The composition according to claim 16 in which the product further comprises a chain extender selected from the group consisting of ZMe2 SiO(Me2 SiO)y SiMe2 Z, ZMe2 Si--X--SiMe2 Z, and ##STR50## where Z is a heterocyclic Si--N group, R is a monovalent hydrocarbon group, X is a divalent hydrocarbon radical selected from the group consisting of --(CR2)m -- or --C6 H4 --, y is ???0, and m is 2 to 6 i