Three series of weakly crosslinked polyurethanes based on a hydroxyterminated polybutadiene, hydroxyterminated butadiene-acrylonitrile copolymer, and hydroxyterminated ethylene-adipate-maleate-fumarate copolymer were prepared while varying the hard segment content between 1.72 and 2.36 mol -NH-COC-/1000 g polymer. Information on the microphase structure and the properties of the synthesized polyurethanes was obtained by differential scanning calorimetry, wide-angle X-ray scattering, and mechanical studies. Up to a urethane concentration of around 2 mol -NH-COC-/1000 g polymer, there is a mixture of hard-hard and hard-soft segments. Above this concentration a large part of the hard-soft segments passes into hard-hard crystallites, liberating the soft segments. The best mechanical properties of the studied polyurethanes were found for a urethane concentration of around 2 mol -NH-COO-/1000 g polymer. (C) 2003 Wiley Periodicals, Inc.