Enhancing Biopolyester Backbone Rigidity with an Asymmetric Furanic Monomer

Abstract

Biobased furanic polyesters can exhibit performance advantages over petroleum-derived polyesters, primarily due to their rigid furan-containing backbones. Herein, we develop two strategies to polymerize methyl 5-hydroxymethyl furanoate to poly-(5-hydroxymethyl furanoate) (PHMF), a furan-based polyester with even greater backbone rigidity than poly-(ethylene furanoate). Thermal, spectroscopic, and computational investigations of PHMF alongside analogous furan-based and phenyl-based polyesters suggest that the high furan content of PHMF leads to its high glass transition temperature, slow crystallization kinetics, and low amorphous mobility. Molecular dynamics simulations suggest that while the backbone of PHMF is exceptionally rigid, its amorphous phase is denser than its phenyl analog due to noncovalent interchain interactions. Together, these results highlight how asymmetric furan-based monomers can modulate key properties in biobased polyesters.

Keywords: chain mobility; crystallization kinetics; furan polyester; hydroxyester polycondensation; molecular dynamics; performance-advantaged bioplastic.

1

(a) Synthesis of PHMF from bio-derived sources via oxidation of HMF, − hydroxymethylation of FA, , and Fischer esterification of HMFA. (b) The chemical structures of the polyesters examined in this study.

2

Synthesis and deconstruction of PHMF. (a) Solution- and melt-based polymerization and methanolysis depolymerization of PHMF. (b) Melt-pressed film fabricated from solution-polymerized PHMF. (c) ¹H NMR spectra of (from bottom to top) virgin MHMF monomer, purified PHMF polymer, and recovered r-MHMF monomer following methanolysis (CDCl₃/trifluoroacetic acid (TFA), 400 MHz).

3

Thermal analysis of polyesters including (a) the first cycle of DSC (10 °C/min) to identify T _m, (b) the second cycle of DSC (10 °C/min) to identify T _g,DSC, (c) DMA (5 °C/min, 1 Hz, 0.1% strain) to identify T _g,tan δ and the maximum of tan δ, and (d) TGA (10 °C/min) to identify T _d,5%.

4

Isothermal crystallization experiments of polyesters: (a) Avrami analysis at intermediate crystallization temperatures (PET: T _c = 185 °C, PEF: T _c = 145 °C, PHMB: T _c = 125 °C, PHMF: T _c = 135 °C), (b) parameters obtained from fitting crystallization curve (a) to the Avrami equation (eq S1), and (c) Hoffman–Weeks analysis to obtain the T _m°.

5

Solid-state NMR spectroscopy of selected polyesters: (a) intensity of carbonyl signal from VCT-CP-MAS NMR spectroscopy of polyesters with fit and (b) extracted T _1ρ and T _CH parameters for each polyester.

6

Computed properties from molecular dynamics simulations on polyesters including (a) amorphous densities (compared to experiment), (b) characteristic ratio, and (c) interchain radial distribution function (RDF) between the carbonyl and adjacent aromatic ring. Error bars represent one standard error from the mean in 25 independent simulations. Shaded regions represent one standard error from the mean in 5 independent simulations (*: p < 0.05, **: p < 0.01, ***: p < 0.001, ****: p < 0.0001).