A hydrogel that is biodegradable under physiological conditions whose hydrogel comprises one or more polymers crosslinked by a linker that decomposes by a beta elimination reaction, (a) wherein the linker, when disposed in the hydrogel, is a residue of the Formula (1) ** Formula ** wherein at least one of R1, R2, R5 is coupled through a first functional group to said one or more polymers and wherein X is coupled to said one or more polymers of the hydrogel through of a second functional group, where m is 0 or 1； wherein one and only one of R1 and R2 may be H or may be alkyl, arylalkyl or heteroarylalkyl, each optionally substituted； at least one or both R1 and R2 are independently CN； NO2； optionally substituted aryl； optionally substituted heteroaryl； optionally substituted alkenyl； optionally substituted alkynyl； COR3 or SOR3 or SO2R3 wherein R3 is H or optionally substituted alkyl； aryl or arylalkyl, each optionally substituted； heteroaryl or heteroarylalkyl, each optionally substituted； or OR9 or NR92 wherein each R9 is independently H or optionally substituted alkyl, or both R9 groups taken together with the nitrogen to which they are attached form a heterocyclic ring； SR4 wherein R4 is optionally substituted alkyl； aryl or arylalkyl, each optionally substituted； or heteroaryl or heteroarylalkyl, each optionally substituted； wherein R1 and R2 can join to form a 3-8 member ring； and each R5 is independently H or is alkyl, alkenylalkyl, alkylalkyl, (OCH2CH2) p O-alkyl wherein p>； = 1- 1000, aryl, arylalkyl, heteroaryl or heteroarylalkyl, each optionally substituted, wherein X is a carbonate, an optionally substituted carbamate, thioether, an ester or phenol wherein X is coupled to the polymer through said second functional group； or (b) wherein said linker is a residue of Formula (2) ** Formula ** wherein at least two of said R1, R2, R5 are coupled through functional groups to said one or more polymers； m and R1, R2 and R5 are defined as in paragraph (a)； n is
