# Metal-Coordinated Polymer-Inorganic Hybrids: Synthesis, Properties, and Application
## 3 Polymer-Inorganic Coordinated Hybrids: Complexation-Induced Properties
### Pyridine-Based Polymers
#### 3.1.1 Thermal Stability of Coordination Complexes of PVPs with Metal Ions
This stabilization energy arises from the strong interaction between the d-electrons of Ru and the surrounding ligands. An important aspect of this coordination involves \(\pi\)-backbonding, where electron density from Ru's filled d-orbitals is donated back into the \(\pi^{\star}\) antibonding orbitals of the carbonyl ligands. This \(\pi\)-backbonding effect stabilizes the coordination complex, lowering the energy of the tgg molecular orbitals and enhancing ligand field splitting (Figure 4b). In addition to polymer-metal interactions, it has been shown that the coordination environment of the complex could impact thermal degradation. [PERSON] et al. [93] demonstrated that the degradation temperatures of the P4 VP-Cu(II) complexes are generally lower than that of pure P4 VP using thermogravimetric analysis (TGA). This reduction is likely due to alterations in the electronic density of the pyridine ring upon complexation, which reduces resonance-induced stabilization. The shift in the CN stretching band to higher wavenumbers, as observed in infrared spectroscopy, confirms these electronic density changes. Furthermore, coordination with Cu(II) ions, varying the anions, such as copper sulfate, chloride, and thiocyanate, significantly alters the degradation temperature of P4 VP. For instance, sulfate complexes showed higher thermal stability compared to chloride and thiocyanate complexes. This is because the Cu(II)-sulfate complex (Figure 4c) coordinates with the nitrogen atoms of the pyridine rings in P4 VP while also interacting with the sulfate ion in a monocoordinated or bridging configuration. This structure results in a symmetry change from the typical tetrahedral (Td) configuration of free sulfate ions to a lower symmetry, either C\({}_{2}\)v or C\({}_{3}\)v, due to coordination. This monocoordination or bridging arrangement stabilizes the complex by reinforcing the polymer's crosslinked network, enhancing rigidity and limiting chain mobility, contributing to higher thermal stability. In contrast, the Cu(II)-thiocyanate complex (Figure 4d) does not form a bridge, leading to a less crosslinked structure compared to the sulfate complex. Similarly, [PERSON] et al. [103] showed that the degradation process of poly(4-vinylpyridine-\(\omega\)-divinylbenzene) block copolymer (PVP-b-DVB) begins at a higher temperature compared to the Cu(II)-complexed version, indicating that Cu(II) ions cause an initial reduction in thermal stability. This effect is thought to result from electronic density changes around the Cu(II) coordination sites, which weaken certain bonds in the polymer and lead to earlier degradation. The PVP-b-DVB copolymer primarily undergoes a two-stage degradation process, whereas the PVP-DVB-Cu(II) complex exhibits a three-stage degradation under a nitrogen atmosphere. This additional degradation stage, observed specifically in the Cu(II)-complexed samples, is associated with the breakdown of sulfate groups and Cu(II)-pyridine coordination bonds. Unlike thermal degradation, the Tg increased concomitantly with greater concentrations of Cu(II) ions, and this is attributed to the formation of crosslinks within the polymer matrix. The preparation of the complex and coordination of Cu(II) ions with pyridine groups are shown in Figure 4e.