Process for the production of a high molecular weight polyester (co)polymer

A process for the production of a (high molecular weight) polyester (co)polymer, comprising

• (a) adding a diphenyl oxalate ester to a starting polyester (co)polymer at least comprising alcohol end groups and units derived from 1,2-diols,
• wherein the phenyl group in the diphenyl oxalate ester is optionally substituted, the substituent being selected from one or more of o-, m- and p-C1-C6 alkoxy, or o-, m- and p-C1-C6 alkyl;
• (b) in case a temperature is not already higher than 220° C., elevating the temperature of the mixture resulting from step (a) to at least 220° C., and reducing the pressure, for a period of time sufficient to obtain a polyester (co)polymer product wherein an amount of oxalate units that remain in the (co)polymer is less than added in the form of the diphenyl oxalate ester;
• and no or less than 1 mole % oxalate units are present, the percentage relative to a total amount of monomer units, the product having a higher molecular weight than that of the starting polyester (co)polymer;
• wherein the process is carried out in the absence of a metal catalyst.