Abstract
Metallo supramolecular assemblies of an 8-arm poly(ethylene glycol) partially substituted with terpyridyl end-groups and the transition metal ions Ni(2+), Fe(2+), Co(2+) and Zn(2+) were studied for their nano-particle formation at dilute conditions and gelation at higher concentrations. The large differences in dissociation rate constants of the metal ligand complexes largely determine the assembly behavior. Thermodynamically stable complexes are generated with Ni(2+) and Fe(2+) chlorides, which lead to distinct particle sizes of ∼200 nm in dilute conditions. The Co(2+) and Zn(2+) chlorides provide multiple size distributions revealing that mono and bis-complexes are present at equilibrium. Upon complexation, terpyridyl groups move to the outer sphere giving aggregates with a charged surface. At polymer concentrations above 5 wt%, crosslinking upon addition of transition metal ions provides hydrogels. Elastic hydrogels were obtained with Ni(2+), Fe(2+) and Co(2+) having storage moduli in excess of 20 kPa, whereas Zn(2+) gels are relatively viscous. Only Zn(2+) gels show a thermoreversible sol to gel transition at a temperature of 25 °C independent of polymer concentration.
Highlights
Polymer gels are networks of polymer molecules that are either covalently or physically crosslinked and expanded throughout their volume by a uid.[1]
The nal ratio of hydroxyl to terpyridyl groups was controlled by the molar feed ratio of the reactants, full conversion of the hydroxyl groups could not be obtained when an excess of the chloro-terpyridine was used
The reaction was performed in such a way that approximately 5 of the 8 hydroxyl groups were substituted with terpyridyl groups
Summary
Polymer gels are networks of polymer molecules that are either covalently or physically crosslinked and expanded throughout their volume by a uid.[1]. Bond strengths in metal ligand complexes can be as high as those of covalent bonds.[12] In certain cases highly stable complexes are formed when multiple Brønsted bases in a molecule donate electrons to a single Lewis acid which is known as the chelate effect.[13,14] Importantly, these interactions can have a high thermodynamic stability, the metal coordination bond can possess a relatively high kinetic lability This combination of relatively high thermodynamic bond stability and kinetic lability makes metal coordination bonds an interesting choice for use as cross-links in polymeric gels. Such systems are currently attracting increasing interest and are termed metallo-supramolecular polymer gels (MSPGs).[15]
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