Method for the production of cyclic diesters, in particular dilactide

A method for the production of cyclic esters of general formula I

• in which
• a) in a first step, a part of a mixture of oligomeric carboxylic acids of formula II
• respectively R being selected, in formulae I to II, from linear or branched aliphatic radicals with 1 to 6 carbon atoms and, in formula II, the average value for n being between 2 and 40, is catalytically converted by cyclising depolymerisation into the cyclic diester of general formula I, wherein the catalyst is a tin containing catalyst, wherein the cyclic depolymerisation is implemented at temperatures between 100 and 300° C., and the thereby produced cyclic diester of general formula I is separated, the part of the oligomeric carboxylic acid of formula II, which is not converted into the cyclic diester of general formula I, is made to react to form a residue during the cyclising depolymerisation, which residue comprises a mixture of oligomeric carboxylic acids of formula II with a higher average value for n than the mixture of oligomeric carboxylic acids of formula II fed into the first step,
• b) in a second step, the residue obtained in a first step is hydrolysed at pressures between 500 mbar and 2 bar and/or at temperatures between 50 and 300° C. to a mixture of oligomeric carboxylic acids of formula II, the average value for n being between 1 and 20, an alpha-hydroxycarboxylic acid of formula III,
• or a mixture of oligomeric carboxylic acids of formula II with an average value for n between 1 and 20 and an alpha-hydroxycarboxylic acid of formula III, and
• c) the hydrolysate obtained in the second step is fed at least partially or completely into the first step,
• wherein the cyclising depolymerisation of the first step is implemented until at most 98% by weight of the mixture of oligomeric carboxylic acids of formula II, which is utilized, is converted by the cyclising depolymerisation into the cyclic diester of general formula I.