Novel segmented poly(urethaneurea) elastomers (TPUU) were prepared through the reaction of poly(tetramethylene oxide)glycols (PTMO-1000 and PTMO-2000) with excess aromatic diisocyanates (TDI and MDI), followed by in situ chain extension with water, through the formation of urea linkages. High molecular weight TPUU polymers with high urea hard segment (HS) contents of up to 44% by weight were obtained. FTIR analysis of the carbonyl region clearly showed the presence of strongly hydrogen bonded urethane and urea groups in the system. Dynamic mechanical analysis indicated the formation of microphase separated morphologies with well-defined PTMO glass transitions, followed by a composition dependent rubbery plateau extending well beyond 200 degrees C. Tapping mode AFM studies clearly indicated the formation of microphase morphology with hard urea domains dispersed or percolated through soft polyether matrix. Stress-strain tests demonstrated the formation of very strong elastomers with tensile strengths of up to 50 MPa. Tensile properties also showed a strong dependence on the soft segment molecular weight, structure of the diisocyanate used and HS content of the copolymer.