## 2 Hypercrosslinked Polystyrene (HPS): Synthesis, Properties, and Applications
The range of manufactured HPS includes neutral sorbents with various porous structure, strong and weakly basic anion exchangers and cation exchangers, sorbents for high-performance liquid chromatography (HPLC) and cartridge fillers for solid-phase extraction. Their combined abilities of quickly absorbing significant amounts of different gases and thermodynamically compatible and incompatible solvents with their capacity for rapid regeneration makes hypercrosslinked polystyrene promising adsorption materials, including for their uses in liquid chromatography and solid-phase extraction [44]. There is a significant amount of research on the application of HPS-based sorbents in medicine. Thus, [PERSON] et al. [45] used HPS Diapak P-3 (Biokhimak, Russia) for the adsorption of catecholamines (adrenaline, norepinephrine, and dopamine), which can serve as biomarker of heart disease, diabetes, schizophrenia, etc. The average recovery of catecholamines from HPS was 89-98%, showing a promise for isolation and concentration of catecholamines from aqueous media (including biological fluids). Hemosorption is an actively developing method of removing toxic substances from humans by pumping the blood stream through a cartridge filled with a sorbent, including one based on HPS [46]. This procedure is used not only for emergencies (poisoning and severe infections), but also for the treatment of chronic diseases (asthma, allergies, pancreatitis, etc.) [26; 47; 48]. For example, in [49] a new adsorbent based on hypercrosslinked polystyrene modified with pyridinyl--HCP (St-DVB-VP)--was used to remove bilirubin and medium/large-molecular toxins (IL-6 and PTH) from a liver. In addition to good adsorption properties towards toxins, this sorbent showed a low rate of hemolysis, a low risk of blood clotting, and acceptable hemocompatibility. [PERSON] et al. [50] isolated nine aromatic microbial metabolites from blood, using the microextraction by packed sorbent (MEPS) method in combination with an HPS-based sorbent. Based on the results obtained, the authors concluded that the use of HPS in commercial MEPS devices is promising for the analysis of biological samples for early diagnosis of sepsis and for monitoring the effectiveness of treatment. The use of HPS for preconcentration of analytes made it possible to reduce the detection limit of antibiotics of the tetracycline group (tetracycline, oxytetracycline, chlortetracycline, and doxycycline) by 90-100 times (0.6-2.0 ng/mL) [51]. [PERSON] et al. assessed the adsorption properties of a magnetic adsorbent based on hypercrosslinked polystyrene (HCPS-Fe\({}_{3}\)O\({}_{4}\)) towards tetracycline antibiotics [52]. The authors showed that HCPS-Fe\({}_{3}\)O\({}_{4}\) retains adsorption properties towards tetracyclines and is easily separated from the solution by applying a magnetic field. It should be noted that materials with magnetic properties, including sorbents and catalysts, increasingly find applications in various fields of research and industry [53; 54; 55; 56; 57]. From this point of view, magnetic HPS also deserve attention, since they combine the technological advantages of magnetic materials and HPS' high adsorption capacities [44; 58]. There are several options for the preparation of magnetic HPS: (1) synthesis of a polymer network in the presence of magnetite nanoparticles (Fe\({}_{3}\)O\({}_{4}\)) [59; 60]; (2) synthesis of magnetic nanoparticles directly in the polymer matrix, for example, via the precipitation of magnetite [61]; and (3) sorption of presynthesized magnetite nanoparticles on the HPS surface [44]. Thus, [PERSON] [62] synthesized magnetic composites using the industrial polymer adsorbents Macronet MN270, 200, 202, and 600 as a basis; these belong to the class of hypercrosslinked polystyrene, styrene-divinylbenzene sorbent Amberlite XAD4 and a hypercrosslinked copolymer of styrene and divinylbenzene. Magnetic sorbents were obtained by immobilizing magnetite nanocrystallites in the pores of HPS via chemical deposition.