Stereochemistry of Lactide Polymerization with Chiral Catalysts: New Opportunities for Stereocontrol Using Polymer Exchange Mechanisms
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- 26 January 2002
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 124 (7), 1316-1326
- https://doi.org/10.1021/ja012052+
Abstract
The synthesis of chiral aluminum and yttrium alkoxides and their application for lactide polymerization are reported. The complexes (SalBinap)MOR [4, M = Al, R = iPr; 5, M = Y, R = (CH2)2NMe2] are synthesized by reacting the ligand (SalBinap)H2 [2,2‘-[(1,1‘-binaphthalene)-2,2‘-diylbis(nitrilomethylidyne)]bisphenol] with the appropriate metal trisalkoxide. While enantiomerically pure yttrium complex 5 did not effect stereocontrol in the polymerization of either meso- or rac-lactide, homochiral 4 was found to exhibit excellent stereocontrol in a range of lactide polymerizations. Enantiomerically pure 4 polymerizes meso-lactide to syndiotactic poly(lactic acid) (PLA), while rac-4 polymerizes meso- and rac-lactide to heterotactic and isotactic stereoblock PLA, respectively. On the basis of the absolute stereochemistry of ring-opening of meso-lactide using (R)-4, a polymer exchange mechanism is proposed to account for the PLA microstructures resulting from rac-4.Keywords
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