A process for polymerizing β-butyrolactone that includes contacting racemic β-butyrolactone or an enantiomer thereof with a catalyst/initiator system which includes a rare earth metal, a chiral ligand, at least one nucleophilic ligand, optionally at least one solvent ligand, and optionally an alkali based co-catalyst. The chiral ligand is an enantiomer of a unit of formula I Each Rz independently is substituted or unsubstituted linear C1-C20 alkyl, or substituted or unsubstituted branched, or cyclic C3-C20 alkyl, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted C5-C20 heteroaryl, or halogen. Each Ra independently is H, Me+, (MeZ)+, wherein Z is a charge equalizing anion, or wherein two Ra together are alkaline earth metal, wherein Me+ is alkali metal or alkaline earth metal.