Abstract: Silicone rubber compositions comprising (a) diorganopolysiloxane with a viscosity at least 1,000,000 centistokes at 25.degree. C, of which from 8 to 80 mole % of the organic groups are monovalent hydrocarbon group having from 2 to 4 carbon atoms, (b) an organopolysiloxane with a viscosity at most 200 centistokes, (c) a silica filler, and (d) an organic peroxide. Their vulcanized form, i.e., silicone rubber elastomers exhibit a high degree of elasticity in an extremely low temperature range, and have superior properties with respect to heat-resistance, oil-resistance, electrical characteristics and compression set.

Abstract: Silicone rubber compositions comprising a diorganopolysiloxane having localized vinyl group distribution, a silica filler with large specific surface area and a curing agent, e.g., an organic peroxide, the diorganopolysiloxane being prepared by the block-copolymerization of a hydroxy-endblocked diorganopolysiloxane without unsaturation and a chlorine-endblocked vinyl-containing diorganopolysiloxane by the dehydrochlorination-condensation. Cured elastomers from silicone rubber compositions have very much improved tear strength with the other properties being as good as those of the conventional silicone rubber elastomers.