. . . . "wherein X is a halogen atom, and R is an alkyl, aryl or cycloalkyl radical containing 1-20 carbon atoms, or products of reaction of said compounds with silicon compounds containing the monomeric unit ##STR1## in which R is the same or different from R', R being an alkyl, aryl, alkoxy or aryloxy radical, and R' being hydrogen, halogen or an alkyl, aryl or cycloalkyl radical containing 1-20 carbon a" .