The Dual-Stereocontrol Mechanism: Heteroselective Polymerization of rac-Lactide and Syndioselective Polymerization of meso-Lactide by Chiral Aluminum Salan Catalysts

Rami Hador, Antonio Botta, Vincenzo Venditto, Sophia Lipstman, Israel Goldberg, Moshe Kol

Research output: Contribution to journalArticlepeer-review

Abstract

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 Tm of 153 °C matching the highest reported value, and the highest crystallinity (ΔHm=56 J g−1) ever reported for syndiotactic PLA.

Original languageEnglish
Pages (from-to)14679-14685
Number of pages7
JournalAngewandte Chemie - International Edition
Volume58
Issue number41
DOIs
StatePublished - 7 Oct 2019

Keywords

  • aluminum
  • poly(lactic acid)
  • stereoregularity
  • stereoselective polymerization
  • syndiotactic polymer

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