The transesterification of polyester with hydroxyl-terminated oligoesters represents a promising route for the chemical upcycling of polyethylene terephthalate waste. The single-stage melt process under investigation yielded value-added PET-based copolyesters containing up to 15 mol.% diethylene glycol and triethylene glycol units. The effect of comonomer content on the thermal properties and crystallization of the copolymers was studied. It was found that an increase in the number of oxyethylene groups in the comonomer leads to a greater decrease in the glass transition temperature and the isothermal crystallization rate. Diethylene glycol was shown to lower the equilibrium melting temperature more than triethylene glycol. Thus, during PET upcycling, the glass transition temperature decreased from 80 to 50 °C, and the melting temperature from 280 to 250 °C.

Open-loop recycling can valorize polylactide (PLA) when mechanical recycling or industrial composting is impractical. This study links PLA glycolysis modeling to UV-curable pressure-sensitive adhesive (PSA) development. A six-reaction transesterification model was calibrated against published data and used to select glycolysis conditions: 120 min at 220 °C and a PLA-repeat-unit:glycerol molar ratio of 1:0.25. Model–experiment discrepancies were 0.2% for hydroxyl value, 10.9% for apparent number-average molar mass, and 3.3% for apparent weight-average molar mass, indicating agreement with the calibration dataset. Oligomers were partially esterified with maleic anhydride (MA; nominal OH:MA ratios of 1:0.2–1:0.7) under ATR-FTIR monitoring and UV-cured at 365 nm with 10–40 wt.% 2-hydroxyethyl acrylate (HEA) on precursor mass. Tack and T-peel resistance increased with HEA loading, whereas apparent lap-shear strength showed precursor-dependent trends, including intermediate-loading maxima. A formulation with OH:MA = 1:0.5 and 30 parts HEA combined an apparent lap-shear strength of 0.30 MPa, T-peel resistance of 0.20 N/mm, and loop-tack force of 9.6 N/25 mm. These findings support glycolysis-derived PLA oligomers as tunable, partially biobased, PSA precursors.