--Z1 --CO--NH--R4 --(NH--CO--)v wherein v=1 or 2, Z1 is oxygen, sulfur or NR5, wherein R5 is hydrogen or lower (C1 -C4) alkyl, Z1 is connected to R1 ; and R4 is a di- or triradical obtained by removing the NCO-groups from an aliphatic, cycloaliphatic or aromatic di- or triisocyanate; Y is: ##STR18## wherein: R6 is: hydrogen, methyl, --COOR5 or --COOR7 OH with the proviso that if R6 is other than h