To clarify whether atom transfer radical polymerization can be extended to free radical ring-opening polymerization, the polymerization of 5,6 -benzo-2-methylene-1,3-dioxepane (BMDO) was carried out in the presence of ethyl alpha -bromobutyrate/CuBr/2,2'-bipyridine. The structure of poly(BMDO) was characterized by H-1 and C-13 NMR spectroscopy as well as gel permeation chromatography. The living polymerization of BMDO was identified by the following facts: a straight line of ln([M]0/[M]) versus polymerization time, linear increase of molecular weight with increasing conversion, and narrowing polydispersity. The mechanism of living free radical ring-opening polymerization was discussed.