A series of iron(III) chloride complexes based upon Schiff base framework have been synthesized and characterized by mass spectra, elemental analysis, and X-ray crystallography. These bench-stable complexes were for the first time capable as highly efficient catalysts for lactide and ε-caprolactone polymerization in the presence of propylene oxide (PO), greatly surpassing conventional aluminum analogies. Electron-withdrawing substituents as well as elevated temperature boosted the activity while a bulky group on salicylaldehyde moieties abnormally produces the same effect, whereas rigid backbone retarded the reactivity. Polylactide tactics ranging from isotactic to hererotactic enchainment were obtained by tuning the ligand backbone and substituents. The stereoselectivity was confirmed to proceed via a chain-end control mechanism by kinetic studies using different isomers of lactide, and the overall polymerization process was also investigated in detail by the oligomer mass spectrum as well as end group (−OCHMeCH 2 Cl) analysis of polymer via 1 H, 13 C, and two-dimensional (2-D) NMR characterizations.

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 P m = 0.98), high activity (typically k p = 15.4 L mol–1 min–1), and low catalyst loading amount (0.01 mol %). The T m 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.

The first versatile gas-controlled polymerization switch based on reversible absorption of CO 2 by organic amidine was developed. The amidine/alcohol system, which originally belonged to CO 2 binding organic liquids, served as an efficient CO 2 -responsive catalyst/initiator system for lactide polymerization. Chemoselective block copolymerization between polylactide and polycarbonate was further explored by opposite gas-controlled amidine and another Cr III catalyst, where an immortal strategy was developed for conjugated alternation of inverse polymerizations on different active species and permits many potential applications.