A polymerization process for the production of a polyester copolymer A comprising simultaneously reacting a polyester (i) with one or more diols (ii) and one or more (hetero) aromatic dicarboxylic acids or any esters thereof (iii), wherein the polyester (i) is polyethylene terephthalate, polyethylene furanoate or polyethylene terephthalate-co-furanoate; and wherein no ethylene glycol is added as a diol (ii); and wherein at least one of the one or more diols (ii) is selected from (bi) cyclic secondary diols; and wherein the components (i) and (ii) are used in sufficient quantities to produce a polyester copolymer A comprising at least 40 mole % of ethylene glycol derived from the starting polyethylene terephthalate, polyethylene furanoate or polyethylene terephthalate-co-furanoate, and at least 5 mole %, of monomers derived from said (bi) cyclic secondary diol(s), the percentages based on the total amount of diol-derived monomer units in polyester copolymer A.