# 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  
Previous research has demonstrated that thermophysical properties are modified synergistically when polymeric ligands occupy sites in the coordination sphere of the transition metal [100; 101]. Many studies investigate the thermal stability of the compounds formed from P4 VP and P4 VP grafted with other polymers coordinated to transition metal salts. [PERSON] et al. [102] investigated the impact of different metal salts, such as zinc (Zn\({}^{2+}\)), copper (Cu\({}^{2+}\)), and magnesium (Mg\({}^{2+}\)), on the glass transition temperature, Tg, of P4 VP. The findings reveal that the type and geometry of the metal ion's coordination with the pyridine rings of P4 VP play a significant role in determining the extent of Tg enhancement. Among the tested metals, Zn\({}^{2+}\) produces the most significant increase in Tg, achieving a maximum 70 \({}^{\circ}\)C rise at a loading of 0.25 molar equivalents, while other metals produce varied effects based on their unique coordination properties. Zn\({}^{2+}\) forms a tetrahedral coordination complex (Figure 4a) with P4 VP, which has a notable effect on the polymer's Tg. This tetrahedral coordination creates a compact and highly crosslinked structure that restricts the mobility of polymer chains, resulting in the largest increase in Tg observed in the study. Cu\({}^{2+}\) increases the Tg of P4 VP, though less effectively than Zn\({}^{2+}\) due to the formation of square planar coordination, which results in a less compact crosslinking structure. This geometry increases the distance between polymer chains, creating a lower level of rigidity and a smaller Tg enhancement compared to Zn\({}^{2+}\). On the other hand, Mg\({}^{2+}\) does not raise the Tg significantly, likely due to its smaller ionic radius and distinct coordination preferences that do not promote the same degree of crosslinking as Zn\({}^{2+}\). Monovalent ions like Na\({}^{+}\) and K\({}^{+}\) have a lesser impact on the Tg, as they form primarily ionic interactions with P4 VP rather than strong coordination bonds. These ions cannot create a crosslinked network within the polymer and thus show minimal Tg enhancement. Figure 4: (**a**) Possible tetrahedral coordination complexation of ZnCl\({}_{2}\) with P4 VP [102]. (**b**) A ruthenium bridge between pyridine ligands on two different polymer chains illustrates the concept of coordination crosslinking [96]. (**c**) Schematic representation of P4 VP–metal complexes: sulfate anions coordinated with Cu(II) in monocordinated and bridged forms. (**d**) Thioocyanate coordinating through nitrogen, showing structural stabilization within the polymer matrix [93]. (**e**) Preparation of the PVP–DVB–Cu(II) complex [103]. Adapted with permission from Elsevier [93; 96; 103]. [PERSON] et al. [96]. examined the effect of coordination between P4 VP and dichlorotri-carbonylruthenium(II) (RuCl\({}_{2}\)(CO)\({}_{3}\))\({}_{2}\). The Ru-based P4 VP complex exhibited an enhancement of Tg by up to 50 \({}^{\circ}\)C [96]. This is driven by a few key mechanisms centered around the unique coordination properties of the Ru ion. Primarily, ruthenium's ability to form strong coordination crosslinks with the nitrogen atoms on the pyridine rings in P4 VP significantly increases the polymer's thermal stability. Each Ru ion has the capacity to bridge two pyridine groups from different polymer chains, creating a network of coordination crosslinks. These crosslinks act as physical restrictions within the polymer matrix, reducing the mobility of the chains and thereby raising the Tg. This restriction of movement at a molecular level forces the system to maintain its solid-like structure until higher temperatures are reached, enhancing the Tg. Additionally, the electronic configuration of the Ru ion plays a crucial role in this effect. Here, ruthenium is in a d\({}^{6}\) octahedral configuration, which is particularly stable due to ligand field stabilization energy.