where R4 is a monovalent hydrocarbon group free of an aliphatic unsaturated bond, X is an organic group having an alkenyl group, a is an integer of from 0 to 3, m is at least 0, provided that a and m being not zero at the same time, n is at least 100, and m+n is such a number that a viscosity of the diorganopolysiloxane at 25 degrees C is 500 mPa???s or higher,