Hydrogels from norbornene-functionalized carboxymethyl cellulose using a UV-initiated thiol-ene click reaction

Chemically crosslinked cellulose hydrogels have wide applications in agriculture and biomedicine, but most crosslinking methods involve potentially toxic crosslinking chemistries or lack significant control over the final modulus of the material. To overcome these challenges, carboxymethyl cellulose (CMC) hydrogels were synthesized utilizing thiol-ene click chemistry. CMC was functionalized with norbornene groups through a base catalyzed, water-borne functionalization reaction with carbic anhydride, yielding a reactive norbornene group. Both reaction pH and anhydride concentration could be used to control the degree of norbornene functionalization of CMC up to 45% norbornene functionalization per CMC repeat unit. This new norbornene functionalized CMC (cCMC) was crosslinked though a UV-light initiated thiol-ene reaction with a 2,2′-(ethylenedioxy)diethanethiol (DEG) crosslinker. Both the ratio of thiols to norbornenes and the irradiation time could be varied at a constant polymer concentration to control the modulus over an order of magnitude. Interestingly, thiol to norbornene ratios of 1:2 and 1:1 yielded the same modulus values, which was attributed to the crosslinking limiting chain mobility early in the reaction and preventing increased crosslink density as the reaction progressed. Hydrolytic degradation of the hydrogels yielded two degradation regimes of initial burst release and continuous daily release. Burst release behavior was tied to the thiol to norbornene ratio used to fabricate the hydrogel, while the daily degradation rate could be correlated to the crosslinking density. Due to its straightforward synthesis and significant control over modulus and degradation rates, cCMC offers high utility for future applications where cellulose derived hydrogels are needed.