Polyesters with indole as sustainable aromatic component

Sammanfattning: Plastics are widely used in our daily lives and industry. Unfortunaltely, most plastics are produced from non-renewable fossil resources. The transition toward sustainable resources such as various biomass is critically important for the future of plastic industry and the whole society. Polyesters consitute a major class of plastics, which are widely used in textiles, packaging and other applications. A major challenge for polyesters to achieve sustainability is to replace the non-sustainable aromatic units, such as terephthalates in polyethylene terephthalate (PET). Alternative sustainable aromatic resources from various biomass resources have therefore attracted growing attention. Indole is a large aromatic compound, which exist widely in nature, animal feces and wastewater streams. It can also be produced by a variety of sustainable ways, such as bacterial fenmentation and natural indigo reduction. Many indole-derivatives such as indole-derived alcohols, amines, carboxylic acids are also produced widely by plants and animals. Therefore, indole can be considered as a potential sustainable replacement for non-renewable terephthalate in PET. An advantage of indole for polyester synthesis is that it has a larger size than benzene or furan, so the resulting polyesters will have enhanced thermal performance which will enable various applications. Another potential advantage for indole-based polyesters is that indole has been widely produced and adapted in natural ecosystem, so these new materials may exhibit suitable biodegradability and reduced eco-toxicity. This thesis is focused on the synthesis and characterization of different types of indole-based polyesters and their structure-property relationships. Firstly, the synthetic feasibility of indole-based sustainable polyesters with suitable molecular weight and intrinsic viscosity was demonstrated. The impacts of the molecular structures such as the di-substitution patterns and the nature of ester groups (aliphatic or aromatic) on the thermal, mechanical and rheological performance were evaluated. Most of the obtained indole-based polyesters were fully amorphous, which can be prepared into transparent films. A few other indole-based polyesters exhibited certain crystallity and opaque appearance. Enhanced thermal stability and glass transition temperatures were achieved for those indole-based polyesters. Enzymatic degradation investigations revealed that indole-based polyesters could be degraded by polyethylene terephthalate hydrolase (PETase) from Ideonella sakaiensis. This was corroborated by the molecular docking simulations. Finally, we have demonstrated that the molecular design of indole-based polyesters with unsubstituted N-H units as hydrogen bond donors could enhance their miscibility with other polyesters with hydrogen bond acceptors, which may facilitate their mechanical recycling.