A polyester copolymer, having a number average molecular weight of equal to or more than 4000 gram/mole and having a glass transition temperature of less than 140° C., containing: (a) in the range from equal to or more than 45 mole % to equal to or less than 50 mole % of one or more bicyclic diol monomer units derived from the group consisting of isosorbide, isoidide, isomannide, 2,3:4,5-di-O-methylenegalactitol and 2,4:3,5-di-O-methylene-D-mannitol; (b) in the range from equal to or more than 25 mole % to equal to or less than 49.9 mole %, of an oxalate monomer unit; (c) in the range from equal to or more than 0.1 mole % to equal to or less than 25 mole % of one or more linear C3-C12 dicarboxylate monomer units; and (d) optionally equal to or more than 0 mole % to equal to or less than 5 mole % of one or more additional monomer units, the percentages based on the total amount of moles of monomer units within the polyester copolymer.