The organopolysiloxane used in such a composition typically is a high molecular weight polysiloxane and comprises units of the general formula Qa Q'SiO.sub.(3-a)/2 and Qb SiO.sub.(4-b)/2 wherein Q denotes a monovalent hydrocarbon or substituted hydrocarbon group having no more than 8 carbon atoms, for example a methyl or a phenyl group, Q'denotes an organic group having olefinic unsaturation, pref