Poly(3-ethyl-N-vinyl-2-pyrrolidone) (C2PVP) is a new type of thermoresponsive polymer which exhibits phase separation in water above the lower critical solution temperature (LCST) at about 26 °C. The thermoresponsive mechanism of C2PVP during the heating–cooling cycle has been investigated by means of temperature-dependent FTIR in combination with a two-dimensional correlation (2Dcos) technique and density functional theory (DFT) calculations. Compared with a secondary-amide thermoresponsive polymer such as poly(N-isopropyl acrylamide) (PNIPAM), the unusual phenomenon of an asymmetrical-to-symmetrical transition of the CO peak shape is observed in the conventional IR spectra of tertiary-amide polymer C2PVP. 2Dcos results confirmed that two species of CO groups (CO⋯2D2O and CO⋯D2O) mainly coexist at the initial heating, and they change to the other two CO groups involving the free CO and CO⋯DOD⋯OC after heating, with the latter predominant above the LCST. A slight hysteresis is observed in the cooling process attributed to the existence of a weak “crosslinking” point originating from the structure of CO⋯DOD⋯OC. Based on 2Dcos results, the dehydration process of different groups could be described in the following order: ethyl groups > CO groups > CH2 groups in ring (“>” means prior to), and the reversible sequence is observed during hydration process. In PNIPAM system, CH2 groups usually change prior to CO groups, while for C2PVP the dehydrated rate of CH2 groups (in ring) lags behind that of CO groups because of the steric hindrance of pyrrolidone structures. It is worth noting that cononsolvency of C2PVP can be found in water/methanol mixed solvents. The LCST of C2PVP in water/methanol mixture shifts from 29.5 to 16 °C as the methanol volume fraction reaches 50% and the phase separation totally disappears when the content of methanol is higher than 60%.

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