We describe the design, synthesis, characterization, and testing of novel polyurethanes (PUs) exhibiting unprecedented combinations of outstanding mechanical properties and oxidative/hydrolytic stabilities. This achievement is due to the use of polyisobutylene (PIB) soft segments plus flexible H-bond acceptor chain extenders (HACEs): the PIB imparts superior oxidative/hydrolytic stability and the HACE produces reinforcing H-bonds, which lead to outstanding mechanicals. Oxidative/hydrolytic stability was quantitated by retention of tensile strength and elongation after exposure to nitric acid. PUs containing 60-70% PIB retain their mechanical properties, whereas Carbothane (R), Bionate (R), and ElastEon (TM), PUs marketed for chemical stability, degrade severely under the same conditions. Various HACEs were identified (e.g., hexaethylene glycol, tripropylene glycol, tributylene glycol, 3,3'-diamino-N-methyl-dipropylamine, etc.) and their effect on mechanical properties was investigated. A PIB- and HACE-containing PU exhibited 29.2 MPa tensile strength, 620% elongation, and 80 Shore A hardness. Properties were analyzed in terms of stress-strain profiles, differential scanning calorimetry traces, dynamic mechanical thermal analysis plots, and oxidative/hydrolytic stability. The properties of various PIB-based rubbers, that is, thermoplastic PUs, SIBSTAR (R), and thermoset butyl rubber are compared. The novel PUs are promising candidates for biomaterials and industrial applications where a combination of mechanical properties and oxidative/hydrolytic stability is of the essence. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 23612371, 2010