Method for producing cyclic ester

A method for producing a cyclic ester, comprising:

• a step (I) of preparing a mixture (I) containing
• an aliphatic polyester,
• a polyalkylene glycol diether represented by Formula (1) below and having a boiling point from 230 to 450° C. and a molecular weight from 150 to 450,
• wherein R1 represents a methylene group or a linear or branched alkylene group having from 2 to 8 carbons, X1 represents a hydrocarbon group, Y represents an alkyl group having from 2 to 20 carbons or an aryl group, p represents an integer of 1 or greater, and in a case where p is 2 or greater, a plurality of R1 moieties may be the same or different, and
• a thermal stabilizer;
• a step (II) of producing a mixture (II) in a state of solution by heating the mixture (I) under normal pressure or reduced pressure to a temperature at which depolymerization reaction of the aliphatic polyester starts;
• a step (III) of producing a distillate (III) by continuing the heating of the mixture (II) and distilling a cyclic ester formed by the depolymerization reaction together with the polyalkylene glycol diether, and
• a step (IV) of recovering the cyclic ester from the distillate (III);
• wherein a polyalkylene glycol monoether represented by Formula (2) below is added to at least one of the mixture (I) and the mixture (II) as a solubilizing agent to enhance solubility of the aliphatic polyester in the polyalkylene glycol diether,
• wherein R2 represents a methylene group or a linear or branched alkylene group having from 2 to 8 carbons, X2 represents a hydrocarbon group, r represents an integer of 1 or greater, and in the case where r is 2 or greater, a plurality of R2 moieties may be the same or different;
• and
• wherein the thermal stabilizer is a sulfonic acid compound, a solubility of the sulfonic acid compound in the polyalkylene glycol diether is 0.04 mol/L or greater, and the distillate (III) further contains the thermal stabilizer.