vinyl groups contained in component (1) above, (4) platinum or a platinum-containing compound in a catalytic amount, and (5) a hydrazine compound having the general formula ##EQU1## where R1, R2, R3 and R4 each represent a hydrogen atom, a phenyl group or an alkyl group having from 1 to 8 carbon atoms, provided always at least one of them is a phenyl or alkyl group, in an amount larger than 10% b