Designing Thermally Stable Organocatalysts for Poly(ethylene terephthalate) Synthesis: Toward a One-Pot, Closed-Loop Chemical Recycling System for PET

Abstract

Organocatalysis provides robust methodology to furnish "greener" routes to polymer synthesis. However, the application toward the synthesis of aromatic polymers via step-growth polymerization is an area that justifies more investigation, as a consequence of the poor thermal stability of many organic catalysts and the high reaction temperatures commonly required. In this study, thermally stable organic salts consisting of an organic base and an organic acid were explored to understand key elements required for the bulk synthesis of poly(ethylene terephthalate) (PET) at 270 °C. The ΔpK a values of the salts played an important role in the thermal stability such that the salts with higher ΔpK a values showed higher stability because of the strong acid−base interactions. The 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) salts with high ΔpK a values (≥16.9) showed the best catalytic activity among the investigated salts in terms of both low amounts of side reactions and discoloration. The thermal and chemical stability of the salts also affected the polymer properties. Dimerization side reactions that lead to defects in the polymer backbone were found to occur more readily in salts containing strong acids as components, particularly as the ΔpK a between the acid−base components decreased. The discoloration of the PET sample was also correlated to the thermal stability of the organic salt catalyst, with a lower stability generally leading to enhanced discoloration likely due to decomposition of base components. Polymerization−depolymerization cycles were also investigated with the TBD:p-toluenesulfonic acid (TSA) salt and the feasibility of simple, closed-loop recycling of PET with the system was established.

Catalyst-Free Single-Step Solution Polycondensation of Polyesters: Toward High Molar Masses and Control over the Molar Mass Range

“…In industry, the workhorse catalysts are often antimony-based (e.g., antimony trioxide), which are hazardous. , Other catalysts have also been investigated and used, such as tin-, zinc-, calcium-, aluminum-, zirconium-, magnesium-, and especially titanium-based catalysts, as well as several organocatalytic systems. Nonetheless, all of them exhibit one or more disadvantages, such as toxicity, discoloration, high cost, low availability, susceptibility to hydrolysis, and/or decreased thermo-oxidative stability of the polymers. ,,,,,,,− Finally, the catalyst system also influences the crystallization rate, making it more difficult to reproducibly study the crystallization behavior of these polymers …”

An efficient cyclodepolymerization route for the chemical recycling of poly(ethylene adipate)

“…Hydrolysis, alcoholysis (glycolysis), and aminolysis are traditional chemical recycling strategies for aromatic polyesters. [15][16][17][18][19][20][21][22][23][24][25][26][27] Due to the high energy cost and waste treatment during the recycling process, the obtained products/ monomers are quite expensive compared to their petroleumbased counterparts. The cyclodepolymerization method can be considered as a fourth way for the chemical recycling of polyesters and polycarbonates, although it is much less studied, where polyesters are converted to corresponding cyclic oligoesters based on ring-chain equilibria in dilute solutions.…”

Hydroxylation-Depolymerization of Oxyphenylene-Based Super Engineering Plastics To Regenerate Arenols

“…Recently, attention has been paid to the degradation of synthesized molecules, especially polymers, into raw chemicals, monomers, or equivalent compounds from the viewpoint of chemical recycling. − In fact, depolymerization reactions have already been developed for commodity polymers and engineering plastics such as polyethylene terephthalate, − polycarbonate, − polyamides, − polyacetals, − and poly­(phenylene ether) . On the other hand, super engineering plastics such as polysulfone (PSU), polyphenylsulfone (PPSU), polyethersulfone (PESU), and polyetheretherketone (PEEK) are known to be highly robust plastics that are composed of stable chemical bonds such as carbon–oxygen bonds in main chains and thus show high thermal stability and chemical resistance (Figure ).…”