Ligand coordination controlled by monomer binding: a hint from DFT for stereoselective lactide polymerization

Stereoselective ring opening polymerization (ROP) of racemic lactide (rac-LA) is a challenging goal because a rationale connecting the catalyst structure and polymer microstructure (as has been established for α-olefin polymerization) is still missing. In this work, we reveal the origin of the stereoselective preference for d and l-lactide with two enantiopure salen-Al complexes, which have so far been claimed as the most efficient in enantiomorphic site control, using Density Functional Theory calculations. We introduce active site reorganization and monomer/chain switching throughout the reaction pathway, unconventional aspects necessitating careful consideration when confronting the intricacies associated with chiral catalyst recognition. We show how the catalytic pocket easily rearranges in the reaction path establishing a novel concept of the ligand coordination controlled by monomer binding. The resulting final picture of PLA stereoselectivity is much more complex than that of α-olefin polymerization catalysis, and a “complete” prediction by brute-force is (currently) hard, but the principles evolving should - even in their incomplete form - be useful in the design of new selective catalysts.