The Science of Polymer Chemical Recycling Catalysis: Uncovering Kinetic and Thermodynamic Linear Free Energy Relationships.

Polymer recycling must accelerate to limit ever-growing wastes and carbon dioxide emissions. Polymer chemical recycling to monomer enables multiple closed recycling loops, tackling the material and property losses endemic to mechanical recycling. Polyesters and polycarbonates, derived from 6- and 7-membered heterocycles, are leading sustainable materials produced by equilibrium polymerizations that can be reversed for selective and efficient chemical recycling to monomer. A systematic understanding of the depolymerization kinetic and thermodynamic structure-recycling relationships is needed; in particular, studies should focus on the low-energy and minimal-chemical additive conditions required for any larger-scale processes. Here, the depolymerization kinetic parameters, including rate constants and transition state energy barriers, are measured for a systematic series of leading aliphatic polyesters and polycarbonates. These recycling experiments are conducted under common conditions using neat polymer melts, at temperatures from 90 to 190 °C and with low loadings (1:100-1000) of a fast, selective, and commercial zinc(II)bis(2-ethylhexanoate) catalyst. The systematic kinetic measurements quantify the influences of different repeat units, substituents, and end-group chemistries on the recycling process. All the polymers conform to a linear free energy relationship between the depolymerization kinetic (Δ) and thermodynamic (Δ) energy differences. The discovery of recycling catalysis linear free energy relationships allows for the rational selection of the lowest temperature (and energy) recycling conditions, operable using neat polymers, to deliver both high monomer conversions and rates. The quantified structure-recycling relationships are also used to efficiently and selectively separate mixtures of structurally similar polymers by their quantitative chemical recycling into pure monomers.