Strong Cationic Radical Initiator‐Based Design of a Thermoresponsive Hydrogel Showing Drastic Volume Transition

In this study, a millimeter‐sized thermoresponsive poly(N‐isopropylacrylamide) (PNIPAAm) gel is prepared by free radical polymerization using 2,2′‐azobis‐[2‐(1,3‐dimethyl‐4,5‐dihydro‐1H‐imidazol‐3‐ium‐2‐yl)]propane triflate (ADIP), a radical initiator possessing a strong cationic group that is newly developed. By comparing this gel with a PNIPAAm gel prepared using a conventional radical initiator, 2,2′‐azobisisobutyronitrile, the following properties are found: 1) the cationic residue of ADIP is introduced in the gel, and 2) the PNIPAAm gel prepared using ADIP exhibits a discontinuous volume transition as a function of temperature and rapid shrinking in response to temperature jump. This thermoresponsive behavior is attributed to the low polymer fraction in the gel prepared using ADIP. The ADIP‐based design of the thermoresponsive hydrogel investigated herein may contribute to the design of functional soft materials. A millimeter‐sized thermoresponsive poly(N‐isopropylacrylamide) gel is prepared by free radical polymerization using 2,2′‐azobis‐[2‐(1,3‐dimethyl‐4,5‐dihydro‐1H‐imidazol‐3‐ium‐2‐yl)]propane triflate (ADIP), a radical initiator possessing a strong cationic group. The obtained thermoresponsive gel shows the following properties: 1) the cationic residue of ADIP introduced in the gel, and 2) a discontinuous volume transition as a function of temperature, and rapid shrinking in response to temperature jump.