Phosphonylated derivatives of a polypentenamer (PP) containing 82% trans and 17% cis double bonds have been prepared by the free-radical addition of dimethylphosphite to the double bonds and subsequent hydrogenation of the residual unsaturation. In this way derivatives were prepared containing approximately 5 and 10 mole % dimethylphosphonate side groups. These side groups were further reacted by hydrolysis or neutralization to form phosphonic acid and cesium phosphonate salts. Reaction conditions were so chosen that no backbone degradation occurred, and side reactions leading to crosslinking were avoided. The derivatives were characterized by gel-permeation chromatography (GPC), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared spectroscopy, and nuclear magnetic resonance spectroscopy. It was found that increasing substituent concentration decreases the melting point Tm in the case of the hydrogenated derivatives and increases the glass transition temperature Tg in the case of the unhydrogenated derivatives. The free acid derivatives are the least thermally stable of all those studied and the hydrogenated derivatives are generally more stable than the unhydrogenated derivatives.