High molecular weight poly(epsilon-caprolactone)s (PCL) were synthesized via zwitterionic polymerization of epsilon-caprolactone initiated with N-heterocyclic carbenes. Ring-opening polymerization of e-caprolactone (1 M) with carbenes 1,3-diisopropy1-4,5-dimethylimidazol-2-ylidene (l.) or 1,3,4,5-tetramethylimidazol-2-ylidene (3) affords cyclic PCL with molecular weights up to M-n = 114 000 g/mol. Crystallization of the cyclic PCL was investigated by synchrotron small.-angle X-ray scattering experiments and differential scanning calorimetry. High molecular weight cyclic poly(epsilon-caprolactone) crystallizes with a similar lamellar thickness and long period spacing as linear poly(epsilon-caprolactone), but the crystallization of cyclic poly(epsilon-caprolactone) is faster than that of linear poly(epsilon-caprolactone) for molecular weights greater than 75 000 g/mol.. These results imply that the polymer topology does not have a significant influence on the crystal structure or morphology but can have a significant influence on the rate of crystallization from the melt.