Breaking the Paradox between Catalytic Activity and Stereoselectivity: rac-Lactide Polymerization by Trinuclear Salen–Al Complexes

Polylactide (PLA), the popular bio-based plastics, has attracted more interest as a benign alternative for oil-based plastics. Lowering the manufacture cost of PLA and improving the performance of PLA products are the essential challenges in PLA study. PLA stereocomplex is a promising choice to improve the thermal and mechanical properties. Efficient preparation of the PLA stereocomplex by the direct ring-opening polymerization (ROP) of inexpensive racemic lactide (rac-LA) requires the ROP catalysts with high activity and stereoselectivity. For this purpose, the novel trinuclear Salen–aluminum complexes were prepared in this work and exerted extraordinary improvement of catalytic activity and stereoselectivity at the same time. These complexes showed the excellent isoselectivity (typically Pm = 0.98), high activity (typically kp = 15.4 L mol–1 min–1), and low catalyst loading amount (0.01 mol %). The Tm of PLA was 219.9 °C, which is the highest value among the reported PLA derived directly from rac-LA until now. It may foreshadow a new vista in the reduction of overall production cost and performance improvement of PLA to be competitive with oil-based commodity or engineering plastics.