Enzymatic bulk synthesis, characterization, rheology, and biodegradability of biobased 2,5-bis(hydroxymethyl)furan polyesters

Abstract

The synthesis of biobased polyesters based on 2,5-bis(hydroxymethyl)furan (BHMF) is challenging due to the limited stability of this interesting furanic monomer. In this work, a series of BHMF-based polyesters were produced via a green and efficient bulk polymerization process, by using either an enzyme (iCALB) or a commercially available catalyst (DBTO). The polymerization methods were compared and shown to both be successful, with values up to 14 000 g mol⁻¹. The number of methylene units in the aliphatic comonomer was varied from 2 to 8, and the influence of this on the thermal behavior and stability of the polymers was investigated. The degree of crystallinity of compression molded discs was found to be in the range from 13 to 27%, and the contact angles were determined to be in the region from 63 to 73°, confirming that these polyesters are hydrophilic. An oscillatory shear rheology investigation demonstrated significant differences between the melt behavior of the BHMF-based polyesters, ranging from a Newtonian liquid to a shear thinning material with a 3 orders of magnitude higher complex viscosity. A clear inversely proportional correlation between the low-frequency complex viscosity and the number of methylene units in the aliphatic segment was observed. Finally, a biodegradability test revealed that the synthesized BHMF-based polyesters had a biodegradable character over time, wherein different biodegradation rates were observed related to the length of aliphatic segments in the repeating units. The sustainability of both synthesis routes was analyzed based on atom economy (AE), reaction mass efficiency (RME), E-factor and EcoScale. This work emphasizes that renewable BHMF-based polyesters can be produced via a solvent-free and sustainable process, which show biodegradable behavior and that their thermal and rheological properties can be tailored by varying the number of methylene groups in the aliphatic unit.