Metal-size influence in Iso-selective lactide polymerization

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a P_ivalue of 0.81-0.84 at 298K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (P_s=0.74, 298K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu. Small is beautiful: Lutetium phospha-salen complexes are developed as highly iso-selective initiators for the ring-opening polymerization of rac-lactide (LA), and show excellent rates and high degrees of polymerization control. The corresponding La derivative exhibits moderate heteroselectivity. The order of rates for the series of complexes is inversely related to metallic covalent radius.