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The Dual‐Stereocontrol Mechanism: Heteroselective Polymerization of rac ‐Lactide and Syndioselective Polymerization of meso ‐Lactide by Chiral Aluminum Salan Catalysts
Author(s) -
Hador Rami,
Botta Antonio,
Venditto Vincenzo,
Lipstman Sophia,
Goldberg Israel,
Kol Moshe
Publication year - 2019
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201906848
Subject(s) - diastereomer , lactide , polymerization , stereocenter , tacticity , catalysis , enantiomer , chemistry , polymer chemistry , stereoselectivity , stereochemistry , materials science , polymer , enantioselective synthesis , organic chemistry
We describe alkoxo‐aluminum catalysts of chiral bipyrrolidine‐based salan ligands that follow the dual‐stereocontrol mechanism wherein a given combination of stereogeneities at the metal site and the proximal center of the last inserted lactidyl (“match”) is active towards lactide having a proximal stereogenic center of the opposite configuration, while the diastereomeric combination of stereogeneities (“mismatch”) is inactive towards any lactide. Polymerization of rac ‐LA by the enantiomerically pure catalysts was sluggish and gave stereoirregular poly(lactic acid) (PLA) because selective insertion to a match diastereomer gives a mismatch diastereomer. The racemic catalysts showed higher activity and led to highly heterotactic PLA following polymeryl exchange between two mismatched catalyst enantiomers. A succession of match diastereomers in selective meso ‐LA insertions led to syndiotactic PLAs reaching a syndiotacticity degree of α =0.96. This polymer featured a T m of 153 °C matching the highest reported value, and the highest crystallinity (Δ H m =56 J g −1 ) ever reported for syndiotactic PLA.