groups other than OH, SH and COOH-groups and olefinic double bonds, a radical ##STR62## or a radical of the formula ##STR63## where R2, R3 and R4 are the same or different and represent a C1 -C12 -alkyl; a C5 -C12 -cycloalkyl or -cycloalkenyl; a C7 -C12 -aralkyl;in addition to which R2 represents optionally substituted aryl and, together with R4 and the central C-atom, may form a 5- to 12-member