composed of RR'SiO units and whose main chain is terminated with R(R )SiO units, (where R is as previously defined, and at least 1 mole percent of R is of vinyl radicals, the balance of R being a methyl or phenyl radical, and R is a methyl, phenyl or vinyl radical), ( 3) an onganohydrogen polysiloxane in which the total number of Si-H bonds is from 50 to 500% of that of vinyl radicals contained