Taking advantage of a melt polycondensation process, a series of copolyesters composed of pure terephthalate acid (PTA), ethylene glycol (EG), and 1,3-propanediol (1,3-PDO) were synthesized. The component, molecular weight, molecular weight distribution, and thermal properties of the copolymers were characterized. The results show that the contents of trimethylene terephthalate (TT) units in the resulting copolyesters are higher than PDO compositions in original diol. Oligomer content in the copolyesters varies with the compositions and attains a minimum value when the TT ingredient is 49.52 mol %. The glass transition temperature (T,,) of the copolyesters varies from 78.5 degrees C for PET (polyethylene terephthalate) to 43.5 degrees C for PTT (polytrimethylene terephthalate) and decreases monotonically with the components. The copolyesters are amorphous copolymers when TT content is in the range of 32.4-40.8 mol %, as calculated from the melting enthalpy (Delta H-m) measured via differential scanning calorimetry. (c) 2006 Wiley Periodicals, Inc.