Iron Tris(bipyridine)-Centered Poly(ethylene glycol)-Poly(lactic acid) Star Block Copolymers

Block copolymers incorporating biocompatible poly(ethylene glycol) (PEG) and poly(lactide) (PLA) polymers and labile metal complexes can serve as stimuli responsive biomaterials. Bipyridine-centered PEG-PLA materials were prepared via ring opening polymerization of ethylene oxide and lactide (d,l and l). Macroligands and their iron complexes [Fe{bpy(PEG-PLA)2}3]Cl2 were characterized by gel permeation chromatography, 1H NMR, and UV−vis spectroscopy. Solvent and counterion effects were also explored by UV−vis spectroscopy. Extinction coefficients drop 3-fold in CH2Cl2; values typical for iron(II) tris(bipyridine) complexes are restored by addition of very small quantities of MeOH. Thermal properties of bpy(PEG-PLA)2 and [Fe{bpy(PEG-PLA)2}3]2+ were investigated by thermogravimetric analysis and differential scanning calorimetry, showing values typical for the polymer blocks and little effect of incorporated ligand or metal sites. Micelle-like nanoparticles of block copolymer macroligands and iron star polymer complexes were prepared via nanoprecipitation. Dynamic light scattering and transmission electron microscopy analyses confirm the presence of spheroid particles (macroligands, 111−253 nm in diameter; iron complexes, 37−75 nm in diameter).