Chemical upcycling of complex PET waste: Upcycling of milder reaction conditions and use for polyurethane as added-value product

• Efficient chemical upcycling of complex PET waste into liquid polyols. • Ethylene carbonate acts simultaneously as reagent and solvent under mild conditions. • Caesium carbonate minimises carbonate hydrolysis, outperforming KOH catalyst. • Polyols with tunable molecular weights (1000–2500 g/mol) used as PU soft segments. • Resulting polyurethanes exhibit high molecular weight and single-Tg behaviour. • Method successfully applied to thermoformed PET, expanding recycling feasibility. This study presents an efficient chemical upcycling route for complex poly(ethylene terephthalate) (PET) waste, including post-consumer bottles, thermoform PET, and the glycolysed monomer bis(2-hydroxyethyl) terephthalate (BHET), transforming them into liquid polyols for the synthesis of polyurethanes. The strategy is based on a catalysed solvolysis using ethylene carbonate (EC) as both a reagent and solvent, operating under mild conditions and at atmospheric pressure. A systematic screening demonstrated that caesium carbonate (Cs 2 CO 3 ) is more efficient catalyst than KOH, as it minimises the hydrolysis of carbonate groups, thereby yielding polyols with a higher content of these valuable units. Kinetic studies revealed the critical importance of reaction time and the transition from a heterogeneous to a solvated and highly swollen polymer state. The resulting polyols, with controlled molecular weights in the range of 1000–2500 g/mol —appropriate for use as soft segments— were successfully employed in the synthesis of polyurethanes. The final materials exhibited high molecular weights and thermal properties that correlate with the chemical composition of the precursor polyol, validating this methodology as a versatile and sustainable alternative to advance towards a circular economy.