Bio-based poly(hexamethylene terephthalate) copolyesters containing cyclic acetalized tartrate units

A cyclic acetal of dimethyl l-tartrate (dimethyl 2,3-di-O-methylene l-threarate, Thx) was used as comonomer of dimethyl terephthalate (DMT) in the polycondensation with 1,6-hexanediol (HD) in the melt. Random copolyesters PHTxThxy with contents in tartrate units up to 50% were obtained with satisfactory molecular weights (Mw between 20,000 and 50,000) and dispersities slightly above 2, and without apparent discoloration. The copolyesters started to decompose above 300 °C. The Tg of the copolyesters oscillated between +9 and −9 °C with values steadily decreasing with increasing contents in Thx units. All the copolyesters as well as the homopolyester entirely made of tartrate units were semicrystalline with Tm falling from ∼145 °C for PHT to ∼70 °C for PHThx with intermediate values decreasing as the content in Thx increased. Copolyesters containing up to 30% of tartrate units were able to crystallize from the melt at crystallization rates that were delayed by the presence of tartrate units. The PHT70Thx30 copolyester displayed significant hydrodegradability when incubated in aqueous buffer and also certain biodegradability when subjected to the action of porcine pancreas lipases.