## 2 Hypercrosslinked Polystyrene (HPS): Synthesis, Properties, and Applications
It has been shown that synthesized composites based on industrial biporous HPS are capable of adsorbing well organic compounds of various classes: aliphatic and aromatic hydrocarbons, alcohols, and ethers. The sorption value of particularly toxic organic compounds--dioxane, benzene, and carbon tetrachloride--reaches 0.8 mL/cm\({}^{3}\). Furthermore, based on the industrial sorbent Macronet MN270, but using a different technique based on the thermal destruction of iron salts, a magnetic composite Fe\({}_{3}\)O\({}_{4}\)/HPS MN270 was synthesized. It was characterized by a high saturation magnetization value (4.5 emu/g) and a magnetite particle size of 40 \(\pm\) 5 nm [63], which was used for the synthesis of magnetic heterogeneous catalysts. A convenient method has been proposed for producing a Fe-containing HPS composite based on a waste foam plastic [64]. This is a fast method that uses readily available reagents and solves the problem of disposing of bulky foam waste. The resulting product can be used for the sorption of hydrogen sulfide and toxic organic solvents, as well as the decomposition products of natural residues--putrescine, cadaverine, indole, and skatole. Over the last decades, science and industry have accumulated significant knowledge in the use of HPS as a sorbent for chromatography and technological processes for the purification of aqueous and organic media. However, new materials appear constantly. Thus, [PERSON] et al. [65] reported the synthesis of hypercrosslinked polystyrenes (sPSs)-HCL-X prepared from syndiotactic polystyrene (sPS) aerogels via Friedel-Crafts alkylation. sPSs have a hierarchical porous structure on three scales: macroropes connecting to macroropes, mesopores, and micropores. HCL-X possesses SSA up to 931 m\({}^{2}\)/g and the ability to instantly absorb large amounts of organic liquids (up to 61 mL/g). The authors presented HCL-X as a good candidate for removing oils from oil-water mixtures and for producing carbonates with hierarchical pores. The functionalization of HPS opens up possibilities for the synthesis of selective sorbents. For example, [PERSON] et al. [66] reported the synthesis of hypercrosslinked microporous poly(para-methoxystyrene) (HCPMOS) functionalized with methoxy groups, capable of selectively extracting Fe\({}^{3+}\) from aqueous solutions even at a very low concentration of 10 ppm. The authors attribute the high selectivity of HCPMOS towards iron cations to their stronger interactions with methoxy groups as compared to cations of other metals. A number of methylamine-modified hypercrosslinked resins have been proposed for the selective sorption of citric acid [67]. During the periodic sorption of citric acid, the methylamine-modified HM-65-2 resin showed a high adsorption capacity (136.3) mg/g and a selectivity of 6.98 (citric acid/glucose), which makes this sorbent a promising candidate for the purification of citric acid and further use in the separation process with simulating a moving bed (SMB). This method is used to separate one or more chemical compounds from one or more other chemicals and allows the production of large quantities of highly purified material at a reduced cost. In the SMB technique instead of moving the bed, the feed inlet, the solvent or eluent inlet, and product exit positions are moved continuously, giving the impression of a moving bed, with continuous flow of solid particles and continuous flow of liquid in the opposite direction of the solid particles. SMB is increasingly applied as a separation technique in the pharmaceutical industry, production of fine chemicals, and in the field of bioengineering [68]. A very promising area of practical application of HPS is the adsorption and storage of gases, primarily carbon dioxide [69].