linear block polymer having a molecular weight of at least 10 4 dalton , comprising urea and urethane groups and ester groups at such a distance from each other that after hydrolysis of these ester groups fragments are created which are so small that they are biologically degradable and can be excreted from a human body and further including an end group selected from primary nh 2 and oh groups , which can be substituted by a monoamine . the polymers are intended to be used as implants .