Abstract
Furandioate-adipate copolyesters are an emerging class of bio-based biodegradable polymers with great potential to replace fossil-derived terephthalic acid-based copolyesters such as poly(butylene adipate-co-terephthalate) (PBAT). Furandioate-adipate polyesters have almost exclusively been prepared with conventional primary (1°) alcohol diols, while secondary (2°) alcohol diol monomers have largely been overlooked until now, despite preliminary observations that using methyl-branched diols increases the Tg of the resultant polyesters. Little is known of what impact the use of 2° alcohol diols has on other properties such as material strength, hydrophobicity, and rate of enzymatic hydrolysis—all key parameters for performance and end-of-life. To ascertain the effects of using 2° diols on the properties of furandioate-adipate copolyesters, a series of polymers from diethyl adipate (DEA) and 2,5-furandicarboxylic acid diethyl ester (FDEE) using different 1° and 2° alcohol diols was prepared. Longer transesterification times and greater excesses of diol (diol/diester molar ratio of 2:1) were found to be necessary to achieve Mws > 20 kDa using 2° alcohol diols. All copolyesters from 2° diols were entirely amorphous and exhibited higher Tgs than their linear equivalents from 1° diols. Compared to linear poly(1,4-butyleneadipate-co-1,4-butylenefurandioate), methyl-branched, poly(2,5-hexamethyleneadipate-co-2,5-hexamethylenefurandioate) (0:7:0.3 furandioate/adipate ratio) displayed both higher modulus (67.8 vs 19.1 MPa) and higher extension at break (89.7 vs 44.5 mm). All other methyl-branched copolyesters displayed lower modulus but retained higher extension at break compared with their linear analogues. Enzymatic hydrolysis studies using Humicola insolens cutinase revealed that copolyesters from 2° alcohol diols have significantly decreased rates of biodegradation than their linear equivalents synthesized using 1° alcohol diols, allowing for fine-tuning of polymer stability. Hydrophobicity, as revealed by water contact angles, was also found to generally increase through the introduction of methyl branching, demonstrating potential for these materials in coatings applications.
Highlights
Various “methyl-branched” polyesters were synthesized from 2,3-butanediol (2,3-BDO), 2,5-hexanediol (2,5-HDO), 2,7-octanediol (2,7-ODO), diethyl adipate (DEA), and 2,5-furandicarboxylic acid diethyl ester (FDEE) via polycondensation
The pathways are broken out in detail in the Scheme itself but, in general, routes a → c and a → e involve Kolbe-type electrochemical syntheses of DEA and 2,7-ODO,[2,6,7] while f → h and i → k comprise the oxidation of CMF to FDEE or hydrolysis of 2,5dimethylfuran and reduction of the resulting dione to 2,5HDO, respectively.[8−10] The longer chain diol 2,7-ODO was found to the be most reactive of the series, giving rise to higher molecular weight (Mw) materials, because of comparatively
The distillate was collected from the Dean−Stark trap a further hour into the experiment and the pressure was reduced to 200 mbar
Summary
Wu and Arnaud et al reported in 2016 and 2017 the synthesis of a series of novel polyesters from secondary (2°) diol monomers derived from 5-(chloromethyl)furfural (CMF), a platform molecule obtainable in high yields directly from the polysaccharides in biomass.[1,2] Various “methyl-branched” polyesters were synthesized from 2,3-butanediol (2,3-BDO), 2,5-hexanediol (2,5-HDO), 2,7-octanediol (2,7-ODO), diethyl adipate (DEA), and 2,5-furandicarboxylic acid diethyl ester (FDEE) via polycondensation. In comparison to poly(1,3-propylene 2,5-furandioate) (PPF), Genovese et al found the methyl-branched poly(neopentyl glycol 2,5-furandioate) to be more hydrophobic, have higher Tg and Tm, higher elastic modulus (E), higher stress at break (σb), and a similar extension at break (εb).[26] Subsequently, Guidotti et al studied a family of poly(butylene/ neopentyl 1,4-cyclohexane dicarboxylate) copolyesters containing butylene-cyclohexane and neopentyl-cyclohexane units where their ratio was used to tune the polymer properties.[27] Here, the methyl branches of the neopentyl moiety disrupted the formation of ordered phases, enhancing the polymer’s flexibility (increasing εb), but decreasing its E and σb, with respect to analogous polyesters with no methyl branching. Of particular importance was the determination of the correct copolymer composition that produced materials capable of being formed into films, where a careful balance between the rigid furan diacid and the more flexible adipate unit would be key to success
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