Efficient monomer-loop recycling of PELT copolyesters: Interplay of theoretical and experimental approaches

Poly(ethylene terephthalate-co-lactic acid) (PELT) copolyesters, incorporating renewable and biodegradable components, offer a promising route to enhance the recyclability of PET. However, the chemical monomer-loop recycling of PELT has not been investigated. Here, an efficient monomer-loop recycling of PELT was reported through a combination of theoretical and experimental approach. Density functional theory calculations reveal the feasibility of this recycling process and identify the nucleophilic attack of HOCH₂CH₂O⁻ on the carbonyl carbon of the PELT chain as the rate-determining step. Charge transfer analysis on the rate-determining step shows that PELT has a significantly smaller HOMO-LUMO gap than PET, leading to a lower Gibbs energy barrier for PELT decomposition (16.6 kcal/mol versus 21.1 kcal/mol). Experimentally, moderate molecular weight PELT was synthesized via ring-opening polymerization of lactide initiated by bis(hydroxyethyl) terephthalate (BHET) followed by polycondensation. Experimental analyses demonstrated that lactide incorporation alters the crystalline structure, which reduces the glass transition temperature, crystallization temperature, and melting temperature. Using PELT-30 as a sample, chemical recycling experiments achieved a 91.5 % BHET yield after 60 minutes at 190°C under HTBD-OAc catalysis. The degradation products were successfully repolymerized into PELT with properties comparable to the original material, confirming the viability of the monomer-loop recycling approach. Life-cycle assessment demonstrated that PELT reduces greenhouse gas (GHG) emissions and primary energy demand (PED) during closed-loop recycling compared with PET. This work advances the sustainable recycling of polyesters and contributes to the development of more eco-friendly materials.