Poly(styrene-co-diethyl vinylphosphonate) copolymers were synthesized by free radical copolymerization. The ester groups of the copolymers were hydrolyzed to phosphonic acid groups, and the sodium and zinc salts ionomers were obtained by neutralization. The structure and the thermal and viscoelastic properties of the copolymers and ionomers were characterized by nuclear magnetic resonance, Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, and small-angle X-ray scattering. The phosphonate ester lowered the glass transition temperature (T-g) of polystyrene. The free acid derivatives and metal phosphonates increased T-g and produced a rubbery plateau region in the viscoelastic properties due to the formation of a physical network. The acid and salt ionomers exhibited microphase-separated morphologies and were thermorheologically complex. The phosphonic acid derivatives absorbed relatively little water, even for materials with ion-exchange capacities greater than 1.0 mEq/g, and were not conductive, which made them unsuitable for application as proton exchange membranes. (C) 2004 Wiley Periodicals, Inc.