R5, R6 or R7 is a hydrocarbon radical, that when only one of the radicals R5, R6 or R7 is present as a hydrocarbon radical this is a C3 -C12 -hydrocarbon radical with the exception of C3 -C12 -n-alkyl, and that when two or three radicals R5, R6 and R7 are present as hydrocarbon radicals these are any C1 -C12 -hydrocarbon radicals, in amounts of 100 to 1 mol %, relative to the total amount of difun