. . . "A sealant composition comprising a prepolymer which is a blend of:a. a silicon terminated vulcanizable polyurethane polymer of the formula ##EQU2## wherein R is a lower alkyl radical of from 1 to 6 carbon atoms; wherein R' is a divalent bridging radical selected from the groups consisting of a divalent hydrocarbon radical, a divalent hydrocarbon ether radical and a divalent hydrocarbon amino radic" .