Energy-Efficient Upcycling of Waste Poly(ethylene terephthalate) via Pre-Annealing-Induced Formation of Reactive Telechelic Oligomers

Polyethylene terephthalate (PET) is a widely used material in single-use consumer products with increasing production demands each year. This study examines a more sustainable method to upcycle PET to produce telechelic oligomers that can function as macromonomers for polymerization or can be transformed into value-added products. The research examines the impact of annealing PET near its melting point on the evolution of the chain structure during depolymerization. These results show that this annealing provides a simple and energy-efficient protocol to successfully obtain dihydroxy-terminated oligomers from consumer waste, with varying molecular weights that are guided by the predepolymerization annealing time. Dihydroxy-terminated PET oligomers are isolated from the depolymerization process more quickly if the PET is annealed before depolymerization, and the molecular weight of the telechelic oligomers can be controlled by adjusting the annealing time. This is because longer annealing times yield higher-molecular-weight precursors with enhanced chemically reactive tie chains formed from stretched amorphous regions. However, increased crystallinity prior to depolymerization, as indicated by differential scanning calorimetry, does not alter the molecular weight of oligomers isolated from depolymerization. The production of the dihydroxy-terminated oligomers also leads to the fragmentation of PET flakes into a powder, providing a visual indication of the isolation of telechelic oligomers via glycolysis. It is hypothesized that the PET flakes fracture when most of the tie chains are severed. Consequently, dihydroxy-terminated oligomers can be more promptly obtained with predepolymerization annealing, reducing reaction time, and eliminating the need for postdepolymerization functionalization steps, thereby improving the overall sustainability of PET upcycling.