Turning natural δ-lactones to thermodynamically stable polymers with triggered recyclability

To extend the use of naturally occurring substituted delta-lactones within the polymer field, their commonly low ceiling temperature and thereby challenging equilibrium behavior needs to be addressed. A synthetic strategy to control the polymerization thermodynamics was therefore developed. This was achieved by copolymerizing delta-decalactone (delta DL) with either epsilon-decalactone (epsilon DL) or epsilon-caprolactone (epsilon CL) at room temperature (RT), with diphenyl phosphate (DPP) as catalyst. The thermodynamic stability of P delta DL-co-epsilon DL and P delta DL-co-epsilon CL increased with increased comonomer ratio in the feed, to 10% and 30% monomeric epsilon DL, respectively, at 110 degrees C. This is in contrast to the P delta DL homopolymer, which under the same conditions depolymerized to 70% monomeric delta DL at equilibrium. The copolymers' macromolecular structure, originating from the copolymerization kinetics, was found to be the crucial factor to mitigate delta DLs equilibrium behavior. To close the loop, designing materials for a circular economy, the recycling of P delta DL-co-epsilon DL was demonstrated, by reaction with benzyl alcohol (BnOH) as an external nucleophile, leading to cyclic monomers or dimers with BnOH at high yield.