Aliphatic polyesters are a class of polymers that offer an attractive alternative to non-degradable, petroleum-based polymers due to their bio-derived monomers and degradable properties. While polyesters are traditionally synthesized under conditions with poor molecular weight control and a limited library of monomers, perfectly alternating ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is an emerging pathway to the preparation of a variety of polyesters in a controlled, chain-growth manner. Rare earth metal catalysts are appealing for this polymerization due to their low toxicity and their ranging Lewis acidity and size. Recently, simple yttrium salts, in combination with a bis(triphenylphosphine)iminium chloride ([PPN]Cl) cocatalyst, were identified to be highly active and controlled for the ROCOP of numerous monomers. Mainly, the cocatalyst cation is proposed to stabilize a carboxylate polymer end group to encourage ring-opening of an epoxide monomer, thereby increasing the rate of polymerization. Herein, we describe the exploration of choline chloride as an inexpensive, sustainable alternative for the [PPN]Cl cocatalyst. Choline chloride is a bio-renewable and abundant molecule, used commonly in deep eutectic solvents. In combination with rare earth metal salts, choline chloride was found to support the ROCOP of epoxides and cyclic anhydrides, albeit at a slower rate than those with a [PPN]Cl cocatalyst. Across the rare earth metal series, trends in polymerization rate for preferred metal catalyst were different than those with [PPN]Cl and varied depending on the monomers used for polymerization. Through rare earth metal catalyst and monomer structure, the impact of choline chloride on polymerization rate and control will be discussed.

Choline chloride as an active cocatalyst in the perfectly alternating copolymerization of epoxides and cyclic anhydrides with rare earth metal catalyst

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