Highly Entangled Hydrogels by Photoiniferter-Mediated RAFT Polymerization

We report the synthesis of ultra-high molecular weight (UHMW) poly(N,N-dimethylacrylamide) (PDMAm) hydrogels with extremely low crosslinking densities by reversible addition fragmentation transfer (RAFT) polymerization using a trithiocarbonate photoiniferter. Fixing the photoiniferter to crosslinker ratio and gradually increasing the targeted degree of polymerization (DPtarget) allowed for simultaneous control over the crosslinking density and the average molecular weight (Mn) of the primary chains, both below and above the critical molecular weight of entanglement (Mc). Interestingly, a plateau in storage moduli (G’) was observed for UHMW PDMAm hydrogels with a sufficiently high DPtarget (> 5,000), indicating a transition to the entanglement-dominated regime, with no contribution from crosslinks to the overall modulus, thus indicating the formation of highly entangled hydrogels. These hydrogels exhibit enhanced properties such as high toughness and resistance to swelling despite their vanishingly small crosslinking densities. Furthermore, even when equipped with cleavable crosslinkers, the UHMW PDMAm hydrogels resist degradation due to dense entanglements which act as transient crosslinks preventing the gels from swelling, while sparse covalent crosslinks help to maintain their structural integrity and avoid disentanglement. This approach allows simple synthesis of elastic and tough hydrogels with a well-defined structure and tuneable contributions from both crosslinks and entanglements.