A new, highly elastomeric ABA triblock copolymer made from epsilon-caprolactone (epsilon-CL) and 1,5-dioxepan-2-one (DXO) has been synthesized by sequential addition of the comonomers in THF or toluene solution initiated by aluminum isopropoxide. Careful purification of the comonomers was essential to obtain high conversion of the final amount of the added monomer. The optimal conditions were found to be polymerization in toluene solution at 25 degrees C with the addition of 1 molar equivalent (with respect to initiator) of pyridine, which gives a slight increase in the rate of polymerization. Under these conditions, poly(epsilon-CL-b-DXO-b-epsilon-CL) triblock copolymers of high molecular weight (M(n) > 10(5) g/mol) with a relatively narrow molecular weight distribution were obtained. Thermal analysis shows a melting peak at 51 degrees C and a glass transition at ca. -38 degrees C, corresponding to the PCL block crystalline phase and the PDXO amorphous region, respectively. The mechanical properties of these materials are highly elastic and can be varied by tailoring the block lengths of the corresponding comonomers. Tensile modulus, tensile strength at break, and elongation at break were found to be 20-30 MPa, 50 MPa, and ca. 1000%, respectively.