Designing polymeric materials for closed-loop material streams is the key to achieving a circular society. Here, a library of macrocyclic carbonates (MCs) was designed by a facile and direct one-pot, two-step synthesis approach without the use of a solvent at a 10 g scale. We demonstrate that anionic polymerization with tert-butoxide enables the ultrafast ring-opening polymerization (ROP) of MCs with high conversion (>97%) within seconds (3-10 s) at ambient temperature. The polymerization rate depends on the odd or even number of methylene groups between the carbonate linkages in the MCs, and not the overall ring size, yielding an "odd-even" effect. This polymerization rate is related to the difference in molecular conformation of the MCs, as determined by X-ray crystallography. The polymers (polypenta-, hexa-, heptamethylene carbonate) were subsequently regenerated back to their original MCs at a high selectivity (95-99 mol %) and good yields (70-85%), hence taking a step toward closing the loop on these long alkyl chain polycarbonates.