Provided is a chelate complex comprising (a) a rare earth metal cation M; (b) a chelate ligand of formula (CL1 A) or (CL1 B) c) at least one anionic nucleophilic ligand LN which is coordinated as a further ligand to the rare earth metal cation; and d) optionally one or more neutral donor ligands LD coordinated as ligands to the rare earth metal cation. Moreover, provided are a process for the preparation of a chelate complex, a process for the preparation of a polymer comprising a polymerization reaction of chiral cyclic ester monomers, and a poly(3-hydroxy butyrate) polymer which can be provided by the process.

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