Comparison of L-lactide polymerization by using magnesium complexes bearing 2-(arylideneamino)phenolate and 2-((arylimino)methyl)phenolate ligands

A series of magnesium complexes bearing 2-(arylideneamino)phenolate (forming 5-membered ring Mg complexes) and 2-((arylimino)methyl)phenolate (forming 6-membered ring Mg complexes) ligands were synthesized, and their application in the ring-opening polymerization of L-lactide (LA) was investigated. The Mg complexes with 5-membered rings exhibit a higher polymerization rate and controllability for polylactide production than those with 6-membered rings do (one-fold to three-fold for LA polymerization). The density function theory calculation results revealed that benzyl alcohol approaches LA through the hydrogen bond with phenolate oxygen, and then initiates LA, with the proton transferring to phenolate oxygen to form the LA-OBn adduct. The LA-OBn adduct was activated using the proton present on phenolate oxygen to open the ring of the LA-OBn adduct. Moreover, the LA polymerization using L^5ThioMg as a catalyst needs to overcome a 17.7 kcal/mol activation free energy that is lower than that of L^6ThioMg (21.0 kcal/mol)