R15 and R16 are also identical or different and denote a halogen atom, such as chlorine, bromine or iodine, C1 -C10 -alkyl, C6 -C10 -aryl, C2 -C10 -alkenyl, C7 -C40 -arylalkyl or C8 -C40 -arylalkenyl, preferably methyl or chlorine, particularly chlorine.