Metal-centered polymers: Using controlled polymerization methodologies for the generation of responsive materials

Controlled polymerization methods were used to prepare highly modular polymeric metal complexes via convergent and divergent strategies. In these materials, the metal center provides a versatile hub for preparing diverse architectures through coordinative bonds. Moreover, the metal complex introduces various properties to the polymer such as luminescence, magnetism, or electroactivity. Suitably functionalized metal complexes have been used for the atom transfer radical polymerization of acrylate and methacrylate monomers by metalloinitiation to generate luminescent biocompatible materials through a divergent synthesis. By cleaving the tert-butyl groups from poly(tert -butyl acrylate), water soluble [Ru(bpyPAA₂)₃] 2+ has been prepared as well as the amphiphilic star block copolymer [Ru{lcub}bpy(PLA-PAA)₂{rcub}₃]2+ (PLA = poly(lactic acid), PAA = poly(acrylic acid); Bipyridine-centered polymeric macroligands may be chelated to a variety of metal salts. The polymer size greatly influences the formation of [Fe(bpy) ₃]2+ centered polymers. As the molecular weight increases (> ∼25 kDa) tris complex formation decreases. Tris(bpy) synthesis is also impacted by chemical composition. BpyPtBA₂ (PtBA = poly(tert-butyl acrylate) generates an iron mono(bpy) complex before giving rise to the bis(bpy) iron complex; no tris complex is observed. In contrast, the combination of bpyPEG₂ (3 equiv) (PEG = (poly(ethylene glycol)) results in the formation of some iron tris(bpy) compound; however, complete tris(bpy) product formation is suppressed, presumably because of the chelating ability of the PEG chains. These examples contrast with other polymeric macroligands such as bpyPS₂, bpyPMMA₂, bpyPCL₂ and bpyPLA ₂ (PS = polystyrene; PMMA = poly(methyl methacrylate); PCL = poly(ϵ-caprolactone); PLA = poly(DL-lactic acid)) for which chelation reactions are facile for low molecular weight macroligands (<15 kDa), with chelation efficiencies (defined as (ϵ_PMC/ϵ_bpy) × 100%) only declining with increased molecular weight.; Finally, a series of bipyridine (bpy)-centered triblock copolymers, BA-bpy-AB, were generated by ring opening polymerization (ROP) and atom transfer radical polymerization (ATRP) mechanisms. Hydroxyl chain ends of poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) precursors, bpyPCL₂ and bpyPLA ₂ respectively, were converted to α-bromoesters for use as macroinitiators for the subsequent addition of second blocks, poly(methyl methacrylate) (PMMA) or poly(t-butyl acrylate) (PtBA). Materials formed by sequential ATRP reactions, bpy(PMMA-PS)₂ and bpy(PS-PMMA)₂, were also produced. New [Fe(bpy(AB)₂)₃]2+ analogues were formed by combination of triblock macroligands with [Fe(OH₂) ₆](BF₄)_s in 3:1 CH₂Cl₂:MeOH solutions. For bpy(PCL-PtBA)₂ and bpy(PLA-PtBA)₂ macroligands, only bis(bpy) materials resulted. These synthetic studies lay the foundation for future investigations of how polymer composition and film morphology affect nanocluster formation in metal-centered star block copolymer templates.