Directly converting waste PET into closed-loop biodegradable plastics

Abstract Polyethylene terephthalate (PET), a prevalent commodity polymer for manufacturing bottles, films, fibers, and engineering plastics, has an annual production of over 74 million tons worldwide1-3. Unfortunately, averaging 70% of PET products are single-use, leading to substantial non-degradable waste that contributes to environmental pollution and the proliferation of microplastics detectable in the human body1,4-8. Efforts to chemically depolymerize PET into its starting monomers using amines, water, and alcohols9, however, have encountered challenges in efficiently extracting the monomers from multimers, hindering its full utilization10-14. Here we demonstrate a closed-loop strategy to directly convert waste PET into biodegradable plastics named PEXT, involving forward and reverse transesterifications between ester groups and hydroxyl, carboxyl, and ester groups. By utilizing commercially available aliphatic X-monomers, e.g., diacids and diols, or their polymers, and leveraging existing industrial equipment, we avoid the inefficient PET depolymerization back to monomers and achieve its complete utilization within a one-off framework. Through precisely controlling the molecular structure, PEXT exhibits superior mechanical performance compared to conventional elastic materials including low-density polyethylene, as well as programmable thermal and biodegradable properties, enabling its extensive applications, e.g., shape memory materials, flexible films, and rigid plastic parts. Additionally, incorporating of PET or X-monomers into PEXT facilitates its closed-loop recycling between different components. This feature, coupled with its biodegradability, effectively balances economic viability with environmental concerns.