Microwave-Assisted Synthesis of Polyethylene Terephthalate as a Fast Screening Method for Catalyst Evaluation and for Evaluating Polymerizability of Monomers Recovered From Degraded Polyesters

Abstract

Microwave irradiation is demonstrated as a screening method for the polymerization of crude terephthalic acid (TA) and crude bis(2-hydroxyethyl) terephthalate (BHET), obtained from the enzymatic hydrolysis and glycolysis of poly(ethylene terephthalate) (PET). The use of microwave irradiation and a reduced pressure of 40 mbar allows for the preparation of medium-long PET with measured Mw values up to 30–40 kg/mol relative to PMMA standards in less than 1 h. The versatility of the screening approach is demonstrated through the evaluation of a range of esterification and polycondensation catalysts using pristine starting materials. It was found that for the reaction between TA and EG using a Ti(OBu)₄ catalyst, it yields the largest polymers, accompanied by minimal coloration. For the polymerization of BHET, the Sb₂O₃ catalyst produced large polymers with little coloration, while the use of the Ti(OBu)₄ catalyst gives highly colored materials. The developed procedure is used to re-polymerize crude TA recovered from enzymatic degradation of PET waste as well as crude BHET recovered from glycolysis of pristine and waste PET. Repolymerization is possible, but the color and molecular weight of the final polymer are found to depend on the type and amount of impurities in the starting materials. The relevance of the formed intermediates as precursors for high molar mass PET was confirmed through solid-state polymerization that yielded high molar mass PET from selected samples.