Multiblock copolymers poly(L-lactic acid)-block-poly(butylene succinate-co-butylene adipate)s (abbr. P(LLA-mb-BSA)s) are synthesized via a polycondensation/chain extension/coupling method using binary chain extenders. A poly(L-lactic acid) (PLLA) prepolymer synthesized via direct melt polycondensation of L-lactic acid is rapidly chain extended with a bis (2-oxazoline) to form hydroxyl terminated dimer, and the dimer is then chain extended/coupled with a hydroxyl terminated poly(butylene succinate-co-butylene adipate) (PBSA) prepolymer by a diisocyanate. The microstructure is characterized with GPC, H-1 NMR and FTIR, and the thermo-mechanical properties are investigated with DSC, TGA, DMA, tensile, and impact testing. In the copolymers, the PLLA hard segments are crystallizable and the PBSA soft segments are amorphous, and only melting of PLLA segment was detected. Two independent glass transitions corresponding to both segments are observed, suggesting incompatibility between the two kinds of segments. The tensile modulus and strength decrease while the elongation at break and impact strength increase with increasing the weight percentage of PBSA segment (phi(w,PBSA)). The mechanical properties can be tuned in wide range by phi(w,PBSA) from toughened thermoplastics with excellent tensile modulus, strength and impact strength to thermoplastic elastomers with high elongation at break.