HCPs are known as one of the most important amorphous polymers possessing great surface areas and low densities, which are synthesized applying disparate chemical procedures (i.e., Friedel − Crafts alkylation chemistry) from other POMs. Moderate reaction conditions, cheap reagents, and ease of scale up are the privileges of HCPs, which make them promising in gas separation, catalysis and removal of aromatic molecules from water (Wood et al., 2008; Xu S. et al., 2013; Masoumi et al., 2020). Wang et al. evaluated the adsorption efficacy of CO₂ applying different types of HCPs functionalized by ethylenediamine (EDA). Based on the influence of amine functionalization, they perceived that the selectivity of amine functionalized HCPs (HCPs-A) significantly enhanced and reached to 85.71 and 8.12 for CO₂/N₂ and CO₂/CH₄ gaseous mixtures due to increased specific surface area and microporosity (Wang et al., 2020).

The prevalent techniques applied for the synthesis of HCPs are (Xu S. et al., 2013; Tan and Tan, 2017; Yang et al., 2020):

A) Post-cross-linking (PCL) of polymers;

The PCL procedure starts by polymeric precursors’ dissolution in solvent. Whenever the swelling process initiates, the polymeric chains are released from a tangled or twisted state and the free area between them is occupied by the solvent. Then, the polymeric chains are exposed to cross-linking process. After solvent removal, the polymeric chains are disassociated using the cross-links, finally eventuating in the preparation of an inter-linked porous polymer. Discovery of Davankov resins has eventuated in an instant advancement in the design, characterization, synthesis and application of HCPs. The most noteworthy privilege of DOP is the direct application of commercial-based accessible polymeric products as precursors for PCL process (Tan and Tan, 2017). Various synthetic procedures consisting of self or co-condensation of chloromethyl/hydroxymethyl-based monomers have been offered for the appropriate synthesis of HCPs. However, all of the abovementioned procedures suffer from certain drawbacks like the need of great amounts of organic reagents and solvents following with energy cost for further purification steps. An outstanding development is ECL strategy in which active formaldehyde dimethyl acetal is used as an external crosslinker to blend simple aromatic components such as benzene with rigid methylene bridges by means of the anhydrous FeCl₃ catalyzed Friedel–Crafts reaction (Tan and Tan, 2017). Application of external cross-linkers as the third synthesis procedure, which has resulted in increasing the variety of HCPs (Germain et al., 2007). Schematic demonstration of hyper cross-linked polystyrene is presented in Figure 2.

PIMs are known as the polymers with amorphous microporosity and rigid main chains. Appropriate solubility in organic solvents, great surface area and their film formation characteristics eventuate in the development of PIMs in industry (McKeown, 2012; Agarwal et al., 2020; Perry et al., 2020; Polak-Kraśna et al., 2021). PIMs are an excellent instance of how changing a co-monomer makes a considerable impact on the porosity of polymer. PIMs are characteristically different from other classes of porous polymeric materials. PIMs have microporosity but do not include designated frameworks (Perry et al., 2020). Figure 2 presents a schematic illustration of the phthalocyanine-based PIM. In an investigation, Budd et al. studied the synthesis process of a hexaazatrinapthylene (Hatn)-based PIM. Based on their investigation (Hatn)-based PIM has a great capability to adsorb approximately 3.9 mmol g^-1 of the metal complex when exposing to an excess of palladium (II) dichloride in chloroform solution (Budd et al., 2003). Ling et al. fabricated a series of metalized PIMs from a carboxyl-based functionalized PIM (C-PIM). They corroborated that C-PIM-Na demonstrated the greatest CO₂ capture capacity of 2.44 mmol g^-1 compared to other metalized PIMs (Ling et al., 2020). Stanovsky et al. conducted an experiment with the aim of purifying flue gas applying the ultra-permeable tetramethyl tetrahydronaphthalene - based PIM coupled with bicyclic triptycene. Acceptable CO₂/N₂ selectivity (in the range of 11–18) encouraged the potential application of this PIM for industrial MCS (Stanovsky et al., 2020). Figure 3 schematically depict the process of PIMs fabrication.

The synthesis of PIMs is an important milestone toward the use of this type of POMs in membrane-based CO₂ separation. PIMs can be synthesized via a polymerization reaction on the basis of a double-aromatic nucleophilic substitution mechanism (Scherf, 1999). This reaction possesses great potential to form two simultaneous covalent bonds with appropriate performance to provide a linking group composed of fused rings (McKeown, 2012). Overally, aromatic nucleophilic substitutions can be of great interest to be done particularly if the halide-including monomer is activated via an electron-withdrawing substituent (i.e., –CN, F, etc.) (Eastmond et al., 2001).

COFs are known as one of the significant members of POMs, which are fabricated via molecular-structured blocks interconnected by covalent bonds. Figure 3 depicts the molecular structure of different COFs. Various features such as custom-made properties (achieved by functionalization), structural versatility and uniform pore size distribution have authorized the COFs to be used in an extensive range of applications like membrane-based gas separation and cancer treatments (Guan et al., 2020; Sharma et al., 2020; Guo et al., 2022). The remarkable development of COFs is prominently justified because of their self-healing capability and thermodynamically manageable covalent chemistry, which eventuate in long-range crystalline structure (Pachfule et al., 2018/01). Compared to MOFs, COFs usually have lower density and therefore show outstanding stability in organic solvents. Moreover, COFs have the ability to tolerate harsh situations and maintain their crystallinity. In comparison with inorganic zeolites and porous silica materials, COFs possess greater efficiency due to higher porosity and tunable pore size, which accelerate the penetrant diffusion (Cooper, 2013). In Figure 4, the molecular structure of different COFs is presented.

Noteworthy advantages of COFs like inherent porosity, adjusted channel structure, low density, great stability and designable functionality have made them suitable for application in different scientific fields like CO₂ separation, optoelectronics, drug delivery and adsorption (Wu and Yang, 2017; Altundal et al., 2020; Ghosh and Banerjee, 2023). In the case of membrane-based CO₂ separation, porous COFs have been of paramount attentions owing to their brilliant ability for the storage and separation of major greenhouse gases (i.e., CO₂), H₂ and methane (Xia and Liu, 2016).

CMPs/CPPs refer to an important category of polymeric materials that mix extended π-conjugation with a microporous structure. Generally, CMPs/CPPs are known as microporous polymeric materials but can be accompanied by great amount of mesoporosity (Lee and Cooper, 2020). CMPs/CPPs possess great potential to be extensively used in disparate applications (i.e., separation processes, heterogeneous catalysis, energy storage and so on) owing to their brilliant properties like high porosity, tunable chemistry, appropriate chemical resistance, and thermal stability (Talapaneni et al., 2016; Wang et al., 2017; Liao et al., 2018; Cao et al., 2022). Although gas separation and storage is regarded as the most prevalent area of investigation for CMPs/CPPs, some drawbacks such as the application of expensive transition metals in their synthesis deteriorates their popularity in large-scale separation applications (Lee and Cooper, 2020). Figure 5 represents a schematic demonstration of the synthesis process of core−shell structured CMPs/CPPs. CMPs can be well identified as the most efficient POMs for the separation of CO₂ greenhouse gas due to their noteworthy advantages including high surface area and tunable properties (Reddy et al., 2021; Zhang et al., 2023). The importance of CMPs is because of the necessary bond conjugation and amorphous morphology. Poly (arylene ethynylene)s were introduced for the first time in 2007 as the synthesized CMPs (Jiang et al., 2007).

CTFs can be defined as an important type of organic polymers fabricated by aromatic 1,3,5-triazine rings with planar π-conjugation properties (Wei et al., 2019; Liao et al., 2023). The occurrence of conjugation between aromatic rings and triazine rings significantly declines overall energy of π-conjugated molecules in the frameworks and therefore, significantly enhances the chemical stability. In the majority of cases, N-containing CTFs frameworks have shown their great potential in adsorption/separation and catalysis (Jadhav et al., 2019; Jiang et al., 2019). Most recently, CTFs have been appeared as a promising type of POMs, which is regarded as an adaptable platform for various applications due to their impressive characteristics such as permanent microporosity and appropriate thermal/mechanical stability and chemical resistance even in the strong acidic/basic environment.

The powerful covalent linkages, intrinsically great content of nitrogen atoms and excellent capability to add hetero-atoms in the structural skeleton have made CTFs versatile for numerous potential applications like gas separation and dye adsorption (Krishnaraj et al., 2020). Additionally, they are reproducible and recyclable, which permits them to be a noteworthy candidate in terms of sustainable materials. Figure 6 represents a schematic demonstration of photocatalytic CO₂ separation from formic acid using CTFs.

In the recent years, the industrial applications of membrane processes for the effective separation of CO₂ pollutant owing to their great potential to overcome the negative disadvantages of conventional technologies like cryogenic distillation and desorption (Dai et al., 2023; Yang et al., 2022/03; Shirazian et al., 2020; Faraji et al., 2020). One of the most efficient membrane-based approaches for the separation of CO₂ is the use of composite membranes. Composite membranes possess brilliant capacity to simultaneously combine the positive points of both inorganic and polymeric membranes, which make them a hotspot for scientific research (Pishnamazi et al., 2020/09). Composite membranes can be considered as mixed matrix membranes (MMMs), which are often used for the efficient separation of CO₂. Due to the presence of disparate challenges toward the use of inorganic or polymeric membranes, composite membranes have been employed to solve the existed limitations of both aforementioned membranes. The industrial use of MMMs is being significantly enhanced due to their ability to combine the compactness of polymeric membranes and great permeability of inorganic membranes (Saqib et al., 2019).

Figure 6 presents the schematic depiction of PAFs and their various applications in industry. PAFs are another category of POMs. PAFs, which are manufactured by carbon-carbon-bond-linked aromatic-based building units (CCBLABU), possess remarkable features such as rigid structures and great surface area. The presence of strong carbon−carbon linkage eventuates in enhancing the chemical resistance of this class of POMs in undesirable chemical environments (Tian and Zhu, 2020). Hence, PAFs illustrate excellent characteristics in chemistry and functionalities in comparison with traditional POMs like zeolites and MOFs. PAFs can be freely functionalized by severe chemical treatments (Tian and Zhu, 2020). Table 2 gives a comprehensive summarization about the characteristics of various POMs investigated in this review paper.