N-Heterocyclic Carbene-Carbodiimide (NHC-CDI) Betaines as Organocatalysts for β-Butyrolactone Polymerization: Synthesis of Green PHB Plasticizers with Tailored Molecular Weights

Abstract

The manufacture of poly(3-hydroxybutyrate) (PHB) stands out as a promising alternative to traditional plastics. This polymer is usually produced by bacteria-based biotechnology or metal-catalyzed ring-opening polymerization (ROP) of β-butyrolactone (β-BL). Nevertheless, commercial PHB suffers from different issues, from poor processability to high production costs. Herein, we report the chemical synthesis of PHB from the ROP of rac-β-butyrolactone promoted by a zwitterionic organocatalyst, an N-heterocyclic carbene-carbodiimide (NHC-CDI) betaine adduct. This reaction has been investigated by a combination of experimental and computational methods. The catalyst cleaves the β-BL via nucleophilic displacement of the C-O(carboxyl) bond followed by intramolecular elimination to give protonated betaine-crotonate ion pairs, the active polymerization initiators. Polymer growth is propagated and controlled by these betaine ion pairs, which render amorphous low-molecular-weight PHBs by an unusual mechanism with rapid chain transfer to the monomer regime, analogous to the well-known mechanism of ethylene oligomerization with late transition-metal catalysts. The resulting material is suitable as a biodegradable and biocompatible plasticizer for commercial bacterial or synthetic PHB. Perfect compatibility between the matrix and the additive and a significant reduction of the crystallinity resulted in a more flexible and resilient material.