Disclosed are linear polyphosphonates produced by a transesterification process. These linear polyphosphonates exhibit a unique and advantageous combination of properties, such as outstanding fire resistance, unexpectedly high Tg's and good toughness at a low molecular weight. The favorable processing characteristics (such as low melt viscosity and excellent melt stability) of these linear polyphosphonates enable economic processing. Also disclosed are polymer compositions that comprise these linear polyphosphonates and at least one other polymer, wherein the resulting polymer compositions exhibit flame retardant properties.