Toward Biomass-Derived Recyclable Polyesters
Abstract: Fossil-based plastics have become a matter of concern due to their negative environmental impacts such as greenhouse gas emissions and threats to the health of living beings. Alternative polymers derived from sustainable biomass resources have thus become an attractive research field since the last decade. Currently, there is a strong demand in the development of new bio-based polymers with enhanced properties (e.g., thermal, mechanical, and barrier properties) and recycling possibilities. This thesis focuses on the design and synthesis of various monomers and polymers using bio-based starting molecules, particularly those derived from lignin fragments such as vanillin. Attention was paid to unravel the impacts of certain molecular structures such as spirocyclic acetals on the polymer properties and recyclability. This thesis includes five papers that describe the important findings of various bio-based polyesters. In Papers I, II, and III, diol and dicarboxylate ester monomers with spirocyclic acetal structures have been synthesized by coupling vanillin with pentaerythritol and derivation by SN2 reactions. Preliminary life cycle assessment (LCA) results indicated that the production of the diol monomer with spiroacetal structures generates low greenhouse gases (Paper I). This diol monomer was used to synthesize copolyesters together with dimethyl terephthalate and 1,6-hexanediol (Paper I) and neopentyl glycol (Paper II). The glass transition temperatures of these polyesters were significantly enhanced by the incorporation of the spiroacetal monomeric units in the backbone. Furthermore, the spiroacetal units could be selectively cleaved under mild acidic conditions, yielding telechelic polymers with aldehyde end groups (Paper II). The resulting telechelic polymers could be conveniently converted back to the original polymers by polyacetalization with pentaerythritol, which showed their potential in the development of a new energy efficient chemical recycling process. We have also demonstrated that polyesters with spiroacetal units could be effectively hydrolyzed using an enzyme Humicola insolens cutinase, which suggested its potential in enzymatic recycling or biodegradation (Paper III). The effectiveness of the same enzyme on a commercial polyester with spiroacetal units (AkestraTM, Paper IV), as well as newly synthesized methoxyhydroquinone-based polyesters without spiroacetal units (Paper V), was also demonstrated.
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