Silicone rubber compositions

An organopolysiloxane composition comprising (a) 100 parts by weight of a linear diorganopolysiloxane with a polymerization degree higher than 3,000 terminated at both chain ends with hydroxy groups directly bonded to the silicon atoms and composed of alternately linked portions of two kinds of blocks, (i) one of which is a vinyl-containing diorganopolysiloxane block composed of randomly linked methylvinylsiloxane units (CH.sub.3) (CH.sub.2 =CH)SiO and diorganosiloxane units R.sub.2 SiO, where R is the same or different substituted or unsubstituted monovalent hydrocarbon group having no aliphatic unsaturation, the total number of said methylvinylsiloxane units and said diorganosiloxane units being from 10 to 400 and the number of said methylvinylsiloxane units in each of the vinyl-containing blocks being at least 3, and (ii) the other of which is a vinyl-free diorganopolysiloxane block composed of at least 400 of diorganosiloxane units R.sub.2 'SiO where R' has the same meaning as R above, (b) from about 20 to about 200 parts by weight of a silica filler with a specific surface area at least 150 m.sup. 2 /g, (c) an organohydrogenpolysiloxane having at least two hydrogen atoms directly bonded to the silicon atoms in a molecule in an amount such that the total number of the hydrogen atoms directly bonded to the silicon atoms is equal to from about 40% to about 400% of the total number of the vinyl groups in the said component (a) defined above, and (d) a catalytic amount of platinum or a platinum compound.