Hydrogels by supramolecular crosslinking of terpyridine end group functionalized 8-arm poly(ethylene glycol)

Abstract

Metallo supramolecular assemblies of an 8-arm poly(ethylene glycol) partially substituted with terpyridyl end-groups and the transition metal ions Ni²⁺, Fe²⁺, Co²⁺ and Zn²⁺ 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 Ni²⁺ and Fe²⁺ chlorides, which lead to distinct particle sizes of ∼200 nm in dilute conditions. The Co²⁺ and Zn²⁺ 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 Ni²⁺, Fe²⁺ and Co²⁺ having storage moduli in excess of 20 kPa, whereas Zn²⁺ gels are relatively viscous. Only Zn²⁺ gels show a thermoreversible sol to gel transition at a temperature of 25 °C independent of polymer concentration.