Isoselective Polymerization of rac-Lactide Catalyzed by Ion-Paired Potassium Amidinate Complexes

Three potassium crown ether complexes supported with bulky amidinate ligands were synthesized for the ring-opening polymerization (ROP) of rac-lactide. The side polymerization reaction initiated directly by ligand anion was suppressed well in the presence of alcohol as our design, and the synthesis of linear polylactide with a molecular weight as high as 117.7 kg/mol was successful together with an isoselectivity value of P_m = 0.88 at -70 °C. In this system, lactide can be deprotonated by amidinate anion to give lactide enolate, which can initiate the ROP of lactide as a side reaction in the absence of alcohol; however, this side reaction can also be suppressed well in the presence of alcohol by a decrease in temperature. An interesting anti-Arrhenius-like behavior in the polymerization was discovered, which can be attributed to the fact that the active catalyst can be converted to a less active lactide enolate potassium complex at a high temperature.