Poly(ethylene terephthalate)-co-poly(ethylene DDP)s [PET-co-poly(ethylene DDP)s], were synthesized by charging 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOP), itaconic acid, terephthalic acid, and ethylene glycol in one reactor to conduct the microaddition reaction (using H 2 PtCl 6 as catalyst), esterification reaction, and polycondensation reaction. H 2 PtCl 6 has demonstrated to be a highly efficient microaddition catalyst to improve the DDP conversion. The microaddition reaction of the phosphorus compound (DOP) with the itaconic acid can be proceeded at a significantly lower temperature (110Β°C) and results in higher conversion (ΟΎ 98%). The use of the H 2 PtCl 6 catalyst makes it possible to charge all the reactants in one reactor to produce high molecular weight phosphorus-containing copolyesters without requiring the presynthesis of the DDP. These resulting copolyesters are identified by Fourier transform infrared spectroscopy, 1 H-NMR, and differential scanning calorimetric analysis. Thermal characteristics, thermal stability, intrinsic viscosity, acid value, and rheological and mechanical properties of these copolyesters were also characterized. The presence of the bulky pendent phosphorus side groups in the copolyester tends to decrease the structural regularity and retards its crystallization. The formation of a protected char layer for the phosphorus-containing copolyester raises the decomposition temperature of the copolyester under an oxygen atmosphere higher than that of PET. The limiting oxygen index values of all phosphorus-containing copolyesters are all higher than 33. Higher phosphorus content results in decreasing crystallinity, lower melting temperature, lower decomposition temperature, as well as lower tensile strength, but increasing residual char after thermal degradation and higher limiting oxygen index value. The rheological behaviors of copolyesters remain similar to that of PET. The glass temperatures of copolyesters are all Ο³ 77Β°C (76.8Β°-77.2Β°C). Incorporation of phosphorus moieties into its molecular chain has a significant effect on thermal and flame retardancy behavior. However, the crystal lattice of all copolyesters do not change with incorporation of the pendent phosphorus side group in the backbone of the copolyester.