A compound of the formula wherein Z is the residue of a macromolecule; L' is the residue of a linker; D is the residue of a drug; X is O or S; A is alkenylene (C 2), aryl or absent; each R 1 and R 2 is independently H; CN; NO 2; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or each R 1 and R 2 is independently COR 3 or SOR 3 or SO 2 R 3 wherein R 3 is H or optionally substituted alkyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or OR or NR 2 wherein each R is independently H or optionally substituted alkyl; or each R 1 and R 2 is independently SR 4 wherein R 4 is optionally substituted alkyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; or optionally substituted alkynyl; wherein the optional substituents are selected from the group consisting of electron-donating groups such as C 1 -C 4 alkyl; C 1 -C 4 alkoxy; C 1 -C 4 alkylthio; amino; alkylamino; and dialkylamino; and electron-withdrawing groups such as halogen; difluoromethyl; trifluoromethyl; nitro; cyano; C R, wherein R is H, C 1 -C 4 alkyl, C 1 -C 4 alkoxy or amino; and SOR and SO 2 R, wherein R is C 1 -C 4 alkyl, aryl or heteroaryl, wherein multiple such optional substituents may be bound to aryl and heteroaryl rings, which multiple substituents may be the same or different; wherein R 1 and R 2 may be joined to form a 3-8 member ring; and wherein both R 1 and R 2 cannot be H; wherein R 5 is H or alkyl (C 1-6). wherein Z is the residue of a macromolecule; L' is the residue of a linker; D is the residue of a drug; X is O or S; A is alkenylene (C 2), aryl or absent; each R 1 and R 2 is independently H; CN; NO 2; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or NO 2; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or each R 1 and R 2 is independently COR 3 or SOR 3 or SO 2 R 3 wherein R 3 is H or optionally substituted alkyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or OR or NR 2 wherein each R is independently H or optionally substituted alkyl; or R 3 is H or optionally substituted alkyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or OR or NR 2 wherein each R is independently H or optionally substituted alkyl; or optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; optionally substituted alkynyl; or OR or NR 2 wherein each R is independently H or optionally substituted alkyl; or each R 1 and R 2 is independently SR 4 wherein R 4 is optionally substituted alkyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; or optionally substituted alkynyl; R 4 is optionally substituted alkyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; or optionally substituted alkynyl; optionally substituted aryl; optionally substituted heteroaryl; optionally substituted alkenyl; or optionally substituted alkynyl; wherein the optional substituents are selected from the group consisting of electron-donating groups such as C 1 -C 4 alkyl; C 1 -C 4 alkoxy; C 1 -C 4 alkylthio; amino; alkylamino; and dialkylamino; and electron-withdrawing groups such as halogen; difluoromethyl; trifluoromethyl; nitro; cyano; C R, wherein R is H, C 1 -C 4 alkyl, C 1 -C 4 alkoxy or amino; and SOR and SO 2 R, wherein R is C 1 -C 4 alkyl, aryl or heteroaryl, wherein multiple such optional substituents may be bound to aryl and heteroaryl rings, which multiple substituents may be the same or different; wherein R 1 and R 2 may be joined to form a 3-8 member ring; and wherein both R 1 and R 2 cannot be H; wherein R 5 is H or alkyl (C 1-6).