The invention relates to a catalytical process for preparing poly(3-hydroxyalkanoates) by copolymerizing an oxirane compound with carbon monoxide in the presence of

• a) at least one single- or multicenter neutral transition metal complex (A) based on a metal of groups 5 to 11 of the Periodic Table of the Elements, in which the metal center(s) is/are present in the formal oxidation state 0, and/or
• b) at least one single- or multicenter anionic transition metal complex (B) based on a metal of groups 5 to 11 of the Periodic Table of the Elements, where multicenter transition metal complexes have metal centers which, without exception, are of the same transition metal,

which comprises carrying out the copolymerization in the presence of at least one nucleophile other than hydroxy-substituted pyridine compounds.