# Metal-Coordinated Polymer-Inorganic Hybrids: Synthesis, Properties, and Application
## 2 Methods of Synthesizing Coordinated Polymer-Inorganic Hybrids
### General Synthetic Route for Supramolecular Coordination Complexes (SCCs)
#### 2.3.1 Vapor Phase Infiltration (VPI)  
In vapor phase infiltration, a gas phase inorganic precursor infiltrates directly into the polymer melt host to form a polymer-inorganic hybrid [55]. VPI originates from atomic layer deposition (ALD); however, here, the precursor is allowed to \"soak\" for a prolonged period to allow infiltration into the polymer. The conversion of the hybrid into an oxide is then conducted by introducing an oxidizing agent, such as water, in a process known as sequential infiltration synthesis (SIS) [56]. In some literature sources, similar processes are introduced as sequential vapor infiltration (SVI), and multiple vapor infiltration (MPI) [57]. The fundamental idea of VPI is chemisorbing a precursor inside a polymer template in a facilitating environment, typically consisting of water vapor [58], ozone [59], or hydrogen peroxide vapor [60]. A few distinct differences between ALD and VPI usually extend to the requirements of the substrate: polar functional groups, relatively long deposition cycles, a high deposition rate, a nano to micro scale growth rate, the choice of the precursor, and the requirements of post-processing steps [57; 61; 62]. VPI is commonly used to enhance the mechanical properties of polymer matrices and etching efficiency [39; 63; 64]. The ALD and SIS processes are shown in Figure 3a [65]. Figure 2: Synthetic approaches for the MPNs. (**A**) Post-assembly polymerization approach: metallacycles or metallacages with multiple reactive sites are first formed through coordination-driven self-assembly. These structures then undergo crosslinking at their reactive sites to create polymer networks with metallacycles/metallacages acting as crosslinking nodes. (**B**) Post-polymerization assembly approach: polymer chains with coordination sites are synthesized first, followed by coordination-driven self-assembly to form crosslinked MPNs. Red represents the metal ion, green represents the organic linker and blue represents the polymer. Adapted with permission from the Royal Society of Chemistry [52]. The infiltration is carried out in three main steps (Figure 3b). First, the polymer template will be exposed to the precursor gas molecules, and sorption of gas molecules into polymers occurs on the surface. This is followed by diffusion of the gas molecules into the polymer matrix, and the concentration of the precursor determines the fraction of material transformed. Precursor-polymer solubility enhances infiltration, but this is not a requirement because VPI is performed after cycles of ultra-high vacuum, outgassing the polymer melt. This increases the free volume in the polymer matrix, permitting the diffusion of gas molecules despite limited solubility. This is a complex physiochemical process that is often simplified with classical continuum thermodynamics. In the case of pure dissolution, [PERSON]'s law is used:  
\[C=S.p. \tag{1}\]  
Here, \(C\), the concentration of a penetrant molecule in a polymer, depends upon the partial pressure (\(p\)) of the penetrant species and its solubility coefficient (\(S\)) [66]. This solubility coefficient scales exponentially with temperature (\(T\)) according to the Van 't Hoff equation [67]:  
\[S=S_{o}\textit{exp}\bigg{[}-\frac{\Delta H_{\mathrm{s}}}{kT}\bigg{]} \tag{2}\]  
where \(S_{o}\) is a scaling constant, \(k\) is Boltzmann's constant, and \(\Delta H_{\mathrm{s}}\) is the partial molar enthalpy of sorption. While these equations do not provide an atomic-scale depiction of the dissolution phenomenology, they are useful for experimentally evaluating sorption processes [68]. This diffusion process is closely related to principles established in gas transport studies, where four primary categories of transport mechanisms are recognized: viscous flow, Knudsen diffusion, molecular sieving, and solution-diffusion [69; 70]. To successfully complete the vapor phase infiltration (VPI) process, precursor molecules must be trapped within the polymer matrix.