Comparative Study of Lactide Polymerization with Lithium, Sodium, Magnesium, and Calcium Complexes of BHT

A series of Li, Na, Mg, and Ca complexes with 2,6-di-tert-butyl-4-methylphenol (BHT) as the ligand has been synthesized, and their reactivity for the ring-opening polymerization of lactide has been studied. The Ca complex with 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (EDBP) as the ligand also has been synthesized to compare with the BHT systems. All complexes, in the presence of benzyl alcohol as initiator, exhibit high activity for the ring-opening polymerization of lactide. Polymerization activities follow the order of Na ≫ Li > Ca ≫ Mg. Additionally, metal–BHT systems are more efficient than metal–EDBP systems, and this superiority was pronounced in THF solution. Four model metal–BHT complexes with coordinated dimethoxyethane (DME) have been characterized by single-crystal X-ray diffraction. These complexes exhibited a wide variation in the Cipso–O–metal angle (159.3° to 180.0°), and DFT calculations suggested that this flexibility allows the metal to vary its electron density and thereby expedite t...