This paper describes the molecular structure dependent thermoresponsive behaviors of pyrrolidone-based water-soluble polymers. A series of well-defined poly[N-(2-methacryloyloxyethyl)pyrrclidone] (PNMEP), poly[N-(3-acryloyloxypropyl)pyrrolidone] (PNAPP), and poly[N-(3-methacryloyloxypropyl)-pyrrolidone] (PNMEP) were synthesized via visible light activating RAFT polymerization at 25 degrees C. Kinetic studies indicate a rapid and well-controlled behavior of this polymerization. Gel permeation chromatography (GPC) and H-1 NMR analysis confirm their intact molecular structure, well-defined molecular weight, and narrow distribution. Laser light scattering and temperature-variable H-1 NMR analyses demonstrate that the cloud point of a PNMEP sample at a degree of polymerization (DP) of 96 is 1.5 degrees C lower than that of PNAPP at a DP = 104. Additional backbone methyl groups in PNMPP lead to a dramatic cloud point lowering, e.g. cloud point of PNMPP at a DP = 100 is 37 degrees C lower than that of PNAPP at a DP = 104. This is contrary to what was observed in poly(N-isopropylacrylamide) (PNIPA) and its polymethacrylamide analogues. These pyrrolidone-based polymers show a dramatic solvent isotopic effect that is different from that of PNIPA; e.g., the cloud point of PNMEP at a DP = 237 is 8.5 degrees C lower in D2O than in H2O. Increasing polymer chain length or hydrophobicity may suppress this solvent isotopic effect. This phase transition is correlated to Hofmeister series but more sensitive than PNIPA. Na2CO3 dramatically lowers cloud point, while NaI significantly improves cloud point, up to hill dissolution in H2O at 95 degrees C. The solvent isotopic effect in NaCl or Na2CO3 solution is the same as what observed in solution absent of salt. Upon heating D2O solution of PNMEP, the polymer first forms the hydrated irregular colloidal aggregates near the cloud point, the phase transition occurs at the fully hydrated state at cloud point, and further heating leads to the dehydration and separation from D2O. However, in NaCl solution, the dehydration or PNMEP occurs subsequently from apolar backbones, spacers, and finally pyrrolidone groups.