||Poly(n-isopropylacrylamide) (PNiPAAm) is one of the well-known stimuli-responsive polymers that respond to their environment by undergoing changes in their chemical and/or physical properties. PNiPAAm has a lower critical solution temperature (LCST) at near the physiological body temperature. Therefore, it has been used for many fields including sensing, drug delivery, tissue engineering, and so one. However, it is difficult to apply to the tumor-targeted drug because the cloud point temperature of the cancer cell is higher than the physiological body temperature. To overcome this drawback, many researchers have tried to develop PNiPAAm-based copolymers. In this study, we synthesized the poly(NiPAAm-co-acrylic acid) [P(NiPAAm-co-AAc)] copolymer and analyzed its phase transition mechanism under change temperature using IR spectroscopy and 2D correlation spectroscopy. The details of this study will be discussed in this presentation.