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

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