Hydrogen Bonding Mechanism in Cononsolvency

Cononsolvency is an extensively debated phenomenon in ternary polymer solutions, where the polymer solubility decreases or even vanishes in a mixture of two good solvents. It is obvious that the cononsolvency phenomenon could not be explained by the Hildebrand solubility parameter-based prediction, which is usually used to describe the polymer solubility in solvents. Based on the Kamlet–Taft (KT) parameters (where α is the H-bond acidity parameter and β is the H-bond basicity parameter), we built a two-parameter model to explain the polymers’ solubilities in a previous work [Du, M.-X. Macromolecules 2022, 55, 4578–4588]. Interestingly, we found that several αβ-type polymers (with both H-bond acidic and H-bond basic groups) can dissolve in both strongly H-bond acidic (αA) solvents A and strongly H-bond basic (βB) solvents B but precipitate in A/B mixed solvents. Based on the two-parameter model, 16 cononsolvency systems have been successfully predicted in this paper. These systems possess the same properties: solvent A is strongly H-bond acidic (αA), solvent B is strongly H-bond basic (βB), and the A/B mixed solvents exhibit a high mixing enthalpy (−6 to −9 kJ/mol). The consistency of the solubility experimental results and the predictions by the two-parameter model analysis (αAβB > 0) suggests that the cononsolvency is caused by the strong H-bond interaction between solvents A and B (αAβB). The H-bond mechanism proposed in this work is expected to predict other cononsolvency phenomena in the future.