Cyclic poly( l -lactide)s were prepared by ring-opening polymerization combined with simultaneous polycondensation (ROPPOC) in bulk at 160 °C with dibutyltin bis(4-cyanophenoxide) as catalyst. It is demonstrated by MALDI TOF mass spectrometry and 1 H NMR end group analyses that cycles are formed by end-to-end cyclization in addition to "back-biting" transesterification. Formation of high molar mass cyclic poly( l -lactide)s by means of several more reactive ROPPOC catalysts presented previously and in new experiments is discussed. These experimental results, together with theoretical arguments, prove that part of the Jacobson-Stockmayer theory is wrong. The critical monomer concentration, above which end-to-end cyclization is seemingly impossible, does not exist and reversible like irreversible polycondensations can theoretically proceed up to 100% conversion, so that finally all reaction products will necessarily adopt a cyclic architecture. The results of this study show that even at the highest initial monomer concentration (IMC) complete cyclization of polymer chains can be achieved at 100% conversion.