## 2 Hypercrosslinked Polystyrene (HPS): Synthesis, Properties, and Applications  
The year 2019 marked half a century since the synthesis of the first HPS samples, which are known as the third generation of polystyrene networks, or \"Davankov-type resins\" [25; 26; 27]. According to the synthesis method proposed by [PERSON], polystyrene chains are crosslinked in solution with rigid bridges (crosslinking agents)--bis-chloromethyl derivatives of aromatic hydrocarbons or monochlorodimethyl ether. The latter reacts with polystyrene in the presence of a Friedel-Crafts catalyst in two stages: (1) introduction of chlorophyll groups into polystyrene and (2) binding of phenyl rings by a methyl group. As a result, a rigid and relatively long bridge is formed, containing a fragment of the crosslinking agent and two phenyl rings of the original polystyrene [26]. Over the decades, the synthesis process has been improved and modified. For example, a one-pot preparation of HPS sorbents was recently proposed, which is based on the in situ formation of monochromodimenthyl ether (MCDE) through the reaction of methylal with acid chlorides of organic and inorganic acids, in particular, CH\({}_{3}\)COCl, (COCl)\({}_{2}\), SO\({}_{2}\)Cl\({}_{2}\), and POCl\({}_{3}\)[28]. The application scope of HPS has now expanded enormously. Methods and strategies for their synthesis have been developed in order to control the microporosity/mesoporosity ratio, to introduce functional groups into the polymer, to improve the selective sorption of gases, organic aqueous pollutants, etc. This new class of microporous materials presents high-performance characteristics [29]. Due its multiple advantages, such as a variety of synthesis methods, ease of functionalization, large specific surface area (SSA), use of inexpensive reagents [30], mild synthesis conditions, and chemical and thermal stability, HPS is increasingly attracting research interests [31]. HPS has become a universal platform for the synthesis of nanoporous materials with specified properties, which, in addition to traditional application in chromatography, gas adsorption, and sorbents for water treatment, have begun to be used in drug delivery systems, sensors, and heterogeneous catalysts [27]. Hypercrosslinked polystyrene is a very rigid polymer network with a special type of porosity (Figure 1). The pore size is quite small (about 20 A) and, according to the IUPAC nomenclature, is on the border of micro- and mesoporosity [32]. The nature of porosity and specific surface area of HPS strongly depend on the characteristics of the initial reagents [33], the method of polymer synthesis and is about 600-2000 m\({}^{2}\)/g or even more [34; 35; 36; 37]. The exceptional sorption properties of HPS are based mainly on hydrophobic and \(\pi\)-\(\pi\) interactions [38], but through functionalization the possibilities of using HPS as a sorbent are significantly expanded [39]. Its chemical modification can be carried out after polymer synthesis, for example, via treatment with concentrated sulfuric acid to sulfonate the surface [25; 40] or by introducing monomers containing functional groups directly during the synthesis of HPS [41; 42; 43]. Figure 1: Schematic presentation of porous structure of hypercrosslinked polystyrene (HPS) and its scanning ion conductance microscopy image. Due to its properties, HPS has gained great commercial popularity over the past decades. Many companies produce it for large-scale sorption processes in the chemical, food, and water treatment industries, for example, Purolite (Hypersol-Macronet, MN series), Dow Chemical (Optiipore), Lanzess (Lewatit VP OC 1163 and S 7768), and Jiangsu N&G Environmental Technology (NG-99 and NG-100), and for analytical purposes, such as International Sorbent Technology (Isolute ENV+), Merck (LiCholnut EN), Spark Holland (HySphene), Macherey Nagel (HR-P), etc. [26; 27].