We present here the thermodynamic investigation of in situ cascade polycondensation-coupling ring-opening polymerization (PROP) for three cyclic aromatic ester monomers, i.e., cyclic oligo(2-methyl-1,3-propylene terephthalate)s (COMPTs), cyclic oligo(neopentylene terephthalate)s (CONTs) and cyclic oligo(2-methyl-2-propyl-1,3-propylene terephthalate)s (COMPPTs). The equibrium monomer to polymer weight ratio in bulk at different polymerization temperatures for each monomer was estimated by the size exclusion chromatography (SEC), and the thermodynamic parameters were estimated by Dainton equation. Quite different from the thermodynamics of aliphatic lactones polymerization, which is an exothermic process with entropy reduction, our results showed the polymerization thermodynamics for three cyclic aromatic ester monomers was a weak exothermic process with slight entropy increment, i.e., a both enthalpy and entropy driving process. Among them, CONTs showed the largest value of enthalpy change, due to its symmetric dimethyl substitution on β-position of propandiol segments.