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Article

Polymerization of Lactide with Zinc and Magnesium β-Diiminate Complexes: Stereocontrol and Mechanism

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Bradley M. Chamberlain, Ming Cheng, David R. Moore, Tina M. Ovitt, Emil B. Lobkovsky, and Geoffrey W. Coates*

Contribution from the Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301

J. Am. Chem. Soc., 2001, 123 (14), pp 3229–3238

DOI: 10.1021/ja003851f

Publication Date (Web): March 16, 2001

Abstract

A series of zinc(II) and magnesium(II) alkoxides based upon a β-diiminate ligand framework has been prepared. [(BDI-1)ZnOⁱPr]₂ [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] exhibited the highest activity and stereoselectivity of the zinc complexes studied for the polymerization of rac- and meso-lactide to poly(lactic acid) (PLA). [(BDI-1)ZnOⁱPr]₂ polymerized (S,S)-lactide to isotactic PLA without epimerization of the monomer, rac-lactide to heterotactic PLA (P_r = 0.94 at 0 °C), and meso-lactide to syndiotactic PLA (P_r = 0.76 at 0 °C). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers in addition to the linear nature of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. The substituents on the β-diiminate ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization. Kinetic studies with [(BDI-1)ZnOⁱPr]₂ indicate that the polymerizations are first order with respect to monomer (rac-lactide) and 1.56 order in catalyst. Polymerization experiments with [(BDI-1)MgOⁱPr]₂ revealed that this complex is extremely fast for the polymerization of rac-lactide, polymerizing 500 equiv in 96% yield in less than 5 min at 20 °C.

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Published In Issue April 11, 2001
Received November 1, 2000

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Article

Polymerization of Lactide with Zinc and Magnesium β-Diiminate Complexes: Stereocontrol and Mechanism

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Controlled and Stereoselective Polymerization of Lactide: Kinetics, Selectivity, and Microstructures

Single-Site β-Diiminate Zinc Catalysts for the Ring-Opening Polymerization of β-Butyrolactone and β-Valerolactone to Poly(3-hydroxyalkanoates)

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