Iron-Centered Star Polymers with Pentablock Bipyridine-Centered PEG-PCL-PLA Macroligands

Elaborate biocompatible block copolymer architectures are achievable via controlled ring opening polymerization (ROP). Selective incorporation of donor groups results in macroligands that can coordinate metals to form site-isolated metallopolymers. A hydroxyl-functionalized bipyridine, bpy(CH2OH)(2), was employed as an initiator for the ROP of ethylene oxide, producing bpyPEG(2). Subsequent reaction with epsilon-caprolactone followed by lactide (D,L- and L-) results in well-defined bpy-centered triblock PCL-PEG-bpy-PEG-PCL and pentablock PLA-PCL-PEG-bpy-PEG-PCL-PLA and PLLA-PCL-PEG-bpy-PEG-PCL-PLLA macroligands, respectively, with low polydispersity indices (PLA = D,L-polylactide, PCL = poly(epsilon-caprolactone), PEG = poly(ethylene glycol), PLLA = L-polylactide). These systems were combined with FeCl2 to produce iron-centered star blocks [Fe{bpy(PEG-PCL)(2)}(3)]Cl-2, [Fe{bpy(PEG-PCL-PLA)(2)}(3)]Cl-2, and [Fe{bpy(PEG-PCL-PLLA)(2)}(3)]Cl-2 with six diblock and triblock arms. Materials were characterized by gel permeation chromatography (GPC), H-1 NMR, and UV-vis spectroscopy, verifying that synthetic targets were achieved. Thermal gravimetric analysis shows decomposition temperatures corresponding to PEG, PCL, and PLA blocks, and melting temperatures are typical for these materials.