A series of aliphatic-aromatic multiblock copolyesters consisting of poly (ethylene-co-1,6-hexene terephthalate) (PEHT) and poly(L-lactic acid) (PLLA) were synthesized successfully by chain-extension reaction of dihydroxyl terminated PEHT-OH prepolymer and dihydroxyl terminated PLLA-OH prepolymer using toluene-2,4-diisoyanate as a chain extender. PEHT-OH prepolymers were prepared by two step reactions using dimethyl terephthalate, ethylene glycol, and 1,6-hexanediol as raw materials. PLLA-OH prepolymers were prepared by direct polycondensation Of L-lactic acid in the presence of 1,4-butanediol. The chemical structures, the molecular weights and the thermal properties of PEHT-OH, PLLA-OH prepolymers, and PEHT-PLLA copolymers were characterized by FTIR, H-1 NMR, GPC, TG, and DSC. This synthetic method has been proved to be very efficient for the synthesis of high-molecular-weight copolyesters (say, higher than M-w = 3 x 10(5) g/mol). Only one glass transition temperature was found in the DSC curves of PEHT-PLLA copolymers, indicating that the PLLA and PEHT segments had good miscibility. TG curves showed that all the copolyesters had good thermal stabilities. The resulting novel aromatic-aliphatic copolyesters are expected to find a potential application in the area of biodegradable polymer materials. (c) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47:5898-5907, 2009