Aromatic-aliphatic random and block copolyesters: synthesis, sequence distribution and thermal properties

A series of poly(butylene terephthalate)-poly(butylene succinate) (PBT-PBS) copolyesters were synthesized by the melt chain-extension reaction of dihydroxyl terminated PBT (HO-PBT-OH) and PBS (HO-PBS-OH) prepolymers using toluene-2,4-diisoyanate (TDI) as a chain extender. Transesterification between PBT and PBS during the chain-extension reaction has been investigated in detail. Both random and block PBT-PBS copolymers can be synthesized by changing the prepolymers' molecular weights at a given reaction time and temperature. The chemical structures and sequence distributions of the copolyesters were analyzed by proton nuclear magnetic resonance (1H-NMR) spectroscopy. The thermal transition behaviours of copolyesters with different sequence distribution were characterized by differential scanning calorimeter (DSC). The number-average molecular weight (Mn) of HO-PBT-OH played a dominant role in the sequence distribution of the resulting copolyesters. The degree of randomness of the copolymer was around 1 when Mn of HO-PBT-OH was 5000 g mol−1 regardless of that of HO-PBS-OH, it decreased to less than 1 when Mn of HO-PBT-OH was more than 10 000 g mol−1, and it gradually decreased with the increase in Mn of HO-PBS-OH. The copolyesters with different degrees of randomness showed different thermal transition and crystallization behaviours.