Organosiloxane compositions yielding tough elastomeric materials

Tough gels and elastomers are prepared using compositions curable by a hydrosilation reaction that are characterized by the presence of a chain extender containing two silicon-bonded hydrogen atoms per molecule and a cross-linking agent containing at least three silicon bonded hydrogen atoms or alkenyl radicals per molecule, with the proviso that the chain extender contains at least 80 percent of the silicon-bonded hydrogen atoms present in the composition or, when said cross-linking agent contains alkenyl radicals, said chain extender contains at least 80 percent of the total moles of silicon-bonded hydrogen and said alkenyl radicals.