Triazabicyclodecene (TBD) has recently been shown to be an effective organocatalyst for the ring-openingpolymerization (ROP) of cyclic esters. Using DFT methods, we have studied possible mechanisms ofthis reaction. Our studies explain not only the narrow polydispersity index (PDI) observed in the ROPof six-membered ring lactones, but also the surprising failure of the ROP for the more reactivebutyrolactone.