No. 428,209 describes and claims such a composition comprising (1) an organopolysiloxane as base siloxane having the formula Li J. wherein n is an integer greater than 1, R is a hydrogen atom or an alkyl radical containing 1 to 4 carbon atoms, at least half of all the radicals R are methyl radicals and each other R is a hydrogen atom or an alkyl, alkenyl or aryl radical, and which may also contain