High Oxygen Barrier Polyester from 3,3′-Bifuran-5,5′-dicarboxylic Acid

An exceptional oxygen barrier polyester prepared from a new biomass-derived monomer, 3,3′-bifuran-5,5′-dicarboxylic acid, is reported. When exposed to air, the furan-based polyester cross-links and gains O2 permeability 2 orders of magnitude lower than initially, resulting in performance comparable to the best polymers in this class, such as ethylene-vinyl alcohol copolymers. The cross-links hinder the crystallization of amorphous samples, also rendering them insoluble. The process was observable via UV–vis measurements, which showed a gradual increase of absorbance between wavelengths of 320 and 520 nm in free-standing films. The structural trigger bringing about these changes appears subtle: the polyester containing 5,5′-disubstituted 3,3′-bifuran moieties cross-linked, whereas the polyester with 5,5′-disubstituted 2,2′-bifuran moieties was inert. The 3,3′-bifuran-based polyester is effectively a semicrystalline thermoplastic, which is slowly converted into a cross-linked material with intriguing material properties once sufficiently exposed to ambient air.


General information
Aluminium TLC plates coated with silica gel 60 were used for TLC. Commercial solvents and chemicals were generally used without further purification unless otherwise specified. Commercial 10% Pd/C pre-wetted with water (50%) was used as a catalyst for coupling reactions. CDCl3 (99.96% D) and (CD3)2SO (99.80% D) were used as NMR solvents, both containing TMS (δ 0.00 ppm) as an internal reference. CF3COOD (99.50% D) was used as a co-solvent for NMR analysis of polyesters. Di(2-ethylhexyl)-3,3'-bifuran-5,5'-dicarboxylate (9): Dicarboxylic acid 3 (0.89 g, 4.0 mmol), crushed into fine powder, was reacted with SOCl2 (0.9 mL, 3 equiv) and few drops of N,N-dimethylformamide in refluxing dichloromethane (10 mL) for 12 h. For the following step, the volatiles were evaporated under reduced pressure. The dried crude diacid chloride was resuspended into dry dichloromethane (40 mL) at 0 °C, to which triethylamine (1.4 mL, 2.5 equiv) was added dropwise. Then, 2-ethylhexanol (1.6 mL, 2.5 equiv) was introduced into the mixture dropwise over 15 min. The mixture was allowed to return to room temperature, with the reaction proceeding for 1 h. After being successively washed with 20 mL saturated NaHCO3 solution, 2x20 mL deionized water, and 20 mL brine, the dichloromethane solution was passed through a short silica plug and evaporated dry under vacuum. The product was further purified by successive filtrations though silica.

Synthesis of precursors and polymers
The product was an off-white solid (1.54 g, 86%). 1  Model compound aging experiment: Diester 9 (0.5 g) was first heated to 100 °C in 10 mL round-bottom flask to maintain it in a molten state. The melt was mixed under air for 7 days using a small PTFE-coated stirring bar. The evolution of the mixture was followed via TLC, which showed new spots appearing after just 1 d.
The resulting mixture was fractionated via column chromatography (silica, ethyl acetate/hexane 1:3) into three initial fractions. The unreacted bifuran (9) eluted first (210.9 mg), followed by a roughly 6:4 molar mixture of Melt-pressing: Carefully dried polymer samples were melt-pressed into films using a heated hydraulic press (Fontijne LabEcon 300). The aluminium press plates (thickness 3 mm) were covered with polytetrafluoroethylene (PTFE) coated glass-fiber mats. The press, with plates pre-heated to 200 °C, was first used to melt the samples without applying compression. After 3 min of melt time, the press was closed with a force of 20 kN (held for 1 min), which was then increased to 40 kN (held for 1 min). The sample was cooled to 30-40 °C using the integrated water circulation, and the films were peeled from the PTFE-coated mat once cooled. Film thickness, controlled by a glass-fiber frame, was 100-200 m. Gage length was 30 mm and cross-head speed 5 mm/min. Attenuated total reflectance FTIR spectroscopy (ATR FTIR): ATR FTIR spectra were acquired using Perkin Elmer Spectrum One. 16 scans were acquired at a resolution of 2 cm -1 .
Water contact angle: The contact angle measurements were conducted using Krüss DSA25 (Germany) Drop Shape Analyzer with high-speed camera and drop analyzing software. During the measurement, a 5 L droplet of water was added on the top of the film and the contact angle was recorded after 10 s. For each sample, five droplets on different locations were analyzed and the results are presented as average.
Cyclic voltammetry: Cyclic voltammetry (CV) was performed with an Gamry reference 600 Potentiostat and analyzed with the Gamry Echem Analyst software (version 6.33). CV was carried out in a five-neck electrochemical cell. A 3.0 mm diameter glassy carbon (GC) electrode was used as the working electrode. A platinum wire was used as an auxiliary electrode. The reference electrode was Ag/Ag + (0.1 M in acetonitrile).
Prior to use, the GC electrode was sanded and polished with alumina slurry on a micro-cloth. The electrode was then rinsed with an ethanol, deionized water and acetonitrile. Monomer (5 mM) was dissolved in supporting electrolyte consisting of tetra-n-butylammonium tetrafluoroborate (TBABF4) thrice recrystallized from absolute ethanol with distilled acetonitrile as solvent. Scan rate of 100 mV/s was used. All experiments were conducted under argon atmosphere.

Discussion:
The storage modulus peak value after the onset cold-crystallization (ca. 105 °C) for 3,3'-PPeBf is affected by cross-linking, which hinders crystallization and results in lower peak value (Fig. 23, Table S4).
Simultaneously, the peak of tan  shifts towards higher temperatures. MPa. The corresponding trends were observer for the peak temperature of tan . In other words, the absence of light slowed down the cross-linking, as did high humidity. Under argon, both incident light and humidity did not appear to have any notable effects, as expected. Cross-linking appeared fastest under dry air and ambient light.
As for the UV-vis measurements (Fig. S24), the sample stored under dry air and ambient light showed the largest change is transmittance, corroborating the results of DMA (Table S4). In contrast, the sample stored under humid air in the dark showed the least yellowing, revealing the accelerating and retarding effects of ambient light and humidity, respectively. Samples stored under argon did not show change in transmittance over the same time period. From these tests, the following conclusions are drawn: 1) O2 is required for the observed process 2) humidity can slow the process 3) ambient light can accelerate the process.
The model compound aging experiment supported the notion that lactones can be formed when 3,3'-BFDCA derivatives are in contact with air. Isolated compounds 10 and 11 appeared to be isomers, and accurate mass measurements yielded the same molecular weight for both: The mass had increased by the mass of a single oxygen atom. Two structurally related compounds with NMR data are presented in Scheme S1 for comparison (data from references 6,7 ; assignment presented is our own) with the above NMR spectra. 3-Phenylfuran-2(5H)-one 7 4-Phenylfuran-2(5H)-one 6 Good matches for the substituent pattern were not available in the literature (and therefore exact chemical shift matches), but the relative differences between the isomers are consistent when compared with those of 10 and 11. The mechanisms by which the products form are still unclear, but the mechanism could involve the initial The initial oxidized intermediate may form due to hydrogen atom abstraction by O2, possibly involving participation from another bifuran that accepts the second oxygen atom. An alternative explanation might be an initial reaction with singlet oxygen, which is known to easily react with furans of various kinds and provide lactones as products. However, singlet oxygen is normally generated using separate photosensitizers and highenergy light sources (i.e., UV light), both of which were absent here. The tentative structure of 10, where the furan ring substituent appears to be located on a different position of the lactone ring, is also curious. Since heat was applied to prevent 9 from crystallizing, the room-temperature degradation of the polyester may follow different routes with different end products.
Cross-linked-like structures could not be isolated after the model compound experiment, but it would appear feasible for the cross-linking to occur due to these types of reactive lactone intermediates. They should result in the build-up of both new carbonyls and new conjugated moieties, increasing the polarity and yellowness of 3,3'-PPeBf. Hydrolysis experiments also revealed a link between the polyester and the model compounds: