substituents selected from alkyl groups comprising up to 5 carbon atoms, alkoxy groups comprising up to 5 carbon atoms, aryl groups comprising up to 10 carbon atoms, halogen atoms, nitro groups and cyano groups, and R7 is a member selected from the group consisting of H and R6 and said iodonium salt is selected from compounds having a nucleus of the following general formula: ##STR39## in which: A