Ligand-Free Magnesium Catalyst System: Immortal Polymerization of l‑Lactide with High Catalyst Efficiency and Structure of Active Intermediates

A simple, inexpensive, and convenient catalyst system consisting of supporting ligand-free Mg<sup><i>n</i></sup>Bu<sub>2</sub> in combination with an alcohol, isopropanol (<sup><i>i</i></sup>PrOH), benzyl methanol (PhCH<sub>2</sub>OH), diphenylmethanol (Ph<sub>2</sub>CHOH), or triphenylmethanol (Ph<sub>3</sub>COH), generates a convenient catalyst system to promote the polymerization of l-LA. In particular, the binary system Mg<sup><i>n</i></sup>Bu<sub>2</sub>/Ph<sub>2</sub>CHOH demonstrates an unprecedentedly high activity in the presence of a large excess amount of Ph<sub>2</sub>CHOH with the [OH]<sub>0</sub>/[Mg]<sub>0</sub> ratio varying from 2 to 500, producing up to 500 polylactide (PLA) chains per Mg center and thus showing a typical nature of immortal polymerization. The molecular weights of the obtained PLAs with a broad range of monomer-to-metal ratios ([l-LA]<sub>0</sub>/[Mg]<sub>0</sub> = 200–5000) are rather accurately controlled by the Ph<sub>2</sub>CHOH loading, relative to [Mg]<sub>0</sub>, while the molecular weight distributions remain nearly constant with polydispersity index (PDI) = 1.08–1.18. Moreover, the active polymerization intermediate has been isolated from the stoichiometric reaction between Mg<sup><i>n</i></sup>Bu<sub>2</sub> and Ph<sub>2</sub>CHOH and structurally characterized as a tetranuclear complex, Mg<sub>4</sub>(Ph<sub>2</sub>CHO)<sub>8</sub>(THF)<sub>2</sub> (<b>1</b>). Complex <b>1</b> remains the tetranuclear structure in solution or in the presence of excess Ph<sub>2</sub>CHOH as determined by 2D DOSY. On the basis of structural information about the active intermediates and polymerization kinetics, a coordination–insertion polymerization mechanism is proposed.