Synthesis of End-Functionalized Polystyrene by Direct Nucleophilic Addition of Polystyryllithium to Bipyridine or Terpyridine

We describe a new approach to synthesize 2,2'-bipyridine- or 2,2':6',2 ''-terpyridine-terminated polystyrene that relies on anionic polymerization and direct end-capping with the desired pyridyl species. End-functionalization occurs by nucleophilic addition of the living polystyryllithium chain to the 6-position of the pyridine ring, followed by termination and oxidative rearomatization. By using an excess of the pyridyl species to avoid coupling of two living chains through addition to the same bipyridine or terpyridine unit, this technique yielded samples consisting of 77-93% singly end-functionalized chains. The functionality of the polymers was determined by nuclear magnetic resonance spectroscopy and chromatographic separation, while molecular weight and polydipersity were determined by size exclusion chromatography. The crude products were easily purified to near-quantitative functionalization by short column chromatography, and the excess pyridyl species could be efficiently recovered and reused. Even though the addition of the polystyrene chain to the 6-position provides some steric hindrance to the ability of pyridyl end-caps to serve as ligands, the terpyridine-functionalized products were found to form bis complexes readily upon addition of 0.5 equiv of iron(II) chloride to a solution of the polymers, as determined by ultraviolet visible spectrophotometry and nuclear magnetic resonance spectroscopy.