The ceiling temperature (Tc) enables the control over monomer-polymer equilibrium, which, in turn, could govern the chemical recycling of polymers. Comprehensive knowledge of the monomer-polymer equilibrium as a function of the reaction medium is highly important to engineer the techniques for chemical recycling to monomers (CRM) by tailoring the Tc of polymers. The polymers with high Tc are less desirable for closed-loop recycling, as this indicates that depolymerization conditions such as high temperature and dilution could activate alternative reaction pathways, presenting selectivity challenges in addition to the high energy input required for depolymerization. To circumvent these issues, the main focus of this study is on how monomer–polymer-solvent interactions promote Tc depression toward selective chemical recycling. Further, we investigated using Hilderbrand solubility parameters to potentially predict and approximate the Tc depression in solvents and compare and explain the Tc values obtained experimentally.
This study examines the alternating ring-opening copolymerization of cyclohexane oxide (CHO) and phthalic anhydride (PA) (CHO-alt-PA), with organocatalyst. The polymerization reaction was performed at different temperatures. Aliquots were removed at regular time intervals until an equilibrium monomer concentration was established and analyzed by ¹H NMR spectroscopy. The empirically determined Tc for neat CHO-alt-PA copolymer is 403 °C. The high Tc value poses technological challenges and limitations for CRM. Thus, to lower the Tc value, we employed dilution with solvent introduction and used various solvents to manipulate the monomer-polymer equilibrium. Then, we calculated the Tc with respect to CHO and PA monomers. The Tc depression was found to be dependent on the Hilderbrand solubility parameter of the solvents, monomers, and polymers. As a result, for example, in chlorobenzene, which has solubility parameter= 19.5 MPa^1/2, the Tc with respect to CHO monomer (solubility parameter = 19.8 MPa^1/2 the Tc,_CHO is 385 °C, at the CHO monomer concentration of 2 M. The comparison of the Tc values between chlorobenzene and DMSO found that decreasing Tc was observed when the solubility parameters of the solvent and monomer were close in values. The knowledge gained on the interplay between the solubility of monomer, polymer, and solvent can be translated to other monomer-polymer systems, inspiring the design of a future fully circular polymer economy.

Ramasinghege Lumbini Ramasinghe

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