thylcyclohexyl)-??-methyethylene, and when a secondary or tertiary amine, X or Y, as the case may be, is preferably of the type ???R4N(R3)(R4)g??? in which R3 is hydrogen or an alkyl group of from 1 to 4 carbon atoms, methyl being especially preferred, and R4 is and alkylene, cycloaliphatic alkylene or aralkylene group, which may include heteroatom(s), an alkylene of less than 10 carbons being esp