. . . . "nd which may be internally interrupted by O, NH, NR5 or S; and A is N, a N-containing polyethylene glycol substituent or a N-containing substituted or unsubstituted C1 -C30 straight chain or cyclic compound which may also be internally interrupted by O, NH, NR5, or S, where R5 is a C1 -C3 alkyl; z is 1 to 10;provided that at least one of R1 and R2 is other than H, at least one of R3 and R4 is othe" .