Di- and triblock copolymers with tetramethyleneoxy and trimethylene urethane repeating units were prepared by sequential polymerization of tetrahydrofuran (THF) and trimethylene urethane (TU) with the monofunctional initiator methyl trifluoromethanesulfonate (TfOMe) and the bifunctional initiator trifluoromethanesulfonic acid anhydride (TfOTf), respectively The block copolymers obtained after purification show a uniform A-B resp. A-B-A microstructure as was deduced from NMR and GPC analyses. The block copolymers are semicrystalline materials with distinct melting points of the poly(TNF) and the poly(TU) domains.