Curable compositions are provided comprising: (a) an isocyanate-functional prepolymer having a weight average molecular weight of 4000 to 15,000; and (b) a curing agent comprising a mixture of polyamines, wherein at least one polyamine has an amine equivalent weight of 125 to 250. The isocyanate-functional prepolymer is a reaction product of a polyisocyanate and either (i) a polyether having primary and/or secondary amino groups or (ii) a mixture of a polyether having primary and/or secondary amino groups and a polyether having hydroxyl groups. Upon curing, the compositions demonstrate a Young's Modulus less than 500 MPa at −40° C., a percent elongation greater than 200% at −40° C., and/or a glass transition temperature less than −20° C., making them ideally suited for use at extreme temperatures.