## 3 Hypercrosslinked Polystyrene in Heterogeneous Catalysis  
One of the promising areas for practical use of porous organic polymers, including hypercrosslinked polystyrenes, is heterogeneous catalytic systems [73]. The large pore volume, large specific surface area, wide possibilities for functionalization, and the ability to control the particle size of the active phase are responsible for the rapid growth in the number of reports on the synthesis of polymer-based catalysts, as evidenced by recent publications [74; 75; 76; 77]. Different catalytic systems based on porous polymers have been reported, like sulfonated solid acid catalysts for hydrolysis, dehydration, and esterification processes [78; 79; 80; 81], systems for photocatalysis [82; 83], hydrogenation [84], and many others. Despite the multiple problems (structural damage, leaching of active metals, swelling, difficult access of large substrate molecules to active centers in micropores, etc. ), it is expected that hypercrosslinked polymers will make a significant contribution to the development of novel heterogeneous catalysts in the coming years [85]. The first reports on the metal nanoparticles stabilized in the microporous structure of HPS were made by [PERSON] et al. [86]. Impregnation of HPS with solutions of cobalt compounds followed by thermolysis at 200 \({}^{\circ}\)C led to the formation of discrete spherical Co nanoparticles with sizes in the range of 1-3 nm. The authors have shown that particle size is controlled by nano sized HPS cavities, which physically limit the increase in their size. One of the first examples of polymer-based hypercrosslinked polystyrene catalysts was a Pt-containing HPS [87]. The microporous structure of HPS made it possible to obtain Pt-nanoparticles of 1.3 nm. The catalyst was used in the oxidation of L-sorpose to 2-keto-L-gulonic acid, demonstrating selectivity for the target product of 98% at 100% conversion. Further studies on the synthesis of catalysts based on commercial HPS, namely micro/macroporous Macronet NM (Purolite, Llantrisant, UK), and different noble metals, showed that the catalytic activity of such systems depends, in particular, on the presence of macropores [88], chemical nature of the precursor of the active phase [89], the presence of functional groups in the HPS [90], and the type of solvent used for the reaction [91]. In general, such catalytic systems have shown higher efficiency as compared to the catalysts with traditional catalytic supports [29]. Further research on HPS-based catalysts is aimed on the synthesis with different active phase, HPS of different porosities and with functional groups, as well as on expanding the scope of reactions. Thus, 1% Pd on HPS was used in the hydrodeoxygenation of stearic acid giving a heptadecane yield of up to 97% [92]. A new method for the synthesis of Pd-containing catalysts based on biporous HPS (Macronet MN200, Purolite, Llantrisant, UK) via the reduction of [Pd(\(\pi\)-allyl)Cl]\({}_{2}\) with hydrogen in supercritical CO\({}_{2}\) has been proposed [93]. The catalyst showed a high activity in the hydrogenation of benzene and was used twelve times without a noticeable decrease in the conversion rate. The use of this catalyst for the hydrogenation of other substrates, like toluene, tetralin, and phenol, is possible. The industrial amine-functionalized crosslinked copolymer MN100 containing Rh nanoclusters showed high catalytic activity in the hydroformylation of olefins in supercritical CO\({}_{2}\) over six catalytic cycles without loss of conversion rate [94]. Ru-containing nanoparticles of mixed composition containing both oxide and metal components dispersed in HPS matrix (Macronet MN200, Purolite, Llantrisant, UK) were tested in the oxidation of D-glucose to D-gluconic acid [95]. The maximum selectivity for D-gluconic acid was 99.8% with a D-glucose conversion of 99%.