The design, synthesis, characterization, and structure-property behavior of polyureas containing novel soft segments of mixed polyisobutylene (PIB)/poly(tetramethylene oxide) (PTMO) chains and conventional hard segments is presented. Modest amounts (12%) of PTMO in the soft PIB phase significantly increase both the tensile strength and elongation of the polyureas. These polyureas exhibit not only oxidative/hydrolytic stabilities far superior to Bionate (R) and Elast-Eon (R) considered the most oxidatively stable polyurethanes on the market but also display mechanical properties (29 MPa tensile strength and 200% elongation) approaching those of conventional thermoplastic polyurethanes. The surfaces of these polyureas are covered/protected by PIB segments, which will lead to excellent biocompatibility. Our results demonstrate that the PTMO segments facilitate stress transfer from the continuous mixed soft phase to the dispersed hard phase, which strengthens and flexibilizes PIB-based polyureas and thus significantly improves elastomeric properties without compromising oxidative and hydrolytic stability. (c) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2787-2797, 2009