Low-molecular-weight HOOC-terminated poly(butylene adipate) prepolymer (PrePBA) and poly(butylene succinate) prepolymer (PrePBS) were synthesized through melt-condensation polymerization from adipic acid or succinic acid with butanediol. The catalyzed chain extension of these prepolymers was carried out at 180-220 degrees C with 2,2'-(1,4-phenylene)-bis(2-oxazoline) as a chain extender and P-toluenesulfonic acid (p-TSA) as a catalyst. Higher molecular weight polyesters were obtained from the catalyzed chain extension than from the noncatalyzed one. However, an improperly high amount of p-TSA and a high temperature caused branching or a crosslinking reaction. Under optimal conditions, chain-extended poly (butylene adipate) (PBA) with a number-average molecular weight up to 29,600 and poly(butylene succinate) (PBS) with an intrinsic viscosity of 0.82 dL/g were synthesized. The chain-extended polyesters were characterized by IR spectroscopy, H-1-NMR spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis, wide-angle X-ray scattering, and tensile testing. DSC, wide-angle X-ray scattering, and thermogravimetric analysis characterization showed that the chain-extended PBA and PBS had lower melting temperatures and crystallinities and slower crystallization rates and were less thermally stable than PrePBA and PrePBS. This deterioration of their properties was not harmful enough to impair their thermal processing properties and should not prevent them from being used as biodegradable thermoplastics. The tensile strength of the chain-extended PBS was about 31.05 MPa. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115:1555-1565,2010