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.
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