2-Cyclohexyl-2-oxo-1,3,2-dioxaphospholane (cyHexPPn), a new monomer for the anionic ring-opening polymerization to poly(ethylene alkyl phosphonate)s is presented. The organo-catalyzed polymerization produces homopolymers with excellent control over molecular weight and narrow molecular weight distributions. The homopolymer was found to exhibit a glass transition at 15 °C, which is 60 °C higher compared to all previously reported poly(ethylene n-alkyl phosphonate)s. Copolymerization with the water-soluble 2-isopropyl-2-oxo-1,3,2-dioxaphospholane (iPrPPn) resulted in water-soluble, well-defined copolymers. The copolymer composition matched the theoretical value in all cases and Tg showed a linear correlation with the amount of (iPrPPn) incorporated. The copolymer was found to exhibit low cell-toxicity towards sensitive murine macrophage-like cells (RAW264.7).