Utilizing lignocellulose-based building blocks to develop recyclable polyesters

Abstract: With the growing global population and the demand for new applications, the production of fossil-based plastics is continuously increasing. This has led to serious environmental concerns about the depletion of fossil resources, land and marine pollution, and generation of greenhouse gas emissions. These undesirable consequences of fossil-based plastics have sparked a significant interest in developing plastics using natural biomass as a sustainable alternative. To successfully compete with the low-cost fossil counterparts, and to mitigate the environmental impacts, biobased plastics with an improved material performance and recyclability are desired. In this context, we have transformed various lignin- and (sugar)cellulose-based building blocks into strategically designed rigid structures for polyester synthesis and investigated efficient pathways for their chemical recycling. This thesis is based on five papers describing key results of new biobased polyesters and their recyclability.In paper 1 and 2, ketones such as ethyl levulinate and ethyl acetoacetate were combined with pentaerythritol through a ketalization reaction to prepare two rigid spirocyclic diesters with a ketal functionality. A preliminary life cycle assessment (LCA) indicated a lower CO2 emission for monomer production from ethyl levulinate and pentaerythritol. The spiro-diesters were used in melt polycondensations with various diols to produce fully aliphatic polyesters, which showed increasing glass transition temperatures with the rigidity of the diol and spiro-diester. In paper 3, the spiro-diester from ethyl levulinate was incorporated into the structures of poly(butylene 2,5-furandicarboxylate) and poly(hexamethylene 2,5-furandicarboxylate), respectively, to improve the chemical recyclability of these aromatic polyesters. The acid-catalyzed cleavage of the ketal groups in the copolyester structures promoted rapid chain scission to small-sized oligomers, which subsequently showed faster hydrolysis into the starting building blocks than the long chain homopolyesters. Additionally, we demonstrated that the ketone-terminated telechelic oligomers obtained after the selective cleavage of the ketal units could be converted back to the original polymer structures through direct polymerization with pentaerythritol. In paper 4, a series of aromatic dicarboxylates possessing varying substitutions of methoxy groups were synthesized by reacting sugar-based methyl 5-chloromethyl-2-furoate with phenolic carboxylates (methyl paraben, methyl vanillate, and methyl syringate). Some of the polyesters prepared using these monomers showed comparable thermal properties to those based on fossil-based terephthalic acid. Moreover, in paper 5, the same sugar-based methyl 5-chloromethyl-2-furoate was combined with potentially biobased dihydroxy benzenes (hydroquinone and resorcinol) to yield two dicarboxylates which were polycondensed with biobased diols using a slightly different method to enable recycling via a telechelic approach. Initially low molecular weight telechelic polyesters with ketone end groups were prepared. Then, chain extension with adipic acid dihydrazide formed high molecular weight polyesters with acylhydrazone bonds as weak links. We demonstrated that acylhydrazone bonds in the polyesters could be selectively hydrolyzed to produce telechelic polyesters, which subsequently could be linked together via a chain extension with dihydrazide to recover the original structure.