Polyesters obtained by the catalytic ring-opening polymerization of macrolactones in many aspects resemble the properties of polyethylene. However, the molecular weight distribution is intrinsically different and equals the molecular weight distribution observed for a step-growth process even though macrolactone ring-opening polymerization follows a chain-growth mechanism. The concurrent occurrence of transesterification reactions leading to the formation of cyclic polymers is responsible for the deviation from the molecular weight distribution characteristic for chain-growth polymerization. To explain and extent on the theoretical principles forming the basis of this peculiar molecular weight distribution, in this work the cyclization process during the polymerization of the 17-membered macrolactone ambrettolide has been analyzed. Liquid chromatography under critical conditions has been applied to semiquantitatively analyze the fractions of cyclic and linear products. In addition, low molecular weight size exclusion chromatography has been used to independently quantify the fractions of the smallest cyclics. Using the combination of these techniques it has been shown that cyclics are present during the whole polymerization process. Furthermore, the thermodynamics of the polymerization reaction were determined. The negligible ¿Hp = 0.9 ± 1.9 kJ·mol–1 and a positive ¿Sp = 38.5 ± 6.5 J·mol–1·K–1 clearly demonstrate the absence of significant ring strain and proofs that the polymerization is driven by entropy. Individual equilibrium concentrations of the cyclics, from monomer to pentamer, were determined and these values were used in combination with the Jacobson and Stockmayer theory to calculate the effective molarity of the cyclic monomer, B = 0.087 M. This value subsequently yields a critical monomer concentration of 0.155 M, for which it was also experimentally determined that polymerizations having a monomer concentration below this value only yield cyclic polymers. Finally, B was used in combination with the monomer and initiator concentration to successfully predict the molecular weight distribution, which shows that real Mn’s are far lower and dispersities far higher than predicted from often-applied theories.