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 (T-d,T-i), refractive index (RI), crystallinity as well as glass transition temperature (T-g). Surprisingly, the T-d,T-i, and RI were not always increasing with the increase of CuSO4 content. Instead, a maximum point happened for both T-d,T-i, and RI at a CuSO4/PVA weight ratio of 1%. Furthermore, the lowest level of crystallinity and a turning point in the T-g similar to 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.

Automated summary

The interaction between macromolecules and metal ions is significant in biological macromolecules, metallo-supramolecular polymers, and polymer/salt composite systems. However, the understanding of coordination bonds in polymer/inorganic salt complexes in a solvent-free state is limited, particularly in differentiating coordinate covalent bonding from ion-dipole interactions.