Isoselective Polymerization of rac‐Lactide by Highly Active Sequential {ONNN} Magnesium Complexes

• Published in: Chemistry : a European journal 2020-12, Vol.26 (71), p.17183-17189
• Main Authors: Rosen, Tomer, Rajpurohit, Jitendrasingh, Lipstman, Sophia, Venditto, Vincenzo, Kol, Moshe
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Chemistry; Coordination compounds; isoselectivity; Ligands; living polymerization; Magnesium; poly(lactic acid); Polymerization; Ring opening polymerization; Stereoselectivity
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202003616

The coordination chemistry and the activities in the ring‐opening polymerization catalysis of racemic lactide (LA) of magnesium complexes of a series of {ONNN}‐type sequential monoanionic ligands are described. All ligands include pyridyl and substituted‐phenolate as peripheral groups. The ligands bearing either chiral or meso‐bipyrrolidine cores led to single diastereomeric complexes, whereas the ligands bearing a diaminoethane core led to diastereomer mixtures. All {ONNN}Mg‐X complexes [X=Cl, HMDS (hexamethyldisilazide)] led to highly active and isoselective catalysts. The complexes bearing the chiral bipyrrolidine core exhibited the highest activities (full consumption of 5000 equiv. of rac‐LA at RT within 5 min) and highest isoselectivities (Pm=0.91), as well as a living character. The complexes of the meso‐bipyrrolidine based ligands were almost as active and slightly less stereoselective, while those of the diaminoethane based ligands exhibited reduced activities and isoselectivities. A complex question: Magnesium complexes of sequential {ONNN}‐type ligands featuring either chiral or meso‐bipyrrolidine cores form as single diastereomers and exhibit exceptionally high activities in rac‐lactide polymerization, fully consuming thousands of monomer equivalents within 5 min, giving poly(lactic acid) of narrow molecular weight distribution and high degree of isotacticity.