Chemical Recycling of Polyesters and Polycarbonates: Why Is Zinc(II) Such an Effective Depolymerization Catalyst?

Effective polymer recycling is essential to reduce plastic pollution; catalytic polymer recycling to monomer is particularly attractive, as it could operate over multiple closed-loop cycles. Aliphatic polyesters and carbonates show properties that compete with current plastics and can be depolymerized to 6- and 7-membered cyclic ester or carbonate monomers. Nonetheless, the rules governing recycling catalyst selection are unclear. Here, Zn(II), Co(II), Mg(II), Sn(II), Ca(II), Ba(II), Y(III) and Bi(III) 2-ethyl hexanoate catalysts are compared for the chemical recycling of 6 different oxygenated polymers, in bulk, at low catalyst loadings (1:100 to 1:1000) and temperatures (90-170 °C). All metals are selective for recycling to monomer but show clear differences in rates; the Zn(II) catalyst is always the most active. Using linear free energy analysis, the depolymerization rate constant directly correlates with the metal's Lewis acidity, as assessed by its hydrolysis constant. The best catalysts comprise metals with intermediate acidity, i.e., Zn(II), Co(II) and Mg(II). The structure-activity correlation applies to polymers that have primary or secondary chain-end group alcohols, 6- or 7-atom repeat units, and those featuring ester or carbonate linkages. Eyring analysis using Zn(II), Co(II), Mg(II) and Sn(II) catalysts shows that the Zn(II) catalysts balance competing transition-state enthalpy (Δ<i>H</i>^‡_d) and entropy (Δ<i>S</i>^‡_d) demands. Density functional theory calculations of key transition states suggest that Zn(II) is particularly effective because it both activates the polymer carbonyl group and labilizes the alkoxide nucleophile. These generally applicable linear free energy relationships are important tools to minimize energy input and maximize performances in future recycling processes.