The polymerization of N-(hydroxypivaloyl)-e-aminocaproic acid lactone [c(HPv-eAC), 1] a) in bulk at 170 8C was found to result in a high molecular weight fraction with M n 20 000 (GPC in DMAC, calibration with polystyrene) and an oligomeric fraction depending on the initiator employed. Separation of the polymeric fraction is unsuccessful by using fractional precipitation, however, by means of dialysis a polymeric fraction with a polydispersity index Q a 1.5 was isolated. NMR spectroscopic analysis of the polymer reveals a strictly alternating microstructure with hydroxypivaloyl and e-aminocaproyl repeating units. Regardless of the initiator used no endgroups were detected in both the polymeric and the oligomeric fraction. Even more, GPC analysis of the well resolved oligomeric series reveals the same elution volumes for a given oligomer, regardless of the initiator used. These results led us to conclude that poly(hydroxypivalatealt-aminocaproate) has cyclic constitution. The glass transition temperature of the semicrystalline polymer is T g = 6,6 8C, the melting point T m = 66,6 8C. A kinetic study of the polymerization reveals that the monomer conversion does not follow a pseudo first order rate law even at very low conversions. A mechanism for the initiation and the polymerization is discussed.