. . . " said hydrocarbon group optionally is substituted with halogen atoms, and R optionally have at least one asymmetric carbon atom,X and Y are independently a group selected from the group consisting of --COO--, --OCO--, --CH2 CH2 --, --CH2 O--, --OCH2, --S--S, --CO--CH2 -- and --CH2 --CO--, or a single bond, A' is a group selected from the group consisting of ##STR382## B' is a group selected from t" . .