TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The process is carried out in: aromatic non-chlorinated solvent comprising toluene, ethylbenzene or xylene; anhydrous medium; and at 25-50 degrees C.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The lactone is epsilon -caprolactone (preferred), delta -valerolactone, beta -butyrolactone or gamma -butyrolactone. The lactam is caprolactam, enantholactam, laurolactam, pyrrolidinone or piperidone. The lactide is in racemic, pure enantiomeric, or meso form. The polymer initiator is a mono or polyhydroxylated oligomer or polymer selected from: (alkoxy) polyalkylene glycols; poly (alkyl)alkylene adipate diols; optionally hydrogenated. alpha -hydroxylated or alpha , omega -hydroxylared polydienes; mono- or polyhydroxylated polyalkylenes; polylactides containing terminal hydroxy functions; polyhydroxyalkanoates; and/or optionally modified polysaccharides. The polymer containing at least one hydroxyl function is preferably selected from: (methoxy)polyethylene glycol, polypropylene glycol, polytetramethylene glycol, poly(2-methyl-1,3-propylene adipate)diol, poly(1,4-butylene) adipate diol, alpha , omega -dihydroxylated polybutadiene, alpha , omega -dihydroxylated polyisoprene, mono or polyhydroxylated polyisobutylene, poly (3-hydroxybutyrate), poly(3-hydroxyvalerate), starch, chitin, chitosan, dextran and/or cellulose. The polymeric initiator is a polymer containing at least one thiol function, preferably selected from polystyrenes, poly(meth)acrylates or polybutadienes, all alpha -thiolated or alpha , omega -thiolated. The polymeric initiator is a vinylic co-oligomer or copolymer of the acrylic, methacrylic, styrenic or dienic family, resulting from co-polymerization of acrylic, methacrylic, styrenic or dienic monomers and functional monomers presenting a hydroxyl or thiol group. The sulfonic acid compound is of formula (R-SO3H) (a). The sulfonic acid compound is (trifluoro) methane sulfonic acid or p-toluene sulfonic acid. The molar ratio of: cyclic monomer to polymer initiator is 5-500, preferably 40-100; and the molar ratio of the sulfonic acid compound to hydroxyl or thiol of polymer initiator is 1.R = 1-20C linear alkyl or 3-20C branched or cyclic alkyl (optionally substituted by oxo or halo (F, Cl, Br or I)), or aryl (optionally substituted by 1 or more substituents chosen from halo (F, Cl, Br or I), nitro,1-20C linear alkyl or 3-20C branched or cyclic alkyl, where the alkyl is optionally substituted by halo (F, Cl, Br or I) or nitro). EXAMPLE - Polyethylene glycol (1.32 g) and toluene (11 ml) were added to epsilon -caprolactone (580 mu l) and trifluoromethane sulfonic acid (12 mu l). The reaction mixture was agitated at 50 degrees C to give the copolymer.