When Bigger is Better: Lanthanum Complexes of Bis(phenoxy-amidine) FAlen Ligands as Unique Catalysts for the Isoselective Ring-Opening Polymerization of Racemic β-Lactones.

Discrete yttrium and lanthanum complexes supported by new bis(phenoxy-amidine) ligands have been prepared in high yields. Some of these so-called FAlen ligands, when coordinated onto the large La(3+) in contrast to the small Y(3+), can adopt an η(3) coordination mode of both amidine moieties. These La-FAlen complexes, either previously isolated or conveniently prepared in situ, act as highly efficient ROP catalysts of simple and functional β-propiolactones rac-BL(R) (R = Me, CH(2)OiPr, CH(2)OSitBuMe(2)) under mild conditions, returning the corresponding linear polymers PBL(R) with high control over the molecular weights and narrow dispersities. More uniquely, when substituents with appropriate bulkiness are installed on the phenoxy and amidine moieties, the polymers all feature high isotacticity (P(m) up to 0.93). High activity and isoselectivity are maintained, no matter the nature and polarity of the solvent used.