As strong as the PET beside it, and it unzips

One polymer is strong in the way a bottle-grade polyester is strong, and it comes apart to its monomers under heat milder than a documented glycolysis of poly(ethylene terephthalate), with a base and catalyst loading that stays catalytic, then goes back together, three times, inside two weeks.

Poly(ethylene terephthalate) is strong enough for bottles and fiber, and it can be glycolyzed back to monomer, but the documented conditions are hot. A weaker polyester that unzips in warm methanol does not replace it. The joint grade is the tensile test against a polyester molded the same day, and a closed chemical loop that does not spend a stoichiometric base to get there.

Cao and co-workers ran catalytic glycolysis of poly(ethylene terephthalate) at 180 °C for 1 hour and reported a bis(2-hydroxyethyl) terephthalate yield approaching 95.5%. That is a real depolymerization of the incumbent polymer. It is above the temperature cap of this problem, so a process that needs 180 °C does not pass, whatever its yield.

Han, Zhang, Zhou, Zhao, Odelius, and Hakkarainen report a triarylmethane polyester, P3Ad, with a maximum tensile stress of 51.66 ± 0.91 MPa and an elongation at break of 167.91 ± 16.39% on injection-molded bars, which they describe as similar to poly(ethylene terephthalate) by comparison with published values. They depolymerized 20 g in refluxing methanol, about 66 °C, in 15 minutes, with 10 wt% potassium hydroxide. Isolated dimethyl adipate was 90%. The triarylmethane monomer was 98% as a crude yield and 88% after recrystallization. One repolymerization with titanium isopropoxide gave a polymer they describe as mechanically indistinguishable from the original. Six mechanical reprocessing cycles retained tensile properties. That is the nearest chemical loop found for this draft.

It does not pass this problem. Ten weight percent potassium hydroxide is 1.78 mmol per gram of polymer. That loading equals 5 mol% only for a repeat unit of about 28 g mol⁻¹. Polyester repeat units are heavier, so the loading is a larger mole fraction than 5 mol%. The tensile numbers were not measured against a poly(ethylene terephthalate) bar molded and pulled the same day on the same machine. The chemical loop was shown once, not three times. The comparison plot against commercial plastics is not a substitute for the side-by-side test.

Tensile specimens of the candidate and of an unfilled bottle-grade poly(ethylene terephthalate) control are molded to the same geometry and tested on the same machine, the same day, by ASTM D638. At least five specimens of each are tested. The graded values are the median ultimate tensile strength and the median elongation at break.

The candidate is then depolymerized at a temperature of at most 120 °C, within 12 hours, to at most two defined monomers. Hydroxide, alkoxide, and any other catalyst, taken together, are at or below 5 mol% relative to repeat units charged. Each monomer is isolated. The isolated mass of each is at least 90% of the mass required by the repeat-unit stoichiometry of the polymer charged. Each isolated monomer is at least 95% pure by quantitative proton NMR, using an internal standard as described by Pauli and co-workers. The monomers are repolymerized by a written procedure to the same polymer, molded, and tested again. This loop is performed three times. After the third repolymerization, the medians are compared with the candidate’s own first medians, not with the polyester control.

The clock starts when the first tensile specimens are molded and ends when the third recycled set is tested. It is at most 14 days. A procedure that cannot finish three chemical loops in that window fails even if one loop looked perfect.

The candidate may be poly(ethylene terephthalate) itself, or another polymer. The joint requirement is the point. A material that is strong and does not unzip, or that unzips and is weak, fails.

The candidate’s first median tensile strength and first median elongation at break are each at least 80% of the poly(ethylene terephthalate) control’s medians. Each cycle returns at least 90% of each monomer at the purity above, at no more than 120 °C, within 12 hours, at a combined base and catalyst loading of at most 5 mol%. After three cycles, both medians remain at least 80% of the candidate’s first medians. The 14-day clock holds.

Eighty percent, 90 percent, 95 percent purity, 5 mol%, 120 °C, 12 hours, three cycles, and 14 days are pass marks defined here. The temperature cap is set below the 180 °C glycolysis cited above, not offered as a record of the mildest depolymerization anyone has run.

The control resin is identified by its grade and by a proton spectrum consistent with poly(ethylene terephthalate). Specimens of both materials are the same thickness. ASTM D638 warns that specimen preparation, speed, and environment change the comparison, which is why the control is molded and tested beside the candidate. Five specimens and the median, rather than the best specimen, are the pass rule defined here.

A depolymerization with no catalyst and no base, if the procedure claims one, is run once on a small charge. It must not itself reach the 90% yield in 12 hours at the same temperature. If it does, the reaction is not the one the procedure describes, and the run is invalid.

Carbon recovered in the isolated monomers, plus identified side products, accounts for at least 90% of the carbon charged. That closure is a pass mark for the mass balance.

Any backbone. Up to two constitutional monomers, as in a diacid and a diol. A catalyst and a base that together stay at or below 5 mol% relative to repeat units. Enzymes, if they meet the 12-hour, yield, and loading rules. Methanol or another solvent, provided the solvent is not counted as a monomer and the 5 mol% cap is still met.

Fillers or fibers that the polyester control does not also contain. Reporting tensile strength of a composite against an unfilled control. Calling an oligomer mixture a monomer. A depolymerization above 120 °C. A stoichiometric hydroxide, including the 10 wt% potassium hydroxide procedure cited above. A cleavage reagent above 5 mol% relative to repeat units, including neat acetic acid or another carboxylic acid used as the solvent. Quoting a literature tensile strength for poly(ethylene terephthalate) instead of the side-by-side test. One successful unzipping with no repolymerization. Mechanical reprocessing counted as a chemical cycle.

The first tensile medians meet the 80% comparison, and one cycle meets the monomer yield, purity, temperature, time, and 5 mol% loading. The later cycles may fall short. One cycle is not a solution. A polymer that unzips and is not remade has not closed the loop. P3Ad, as reported, does not meet this lesser bar either, because of the base loading and the missing side-by-side polyester control.

ASTM D638-22, the standard test method for tensile properties of plastics, astm.org/d0638-22.html (opens in a new tab). The method says tensile properties vary with specimen preparation, speed, and environment, and that comparative tests have to control those factors. Cao, Liang, Yang, Zhu, Deng, Li, Elimelech, and Lu, Nature Communications, 2024, 15, 6266, doi:10.1038/s41467-024-50702-5 (opens in a new tab), for glycolysis at 180 °C and the yield approaching 95.5%. Han, Zhang, Zhou, Zhao, Odelius, and Hakkarainen, Angewandte Chemie, 2025, doi:10.1002/ange.202421431 (opens in a new tab), for the P3Ad tensile figures, the 10 wt% potassium hydroxide methanolysis at about 66 °C, the monomer yields, and the single chemical repolymerization. Monomer purity uses the quantitative proton NMR assay of Pauli and co-workers, Journal of Medicinal Chemistry, 2014, doi:10.1021/jm500734a (opens in a new tab). The conversion of 10 wt% KOH into 1.78 mmol g⁻¹, and the statement that this equals 5 mol% only near 28 g mol⁻¹, is arithmetic on that loading. The 80%, 90%, 5 mol%, 120 °C, 12-hour, three-cycle, and 14-day requirements are pass marks defined here.