# 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)
This entrapment typically occurs through chemical reactions between the precursor and functional groups in the polymer. However, other methods, such as reactions with a secondary precursor or physical trapping, can also be effective. The optimal scenario for VPI involves a tradeoff between diffusion and chemical bonding of the precursor molecules for complete infiltration into the polymer. Another strategy to achieve this moderate reactivity is by using polymer blends or copolymers, which combine highly reactive and non-reactive monomers, providing better control over the precursor's diffusion and eventual entrapment in the polymer matrix [68]. The choices of primary precursors and secondary precursors are critical in VPI. The polymer must have polar sites where the precursor can be chemisorbed [71]. Furthermore, the operating temperature must be below that of the glass transition (Tg) of the selected polymer. This puts a constraint on the primary precursor, as it should be able to maintain very high vapor pressure below the respective Tg values [72]. These temperatures typically vary between 100 and 150 \({}^{\circ}\)C.  
One of the common precursors used in VPI is trimethylaluminum (TMA), to synthesize aluminum oxide structures [58; 73]. Metal halides such as TiCl\({}_{4}\), MoCl\({}_{5}\), SnCl\({}_{4}\), alkoxides such as TIP, and metal alkyls like diethylzinc (DEZ) are also used in addition to TMA [55]. Polar polymers that can host such metal precursors range from biopolymers such as spider silk [63], egg collagen [64], polysaccharides [74], and cellulose [39], synthetic polymers such as Kevlar [75], epoxy-based photoresists [76], polyurethane [77], polyaniline [78], P3 HT, PMMA, PAA [38], and poly(vinylpyrrolidone), and block copolymers (BCPs) such as PS-b-PMMA [79] and PS-b-P4 VP [43]. A major advantage in VPI is the area selectivity of the precursor due to chemical selectivity. Block copolymers (BCPs) are a common candidate for area-selective VPI and have been utilized for making 3D porous materials due to their ability to assemble into hierarchical structures [80; 81; 82]. Infiltrating such structures with inorganic precursors makes inorganic patterns and porous structures obtainable (Figure 3c). One of the limitations of VPI lies in its reliance on the polymer template's uniformity. Unlike ALD, defects in the polymer may transfer into the final complex. As mentioned above, shrinkage of the porous structures could also be an issue for the quality of the final product. Some polymers may not have high infiltration dynamics, and additional pores are introduced by swelling [43]. These are prone to shrinkage in final processing due to mechanical instability. However, the major limiting factor is the Tg of the polymer materials. Due to the Tg ranging between 100 and 150 C for most polymers, the inorganic precursors must have high vapor pressure around the same temperature regime. This limits the library of inorganic precursors that can be used to infiltrate polymers using VPI [55].