Functionalized non-halogenated flame retardants are disclosed having improved hydrolytic stability, represented by decreased conductivity measured in units of ? S/cm when immersed in distilled water and heated to 80 C. The functionalized flame retardant comprises polyphosphonate functionalized by epoxy-functional styrene-acrylic oligomer or aromatic carbodiimide, and optionally antioxidant. Also disclosed are polymer compounds, comprising the functionalized flame retardant described above and a second polymer resin; and flame retardant plastic articles made from such polymer compounds.