Polyesters and deep eutectic solvents: From synthesis through modification to depolymerization

• Polyester processing benefits from greener alternatives like DESs instead of IL. • Eutectic monomer mixtures enable solvent-free, autocatalyzed polyester polycondensation. • DES-modified polyester surfaces can enhance their compatibility or biodegradability. • Closed-loop reactions of polyesters can be catalyzed with DESs. • Zinc salts-based DESs are cheaper catalysts than pure zinc salts. Thermoplastic polyesters constitute an important class of materials in today's world due to their unique combination of properties, versatility, recyclability, sustainability, and other advantages. A wide range of monomers used in polyesters synthesis lead to their usage in various industries, such as packaging, automotive, or electronics. Poly(ethylene terephthalate) (PET) and other thermoplastic polyesters have been around for decades, however, nowadays, with growing problems such as microplastic migration, growth of landfills, and decreasing sources of fossil fuels, the lack of their biodegradability or the high cost of biodegradable ones make it necessary to search for greener solutions. A novel group of media: deep eutectic solvents (DESs) that have found applications in many areas of science, can also be applied in polyester technology. This review is a holistic approach presenting polyesters in every step of their technology. DESs as easy-to-prepare, green, and cheap alternatives to the organic solvents, metal salts, and ionic liquids employed as reaction media or catalysts. In polyester synthesis, DESs serve as monomer sources, reaction media, and catalysts, i.e. monomeric DESs facilitate solvent-free, autocatalyzed polymerization and production of safe and biodegradable materials that can be applied, for example, in pharmaceutical or medicine engineering. Some DESs cannot depolymerize polyesters, but can render their surfaces more hydrophilic without affecting crystallinity and thus hold promise as functional additives (interfacial/active agents, plasticizers and compatibilizers) for polyesters and their blends. DESs have been widely used in the depolymerization of polyesters (mainly PET but also poly(lactic acid) and poly(ethylene 2,5-furanoate)) as cheaper or greener catalysts or reaction media (or both) with conversion up to 100% and high yield of monomer. In this paper, we consider polyesters and DES issue from the “cradle-to-grave” or even "cradle-to-grave-to-cradle" viewpoint emphasizing the importance of solvolysis as a chemical recycling method. Finally, we present the future perspectives and possibilities of DES usage in polyester technology.