Hydrogels with biodegradable crosslinking

1. A biodegradable hydrogel obtainable by

(a) providing a first four-armed polyethylene glycol (PEG) polymer wherein each arm is terminated by a group comprising orthogonal first and second functional groups, which first orthogonal functional group is different from said second orthogonal functional group; and

(b) reacting said first four-armed polymer of (a) with a crosslinker coupled to a second four-armed PEG polymer wherein said crosslinker comprises a functional group that reacts only with the second orthogonal functional group to obtain a crosslinked polymer as said hydrogel;

wherein the crosslinker is degradable by an elimination reaction and is of Formula (1)

which has the structure:

wherein X or one of R 1 and R 5 comprises a functional group and X or one of R 1 and R 5 is coupled to said second four-armed polymer with the proviso that X must either comprise said functional group or be coupled to said second four-armed PEG polymer;

m=0 or 1;

R 1 is CN or SO 2 R 3 wherein R 3 is H or is alkyl, aryl or arylalkyl, each optionally substituted or R 3 is NR 9 2 wherein each R 9 is independently H or optionally substituted alkyl, or both R 9 groups taken together with the nitrogen to which they are attached form a heterocyclic ring;

R 2 is H; and

each R 5 is independently H, alkyl, alkenylalkyl or alkynylalkyl;

wherein each functional group independently comprises N 3 , NH 2 , NH—CO 2 t Bu, SH, S t Bu, maleimide, CO 2 H, CO 2 t Bu, 1,3-diene, cyclopentadiene, furan, alkyne, cyclooctyne, acrylate, or acrylamide; and wherein when one orthogonal functional group comprises N 3 the other does not comprise alkyne or cyclooctyne; when one orthogonal functional group comprises SH the other does not comprise maleimide, acrylate, or acrylamide; when one orthogonal functional group comprises NH 2 the other does not comprise CO 2 H; and when one orthogonal functional group comprises 1,3-diene or cyclopentadiene the other does not comprise furan.