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Abstract

A process for preparing linear copolyesters which contain omega-hydrocarboxylic acid units, from polyesters that contain at least 90 mol % polyethylene terephthalate units, as well as the usual amount of diethylene glycol units and possibly the known catalysts and stabilizers, includes adding to the already prepared polyester melt at least one compound of the group of lactones, optionally along with further additives but without additional catalysts in a system sealed off from gas exchange and at a higher pressure than the lactone vapor pressure in a proportion of up to 10 mol %, referred to the final melt, and mixed intensively with the melt. The resultant copolyesters, after a total dwell time of less than 30 minutes and a temperature in the range of 265.degree. to 310.degree. C., are further processed or discharged, granulated and subsequently further processed, wherein the copolyesters are identical in viscosity to the starting polyesters and contain randomly distributed omega-hydroxycarboxylic acid units in the polymer chains and whereby the sealed system is a pipe which is under pressure and is provided with a dosing opening and with a mixer downstream of the dosing opening.

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