Synthesis of Aliphatic-Aromatic Copolyesters by a Melting Bulk Reaction Between Poly(butylene terephthalate) and DL-Oligo(lactic acid)

DL-Oligo(lactic acid) (OLA) readily reacts with poly(1,4-butylene terephthalate) (PBT) in the melt leading to the formation of high molecular mass poly(1,4-butylene terephthalate-co-DL-lactide) (BLA) copolymers. The analysis from 1H NMR reveals that transesterification between butylene terephthalate (BT) and lactide (LA) segments during synthesis has occurred. The BLA copolymers are segmented copolyesters with random arrangement, as shown by the sequence structure of BLA copolyester chains and as confirmed by their thermal behavior. The BLA copolyesters show only one melting temperature ( Tm), crystallization temperature ( Tc), and thermal decomposition temperature ( T d) when the reaction between PBT and OLA takes place at 280°C for more than 0.5 h. With increased OLA composition, the Tm, Tc and Td decrease due to fewer aromatic sequences. The copolyester with a PBT/OLA starting molar ratio of 5.9/94.1 is soluble in chloroform, while pristine PBT is only soluble in mixed chloroform/phenol solvent. Conventional size-exclusion chromatography (SEC) polystyrene calibration determined weight-average molecular weight of the copolyester (5.9/94.1 mol mol-1 PBT/OLA) to be 11200 g mol-1.