Yttrium Phosphasalen Initiators for <i>rac</i>-Lactide Polymerization: Excellent Rates and High Iso-Selectivities

Abstract

Highly active yttrium phosphasalen initiators for the stereocontrolled ring-opening polymerization of <i>rac</i>-lactide are reported. The initiators are coordinated by a new class of ancillary ligand: an iminophosphorane derivative of the popular “salen” ligand, termed “phosphasalen”. Changing the phosphasalen structure enables access to high iso-selectivities (<i>P</i>_i = 0.84) or hetero-selectivities (<i>P</i>_s = 0.87). The initiators also show very high rates, excellent polymerization control, and tolerance to low loadings; furthermore, no chiral auxiliaries/ligands are needed for the stereocontrol. The combination of such high rates with high iso-selectivities is very unusual