More than ring-strain: revisiting the definition of enthalpy in ring-opening polymerization.

The thermodynamics of ring-opening polymerization (ROP) are central when predicting the chemical recyclability of aliphatic polyesters and polycarbonates. Conceptually, the enthalpy of polymerization, , is widely understood as a measure of ring-strain for a given monomer. However, the ring-strain is commonly larger than , generating the question of how the release of ring-strain energy during ring-opening transforms. In this work, we propose that is the sum of the energy released by the ring-strain and the energy absorbed by the polymer conformations . Owing to the similar ring-strain, but vastly different values, δ-valerolactone, δ-caprolactone, δ-octalactone, and δ-decalactone were used as model compounds to evaluate the energy cost of polymer conformational freedom. Polymer conformation, measured by ¹³C NMR, DSC, and molecular dynamics, results are in good agreement with the hypothesis and can explain previous literature observations i.e. positive for systems with ring-strain, substituent effects, and solvent effects, that are difficult to understand when only using the analogy of ring-strain and . We believe that our results provide a deeper understanding of the underlying thermodynamics and their interpretation in ROP.