Isoselective mechanism of the ring-opening polymerization of rac-lactide catalyzed by chiral potassium binolates

In this work, two enantiopure potassium complexes were synthesized and characterized. These two complexes show good isoselectivity for the ring-opening polymerization (ROP) of rac-lactide (Pm = 0.80 at room temperature for complex 1 and P-m = 0.80 at -60 degrees C for complex 2) and display very different activities for the ROP of rac-lactide due to the presence of an intramolecular hydrogen bond in complex 1. Meanwhile, a ligand assisted monomer activated mechanism for the ROP of lactide is rationalized in the presence of alcohol for this system. The stereoselective mechanism is ascribed to a chain end control mechanism via three experimental methods (the analysis of the stereo microstructure of the polymer, the activity of one enantiopure catalyst towards the ROP of L-lactide, D-lactide, and rac-lactide, and the enantiomeric excess values of the residual monomers and polymers via optical rotation measurements at different ROP conversions).