The thermodynamics of ring-opening polymerization (ROP) are central when predicting the chemical recyclability of aliphatic polyesters and polycarbonates. Conceptually, the enthalpy of polymerization, DH p, is widely understood as a measure of ring-strain for a given monomer. However, the ring-strain is commonly larger than DH p, generating the question of how the release of ring-strain energy during ring-opening transforms. In this work, we propose that DH p is the sum of the energy released by the ring-strain ðDH ring-strainÞ and the energy absorbed by the polymer conformations ðDH confÞ. Owing to the similar ring-strain, but vastly different DH p values, d-valerolactone, d-caprolactone, d-octalactone, and d-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 DH p for systems with ring-strain, substituent effects, and solvent effects, that are difficult to understand when only using the analogy of ring-strain and DH p. We believe that our results provide a deeper understanding of the underlying thermodynamics and their interpretation in ROP.