It is imperative to develop high-performance ocean-degradable polymers to prevent further accumulation of non-degradable, petrochemical plastics in the ocean. At the forefront of potential candidates is (R)-poly(3-hydroxybutyrate) (PHB), a material naturally made and degraded by bacteria. However, biological or chemocatalytic synthesis of PHB has yet to be demonstrated on large scale at a low cost, preventing widescale production and implementation. Herein, we report the enantioselective polymerization of rac-β-butyrolactone (β-BL) by an earth abundant metal pre-catalyst, (S,S)-prophenolMg₂(μ-OⁿBu)(THF)₂ ( prophenol = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol). We show that this catalyst system can synthesize isotactic (i) 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. Mechanistic investigations reveal that, despite having a large preference for (R)-β-BL (k_R/k_S = 140) during polymerization, (S,S)-prophenolMg₂(μ-OⁿBu)(THF)₂ must be activated by (S)-β-BL to achieve high enantioselectivity during polymerization. This research contributes to the design and understanding of future enantioselective and earth-abundant metal catalysts for the ring-opening polymerization of β-lactones.

Anne LaPointe

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