Investigations of the kinetics of zwitterionic ring-opening polymerization of E-caprolactone by N-heterocyclic carbenes (NHC) were carried out to illuminate the key reaction steps responsible for the formation of high molecular weight cyclic poly(caprolactones). Modeling of both the decay in monomer concentration as well as the evolution of molecular weights and polydispersities were necessary to identify the key reaction steps responsible for initiation, propagation, cyclization and chain-transfer. Nucleophilic attack of the NHC on E-caprolactone to generate reactive zwitterions is slow and reversible. The modeling indicates that less than 60% of the carbenes are transformed to active zwitterions, but that these zwitterions rapidly add monomer and cyclize by intramolecular backbiting of the terminal alkoxides on internal esters of the zwitterions. This cyclization event maintains the concentration of active zwitterions. The reactivation of cyclized chains by active zwitterions is a key step that leads to high molecular weight poly(caprolactones).