Well-Defined Polymers Containing High Density of Pendant Viologen Groups

Atom transfer radical polymerization (ATRP) of a viologen-containing methacrylate, 1-propyl-1-[2-(methacryloyloxy)ethyl]-4,4-bipyridinium dihexafluorophosphate, is reported. To achieve good polymerization control, it was essential to use the viologen-based monomer with a hexafluorophosphate instead of halide counterion, and 2,2-bipyridine as the ligand for the Cu-based ATRP catalyst. The solubility of produced cationic polymers could be tuned by anion metathesis: the polymers with hexafluorophosphate counterions were soluble in organic solvents (e.g., acetone, DMF), and those with chloride counterions were water-soluble. In aqueous solutions, the polymers (chloride salts) formed large aggregates, the sizes of which ranged from about 200 to about 400 nm (based on dynamic light scattering measurements) depending on the molecular weight. Upon addition of electrolytes (e.g., NaCl), the aggregates underwent dissociation. The apparent diffusion coefficients of the aggregates existing in aqueous solutions and the products of their electrolyte-induced dissociation were measured by diffusion-ordered NMR spectroscopy. The association-dissociation processes were also studied by fluorescence spectroscopy: the aqueous polymer solutions, which were originally fluorescent (lambda(em)=402 nm at lambda(ex)=350 nm), lost their fluorescence in the presence of NaCl. The addition of small amounts of the viologen-containing polyelectrolytes to solutions of inorganic salts (NaCl) altered the crystal morphology of the salts due to interaction of the multiple charged pendant groups with small ions. In the presence of reducing agents, the pendant viologen groups were converted to viologen radical-cations, which are prone to dimerize reversibly in aqueous solutions. Indeed, marked dimerization of viologen radical cations (with absorbance maxima at 520 and 870 nm) was observed in relatively dilute aqueous solutions (4 mg mL(-1)) upon addition of reducing agents (hydrazine). (c) 2016 Wiley Periodicals, Inc.