Polyvinyl pyrrolidone is a vinyl polymer built from N-vinylpyrrolidone units, in which a lactam ring hangs from each backbone carbon [1]. The amide carbonyl and the ring nitrogen make the polymer a strong hydrogen-bond acceptor and strongly hygroscopic, and the same polarity makes it soluble in water and in alcohols while insoluble in hydrocarbons [2]. Because the lactam is non-ionic, the polymer carries no charge at any pH and its solutions stay stable across the pH range [1]. The ring also complexes iodine and phenolic compounds, and the chain adsorbs onto polar surfaces through the carbonyl oxygen [3]. Molar mass spans several orders of magnitude, and the chain-length grade rather than the chemistry sets the viscosity of a solution [2].