ts of the unit formula H2 C???CROOR'SiO1.5 where R is a methyl radical or hydrogen and R' is a divalent alkylene radical of from 1 to 4 inclusive carbon atoms and containing siloxane units possessing aliphatically unsaturated hydrocarbon radicals selected from the group consisting of vinyl, allyl and butenylene radicals, the remaining silicon-bonded substituents in addition to divalent oxygen radi