The vinylated organopolysiloxane (A) can be an organopolysiloxane exhibiting siloxyl units of formula: Y a ??? Z b ??? SiO ( 4 - a - b 2 ) ( 1 ) in which Y is a vinyl group, Z is a monovalent hydrocarbonaceous group not having an unfavourable effect on the activity of the catalyst, Z generally being chosen from alkyl groups having from 1 to 8 carbon atoms inclusive, such as the methyl, ethyl, prop