Chemistry Reference
In-Depth Information
addition of 0.004 wt% NaCl yielded a 200-fold drop in viscosity. With only slightly
more cross-linker (1.0 wt%), the same amount of NaCl only halves the viscosity.
Mixing different cross-linkers (19b and 19c) yielded systems with a strong gel -
weak gel transition, rather than a distinct sol -gel shift. When the concentration of
each of the cross-linkers was above the critical percolation threshold, the kinetically
slower cross-linker (19c) dictated the gel properties. Upon addition of enough of a
competitive binding additive, such as DMAP, to drop the concentration of the
“active” cross-linking units below their individual percolation thresholds but still
allowing the total amount of both active cross-linkers (19b and 19c)tobeabove
the percolation threshold results in a gel whose properties are now controlled by
the kinetically faster cross-linker (19c).
If the polymeric macromonomer contains only one metal-binding center within its
backbone, then addition of an appropriate metal ion will result in the self-assembly of
a metallosupramolecular star polymer. Johnson and Fraser (2004) synthesized bipy-
centered block copolymers that, upon addition of Fe(BF
4
)
2
, reversibly self-assemble
into a hexa-arm star. Bender et al. (2002) also prepared star polymers utilizing Eu
III
and the b-diketonate anion. A tri-arm star polymer could be self-assembled via
complexation of Eu
III
to three monotopic poly(lactic acid) macromonomers. The
large coordination sphere of the Eu
III
allows the further addition of a neutral bipy-
containing poly(1-caprolactone). The result is a penta-arm star, which exhibits
block-copolymer-like properties, namely, phase segregation in the solid state.
Atomic force microscopy studies show the formation of a lamellar structure compris-
ing poly(1-caprolactone) and poly(lactic acid) with the metal ions placed at the inter-
face. Preliminary work on this system suggests that the temperature-sensitive nature
of the film's morphology is partly a consequence of the dissociation of the kinetically
labile bipy ligand at high temperature.
The above examples show one of the most appealing aspects of MSPs, namely that
the dynamic nature of the metal -ligand bond potentially allows the development of
stimuli-responsive materials, which exhibit dramatic changes in macroscopic proper-
ties upon exposure to environmental stresses. An elegant example of a linear stimuli-
responsive MSP was reported by Paulusse et al. (2006). Mechanoresponsive poly-
mers were synthesized from ditopic phosphine-terminated poly(tetrahydrofuran)
monomers and PdCl
2
(Fig. 7.14a, 20). In this system, they showed that ultrasound
sonication can be used to induce chain scission of the MSP (21 and 22). The proposed
mechanism of depolymerization involves shear-induced breakage of the coordination
bonds caused by the collapse of the gas bubbles formed during sonication. This
method provides a potentially facile means of processing, as chain scission only
takes place at the reversible Pd
II
-phosphine bonds. When the shear stress (or in
this case sonication) is removed, the polymeric aggregates reform. In addition to
the ease of processing afforded by this technique, potential catalytic sites are
offered by the accumulation of unbound palladium chain ends, opening the door to
mechanochemical reactions using similar systems. Replacing the Pd
II
in these
systems with metals ions that have up to four free coordination sites, for example,
Ir
I
or Rh
I
, and using phosphinite rather than phosphine ligands (23), ultrasound-
responsive gels have been prepared (Fig. 7.14b; Paulusse et al. 2007). These two
