Homopolymerization of the unsaturated cyclic ester, 6,7-dihydro-2(5H)-oxepinone (DHO), and copolymerization with epsilon -caprolactone (epsilon CL) are an easy way to produce unsaturated aliphatic polyesters in a controlled manner, Polymerization of DHO initiated by Al((OPr)-Pr-i)(3) in toluene at room temperature proceeds by a coordination-insertion mechanism and is living as certified by the agreement between the experimental molecular weight at total monomer conversion and the value predicted from the initial monomer to initiator molar ratio. The polydispersity is also low (M-w/M-n less than or equal to 1.2). Random copolymerization of DHO and epsilon CL leads to the same conclusion. Beyond the complete monomer conversion, the poly(DHO) chains experience mainly intermolecular transesterification reactions at the same rate as poly(epsilon CL) under the same conditions. The livingness of the ROP of both DHO and epsilon CL allows diblock copolymers to be synthesized with predicted composition and molecular weight. Homopoly(DHO) is semicrystalline with T-m at 35 degreesC and T-g at -50 degreesC. T-m and thermal stability can be modulated by copolymerization of DHO with epsilon CL.