Preferably, such a first vinyl-containing gum has the formula ##STR1## wherein in the above formula, Vi is vinyl, R is selected from C1-8 monovalent hydrocarbon radicals free of aliphatic unsaturation, and R1 is a C1-8 monovalent hydrocarbon radical, and x and t vary such that the viscosity of the gum varies from 1???106 to 20???107.