Efficient Polymerization of Methyl-ϵ-Caprolactone Mixtures to Access Sustainable Aliphatic Polyesters

Aliphatic polyesters are a versatile class of materials that can be sourced from bioderived feedstocks. Poly(γ-methyl-ϵ-caprolactone) (PγMCL) in particular can be used to make degradable thermoplastic elastomers with outstanding mechanical properties. PγMCL can potentially be manufactured economically from p-cresol, a component of lignin bio-oils. A complication is that additional manufacturing processes are necessary to isolate pure cresol isomers. Using mixed feedstocks of cresol isomers to access the corresponding methyl-substituted ϵ-caprolactone (MCL) monomer mixtures would convey economic advantages to sourcing these materials sustainably. Moreover, the use of organocatalysts in lieu of traditional tin-based catalysts averts issues with potential environmental and human toxicity. With these motivations in mind, we explored the ring-opening transesterification polymerization (ROTEP) of MCL mixtures and characterized the molecular, thermal, and rheological properties of the resulting copolymers. The molar mass of MCL mixtures that would be obtained from meta- and para-cresol can be readily modulated. The thermal and rheological properties of these statistical copolymers and terpolymers are at parity with pure PγMCL homopolymer. The use of diphenyl phosphate (DPP) and dimethyl phosphate (DMP) as organocatalysts enabled access to these materials that have potential to improve sustainability in the synthesis of these polyesters.