Poly(N-isopropylacrylamide) hydrogels with improved shrinking kinetics by RAFT polymerization

Functional poly(N-isopropylacrylamide) (PNIPAM) hydrogels were prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) in the presence of N,N-methylenebisacylamide (BIS) as a cross-linker and 4-cyanopentanoic acid dithiobenzoate as chain transfer reagent (CTA). The swelling behaviors were investigated and the hydrogels by RAFT polymerization (RAFT gels) showed accelerated shrinking kinetics and higher swelling ratio comparing with conventional hydrogel (CG). It could be attributed to the presence of dangling chains mainly caused by CTA, which could retard the crosslinking reaction rate greatly. Another CTA, 3-(trithiocarbonyl) propanoic acid, was adopted to further investigate the effect of CTA. It showed the similar effect except the different accelerated degree to the shrinking kinetics. Furthermore, the living character of the RAFT process was used to polymerize a new batch of monomer (NIPAM) from functional RAFT gels to introduce grafted structure. The PNIPAM-g-PNIPAM hydrogels indicted further accelerated shrinking kinetics than functional backbone hydrogels.