Chemistry Reference
In-Depth Information
of polymer chains, suggesting an entropically controlled ring-opening polymerization
is occurring.
One key aspect of any MSP is the metal ion-ligand stoichiometry (Chen and
Dormidontova 2003). In Vermonden et al.'s (2003) system, two ligands can bind
to one Zn
II
ion; thus, a 1:1 ratio of 2 to Zn(ClO
4
)
2
is required to obtain high molecular
weight aggregates. Deviation from this stoichiometry would result in an excess of one
of the components, yielding chain ends and a significant decrease in the molecular
weight. Using a combination of viscosity as a probe for molecular weight and theor-
etical models, Vermonden's group investigated how the length of the ethylene oxide
core (n ΒΌ 4 or 6) and metal -ligand stoichiometry influenced the ring chain equili-
bria. It would be expected that a maximum in the reduced viscosity would be
observed at a 1:1 metal -ligand ratio. For 2a
.
Zn(ClO
4
)
2
this is indeed what is
observed at monomer concentrations of 19.2 and 37.8 mM. For 2b
.
Zn(ClO
4
)
2
,
such a maximum is observed at a higher monomer concentration (34.5 mM).
However, at lower concentrations (17.8 mM) a dip in the reduced viscosity occurs
at a 1:1 metal -ligand ratio. Theoretical studies suggest that this occurs as a conse-
quence of the longer ethylene oxide core, which allows a significant portion of the
monomer units to form rings and reduces the viscosity of the system. This behavior
is not observed in 2a
.
Zn(ClO
4
)
2
as the shorter core hinders the formation of small
macrocycles at the lower concentration.
Ring formation can also be induced by careful monomer design. Various groups
have synthesized terpy-containing multifunctional ligands with geometries that favor
ring formation. For example, Constable and colleagues (2003) tailored the length of a
polypeptide core unit to suit a thermodynamically favorable ring size. Newkome's
group (Wang et al. 2005) synthesized bent ditopic monomers that predispose the
system to ring formation upon addition of metal ions, also yielding macrocyclic
structures (Fig. 7.6). Attaching reactive groups onto these bent ditopic ligands (3)
yielded a method for accessing large macrocycles using a templating methodology
(Wang et al. 2005).
Most MSPs are polyelectrolytes; as such, the nature of the counterions should be
considered. For example, counterions can potentially act as competitive ligands
for the metal ion and have a significant effect on the formation of an MSP.
Schmatloch et al. (2004) demonstrated that the degree of complexation and sub-
sequently the size of the MSP in aqueous solutions formed from Fe
II
and terpy
end-capped PEO depends on the counterion present: chloride . sulfate ..
acetate. Lipophilic counterions offer the ability to access amphiphilic MSPs, which
in turn potentially allow the polymeric materials themselves to assemble into hier-
archical structures. Liu and coworkers (2004) used such a material to create higher
ordered structures in a variety of different ways. For example, addition of long
alkyl chain counterions, such as dihexadecyl phosphate (DHP), to a preformed
MSP, based on a terpy/Fe
II
system (4
.
Fe(OAc)
2
), results in counterion exchange
and organization of the DHP around the metal-ligand binding site. These amphiphilic
MSPs can assemble into monolayers at the air-water interface or into straight rods
(length
200 nm) on a graphite surface in the presence of long chain alkanes.
These amphiphilic MSPs also assemble in the solid state to form thermotropic
