Block copoly(ether urea)s with uniform hard blocks consisting of two urea groups possess appealing elastomeric properties. The crystal structure of a model bisurea illustrates the formation of long stacks of hydrogen-bonded urea groups. Thermal anal. of these polymers demonstrates the reversible melting of the hard blocks, causing the material to flow. The low glass transition temp. ensures excellent low-temp. flexibility. The morphol. of the material consists of long stacks of assocd. hard blocks embedded in the soft phase. Elongation of the materials demonstrates their highly elastic behavior, with a strain at break ranging from 1000 to 2100%. During tensile testing, irreversible deformation and reorganization of the hard blocks occur, resulting in a significant amt. of tensile set. These well-defined polymers proved to be superior compared to a less-defined analog having a polydisperse hard block.