Highly Enantioselective Polymerization of β-Butyrolactone by a Bimetallic Magnesium Catalyst: An Interdependent Relationship Between Favored and Unfavored Enantiomers.

Herein, we report that (,)-prophenolMg(μ-OBu)(THF) ((,)-, prophenol = (,)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol) is a highly enantioselective (/ = 140) precatalyst for ring-opening polymerization of -β-butyrolactone (β-BL) to isotactic poly(3-hydroxybutyrate) (-PHB), a high performance, biodegradable polyester. Precatalyst (,)- polymerizes ()-β-BL with an inversion of stereochemistry to ()PHB with a % (percentage of adjacent linkages with a configuration) of 98% at 41% conversion and of 165 °C under a variety of conditions. Complex (,)- demonstrates unique polymerization kinetics, as it does not polymerize the preferred enantiomer, ()-β-BL, alone. Mechanistic studies revealed that ()-β-BL is needed to convert (,)- into the active enantioselective polymerization catalyst. To the best of our knowledge, (,)- produces -PHB with the highest degree of isotacticity observed from a polymerization of -β-BL. This study informs the design and understanding of future enantioselective and earth-abundant metal catalysts for ring-opening polymerization of β-lactones.