Thermosensitivity of poly(N-vinylpyrrolidone): Effect of transition-metal halides

The effect of the halides of a series of transition metals on the properties of aqueous solutions of poly(N-vinylpyrrolidone) is studied. It is shown that the introduction of ZnCl2 into solutions of poly(N-vinylpyrrolidone) causes phase separation with an increase in temperature. In the presence of halides of other transition metals (CdCl2, CuCl2), this phenomenon is not observed. However, the addition of HCl and/or alkali-metal halides to these systems likewise gives rise to phase separation, and in the ZnCl2-based systems, the cloud point of solution substantially decreases with the introduction of alkali-metal salts. The same situation is observed in the case of polyvinylcaprolactam. Thus, it is concluded that the observed phenomena are associated with the formation of MtCl 4 2− complex anions in solutions that promote the globulization of polymer chains with participation of water molecules surrounding polymer chains and cations of transition-metal salts bound to complexes with C=O groups of lactam rings of the polymers.