Pathway to fully-renewable biobased polyesters derived from HMF and phenols

Abstract

Building on previous work where 5-hydroxymethylfurfural (HMF) was selectively functionalized by etherification with phenols, we demonstrated that the oxidized versions of these HMF ethers can be converted to functionalized δ-hexalactones (FDHLs) that can subsequently undergo ring-opening polymerization (ROP) to form polyesters. The key step in FDHL production is Ru-catalyzed selective hydrogenolysis of the C–O bond of functionalized-2-furan carboxylic acids (FFCAs). We found that the combination of a TiO₂ support and a polar, aprotic solvent leads to high selectivity towards the lactone product. Under the optimized conditions, we achieved a 60% yield of FDHL at 150 °C with 1.5% Ru/TiO₂ in 1,4-dioxane using 5-phenoxy-2-furan carboxylic acid as a model reactant. ROP of six-membered lactone monomers bearing either a methoxy (MDHL) or a phenolic (PDHL) pendant group resulted in polymers ranging from 5 to 30 kg mol⁻¹ with narrow dispersity. The polymerizations were carried out at room temperature using diphenyl phosphate (DPP) and triazabicyclodecene (TBD) as organocatalysts. Typical equilibrium polymerization behavior was observed at room temperature, and the reaction was observed to be pseudo-first order with respect to monomer concentration in solution. Poly(PDHL) had a significantly higher glass transition temperature (6 °C) than unsubstituted poly(valerolactone) due to the presence of the bulky phenolic group off the polymer backbone.