Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β-lactones, namely 4-alkoxymethylene-β-propiolactones (BPL s). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand {ONOO } , which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R' substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (P up to 0.91 with R'=cumyl) to very high isoselectivity (P up to 0.93 with R'=Cl), as supported by DFT insights. This is the first example of a highly isoselective ROP of a racemic chiral β-lactone.

Origins of stereoselectivity in ROP of racemic chiral cyclic esters promoted by achiral yttrium complexes, resulting in the formation of highly heterotactic polylactide, and highly syndiotactic or, more uniquely, highly isotactic poly(3-hydroxybutyrate)s, are discussed. A close interplay between the nature of the cyclic ester, most particularly of the exocyclic functional chain installed on the chiral center of β-lactones, and the ortho-substituents installed on the phenolate rings of the ligand, results in various determining secondary interactions of steric and also electronic nature.