A batch of poly(N-isopropylacrylamide-co-acrylic acid) synthesized in benzene shows distributions in molecular weight as well as polymer composition. The copolymer turns out to be a blend of polymers having various lon er critical solution temperatures. An aqueous solution of the copolymer above a critical concentration experiences four distinct phases upon heating. One of the phases is a gel phase that shows reversibility in gel-sol transition without noticeable hysteresis or syneresis. Once the gel is formed, it does not dissolve or change in water content until the temperature reaches another transition point. This particular process is related to the molecular weight, the chain entanglement, the molecular transition at elevated temperatures from expanded coils to collapsed globules of a portion of the polymers, and the subsequent aggregation of the collapsed globules to make weak physical junctions. The junctions can be discriminated from the physical cross-linking involved in gelation processes known so far. The thermally induced molecular aggregation of the collapsed polymer chains may be responsible for the gel opacity, negligible gel induction time, lack of hysteresis, and non-elastic deformation.