Revisiting coordinate bonding in non-aqueous polymer/metal ions complex

The interaction between macromolecules and metal ions plays an important role in biological macromolecules, Metallo-supramolecular polymers, and polymer/salt composite systems. However, the detailed information about the coordination bonds in polymer/inorganic salt complexes at solvent-free state was poorly understood. Particularly, studies of the differentiation between coordinate covalent bonding and ion-dipole interactions are lacking. Herein, we prepared a series of PVA-CuSO4 composite films with varying CuSO4 content via solvent casting to probe the effects of CuSO4 content on the properties of the PVA-CuSO4 composite films in terms of initial thermal degradation temperature (Td,i), refractive index (RI), crystallinity as well as glass transition temperature (Tg). Surprisingly, the Td,i, and RI were not always increasing with the increase of CuSO4 content. Instead, a maximum point happened for both Td,i, and RI at a CuSO4/PVA weight ratio of 1%. Furthermore, the lowest level of crystallinity and a turning point in the Tg ∼ CuSO4 content curve were observed when the CuSO4/PVA weight ratio was 1%. These results consistently indicated that the composite with a CuSO4/PVA weight ratio of 1% formed the largest amount of coordinate bonds, while the further increase of CuSO4 content might lead to ion-dipole interactions, which could destroy the formation of coordinate bonds. Moreover, both XPS and UV–visible absorption spectrum showed that copper (II) ions were at different bonding states when CuSO4/PVA weight ratio was below and above 1%, proving the proposition that coordinate bonds in PVA-Cu2+ composite saturated at a CuSO4/PVA weight ratio of 1%. Taken together, our results provided evidences for using a novel perspective to understand the coordination bonds formed in polymer/inorganic salt complexes at solvent-free state. Such findings could not only help to enrich the fundamental research but can also guide the future development of advanced polymer/salt complexes.