The present disclosure is a polyester copolymer, having a number average molecular weight of equal to or more than 5000 grams/mole and having a glass transition temperature of less than 160° C., comprising:—in the range from equal to or more than 25.0 mole % to equal to or less than 49.9 mole %, based on the total amount of moles of monomer units within the polyester copolymer, of one or more bicyclic diol monomer units, wherein such one or more bicyclic diol monomer units is/are derived from one or more bicyclic diols chosen from the group consisting of isosorbide, isoidide, isomannide, 2,3:4,5-di-O-methylene-galactitol and 2,4:3,5-di-O-methylene-D-mannitol;—in the range from equal to or more than 45.0 mole % to equal to or less than 50.0 mole %, based on the total amount of moles of monomer units within the polyester copolymer, of an oxalate monomer unit; and/or—in the range from equal to or more than 0.1 mole % to equal to or less than 25.0 mole %, based on the total amount of moles of monomer units within the polyester copolymer, of one or more linear C2-C12 diol monomer units, wherein such one or more linear C2-C12 diol monomer units is/are derived from one or more linear C2-C12 diols; and/or—optionally equal to or more than 0.0 mole % to equal to or less than 5.0 mole %, based on the total amount of moles of monomer units within the polyester copolymer, of one or more additional monomer units.