Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-Opening Polymerization of Functional Racemic β-Lactones

Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional beta-lactones, namely 4-alkoxymethylene-beta-propiolactones (BPL(OR)s). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand {ONOOR'(2)}(2-), which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R' substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (P-s up to 0.91 with R'=cumyl) to very high isoselectivity (P-i up to 0.93 with R'=Cl), as supported by DFT insights. This is the first example of a highly isoselective ROP of a racemic chiral beta-lactone.