Macromolecular engineering of cyclic polycaprolactones has been carried out by a ring-expansion procedure catalyzed by a series of alkyl-organoaluminum initiators. The single-site nature of the initiators allows a very well-controlled macrolactonization process to give moderate to high molecular weight cyclic polymers with narrow polydispersities. Cyclic architectures are supported by a combination of techniques such as viscosity measurements, NMR, and MALDI-TOF MS analysis.