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

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• 作者：Yang Wang ; Wei Zhao ; Xinli Liu ; Dongmei Cui ; Eugene Y.-X. Chen
• 刊名：Macromolecules
• 出版年：2012
• 出版时间：September 11, 2012
• 年：2012
• 卷：45
• 期：17
• 页码：6957-6965
• 全文大小：477K
• 年卷期：v.45,no.17(September 11, 2012)
• ISSN：1520-5835

文摘

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鈥?000) 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鈥?.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鈥搃nsertion polymerization mechanism is proposed.