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Heteroselective Polymerization of rac-Lactide in the Presence of Dialkylgallium Alkoxides: The Effect of Lewis Base on Polymerization Stereoselectivity

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posted on 13.09.2010, 00:00 by Paweł Horeglad, Przemysław Kruk, Jacques Pécaut
Well-defined dialkylgallium alkoxides derived from (S)-methyl lactate, (S,S)-[Me2Ga(μ-OCH(Me)CO2Me)]2 (1), and 2-methoxyethanol, [Me2Ga(μ-OCH2CH2OMe)]2 (2), have been used as model complexes for the ring-opening polymerization of lactide. (S,S)-[Me2Ga(μ-OCH(Me)CO2Me)]2 (1) can polymerize rac-lactide in solution in a living manner to give atactic polylactide (PLA). The addition of Lewis base to 1 gives heterotactically enriched polylactide at 40 °C, in CH2Cl2 with 6 equiv of THF (Pr = 0.60, Pr = probability of heterotactic enchainment) and in THF (Pr = 0.68) or in CH2Cl2 with 6 equiv of γ-picoline (Pr = 0.78), which reveals that heteroselectivity is strongly related to coordination efficiency of a Lewis base to gallium. The polymerization with 1 at 40 °C is living, as evidenced by the narrow polydispersities that remain essentially constant during the polymerizations. A temperature increase to 70 °C leads to transesterification reactions and to a decrease in heteroselectivity. Despite the latter, heterotacticly enriched PLA can be obtained in the melt at 130 °C. [Me2Ga(μ-OCH2CH2OMe)]2 (2) with achiral alkoxide ligands revealed essentially the same activity and stereoselectivity as 1, indicating a chain-end control mechanism. The application of gallium complexes for the controlled and stereoselective polymerization of rac-lactide and the effect of Lewis base on the stereoselectivity were discussed.