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 MgⁿBu₂ in combination with an alcohol, isopropanol (ⁱPrOH), benzyl methanol (PhCH₂OH), diphenylmethanol (Ph₂CHOH), or triphenylmethanol (Ph₃COH), generates a convenient catalyst system to promote the polymerization of l-LA. In particular, the binary system MgⁿBu₂/Ph₂CHOH demonstrates an unprecedentedly high activity in the presence of a large excess amount of Ph₂CHOH with the [OH]₀/[Mg]₀ 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]₀/[Mg]₀ = 200–5000) are rather accurately controlled by the Ph₂CHOH loading, relative to [Mg]₀, 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 MgⁿBu₂ and Ph₂CHOH and structurally characterized as a tetranuclear complex, Mg₄(Ph₂CHO)₈(THF)₂ (1). Complex 1 remains the tetranuclear structure in solution or in the presence of excess Ph₂CHOH 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.