349Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... DOI: 10.17344/acsi.2020.5901 Feature article Post Polymerisation Hypercrosslinking with Emulsion Templating for Hierarchical and Multi-Level Porous Polymers Amadeja Koler,1 Irena Pulko2 and Peter Krajnc1,* 1 University of Maribor, Faculty of Chemistry and Chemical Engineering, PolyOrgLab, Smetanova 17, Maribor, Slovenia 2 Faculty of Polymer Technology, Ozare 19, Slovenj Gradec, Slovenia * Corresponding author: E-mail: peter.krajnc@um.si Received: 02-11-2020 Abstract Porosity in polymers and polymeric materials adds to their functionality due to achieving the desired tailored charac- teristics porosity offers, such as improved mass transfer through the material, improved accessibility of reactive sites, reduced overall mass, tunable separation properties, etc. Therefore, applications in many fields, e.g. catalysis, separation, solid phase synthesis, adsorption, sensing, biomedical devices etc., drive the development of polymers with controlled morphology in terms of pore size, shape, interconnectivity and pore size distribution. Of particular interest are polymers with distinct bimodal or hierarchical pore distribution as this enables uses in applications where pore sizes on multiple levels are needed. Emulsion templating can be used for the preparation of polymers with included interconnected spher- ical pores on the micrometre level while post polymerisation crosslinking adds micro porosity. Combined use of both techniques yields multi-level and hierarchically porous materials with great application potential. Keywords: PolyHIPE; hypercrosslinking; porous polymers; porosity; emulsion templating; hierarchical polymers 1. Introduction Methods for generation of porosity in polymers can be generally divided into chemical and physical. Among physical methods, various templating can be used while post polymerisation crosslinking and phase separation in- duced syneresis are examples of chemical methods (Figure 1). According to IUPAC guidelines,1 pores are referred to as macro (diameters over 50 nm), meso (diameters be- tween 2 and 50 nm) and micro (diameters less than 2 nm). In terms of pore size distribution, it can be statistical how- ever materials with distinct bimodal or hierarchical pore distribution can be prepared meaning that micro and mac- ro pores are present or that pore size distribution follows a hierarchical concept where a multi-level porous material is produced with pore size levels following one after another. Among templating methods, emulsion templating is wide- ly used.2–7 Both water-in-oil and oil-in-water emulsions can be used for the purpose of macro porosity induction during the polymerisation process. When a high concen- tration of the droplet phase is used, droplets’ shapes be- come distorted and a dispersion of droplet size is observed (Figure 2). In the case of inclusion of monomers into the continuous phase the polymerisation results in a mono- lithic porous material, typically with an interconnected porosity which is the result of the shrinkage of continuous phase volume at the sol-gel transition. In the case of uni- form packing of monodisperse spherical droplets, the vol- ume of the droplet phase accounts for 74,05% of the total emulsion volume while at random packing this share is lower, namely 64%.8 Polymers prepared from emulsions with droplet phase volume shares higher than these border values are termed polyHIPEs, following the abbreviation for high internal phase emulsion.9 The internal topology of so prepared polymeric material features two levels of pores, the primary pores, termed cavities and secondary, interconnecting pores (Figure 3). The size of primary pores follows the size of the droplets prior to polymerisa- tion as demonstrated by a series of experiments containing emulsion aging and room temperature polymerisation in- itiation with a redox initiation pair.10 Therefore, the con- trol of emulsion droplet size prior to polymerisation is the main control also for the primary pore size. The main fac- tors controlling emulsion droplet size include emulsion 350 Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... stabilization by surfactant molecules and energy input at emulsion preparation. On the other hand, the frequency and size of inter- connecting pores determine the connectivity of porous structure or what could be defined as the openness of the structure (Figure 4).11 Figure 1: Methods for porosity creation in polymers Figure 2: Droplet size and shape change at emulsion concentration Figure 4: Interconnectivity of porous structure in polyHIPEs Figure 3: PolyHIPE morphology The thickness of the film of continuous phase be- tween the adjacent droplets seems to be the main factor affecting the size and frequency of the interconnecting pores. This is mainly determined by the droplet to contin- uous phase volume ratio and by the concentration and structure of the surfactant(s). In summary, main factors determining the morphological features of polyHIPEs are volume ratio of droplet/continuous phase, energy input at preparation, and surfactant concentration and structure. Many other variables were considered and their role can be important. This makes a polymerizable HIPE a multi variable system and careful experimental consideration must be involved when planning a particular structure (Figure 5, Table 1). 351Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... 2. Recent Advances in PolyHIPE Synthesis and Creation of Multi-Level Porosity While water-in-oil high internal phase emulsions are most commonly used for the preparation of polyHIPE polymers, other emulsion based systems have also been described. Both oil-in-water and oil-in-oil high internal phase emulsions can be applied.12 For water soluble or hy- drophilic monomers, solvents such as hydrocarbons or benzene derivatives were used as the droplet phase. In such manner, polyHIPEs were prepared from acrylic acid,13 2-hydroxyethyl methacrylate,14–18 N-isopropyl acrylamide (NiPAAm),19,20 acrylamide,21 1-vinyl-5-ami- notetrazole,22,23 and dimethylaminoethyl methacrylate.24 Furthermore, combination of emulsion templating and other porosity induction techniques, have yielded hi- erarchically porous polymer materials. Sušec et al.25 and Johnson et al.26 have demonstrated the principle of apply- ing a high internal phase emulsion within a stereophoto- lithographic based additive manufacturing setup. With such a system, a three-dimensional object can be built us- ing a lithographic photo polymerizable system. Due to the use of a high internal phase emulsion with photo polymer- izable monomers in the continuous phase, the object has an internal polyHIPE structure. Thus, another level of po- rosity is created using the lithographic process while pores of smaller dimensions are created as a result of emulsion templating. Monomer mixtures particularly suitable for such photo polymerisation were found to be multifunc- tional thiols and alkenes producing polymer networks via the thiol-ene click reaction.27 Another recent example of adding structure com- plexity is the use of hard sphere templating. Within this approach, spherical particles (typically polymeric) are fused together, to construct a monolithic porous network with interconnected porosity. So constructed material is then impregnated with a monomer mixture, polymerized while the previously constructed template dissolved. Mac- roporous polymethacrylates prepared in this way have open interconnected porosity and have been used as scaf- folds in tissue engineering applications.28–31 We have shown that a combination of this hard sphere templating and high internal phase emulsion templating can yield polymers with open interconnected and hierarchical po- rosity which is especially advantageous in biomedical ap- plications such as tissue constructs.32 Primary monolithic template was formed by sintering polymethyl methacrylate beads and subsequently filled with a high internal phase emulsion containing thiols and alkenes as monomers in the continuous phase (Figure 6). Photopolymerisation yielded hierarchically structured polymer network with open porosity and biodegradability was introduced by the use of thiol monomers with ester groups. (Figure 7) So prepared multi-level porous polymers were successfully applied as scaffolds for human bone cell growth. Figure 5: polyHIPE preparation Figure 6: Combination of hard sphere and emulsion templating Table 1: Experimental factors and their effect on HIPE stability and polyHIPE morphology Experimental factor Effect on HIPE stability Effect of polyHIPE morphology Rate of stirring Higher rates-increase in stability Smaller cavities Time of stirring Prolonged stirring-increase in stability Reduced dispersion of cavities Viscosity Increase in viscosity-increase in stability Smaller cavities Surfactant content Lowers the interfacial tension Increase in interconnectivity Temperature Increase in temperature-enhances the coalescence, Increase in cavity size, increased reduces stability dispersion. stabilizing salts Enhance the rigidity of the interfacial film-enhances stability. Reduces cavity size Inhibiting Ostwald ripening. 352 Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... While electrospinning is in itself a method produc- ing porous fibrous structures from polymer solutions or melts,33,34 high internal phase emulsion templating has been used in combination with electrospinning to add a level of porosity to the product.35,36 Using this combina- tion, fibres with bicontinuous morphology were obtained by electrospinning HIPEs consisting of aqueous poly(vinyl alcohol) solutions dispersed within polycaprolac- tone-in-toluene solutions.35 Similarly, Dikici et al.36 used a polycaprolactone barrier membranes to form bilayers by combining electrospinning and emulsion templating tech- niques and applied them for guided bone regeneration. The electrospunn fibres had a mixture of open and closed cell porous polyHIPE type morphology. 3. Inducing Meso and Micro Porosity Within PolyHIPEs Due to relatively large pores induced by the droplet phase in polyHIPE preparation, the result is a macroporo- us material with primary pore sizes typically between 500 nm and 100 µm. Consequently, specific surface areas of polyHIPEs are low, usually below 50 m2/g. This is the re- sult of the lack of meso and micro porosity. For many ap- plications surface area of the polymer support plays a vital role. Early attempts of improving specific surface areas of polyHIPEs mostly included the addition of porogenic sol- vents to the monomer containing continuous phase37 and increasing the crosslinking degree38 thus producing poly- HIPEs with specific surface areas up to 550 m2/g. Intro- duction of porogenic solvent into the monomer contain- ing continuous phase induces meso porosity via either early or late phase separation within the gelation process during the polymerisation.39 Surfactant concentration and structure also affect the surface area of styrene based poly- HIPEs.40 However, both methods, namely adding poro- genic solvent and surfactant to the monomer containing continuous phase increase the nominal porosity which results in sacrificing the material mechanical properties in terms of elastic modulus and brittleness. The morphology of polymers prepared by the addition of porogenic sol- vents include a fused-bead, cauliflower-like features which is not optimal for mechanical properties.39 Our study comparing materials with fused-bead and polyHIPE mor- phology showed that polyHIPE structure is superior to fused-bead morphology allowing for the preparation of materials with overall porosities higher than 75%.32 Com- bining high porosity with sufficient mechanical stability is an important materials feature with applications in mind. For example, permeable open cellular polyHIPEs for chro- matography stationary phases and membranes with signif- icantly higher porosity compared to commercially availa- ble monolithic columns facilitate lower back pressures and thus the efficiency of separation.41–50 4. Post Polymerisation Crosslinking In order to avoid the formation of fused bead type morphology within the formulating polymer film of the continuous phase of a high internal phase emulsion, a post polymerisation crosslinking of already formed polyHIPE material can be attempted. Post polymerisation crosslinking, in this description referred to as hypercrosslinking, is a method of polymer chain crosslinking and the result is the creation of numer- ous new pores at meso and micro size scale and thus the creation of meso and/or microporous polymer materi- al.51,52 The porous profile of hypercrosslinked polymers differs from the porous profile of polymers prepared by free radical polymerisation of monomers and crosslinkers. The porosity of polymers prepared by conventional copol- ymerisation is the result of phase separation during po- lymerisation in the presence of an inert diluent, which may be either a non-solvent or a thermodynamically good sol- vent. The non-solvent does not dissolve the growing crosslinked chains, so the network shrinks and precipitates into micro spheres. These non-porous nodules aggregate and agglomerate in a cauliflower-like structure, and the prepared material is macroporous.53–55 When a good sol- vent is used, the polymer network swells, but at high crosslinking degrees it can no longer adsorb the diluent. This results in phase separation in the form of micro and macrosyneresis.56 In the case of macrosyneresis, a macro porous network is formed as the resulting gel decomposes and thus a microgel is formed, which behaves as a contin- uous phase in the reaction mixture. These particles then agglomerate during the polymerisation to form a macrop- orous network. While in microsyneresis the diluent is dis- tributed over the gel. The result of the whole texture is a macroporous network with cauliflower-like structure, but Figure 7: Scanning electron micrograph of hierarchically porous polymer prepared by combining hard sphere and emulsion templat- ing 353Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... having micro pores due to the primary polymer nod- ules.53–55 By using a diluent during radical crosslinking even in the continuous phase of high internal phase emul- sions, macroporous materials are obtained. Microsynere- sis also introduces micro pores, thus influencing the in- crease of specific surface area. However, frequently, due to strong capillary forces, the micro pores in polyHIPE col- lapse during solvent removal. Unlike traditional crosslinking polymerisation, hy- percrosslinked polymers are prepared by post polymerisa- tion crosslinking of long polymer chains in a semi-solu- tion state creating many new bridges. This does not result in phase separation because the polymer chains are dis- tributed throughout the solution and are strongly solvated throughout the hypercrosslinking. A typical example of such process is heating of solvated chloromethylated poly- styrene in the presence of a Friedel-Crafts catalyst. This creates new connections by converting chloromethyl groups into methylene bridges that interconnect polymer chains. When the swelling solvent is removed by drying, the additional crosslinking prevents complete collapse of the polymer network and the resulting polymers exhibit extensive microporosity even in the dry state. Initially, the formation of methylene bridges is fast because the mobili- ty of the polymer chains in the swollen polymer is higher than later in the reaction, when the polymer chains are al- ready connected to the newly formed methylene bridges. By introducing new methylene bridges into the network, pores are formed as spaces between highly cross-linked nodules. In further stages of hypercrosslinking, the rigidi- ty of the nodules increases and therefore, after solvent re- moval, the morphology results in a stable microporous network.52,57 Hypercrosslinked polymers prepared from gel-type precursors contain only the micro pores. If, on the other hand macroporous polymer is used as the precursor, the product then contains beside the macro pores also mi- cro pores, and a polymer with a bimodal pore distribution is formed. Hypercrosslinked polymers contain a very high density of crosslinks creating micro pores and exhibit high surface areas up to 2000 m2/g.58,59 After the removal of the solvent, micro pores remain, which increases compatibili- ty with both polar and non-polar solvents,60,61 what is ex- tremely important for applications. The chemical nature of the conventionally prepared STY/DVB polymer is very similar to the hypercrosslinked polymers that have many methylene bridges between polymer chains. However, these materials differ crucially in terms of topology and mechanical properties. STY / DVB copolymers are pre- pared without the addition of solvent, which means that their polymer chains are very densely packed in dry state, due to the strong attraction between them. Under these conditions, the polymers swell in thermodynamically good solvents, since the polymer-polymer interactions are replaced by stronger polymer-solvent interactions. Hyper- crosslinked polymers, however, are prepared in the pres- ence of an excess of good solvent, and if the crosslinking rate is high and conformational rigid connections are es- tablished, then the polymer chains are not densely packed after removal of the solvent. The final material in the dry state has high free volume and significantly reduced poly- mer-polymer interactions. It is essential that due to the affinity between the polymer fragments, the rigid structure of the hypercrosslinked polymer causes high inner stresses in the polymer chains of the network. Because of this, the hypercrosslinked materials tend to release inner stresses, which happens when the network is expanded, that is, when it swells. Swelling is possible on contacting any liq- uid, regardless of thermodynamic affinity with the poly- mer which means that the hypercrosslinked polymers are compatible with both thermodynamically good and bad solvents.60,62 Hypercrosslinking can be achieved by different chemical methods and can be divided into: post polymer- isation crosslinking (hypercrosslinking using polystyrene precursors, hypercrosslinking using VBC / DVB precur- sors), direct one-step polycondensation of functional monomers and hypercrosslinking by the so called knitting method. 4. 1. Hypercrosslinking of Polystyrene Precursurs (Davankov Resins) Introduction of hypercrosslinking of polymer chains dates into 70’s when hypercrosslinked polystyrene (PS) was demonstrated, using linear PS or gel-type swollen pol- ystyrene-co-divinylbenzene and external crosslinkers in the presence of Lewis acid catalyst and solvents.63 External crosslinkers create new covalent bonds between polysty- rene chains applying the Friedel-Crafts reaction61,64,65 (Scheme 1). This reaction achieves short and rigid connec- tions and forms a rigid three-dimensional polymer net- work. Almost all aromatic rings can be consumed in this reaction, which results in a high degree of hypercrosslink- ing, and consequently in a large number of newly formed links and a high specific surface area of the materials.51 Typical protocol for hypercrosslinking of linear polysty- rene or the STY / DVB copolymer contains introducing a sufficient amount of external crosslinker into the dissolved linear polystyrene or swollen polystyrene network and adding the Lewis acid while cooling the reaction medium to achieve homogeneous distribution of the catalyst before gelation of the mixture. The mixture is then heated, allow- ing high conversion of the reactive linker groups.66 As ex- ternal linkers, chloroalkanes are most commonly used; usually dichloroethane, as it plays two roles - as an external linker and a good solvent for PS. Chloromethyl ether was used predominantly for the hypercrosslinking of polystyrene, but was replaced by monochlorodimethyl ether,60,67–69 carbon tetrachlo- ride,70,71 dichloroxylene,64,72 4,4’-bis (chloromethyl)-bi- phenyl,64,72 trifunctional tris-(chloromethyl)-mesitylene,51 4,4’-bis-chloromethyl-1,4-diphenylbutane,51 formalde- 354 Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... hyde dimethyl acetal71–74 or dichloroethane51,75,76 and oth- er dichloroalkanes,76 due to its adverse health effects. It should be noted that the length of the external crosslinker affects the rigidity of the hypercrosslinked material, which also affects the morphology of the material. All the com- pounds form bridges of a limited conformational mobility in the final network except for diphenylbutane. In this case, the diphenylmethane crosslinker type are most influ- enced by the rigidity of the structure. Another limitation in mobility is tris-(chloromethyl)-mesitylene because it links three polystyrene chains at one point. Scheme 1: Post polymerisation hypercrosslinking: Hypercrosslink- ing using polystyrene precursurs (Davankov resins) When PS with a low degree of initial crosslinking (0.3–2%) is used for hypercrosslinking, intrinsic micropo- rosity is formed. However, if the initial degree of crosslink- ing is increased, the macroporous network is formed prior to hypercrosslinking, so the final pore distribution is bi- modal.61,77 Due to their high surface area, good solvent compat- ibility and good mechanical properties, bimodal porosity and interpenetrating network, hypercrosslinked polysty- renes are often used as adsorbents for gases78,79 and vari- ous organic molecules,57,80,81 for chromatographic separa- tion,82–84 or for adsorbents for blood purification.85 4. 2. Hypercrosslinking Using Chloromethylated Groups of Vinylbenzyl Chloride Hypercrosslinking can be performed on polyvinylb- enzyl chloride or its copolymers utilizing vinylbenzyl chloride moieties in the polymer chains as internal elec- trophiles in the Friedel-Crafts reaction, without the addi- tion of external linkers (Scheme 2).58,61 In this reaction, the chloromethyl groups are converted to methylene bridges and thus new links are created. The aromatic ring which is to be substituted is electron-rich, resulting in a formation of six-membered ring following the cyclization reaction. As with Davankov’s type of hypercrosslinking, FeCl3 is most commonly used as a Friedel-Crafts catalyst because it has good solubility in the usual solvents used and does not cause steric hindrance.58 Due to the similari- ty of the polystyrene network to VBC / DVB network, the same solvents are used, most commonly DCE. The mor- phology of polymers hypercrosslinked by this post-polym- erisation approach is similar to Davankov type resins meaning that hypercrosslinking induces micro pores and results in a significant increase of specific surface area. This was demonstrated by hypercrosslinking several poly (VBC-DVB) copolymers with DVB content between 2% and 20%.58 The specific surface area of hypercrossliked products depended on the initial crosslinking and reaction time, being highest with lowest initial crosslinking and in- creasing with reaction time up to 2 hours while further elongation of reaction time had no effect. Decrease in the chloride content of the polymer coincides with the drastic increase in the specific surface area. The importance of initial crosslinking for efficiency of hypercrosslinking and final porous structure was con- firmed in another report.86 The maximum specific surface area was achieved after hypercrosslinking of polymer con- taining 2% of DVB (2060 m2/g). By increasing the DVB amount, a bimodal structure was formed, with well-de- fined macro and micro pores, while at 7% of DVB the structure was completely micro porous. Increasing the surface area after hypercrosslinking of VBC/DVB poly- mers at a lower DVB content occurs because the macro- molecular chains of the polymer are still very loose and can orient more favorably in the presence of solvent in the hypercrosslinking process, thus forming more methylene bridges than in the case of higher crosslinked poly (VBC / DVB). One of the most advantageous consequences of hy- percrosslinking VBC / DVB polymers is their improved sorption properties. Unlike non-functionalized hyper- crosslinked polystyrene, which is a good sorbent due to hydrophobic π-π interactions,82 functional groups can un- doubtedly improve the adsorption properties, which in turn can affect the development of many applications.87–91 Scheme 2: Post polymerisation hypercrosslinking : Hypercrosslink- ing using chloromethylated groups of vinylbenzyl chloride 4. 3. Direct Hypercrosslinking Arising from Polycondensation Hypercrosslinked polymers can also be produced by the direct polycondensation of small molecule monomers without the need to make the precursor crosslinked poly- mer. However, the synthesis of polymer precursors is 355Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... time-consuming and limited functional monomers can be selected to satisfy the combined conditions from reactions of radical polymerisation and Friedel–Crafts alkylation. Similarly to the other hypercrosslinking approaches, these use DCE as a solvent and FeCl3 as a Friedel-Crafts cata- lysts. The resulting networks can be considered the ana- logues of the Friedel-Crafts linked PS materials. This direct approach creates microporous organic networks and uses bis(chloromethyl) aromatic monomers such as dichlorox- ylene,92 bis(chloromethyl)biphenyl,75,93 and bis(chloro- methyl) anthracene.75,93 By using Lewis acid as a catalyst, the chloromethylene groups react with adjacent phenyl rings. This results in the formation of rigid methylene bonds between the rings, which in turn produces micro pores and high specific surface areas up to 2000 m2/g.93 Due to high specific surface areas hypercrosslinked poly- mers using polycondensation have good gas adsorption capacity.75,93–95 The use of o-DCX isomers for condensa- tion with m-DCX or p-DCX has been found to have an adverse effect on the growth of specific surface areas, while m-DCX and p-DCX provide materials with comparable specific surface areas.75 For well-defined micro porous polymers, DCX and BCMBP were used as crosslinkers in order to connect heterocyclic (carbazole), metal-doped (ferrocene) and highly rigid (triptycene) building blocks. It was also found that the length of crosslinkers can affect the porosity of the resulting polymer. For example, longer crosslinker molecules affect larger pores, while shorter molecules create micro pores, thereby contributing to an increase in the specific surface area of the polymer.96 Fluo- rene derivatives (fluorene, 9,90-spirobi(fluorene), diben- zofuran and dibenzothiophene) were also used as non-functional aromatic precursors, which showed good microstructure in condensation with BCMBP under Friedel-Crafts catalytic conditions. The highest surface area of up to 1800 m2/g was obtained from dibenzofurane monomers with 10% molar fraction.95 Aromatic precursors used in addition to benzene97,98 were polyaniline,95 polypyrrole,94 polythiophene, polyfu- rane, aniline, carbazole,99,100 aminobenzene,101 bishydrox- ymethyl monomers,102 and were found to form hyper- crosslinked polymers. 4. 4. Knitting Aromatic Compound Polymers Using an External Crosslinker A special type of one-step polycondensation, howev- er, is the “knitting” method for hypercrosslinking with an external crosslinker-formaldehyde dimethyl acetal FDA, which is more environmentally friendly as it has no dan- gerous by-products during the Friedel-Crafts reaction. The mechanism of the reaction is proposed as: Lewis acid first complexes with the crosslinking molecule, which reduces the interaction between the methoxyl group and the cen- tral carbon atom, and then produces a large number of in- termediate carbocations in the DCE (Scheme 3).98 The carbocations then react with the phenyl ring and the addi- tion of the multi - methoxymethyl groups to the aromatic ring proceeds, releasing methanol. The methoxymethyl groups are then converted to methylene links and reacted with other phenyl rings to form a rigid crosslinked struc- ture. Scheme 3: Typical hypercrosslinking by the knitting method from benzene monomers98 Typically, in this one-step approach, the aromatic monomer (including benzene, phenol or chlorobenzene), the crosslinker (FDA) and the catalyst (FeCl3) are dis- solved in DCE to complete the condensation.98 Increasing the FDA crosslinker content of hypercrosslinked materials obtained using tetraphenylmethane blocks also increases the specific surface area up to 1314 m2/g.103 Similar ap- proaches using FDA as external crosslinker were shown with different monomers such as aromatic heterocy- cles,104,105 hydroxymethylated aromatic molecules,102 ani- line and benzene,72,97 styrene105 and tetrahedral mono- mers,106,107 among others. The “knitting” method is used for the design and synthesis of microporous polymers based on various rigid aromatic building blocks, including nonhalogenated monomers. 1,4-dimethoxybenzene was also used as an external crosslinker.108 5. Hypercrosslinking of PolyHIPEs Hypercrosslinking of polyHIPE polymers results in rigid polymers with induced meso and microporosity (ter- tiary pores) in macroporous material which leads to very high specific surface areas due to induction of micro and meso pores. Research so far shows that the morphology of polyHIPEs does not change significantly after hyper- crosslinking and the typical open cellular macroporous structure is retained.59 Rigid connections created during hypercrosslinking make better compatibility with both thermodynamically good and bad solvents and thus better accessibility of reactive sites. As a result, hierarchically po- rous material is obtained with macro pores that allow con- vective transfer and reduce back pressure in flow systems, and at the same time with high specific surface area due to the presence of micro and meso pores allowing good ac- cessibility of reactive sites. PolyHIPEs with hierarchical or 356 Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... bimodal porosity can be very useful in many applications as demonstrated by various research groups. Schwab et al.77 synthesized VBC / DVB polyHIPE monoliths, and the DVB molar fraction varied between 2.5 and 40%. Materials were hypercrosslinked by Friedel- Crafts alkylation reaction in the presence of a Lewis base to give monoliths with specific surface areas up to 1200 m2/g, while maintaining the morphology of precursor pol- yHIPE. Due to hypercrosslinking, the non-porous walls of polyHIPEs have become highly microporous. Such mono- lithic VBC based polyHIPE polymers with bimodal struc- ture were proven to be very promising for n-butane stor- age and the results were comparable to the commercially available Sorbonorit powder. In another report, VBC based polyHIPEs with 2 mol% DVB content were used for controlled hypercrosslinking to leave some unreacted ben- zyl chloride groups for further binding of methyl amino pyridine. Functionalized hypercrosslinked polyHIPE was used as a very effective nucleophilic catalyst for the alkyla- tion of methylcyclohexanol with acetic anhydride due to its hierarchical porosity and high specific surface area. It was found that after 3 hours, 100% alkylation conversion was achieved using hypercrosslinked polyHIPE-MAP while non hypercrosslinked materials performed signifi- cantly worse demonstrating the advantage of the bimodal pore structure with facilitated mass transfer.59 In addition to hypercrosslinking styrene type poly- HIPEs with an internal crosslinker, Friedel-Crafts reaction with an external crosslinker was used. Crosslinked STY / DVB polyHIPEs were used for hypercrosslinking using formaldehyde dimethyl acetal as the external crosslinker applying the knitting approach. This hypercrosslinking method resulted in polyHIPE monoliths with specific sur- face areas between up to 595 m2/g. Due to their extremely hydrophobic surface, the hypercrosslinked STY / DVB polyHIPE materials have shown good absorption capacity for oils, and could be used for oil-spill cleaning.109 Knitting type hypercrosslinking was also used to synthesize porous carbon foams, which were produced by carbonizing the STY / DVB hypercrosslinked polyHIPEs. Dimethoxymethane was used as an external crosslinker for hypercrosslinking. It was found that the STY / DVB ratio of polyHIPE precursors is strongly influenced by char yield, micro pore volume and BET surface area of car- bonized polyHIPEs.110 Silverstein et al. synthesized porous carbons with high specific surface areas and hierarchical porous struc- ture by pyrolysis of hypercrosslinked VBC/DVB poly- HIPEs. 111 Hypercrosslinking generated new links which limited the degradation of polyHIPE morphology after pyrolysis. Surface areas of pyrolyzed hypercrosslinked pol- yHIPEs were as high as 553 m2/g, which meant less than 40% reduction compared to precursor polymers. Hyper- crosslinking via FeCl3 catalysis was further used to synthe- size acrylonitrile-DVB polyHIPEs, which were used for pyrolysis to produce nitrogen- and oxygen-codoped car- bo-polyHIPEs with interconnected macro pores and mi- cro / mesoporous carbon skeleton.112 Such carbo-poly- HIPE was applied as a solid-state support for Pt and Ru bimetal nanoparticles, which, in turn, demonstrated a re- markable electrocatalytic ability to methanol electrooxida- tion. The specific surface area of carbo-HIPE was increased to 417 m2/g after hypercrosslinking, which proved to be important for improving electrocatalytic performance. Sil- ica particle stabilized polyDVB polyHIPE was used as a porous solid acid catalyst for the production of hydroxym- ethyl furfural from cellulose in the presence of 1-ethyl-3-methyl-imidazolium chloride. For comparison, basic polyDVB was prepared, grafted with a -SO3H sul- fonation process, and PDVB-co-SS polyHIPE, which was also sulfonated. This polymer was then hypercrosslinked and used as a solid state catalyst which, in addition to the macro pores in its skeleton, also had micro pores and, con- sequently, high specific surface area. Large specific surface area of polyHIPE and super-strong acid sites have been found to be crucial for cellulose conversion.113 Sevšek et al.114 synthesized STY/DVB polyHIPE monoliths with high DVB content. The remaining vinyl groups of DVB in STY/DVB monoliths were used for post-polymerisation crosslinking using the radical initiator di-tert-butyl perox- ide in toluene and acetonitrile. The surface area in both solvents was found to be much larger after hypercrosslink- ing (up to 355 m2/g), and the nitrogen adsorption / deso- rption method showed an increase in the number of micro pores after hypercrosslinking, which coincides with an in- crease in specific surface area. Pyridine containing poly- HIPE could be hypercrosslinked by the second stage radi- cal crosslinking of remaining vinyl groups.115 For the pur- poses of solid state support for catalysts, vinyl pyri- dine-DVB polyHIPE was prepared. Pyridine ring nitrogen was used for further functionalization – for the Cu (II) coordinate linker. This functionalized polyHIPE has been used as a solid state support for catalysts for a cycloaddi- tion click reaction. Post polymerisation radical treatment (using di-tert-butyl peroxide) increased the specific sur- face area and created a multi modal porous profile, which was crucial for the success of the cycloaddition reaction. Hypercrosslinking was demonstrated also on non- styrene-type polyHIPEs. Mezhoud et al.116 synthesized poly (2-hydroxyethyl methacrylate-co-N,N’-methylenebi- sacrylamide) polyHIPE and functionalized it with ally- lamine and propargylamine to create free double bonds that were used for hypercrosslinking with di- or tetra-thi- ols via thiol-ene click reactions. After the treatment sur- face areas of up to 1500 m2/g ware measured. The mono- liths were then used for Au-nanoparticle decorated cata- lytic support to reduce nitrophenol and Eosin Y. Another similar method is described as in situ hy- percrosslinking of GMA-based polyHIPEs with multi- functional amines, where the amino-epoxy reaction is running parallel to the polymerisation. The result is a highly porous material with surface areas up to 63 m2/g 357Acta Chim. Slov. 2020, 67, 349–360 Koler et al.: Post Polymerisation Hypercrosslinking with Emulsion ... and with good accessibility of reactive sites, but this system has disadvantages in terms of HIPE stability.117 6. Conclusion and Outlook Porous polymers in different formats e.g. particles, monoliths, membranes, have a wide range of application fields; in separation, catalysis, synthesis, purification, in biomedical fields as supports for cell and tissue cultiva- tion etc. Control of pore geometry, interconnectivity and size is of utmost importance for the desired perfor- mance. Not only is the control to narrow the pore size distribution important but the possibility to create mate- rials with bimodal and hierarchical pore size distribu- tion is very desired. While there are many methods for creating macroporosity in polymers, either during the polymerisation or after, high internal phase emulsion templating offers easily scalable straightforward tech- nique for the synthesis of polymers with spherical inter- connected pores with micrometer dimensions and high pore volume. Polymerisation parameters, droplet phase volume share and surfactants are key factors deciding the final structure. While good control of macroporosity in polyHIPEs is possible, the introduction of meso and microporosity is less trivial. Addition of porogenic sol- vents into the continuous phase can induce meso and micro porosity however the prevalence of cauliflow- er-like morphology and increase of pore volume signifi- cantly decreases the mechanical properties and is there- fore in many cases unpractical. On the other hand, the post polymerisation hypercrosslinking enables the crea- tion of meso and microporosity in already formed mac- roporous polyHIPEs without sacrificing the mechanical properties or even improving them in many aspects. Im- proved accessibility of reactive sites in the interior of the bulk of the material, increased surface area and wide sol- vent compatibility are further advantages of hyper- crosslinked polyHIPEs. It is therefore expected that such materials with bimodal and hierarchical pore distribu- tion will play an increasingly important role in various application fields in the future. List of abbreviations used BCMBP 4,4’-bis(chloromethyl)biphenyl BET Brunauer-Emmett-Teller DCE dichloroethane DCX dichloroxylene DVB divinylbenzene FDA formaldehyde dimethyl acetal GMA glycidyl methacrylate HIPE high internal phase emulsion MAP 4-(N-methylamino)pyridine NiPAAm N-isopropyl acrylamide PS polystyrene STY styrene SS sodium p-styrene sulfonate VBC vinylbenzyl chloride Author biographies Amadeja Koler is a PhD student in PolyOrgLab at the Faculty of Chemistry and Chemical Engineering, Uni- versity of Maribor. She studies the preparation of multi-lev- el porous macromolecules. Her research includes the in- troduction of new reagents for controlled RAFT polymer- ization as well as colloidal precursor methods for forma- tion od macroporosity and incorporation of hypercross- linking for microporosity in macroporous materials. Irena Pulko is a professor of chemistry of materials at the Faculty of Polymer Technology in Slovenj Gradec. She studied for PhD in the PolyOrgLab and in the research groups of Prof. Neil Cameron at Durham University and Prof. Christian Slugovc at Graz University of Technology. Her research includes porous materials and polymer-based materials from renewable resources. Peter Krajnc runs PolyOrgLab at the Faculty of Chemistry and Chemical Engineering, University of Mari- bor and is the vice dean at the faculty. He did his PhD in the group of Prof. Marko Zupan at the Faculty of Chemis- try and Chemical Technology of the University of Ljublja- na. 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DOI:10.1007/s00396-018-4455-z Except when otherwise noted, articles in this journal are published under the terms and conditions of the  Creative Commons Attribution 4.0 International License Povzetek Poroznost v polimerih in polimenih materialih je zelo pomembna, saj jim le ta daje posebne funkcionalnosti, kot so izbol- jšani prenos snovi skozi material, izboljšana dosegljivost reaktivnih mest, znižana skupna masa, prilagojene separacijske lastnosti, itd.. Razvoj na področju polimerov s kontrolirano morfologijo, kar se tiče velikosti in oblike por, povezovalnosti por in njihove porazdelitve velikosti pomembno vpliva na uporabnost teh polimerov na področju katalize, separacije, sinteze na trdni fazi, adsorpcije, senzorjev, biomedicinskih pripomočkov in mnogih drugih. Zlasti so zanimivi polimeri z izrazito bimodalno ali hierarhično porazdelitvijo por, saj to omogoča uporabo v aplikacijah, kjer so potrebne velikosti por na več ravneh. Emulzije lahko uporabimo za pripravo polimerov z vključenimi medsebojno povezanimi sferičnimi porami na mikrometrski ravni, medtem ko postpolimerizacijsko zamreženje vpliva na mikro poroznost. S kombinirano uporabo obeh tehnik dobimo materiale z večnivojsko in hierarhično poroznostjo z velikim potencialom uporabe.