Organocatalytic Synthesis of Poly(hydroxymethylfuroate) via Ring-Opening Polymerization of HMF-Based Cyclic Oligoesters

The synthesis of hydroxymethylfuroate macrocyclic oligoesters c(HMF)n promoted by an N-heterocyclic carbene (NHC) organocatalyst1 is herein presented together with the subsequent organocatalytic, entropically-driven ring-opening polymerization (ED-ROP)2 leading to the fully furan-based poly(hydroxymethylfuroate) (PHMF). The target macrocycles (mostly trimer and tetramer species) have been obtained directly from the platform chemical HMF (77% isolated yield) under high dilution conditions using a quinone as the external oxidant and the green solvent Me-THF. The ED-ROP of c(HMF)n has been optimized at 160 °C (melt condensation technique) with the couple triazabicyclodecene (TBD)/n-octanol (1:1) as catalyst/initiator of the polymerization process in the presence of commercial antioxidants Irganox 1010 (0.1% w/w) and Irgafos 126 (0.3% w/w) to suppress degradation side reactions. Under these conditions, the bio-based PHMF (poly-HMF) was obtained as a color-free polymer with number-average molecular weight up to 48600 g mol-1 and dispersity between 1.5 and 1.9 as determined by NMR and GPC analyses. The thermal behavior of the novel furan-based polyester PHMF was investigated (TGA and DSC analyses) observing a good thermal stability (onset temperature of degradation 310°C) and a semicrystalline structure with melting temperature above 160°C when processed from solvent, thus making PHMF a promising material for processing as others commercial polyesters.