP interactions (percolation, or the gel point), the polymer
network becomes effectively infinite and the diffusion of intact components is
negligible. Supramolecular dissociation is expected to dominate the mechanical
properties landscape.
These characteristics are illustrated by studies on SP networks formed through the
same van Koten type pincer coordination complexes described above. We focus the
discussion on SPs formed from poly(4-vinylpyridine) (PVP) that is cross-linked in
DMSO by bis(M(II)-pincer) compounds 4a-d (Fig. 3.5; Yount et al. 2005a,
2005b). The same simple steric effect in the pincer alkylamino ligands is used,
Figure 3.6 Schematic of a supramolecular polymer network.
