## 2 HCL Polystyrenes
### External Crosslinkers, Catalysts and Solvents  
The first external crosslinker that was used in the hyper-crosslinking step was chloromethyl ether. Other reagents, such as carbon tetrachloride, monochlorodimently ether, dichloroxylene, or dichloroethane, which can act both as an external crosslinker and a solvent, were also reported [51, 52, 53, 54, 55]. More recently, [PERSON] et al. [56] proposed the use of formaldehyde dimethyl acetal, which gives methanol as a by-productinstead of chlorinated compounds. The procedure proposed allowed for preparing HCP in a one-step procedure, dissolving the aromatic monomers, FDA, and the Friedel-Crafts catalyst in the proper solvent. By adjusting the type of monomers and the amount of FDA, the largest SSA value achieved was \(\sim\)1400 m\({}^{2}\)/g, for a FDA:benzene molar ratio of 3:1. Through the knitting technique, [PERSON] and co-workers [57] obtained high surface area flexible polystyrene membranes, starting from commercial PS membranes, using FDA as a crosslinker and FeCl\({}_{3}\) as a catalyst. Their procedure was based on the evidence that the external layers of the PS membranes first react very fast with FDA, producing a microporous insoluble shell, and then hyper-crosslinking proceeds inside the membrane, while part of the PS dissolves and moves from the core to the shell, finally creating a sandwich-structured polymeric molecular sieve membrane with a hierarchical and tailorable porosity (see Figure 3). The hyper-crosslinked PS membranes displayed specific surface areas from 260 to 792 m\({}^{2}\)/g, increasing as the membrane thickness decreased from 109 to 86 \(\upmu\)m. In fact, as the hyper-crosslinking process begins from the outside of the membrane, it proceeds more easily on thinner ones, due to a faster mass transport, resulting in higher specific surface area. Moreover, all of the membranes showed hierarchical porosity and predominant microporosity. In particular, they showed similar pore size distribution, with a dominant pore size of about 5 A, and mesopores size in the range 2-4.4 nm. Figure 3: Synthesis of the hyper-crosslinked porous polymeric membranes. (**a**) Schematic illustration of the preparation procedure for the porous polymeric membranes; (**b**) the non-equilibrium diffusion at the interface (the reaction is only shown for one side of the membrane); and, (**c**) SEM images of the cross-section of the porous polymeric membranes crosslinked for 24 h (scale bar 20 \(\upmu\)m). Reprinted by permission from Macmillan Publishers Ltd. (Basingstoke, UK): Nature Communications [57], copyright 2014. Another type of hyper-crosslinked PS membrane was prepared by [PERSON] and co-workers [58]. They obtained soluble HCP by inducing intramolecular crosslinking of PS. They exploited the fact that at very low concentrations the polystyrene precursor is highly dispersed and expanded, which favours the intramolecular crosslinking with respect to intermolecular knitting. This soluble HCP showed relatively lower surface area but, on the other hand, was solution-processable to produce thin hyper-crosslinked membranes. Friedel-Crafts catalysts that are suitable for the hyper-crosslinking reactions are Lewis acids or protonic acids. In principle, all of the Lewis acids could be used, but poor solubility and steric hindrance of catalysts are some factors that can limit an extensive hyper-crosslinking. The latter is a particularly relevant issue when considering that, after the first crosslinks form, larger catalysts may be obstructed from accessing the chloromethyl moieties and activate the formation of further crosslinks [59]. Hyper-crosslinking is typically performed in a thermodynamically favourable solvent for the polymer, in which the precursor resin may dissolve or swell, depending on the linear or lightly crosslinked structure. The extensive crosslinking locks the polymer chains while they are in an expanded form, preventing them from collapsing after solvent removal, thus creating a three-dimensional structure that is made of interconnected pores.