Romain Poupart

Daniel Grande

Benjamin Carbonnier

Dr Benjamin Le Droumaguet
email: ledroumaguet@icmpe.cnrs.fr

Porous Polymers and Metallic Nanoparticles: a Hybrid Wedding as a Robust Way Toward Efficient Supported Catalytic Systems

Keywords: Porous polymers, Metallic nanoparticles, Hybrid materials, Supported heterogeneous catalysis

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Porous polymers and metallic nanoparticles: A hybrid wedding as a robust method toward efficient supported catalytic systems

Introduction

A catalyst is commonly described by the International Union of Pure and Applied Chemistry (IUPAC) as a substance that increases the rate of a reaction without modifying the overall standard Gibbs energy change in the reaction. Two different types of catalytic processes actually exist: (i) one type based on homogeneous catalysis, namely the catalyst is present in the same phase as the reactants, mostly a liquid phase [START_REF] Van Leeuwen | Homogeneous catalysis: understanding the art[END_REF], while (ii) the other type is named as heterogeneous when the catalyst and the reactants are in different phases.

Though, the frontier between both kinds of processes can be sometimes really thin in particular cases [START_REF] Widegren | A review of the problem of distinguishing true homogeneous catalysis from soluble or other metal-particle heterogeneous catalysis under reducing conditions[END_REF]. The metal-based catalyst is mostly in the solid state, finely divided, and the reactants are in the liquid or gaseous one. Nevertheless, some examples of heterogeneous biphasic catalysis reported that the catalyst remains in one liquid phase (i.e. water) and reactants in another one (i.e. oil), such as for the hydroformylation of propene, for instance [START_REF] Herrmann | Watersoluble metal complexes and catalysts[END_REF]. One such example illustrates very well the possibility to perform catalysis by phase transfer. Recent developments in this research area notably demonstrated that catalytic reactions, especially organometallic-based ones, can be carried out in more environmentallyfriendly conditions than a few years ago [START_REF] Pirkanniemi | Heterogeneous water phase catalysis as an environmental application: a review[END_REF]. Such catalytic reactions more and more call upon green and sustainable chemistry, and efforts have been put forward in this direction to address specific issues related to the recycling of the catalysts.

In this context, supported metallic nanoparticles present some undeniable advantages regarding heterogeneous catalysis. They have actually shown an increasing interest over the last decade. Such nanometer-sized metallic particles, immobilized on high surface area materials, can now be relatively well characterized by different techniques and have shown some great catalytic performances. The preparation of such hybrid materials involves the design and development of still novel and efficient catalysts, and thus the improvement of (in)organic supports in terms of specific surface area, porosity, surface functionality and chemical inertness towards a wide variety of versatile or harsh reaction conditions [START_REF] Bell | The Impact of Nanoscience on Heterogeneous Catalysis[END_REF].

Catalytic systems are commonly used in various industrial techniques [START_REF] Copéret | Homogeneous and Heterogeneous Catalysis: Bridging the Gap through Surface Organometallic Chemistry[END_REF] as well as in our common life, as demonstrated by automotive catalytic converters for instance [START_REF] Bell | The Impact of Nanoscience on Heterogeneous Catalysis[END_REF]. While a serious drawback has been encountered with suspended nanoparticles, i.e. the recycling of the nanometal catalyst through time-consuming and non-environmentally friendly purification processes, miscellaneous solutions exists. First, a recently reported solution to address this issue relies on the use of nanoparticles bearing a magnetic core, generally γ-Fe2O3. In this case, the nanometal recovery step can be achieved by harvesting with the help of a magnet. However, a coating (silica, carbon, or polymer) of these magnetic metallic nanoparticles is generally required to allow for their suspension in a stable fashion, thus avoiding their coalescence, and protecting them from the surrounding environment. When a polymer coating is implemented for such magnetic nanoparticles, the resulting coated particles are not stable in harsh reaction conditions, such as at high temperature. The same observations have been made with silica coatings that are porous. Such an inorganic coating has a very low stability in harsh pH conditions. Finally, when carbon-based coatings are used for magnetic nanoparticles, they aim at agglomerating and form clusters, which would lead to a decrease of specific surface area of the catalysts [START_REF] Lu | Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application[END_REF]. Then, another solution consists in using macroscopic porous matrices as supports for the immobilization of metallic nanoparticles at the pore surface. The robust and straightforward immobilization of nanoparticles at the pore surface of suitable supports, notably by tuning the nature of the interface, enables to further avoid tedious purification processes, as the supported catalyst can be readily removed from the reaction mixture by mere filtration.

Hybrid catalysts based on supported metallic nanoparticles generally consist of (in)organic/hybrid porous frameworks presenting a rather high specific surface area that allows for a large amount of metallic nanoparticles to be immobilized in a straightforward and robust fashion. Among such high specific surface area supports, inorganic materials like zeolites [START_REF] Sachtler | Zeolite-Supported Transition Metal Catalysts[END_REF] or ordered mesoporous silicas [START_REF] Opanasenko | Heterogeneous Pd catalysts supported on silica matrices[END_REF] can be found. Zeolites possess pores within the microporous range, enabling them to be used as catalyst supports in liquid or gas reaction conditions. Even though zeolites can be synthetized in rather mild temperature conditions (90-180 °C), their preparation requires pressures up to 15 bars in autoclaves. Other inorganic supports like mesoporous silicas can be prepared in a straightforward fashion by reacting in a first step a surfactant typically arising from the Tween ® or Pluronic ® family and a precursor mainly tetraethyl orthosilicate (TEOS) in mild conditions [START_REF] Zhao | Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures[END_REF]. However, the major concern of such porous inorganic supports relies on the harsh reaction conditions required in the calcination step that enables for the disappearance of the organic surfactant and thus the generation of the porosity. The calcination step is indeed performed at very high temperatures, i.e. hundreds of °C, for several hours, which is highly energy-and time-consuming. On the other hand, organic polymer-based supports as well as modified carbon nanotubes [START_REF] Wildgoose | Metal Nanoparticles and Related Materials Supported on Carbon Nanotubes: Methods and Applications[END_REF] have been recently developed. Finally, hybrid structures, i.e. Metal-Organic Frameworks [START_REF] Liu | Applications of metal-organic frameworks in heterogeneous supramolecular catalysis[END_REF] (MOFs) have been more recently deeply investigated in heterogeneous supported catalysis applications, due to their versatility and remarkably high surface area. Unfortunately, this class of hybrid materials suffers from high fabrication costs, poor selectivity, low capacity, and difficulty in recycling/regeneration [START_REF] Vikrant | Metal-organic frameworks (MOFs): potential and challenges for capture and abatement of ammonia[END_REF].

In the case of polymeric materials, some advantages rapidly come to mind. They can first be easily functionalized so as to tune the pore surface chemistry, which is of upmost importance for further adsorption of chemical species or metallic nanoparticles. One can also easily play with the hydrophilic/hydrophobic nature of the polymer interface that will have some consequences on the interaction with the surrounding fluid, notably in terms of wettability. The porosity can also be readily varied in terms of pore size, ratio, and shape. Such polymeric materials are generally cross-linked, allowing for a better stability of the resulting hybrids in miscellaneous experimental conditions such as harsh pH or temperature.

Finally, such polymer-based porous supports have mechanical properties tunable in a useful range, and their production cost is lower than that of their inorganic analogues. On the opposite, some drawbacks can be noticed with such porous polymers: they cannot generally resist to high pressure and temperature, rendering some catalytic chemical reactions on these supports difficult to envision. However, they still remain common porous supports for metallic nanoparticles immobilization and are thus the subject of widespread interest in the field of heterogeneous catalysis.

In light of this general introduction, this review will focus on the design and synthesis of hybrid materials consisting of metallic nanoparticles immobilized at the pore surface of porous polymers for catalytic reaction purposes. A first section will be devoted to general features about porous polymer-based materials; then the main strategies to prepare such porous supports and the associated techniques of characterization will be presented. The reader should bear in mind that this section will not give a full overview of all strategies implemented for the synthesis of porous polymers, but it will rather focus on those mainly used for the preparation of porous polymeric systems meant for supported catalysis applications. It is thus recommended for casual readers to refer to more general reviews on porous polymers to get a full overview of their preparation routes [START_REF] Wu | Design and Preparation of Porous Polymers[END_REF]. A second section will then be directed towards the use of hybrid porous materials obtained after metallic nanoparticles immobilization at the pore surface of polymeric supports, and their further implementation in heterogeneous catalysis.

Porous polymers: general features, synthesis, and characterization

Basics on porous polymers

According to the IUPAC [START_REF] Vikrant | Metal-organic frameworks (MOFs): potential and challenges for capture and abatement of ammonia[END_REF], porous solid materials can be classified into three main categories. Firstly, microporous materials are characterized by pore diameters below 2 nm.

Secondly, the term "mesoporous" is used to qualify materials with pore diameters between 2 and 50 nm. Finally, macroporous materials can be distinguished by a pore diameter larger than 50 nm. It is at this stage very important to note that this classification is the only one that is commonly admitted by the scientific community. However, in the areas of materials science and nanotechnologies, the term "nanoporous" is commonly applied to materials containing pore sizes lower than 100 nm, even though such a terminology can be somehow confusing for casual readers and even experts in the field. Likewise, materials with porosity in the micrometer range (or more) are often called macroporous materials. This general classification can be applied to any type of porous material, namely inorganic, hybrid or organic ones.

Macroporous polymers

Macroporous materials may be prepared by different techniques through the use of miscellaneous porogens. In 1967, Seidl et al. [START_REF] Seidl | Makroporöse Styrol-Divinylbenzol-Copolymere und ihre Verwendung in der Chromatographie und zur Darstellung von Ionenaustauschern[END_REF] distinguished three main synthetic strategies to prepare porous polymeric matrices: (i) by using a porogenic solvent, (ii) by using a non-solvent as the porogen or (iii) by adding a linear polymer as a macromolecular porogen.

According to Švec and Fréchet, these synthetic strategies are the most commonly used, but above all they are easy to implement [START_REF] Svec | New Designs of Macroporous Polymers and Supports: From Separation to Biocatalysis[END_REF].

When a solvent is used as a porogenic agent, the initiator, monomer(s) and crosslinker are dissolved in a solvent or in a solvent mixture. When the polymerization is triggered by photochemical or thermal decomposition of the initiator, polymeric particles nucleate, grow, and coalesce in the solvent. Depending on the affinity of the solvent for the growing cross-linked polymer, the former will be ejected more or less promptly from the polymer matrix through a syneresis phenomenon, creating voids filled with solvent. Upon porogenic solvent removal, these voids generate the porosity within the polymeric material. As the moment of the solvent ejection is closely related to the solvent and polymer compatibility, the pore size of the resulting materials can be thus easily tuned by merely changing the solvent polarity. In addition, the porosity ratio will also be dictated by the comonomers/(co)solvent(s) volume ratio. Finally, the porosity can be open or closed, depending on the amount of porogenic agent chosen. For very low porogenic solvent(s) to (co)monomers ratio, it is particularly true.

Such a way of generating porosity within polymeric materials has been widely implemented notably for preparing polymer-based monolithic columns with an interconnected porosity. This enables liquids or gases to easily flow through such monolithic columns, depending on the average pore size of the material as well as on the viscosity of the solvent, to avoid too high back pressures. Different monolithic columns have been prepared so far following this synthetic strategy, whatever the nature of the monomer used. Generally, the monomer is functional, that is to say it possesses a chemical moiety that can be easily chemically modified through a post-polymerization step consisting of a reaction occurring at the interface of the pore with the surrounding fluid, allowing for the interfacial properties of the pore surface to be easily tuned. Historically, the first monolithic capillaries were prepared by Švec's research group in the mid-1990's using glycidyl methacrylate (GMA) as a functional monomer [START_REF] Svec | Modified poly(glycidyl methacrylate-co-ethylene dimethacrylate) continuous rod columns for preparative-scale ion-exchange chromatography of proteins[END_REF]. GMA bears an epoxide moiety that can be easily functionalized through ring-opening reaction with rather strong nucleophiles like amines [START_REF] Svec | Modified poly(glycidyl methacrylate-co-ethylene dimethacrylate) continuous rod columns for preparative-scale ion-exchange chromatography of proteins[END_REF]. Later on, the same research group has deeply expanded his pioneering works on the exploitation of GMA monomer, notably for chromatographic applications [START_REF] Xu | Porous Polymer Monolithic Column with Surface-Bound Gold Nanoparticles for the Capture and Separation of Cysteine-Containing Peptides[END_REF]. It is now used in other laboratories [START_REF] Luo | Chromatographic separation of proteins on metal immobilized iminodiacetic acid-bound molded monolithic rods of macroporous poly(glycidyl methacrylate-co-ethylene dimethacrylate)[END_REF][START_REF] Floris | Agglomerated polymer monoliths with bimetallic nano-particles as flow-through micro-reactors[END_REF], allowing for a plethora of potential applications to be envisioned. In the late 1990's, 4-chloromethyl styrene, also known as 4-vinylbenzyl chloride (4-VBC or VBC), was investigated in applications related to monolithic columns [START_REF] Gusev | Capillary columns with in situ formed porous monolithic packing for micro high-performance liquid chromatography and capillary electrochromatography[END_REF]. This styrenic monomer gave birth to highly hydrophobic columns, while the pore surface of the resulting materials can be easily tuned by nucleophilic substitution of the benzylic chlorine. It is worth mentioning that such functionalization reactions can lead to hypercrosslinked materials, provided that the chemical graft to anchor possesses two identical reactive groups [START_REF] Lv | Hypercrosslinked Large Surface Area Porous Polymer Monoliths for Hydrophilic Interaction Liquid Chromatography of Small Molecules Featuring Zwitterionic Functionalities Attached to Gold Nanoparticles Held in Layered Structure[END_REF]. 2-Vinyl-4,4dimethylazlactone (VDMA) is another interesting monomer used for the preparation of porous materials [START_REF] Xie | Design of reactive porous polymer supports for high throughput bioreactors: Poly(2-vinyl-4,4-dimethylazlactone-co-acrylamide-co-ethylene dimethacrylate) monoliths[END_REF]. Indeed, VDMA can be readily incorporated into the composition of polymerization mixtures in conjunction with diverse hydrophilic monomers, e.g. 2-

hydroxyethyl methacrylate (HEMA) and acrylamide (AM), to prepare functional in-capillary monoliths that can be functionalized with amine bearing bio(macro)molecules [START_REF] Xie | Design of reactive porous polymer supports for high throughput bioreactors: Poly(2-vinyl-4,4-dimethylazlactone-co-acrylamide-co-ethylene dimethacrylate) monoliths[END_REF]. Finally, N-acryloxysuccinimide (NAS) and glycidyl carbonate methacrylate (GCMA) have been more

recently implemented for the design of innovative functional porous in-capillary columns, as shown in Fig. 1A [START_REF] Guerrouache | In situ functionalization of N-acryloxysuccinimide-based monolith for reversed-phase electrochromatography[END_REF]. NAS can undergo nucleophilic substitution due to the presence of pendant activated ester moieties and has been widely used for chromatographic applications, such as capillary electrochromatography (CEC) separations [START_REF] Guerrouache | Functionalization of Macroporous Organic Polymer Monolith Based on Succinimide Ester Reactivity for Chiral Capillary Chromatography: A Cyclodextrin Click Approach[END_REF][START_REF] Guerrouache | Capillary columns for reversed-phase CEC prepared via surface functionalization of polymer monolith with aromatic selectors[END_REF][START_REF] Dao | Thiol-yne Click Adamantane Monolithic Stationary Phase for Capillary Electrochromatography[END_REF][START_REF] Tijunelyte | Hydrophilic monolith with ethylene glycol-based grafts prepared via surface confined thiol-ene click photoaddition[END_REF][START_REF] Kebe | Thiol-ene click chemistry for the design of diol porous monoliths with hydrophilic surface interaction ability: a capillary electrochromatography study[END_REF][START_REF] Mekhalif | Novel Monolithic Stationary Phase with Surface-Grafted Triphenyl Selector for Reversed-Phase Capillary Electrochromatography[END_REF] or for flow through catalysis applications [START_REF] Poupart | Copper nanoparticles supported on permeable monolith with carboxylic acid surface functionality: Stability and catalytic properties under reductive conditions[END_REF]. Alternatively, oligomeric or polymeric chains have also been used as porogens in other studies (Fig. 1B) [START_REF] Lav | Porous polystyrene-based monolithic materials templated by semi-interpenetrating polymer networks for capillary electrochromatography[END_REF][START_REF] Lav | Porous Poly(styrene-codivinylbenzene) Neutral Monolith: From Design and Characterization to Reversed-Phase Capillary Electrochromatography Applications[END_REF]. Other porogens than those defined by Seidl and coworkers are nowadays commonly used for different purposes. Especially, macroporogen templating has gained a tremendous interest in the last years. It relies on the use of a so-called template which acts as a macroporogen. It is added to the initial polymerization mixture (consisting of the initiator, the (co)monomer(s) and the cross-linker(s)) and immediately removed after the polymerization completion. The judicious choice of this porogenic template notably allows for tuning the pore morphology as the pores will present a shape that perfectly matches the template imprint.

Such porogenic templates are based on solid, mostly inorganic crystal particle, such as sodium chloride (NaCl) [START_REF] Lin | Preparation of macroporous biodegradable PLGA scaffolds for cell attachment with the use of mixed salts as porogen additives[END_REF], calcium carbonate (CaCO3) [START_REF] Chow | Novel Fabrication of Open-Pore Chitin Matrixes[END_REF] or ammonium bicarbonate (NH4HCO3) [START_REF] Lin | Preparation of macroporous biodegradable PLGA scaffolds for cell attachment with the use of mixed salts as porogen additives[END_REF] particles, for instance. The use of such a methodology presents some non-negligible advantages as it permits to vary the size (by particle sieving) and morphology (depending on the shape of selected porogen) of the pores, while their removal is generally simple to achieve through easy template leaching into an appropriate aqueous solution. In fact, they are usually dissolved in pure water through particle leaching, such as for the extraction of NaCl particles.

Alternatively, CaCO3 particle-based templates require an acidic aqueous solution to be removed from the polymer matrix. It is worth mentioning that the porogenic template could also be prepared with the desired shape [START_REF] Zhang | A comparative study of porous scaffolds with cubic and spherical macropores[END_REF] (Fig. 1C). As a matter of fact, other investigations reported on the use of different sacrificial templates derived from organic (macro)molecules, such as paraffin or poly(methyl methacrylate) (PMMA) beads. In this case, the porogen could also be dissolved via Soxhlet extraction with an appropriate organic solvent. LaNasa et al. [START_REF] Lanasa | Presence of pores and hydrogel composition influence tensile properties of scaffolds fabricated from well-defined sphere templates[END_REF] and Le Droumaguet et al. [START_REF] Droumaguet | Engineering functional doubly porous PHEMA-based materials[END_REF] independently demonstrated that it is possible to successfully use sintered polymeric PMMA beads as an original porogenic template (Fig. 1D). Such sintered PMMA beads could further be extracted in organic solvent(s), while the porous polymeric matrix remains intact, due to permanent cross-linking.

These sintered spherical beads allowed for the generation of interconnected spherical pores upon removal of the macroporogen.

Nanoporous polymers

Miscellaneous approaches have been developed so far to prepare nanoporous polymerbased materials. These materials are mostly used as filtration membranes. In this particular case, the process is subtler than for the preparation of macroporous materials, and it usually involves the removal of a sacrificial polymer segment from nanostructured precursors, thus generating pores at the nanoscale level. Techniques like track-etching which lead to nanopores will not be addressed here, but reviews discussing the subject may be easily found [START_REF] Apel | Track etching technique in membrane technology[END_REF].

A largely investigated strategy consists in specifically removing one sacrificial block from oriented block copolymers (BCPs), thus leading to ordered nanoporous materials via a process milder than that used with other strategies. BCPs and especially diblock copolymers develop very precise equilibrium domain morphologies depending on three main critical parameters, i.e. the volume fraction of both blocks f, the number of repeating units N in the copolymer, and χAB the Flory-Huggins interaction parameter between the two different blocks [START_REF] Matsen | Unifying Weak-and Strong-Segregation Block Copolymer Theories[END_REF]. According to theoretical phase diagrams, body-centered spheres, hexagonally closepacked cylinders, bicontinuous gyroids or alternating lamellae can be obtained (Fig. 2) [START_REF] Bates | Block copolymers-designer soft materials[END_REF][START_REF] Cochran | Stability of the Gyroid Phase in Diblock Copolymers at Strong Segregation[END_REF]. Polydispersity of both blocks is also a key parameter to precisely control the morphology of the block copolymers after orientation of the nanodomains. Indeed, a high polydispersity of the minority block has been shown to lead to a change of morphology due to a larger interfacial curvature [START_REF] Lynd | Influence of polydispersity on the self-assembly of diblock copolymers[END_REF]. On the opposite, a high polydispersity of the majority one was demonstrated to lead to a change of morphology due to a smaller interfacial curvature. So far, different natures of sacrificial blocks have been implemented to prepare nanoporous materials from this strategy. Historically, the first diblock copolymer precursors used were constituted of a degradable polyisoprene (PI) block and of a stable polystyrene-derived (4vinylphenyl)dimethyl-2-propoxysilane) block, both synthetized by anionic polymerization.

The former block was removed by ozonolysis that selectively cleaved the carbon-carbon double bonds of the isoprene units, while the polystyrene (PS) segment was simultaneously crosslinked, thus revealing nanoporous PS-based networks [START_REF] Lee | Polymerization of monomers containing functional silyl groups. 5. Synthesis of new porous membranes with functional groups[END_REF]. Later on, other research groups have successfully developed other sacrificial blocks by varying the conditions of etching [START_REF] Grande | Design of functional nanoporous polymeric materials from sel-organized block copolymers[END_REF]. Herein, we will focus on poly(D,L-lactide) (PLA), which has been widely used in the area of nanoporous polymers. PLA can be synthetized via anionic or coordinative ring-opening polymerization (ROP) of 3,6-dimethyl-1,4-dioxane-2,5-dione (usually called D,L-lactide), from alcohol- [START_REF] Gorzolnik | Novel Functional Mesoporous Materials Obtained from Nanostructured Diblock Copolymers[END_REF] or amine-based [START_REF] Gorzolnik | Design of Mesoporous Materials with Controlled Porosity and Functionality from Nanostructured Diblock Copolymers[END_REF] initiators in the presence of organic [START_REF] Chuma | The Reaction Mechanism for the Organocatalytic Ring-Opening Polymerization of l-Lactide Using a Guanidine-Based Catalyst: Hydrogen-Bonded or Covalently Bound?[END_REF] or organo-metallic [START_REF] Cross | Aluminum salen and salan complexes in the ring-opening polymerization of cyclic esters: Controlled immortal and copolymerization of rac-β-butyrolactone and rac-lactide[END_REF] catalysts. PLA is generally etched in mild conditions, namely in alkaline conditions and especially in NaOH or KOH hydro-alcoholic solutions, as described by numerous studies from Hillmyer and coworkers first in 2001 [START_REF] Zalusky | Mesoporous Polystyrene Monoliths[END_REF] or later on from our research group [START_REF] Grande | Functionalized ordered nanoporous polymeric materials: From the synthesis of diblock copolymers to their nanostructuration and their selective degradation[END_REF][START_REF] Majdoub | Original route to polylactide-polystyrene diblock copolymers containing a sulfonyl group at the junction between both blocks as precursors to functional nanoporous materials[END_REF] (Fig. 3A). It is worth noticing that PLA can also be degraded in acidic conditions. It is not until recently that such etching conditions have been reported in the literature for PS-b-PLA [START_REF] Sarkar | Robust porous polymers enabled by a fast trifluoroacetic acid etch with improved selectivity for polylactide[END_REF]. Another somehow smarter strategy to etch the sacrificial block from BCPs relies on the selective cleavage of the junction present between both block. Indeed, the degradable character of such a junction can lead to easy and straightforward removal of the entire sacrificial block in a non-solvent of the remaining block. Such a strategy notably permits to carry out the sacrificial block removal in milder conditions than those employed for the chemical degradation of the block itself and to ensure the presence of a well-defined functional group at the pore surface. Russell's group pioneered this elegant strategy by precisely positioning an anthracene photodimer at the junction between both blocks of a polystyrene-block-poly(methyl methacrylate), the PMMA sacrificial block being further released from the oriented diblock copolymers by dissociation of the photodimer under UV or thermal stimuli, thus revealing nanopores [START_REF] Goldbach | Synthesis and Thin Film Characterization of Poly(styrene-block-methyl methacrylate) Containing an Anthracene Dimer Photocleavable Junction Point[END_REF]. Later on, different selectively cleavable chemical junctions have been used in diblock copolymers. They can be categorized into three main sub-classes. The first one relies on using an irreversibly cleavable junction between both blocks (Table 1). Trityl ether [START_REF] Yurt | Scission of Diblock Copolymers into Their Constituent Blocks[END_REF][START_REF] Zhang | Highly Ordered Nanoporous Thin Films from Cleavable Polystyrene-block-poly(ethylene oxide)[END_REF] which is easily cleavable by trifluoroacetic acid (TFA), o-nitrobenzyl ester [START_REF] Kang | Synthesis of Photocleavable Poly(styrene-block-ethylene oxide) and Its Self-Assembly into Nanoporous Thin Films[END_REF][START_REF] Schumers | A versatile strategy for the synthesis of block copolymers bearing a photocleavable junction[END_REF][START_REF] Zhao | Highly Ordered Nanoporous Thin Films from Photocleavable Block Copolymers[END_REF] or carbamate [START_REF] Gamys | Amine-functionalized nanoporous thin films from a poly(ethylene oxide)-block-polystyrene diblock copolymer bearing a photocleavable o-nitrobenzyl carbamate junction[END_REF] derivative, hemiacetal junction [START_REF] Ouchi | Consecutive living polymerization from cationic to radical: a straightforward yet versatile methodology for the precision synthesis of "cleavable" block copolymers with a hemiacetal ester junction[END_REF] or acetal moiety [START_REF] Poupart | Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(d,l-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes[END_REF][START_REF] Satoh | A facile synthesis of clickable and acid-cleavable PEO for acid-degradable block copolymers[END_REF] for instance have been successfully implemented in this context. Other strategies involving reversible junctions have also been put forward, such as the aforementioned [4π+4π] anthracene photodimer [START_REF] Goldbach | Synthesis and Thin Film Characterization of Poly(styrene-block-methyl methacrylate) Containing an Anthracene Dimer Photocleavable Junction Point[END_REF][START_REF] Goldbach | Nano-to Macro-Sized Heterogeneities Using Cleavable Diblock Copolymers[END_REF], disulfide bridges, [START_REF] Droumaguet | Clickable" thiol-functionalized nanoporous polymers: from their synthesis to further adsorption of gold nanoparticles and subsequent use as efficient catalytic supports[END_REF][START_REF] Ryu | Highly Ordered Gold Nanotubes Using Thiols at a Cleavable Block Copolymer Interface[END_REF] oxiimines [START_REF] Rao | Synthesis and self-assembly of dynamic covalent block copolymers: towards a general route to pore-functionalized membranes[END_REF] or hetero Diels-Alder adducts arising from RAFT agents [START_REF] Glassner | Formation of nanoporous materials via mild retro-Diels-Alder chemistry[END_REF], for instance, as depicted in Table 2. Finally, other investigations reported on the possibility to implement supramolecular junctions to link both blocks in a non-covalent manner, as shown in Table 3.

We can notably mention the use of terpyridine-ruthenium [START_REF] Fustin | Nanoporous Thin Films from Self-Assembled Metallo-Supramolecular Block Copolymers[END_REF] or terpyridine-nickel [START_REF] Mugemana | Functionalized Nanoporous Thin Films from Metallo-Supramolecular Diblock Copolymers[END_REF] complexes, ionic interactions [START_REF] Yu | Nanoporous thin films from ionically connected diblock copolymers[END_REF] or even hydrogen bonds [START_REF] Montarnal | Highly Ordered Nanoporous Films from Supramolecular Diblock Copolymers with Hydrogen-Bonding Junctions[END_REF] taking place between the two adjacent blocks of oriented copolymers. Finally, some polymeric materials develop intrinsic free volume that can be viewed as micropores; this is the so-called intrinsic microporosity. Such micropore sizes notably allow such types of material to be used for gas-related applications [START_REF] Dawson | Nanoporous organic polymer networks[END_REF][START_REF] Mckeown | Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage[END_REF][START_REF] Sakaushi | Carbon-and Nitrogen-Based Organic Frameworks[END_REF] especially for separation sciences, gas storage or heterogeneous catalysis in the gaseous phase. Only monomers that offer a good rigidity to the polymer network can prevent them from pore collapse and enable a permanent microporosity. Polymeric microporous polycyanurate [START_REF] Shen | Synthesis of 1,3,5,7-tetrakis(4-cyanatophenyl)adamantane and its microporous polycyanurate network for adsorption of organic vapors, hydrogen and carbon dioxide[END_REF], polyisocyanurate [START_REF] Zhang | Microporous Polyisocyanurate and Its Application in Heterogeneous Catalysis[END_REF], polyurethane [START_REF] Dey | Microporous polyurethane material for size selective heterogeneous catalysis of the Knoevenagel reaction[END_REF], polytriazine [START_REF] Kuhn | Porous, Covalent Triazine-Based Frameworks Prepared by Ionothermal Synthesis[END_REF] (Fig. 3B), based on Tröger's base [START_REF] Carta | Heterogeneous organocatalysts composed of microporous polymer networks assembled by Tröger's base formation[END_REF] or porous aromatics frameworks [START_REF] Ben | Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area[END_REF][START_REF] Verde-Sesto | First Pre-Functionalised Polymeric Aromatic Framework from Mononitrotetrakis(iodophenyl)methane and its Applications[END_REF] can be found among such innovative materials.

Biporous polymers

Biporous materials have gained a tremendous interest within the last decades in different research area, such as civil engineering, tissue engineering or even drug delivery.

Scientists are now able to prepare porous polymers possessing at least two distinct levels of porosity. Several methods have been so far developed to prepare biporous materials, including rapid prototyping (also called solid free-form fabrication or additive manufacturing) which is undergone by 3D printing [START_REF] Zeltinger | Effect of pore size and void fraction on cellular adhesion, proliferation, and matrix deposition[END_REF], gas foaming [START_REF] Ruiz | Two-step micro cellular foaming of amorphous polymers in supercritical CO2[END_REF], temperature-induced phase separation (TIPS) [START_REF] Ly | Tailoring doubly porous poly(2hydroxyethyl methacrylate)-based materials via thermally induced phase separation[END_REF], the polyHIPE technique or the double porogen templating approach. Due to purpose considerations, this part will essentially focus on electropsun materials, polyHIPEs, and doubly porous materials derived from the double porogen templating approach, as these three strategies were recently implemented in the literature for the preparation of hybrid polymer-based materials dedicated to supported catalysis.

Closely related to the use of a 3-D printer (i.e. rapid prototyping) for constructing biporous materials, electrospinning is a powerful technique that allows for the formation of highly porous scaffolds from solutions of polymeric materials [START_REF] Huang | A review on polymer nanofibers by electrospinning and their applications in nanocomposites[END_REF]. The electrospinning process was patented by Anton Formhals in 1934 [START_REF] Formhals | Process and apparatus for preparing artificial threads[END_REF], and it was intensively developed by

Reneker's research group [START_REF] Reneker | Nanometre diameter fibres of polymer, produced by electrospinning[END_REF][START_REF] Reneker | Electrospinning of Nanofibers from Polymer Solutions and Melts[END_REF] in the 1990's and 2000's with the emergency of nanotechnologies. Electrospun materials are prepared by applying a high voltage electrostatic field (usually in the 10-30 kV range) between a syringe containing a viscous polymeric solution and a collector for the deposition of polymeric fibers. Due to the fiber packing, pores with a wide pore size distribution are generated between fibers, thus allowing for the production of macroporous materials with highly interconnected voids and a large ratio of surface area to volume [START_REF] Villarreal-Gómez | Electrospinning as a powerful technique for biomedical applications: a critically selected survey[END_REF]. Electrospun polymeric fibers present morphological similarities as natural collagen fibrils, and their morphologies can be easily tuned by varying different parameters, such as the voltage, the syringe needle-to-collector distance, the polymer solution flow rate, the solvent volatility and polarity, the polymer solution viscosity and conductivity, etc. Therefore, choosing appropriate (co)solvents and electrospinning parameters are crucial to finely control the porous features of resulting polymeric materials (Fig. 4A) [START_REF] Bognitzki | Nanostructured Fibers via Electrospinning[END_REF].

Alternatively, polymerization of High Internal Phase Emulsions (HIPEs) is a technique of choice to prepare biporous polymeric frameworks [START_REF] Cameron | Non-aqueous high internal phase emulsions. Preparation and stability[END_REF][START_REF] Silverstein | PolyHIPEs -Porous Polymers from High Internal Phase Emulsions[END_REF]. Such materials were first designed by Barby and Haq from Unilever in the early 1980's [START_REF] Barby | Low density porous cross-linked polymeric materials and their preparation[END_REF]. In this patent, the authors reported on the preparation of a biporous polymer network obtained after the polymerization of a high internal phase emulsion, the porous polymer being subsequently called polyHIPE. In such polyHIPE-based scaffolds, the higher porosity level arises from drops of the discontinuous phase of the emulsion, while the lower one originates from the interconnections between adjacent pores. PolyHIPEs can be prepared from water-in-oil (w/o) [START_REF] Krajnc | Monolithic Scavenger Resins by Amine Functionalizations of Poly(4-vinylbenzyl chloride-co-divinylbenzene) PolyHIPE Materials[END_REF], oil-in-water (o/w) [START_REF] Krajnc | Acrylic Acid "Reversed" PolyHIPEs[END_REF] emulsions or from emulsified biphasic systems constituted by two immiscible liquids [START_REF] Kot | Non-aqueous high internal phase emulsion templates for synthesis of macroporous polymers in situ filled with cyclic carbonate electrolytes[END_REF] in the presence of surfactants that help to the emulsion stabilization. Due to monomers and cross-linkers remaining in the external continuous phase, w/o emulsions are appropriate to yield hydrophobic polymers such as styrenic polyHIPEs [START_REF] Cameron | Preparation and glass transition temperatures of elastomeric PolyHIPE materials[END_REF] while o/w emulsions are suitable for the production of more hydrophilic polymers (Fig. 4B) [START_REF] Krajnc | Acrylic Acid "Reversed" PolyHIPEs[END_REF]. Reproduced with permission from Springer.

Last but not least, the double porogen templating approach consists in using two different types of porogenic agents, i.e. one for the generation of pores within the micrometer range and the other one to obtain pores within the nanometer range. For instance, the use of electrospinning and particle leaching [START_REF] Mehraban | Preparation of porous nanofibers from electrospun polyacrylonitrile/calcium carbonate composite nanofibers using porogen leaching technique[END_REF] (Fig. 5A) allowed for the preparation of materials with a macroporosity generated by the fibers organization and a nanoporosity revealed by calcium carbonate particle leaching using 1, 3, and 5% v/v of HCl aqueous solution, increasing the specific surface area of the porous scaffolds. Another interesting approach using two different porogens was described by Ly et al. Poly(HEMA-co-EGDMA) monoliths were synthetized using macroporogenic agents consisting of fused PMMA beads [START_REF] Ly | Designing and modeling doubly porous polymeric materials[END_REF] or

NaCl particles [START_REF] Ly | Designing and modeling doubly porous polymeric materials[END_REF][START_REF] Ly | Facile fabrication of doubly porous polymeric materials with controlled nano-and macro-porosity[END_REF][START_REF] Ly | Biporous Crosslinked Polymers With Controlled Pore Size and Connectivity[END_REF][START_REF] Ly | Functionalized Doubly Porous Networks: From Synthesis to Application in Heterogeneous Catalysis[END_REF][START_REF] Ly | Versatile functionalization platform of biporous poly(2-hydroxyethyl methacrylate)-based materials: Application in heterogeneous supported catalysis[END_REF] and different porogenic solvents (Fig. 5B,C) [START_REF] Ly | Facile fabrication of doubly porous polymeric materials with controlled nano-and macro-porosity[END_REF]. The fused macroporogens allow for the pores generated after particle leaching to be interconnected, while the porogenic solvent gives rise to a lower porosity level, thus enhancing the specific surface area of the resulting doubly porous materials. Different experimental conditions such as the particle sintering conditions (Spark Plasma Sintering vs. vacuum oven), the particle morphology (spherical vs. cubic) as well as the porogenic solvent nature were carefully investigated. Such studies have led to optimized materials on which were immobilized gold nanoparticles, thus leading to porous hybrid materials meant for heterogeneous supported catalysis [START_REF] Ly | Functionalized Doubly Porous Networks: From Synthesis to Application in Heterogeneous Catalysis[END_REF][START_REF] Ly | Versatile functionalization platform of biporous poly(2-hydroxyethyl methacrylate)-based materials: Application in heterogeneous supported catalysis[END_REF].

Characterization techniques of porous materials

Different techniques have been so far developed for the fine characterization of porous materials. In this way, critical information, including pore size, pore size distribution, pore connectivity (open vs. closed, i.e. presence or absence of interconnections between adjacent pores), and specific surface area, can be accurately determined using complementary physicochemical techniques in the laboratory.

Two different techniques are mainly used to determine the pore size of porous polymeric materials. Such techniques rely on the type of porous materials under investigation.

Mercury intrusion porosimetry (also shortened as MIP, Table 4) is a specific technique consisting in intruding a non-wetting liquid, i.e. mercury, into a porous sample placed within a penetrometer. Upon applying an increasing pressure, mercury is forced to intrude into the pores of the material [START_REF] Webb | Analytical methods in fine particle technology: Micromeritics Instrument Corp[END_REF]. Mercury does not wet materials, and so it will not penetrate pores by capillary action, excepted if it is forced to do so by applying the said pressure. The pore size can then be determined by correlating the pressure required for mercury intrusion into the pores to the pore size via the Washburn equation (Equation 1) [108]:

𝑃 = -4𝛾 cos 𝜃 𝐷 ( 1
)
where P is the pressure applied for the mercury intrusion, γ is the mercury surface tension, θ is the contact angle between the mercury and the pore wall, and D stands for the diameter of the pore being intruded.

It is noteworthy that the pores are supposed to by cylindrical in this equation, while most of the real porosity is not. As said before, for MIP analyses, the porosimeter apply a pressure to force mercury intruding the pores, and the Washburn equation allows for determining the pore diameter [START_REF] Moura | Mercury intrusion porosimetry in pulp and paper technology[END_REF]. Applying an increasing pressure indeed pushes known amounts of mercury into the porosity of the material, and it thus allows for determining the pore volume for a precise pore size. Additionally, knowing the distribution of the pore volume with respect to its pore size provides the pore size distribution [110]. However, this technique has some limitations especially regarding pore sizes in the nanometer range.

For materials exhibiting pore sizes that range from 2 to 300 nm, gas sorption is much more appropriate, especially for accuracy reasons [START_REF] Rouquerol | Recommendations for the characterization of porous solids (Technical Report)[END_REF]. Gas sorption measurements can use different gases: mostly N2 [START_REF] Feller | N2-BET Specific Surface Areas of Some Low Activity Clay Soils and Their Relationships With Secondary Constituents and Organic Matter Contents[END_REF] is used, but CO2 [START_REF] Kim | Applicability of using CO2 adsorption isotherms to determine BET surface areas of microporous materials[END_REF] or Kr [START_REF] Yanazawa | Precision Evaluation in Kr Adsorption for Small BET Surface Area Measurements of Less Than 1 m2[END_REF] can also be employed at a precise temperature to obtain isotherms. Gas sorption porosimetry is now routinely used to determine the specific surface area of porous materials, using the Brunauer-Emmett-Teller (BET) method [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF], whose equation is given below (Equation 2):

𝑃 𝑛 𝑎 (𝑃 0 -𝑃) = 1 𝑛 𝑚 𝑎 𝐶 + (𝐶-1) 𝑛 𝑚 𝑎 𝐶 × 𝑃 𝑃 0 (2)
where n a is the gas amount adsorbed at the relative pressure

𝐴(𝐵𝐸𝑇) = 𝑛 𝑚 𝑎 × 𝐿 × 𝑎 𝑚 (3)
where A(BET) is the specific surface area, L the Avogadro constant, and am the average area occupied by each adsorbed molecule in the complete monolayer (i.e. the molecular crosssectional area).

Gas sorption measurements can also give pore sizes for porous materials exhibiting a porosity from 2 to 300 nm using the so-called Barrett-Joyner-Halenda (BJH) method (Table 4) [START_REF] Barrett | The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms[END_REF]. Similarly to MIP, this calculation method leads to a pore size distribution, but it is limited for materials exhibiting pores higher than 0.1 µm.

Alternatively, a less widespread characterization technique can be used for the porosity characterization of mesoporous materials, namely thermoporometry (Table 4), based on differential scanning calorimetry (DSC) [START_REF] Iza | Determination of pore size distribution for mesoporous materials and polymeric gels by means of DSC measurements: thermoporometry[END_REF]. This technique relies on the Gibbs-Thomson equation (Equation 4) [START_REF] Kim | Applicability of using CO2 adsorption isotherms to determine BET surface areas of microporous materials[END_REF]:

𝐷 𝑝 = 2 (𝐴 + 𝐵 𝑇 𝑚 -𝑇 𝑚0 ) ( 4
)
where Dp is the pore diameter, A and B are constants depending on saturating solvent, pore geometry and measurements on cooling or heating, Tm and Tm0 are the melting temperatures of confined liquid and bulk liquid, respectively.

If a porous material is filled with a liquid, and then the latter is frozen, the melting temperature Tm of the liquid will not be the same for that confined in the pores and that in the bulk. Tm for the liquid in the pores will be lower, and the difference between Tm and the bulk liquid melting temperature provides the pore diameter according to Equation 4. Besides, comparing data from freezing and melting phenomena leads to precious information regarding the pore shape. The limitation of this technique is due to its principle: if the pore size is too high, the liquid confined into the pores will act as a bulk liquid. Therefore, the melting peak of the confined liquid will be hidden into the profile of the bulk liquid melting peak and no differences will be observable. However, DSC-based thermoporometry has been proved to be effective using various solvents, like water [START_REF] Brun | A new method for the simultaneous determination of the size and shape of pores: the thermoporometry[END_REF], benzene [START_REF] Brun | A new method for the simultaneous determination of the size and shape of pores: the thermoporometry[END_REF], cyclohexane [START_REF] Jones | Nanoporous Materials Derived from Polymeric Bicontinuous Microemulsions[END_REF] or acetonitrile [START_REF] Wulff | Pore size determination by thermoporometry using acetonitrile[END_REF] that are commonly used solvents. Another variation of the technique relies on the use of nuclear magnetic resonance (NMR) [START_REF] Strange | Pore surface exploration by NMR[END_REF]. Finally, pycnometry can be used to determine an "apparent density" (more precisely a volume) of a porous solid, which is defined as the ratio between its mass and the total volume enclosed by an envelope of fluid. Pycnometry mostly uses gases, such as helium (He), but it can also be achieved with liquids, such as water or xylene. A typical pycnometer consists of two sealed chambers connected between them by a valve. The first chamber is used as a reference and the second one holds the sample. The sample chamber is filled with the fluid, while the other one is still under vacuum. Then, the valve is opened and the fluid is allowed to expand into the second chamber at a precise temperature while the pressure is measured, thus giving the volume of the sample VS, i.e. the open porosity volume using the Boyle-Mariotte law (Equation 5) [START_REF] Dlapka | Defining the pores in PM components[END_REF]:

𝑉 𝑆 = 𝑉 𝐶 - 𝑉 𝑟 1- 𝑃 1 𝑃 2 (5)
where VC is the volume of the empty sample chamber, Vr is the volume of the second chamber, P1 is the first pressure in the sample chamber and P2 is the pressure after expansion into the combined volume of the chambers.

It is important to notice that pycnometry gives the volume of the open porosity. In the case of a closed porosity in the sample, the density will be an effective one. To give access to the closed porosity, two measurements are required: one with a porous system and another with a bulk system. The comparison between both gives access to the volume of the closed porosity [START_REF] Vakifahmetoglu | A Direct Method for the Fabrication of Macro-Porous SiOC Ceramics from Preceramic Polymers[END_REF]. This method has been extended to other porous polymeric systems to enable the characterization of polymer gels for instance.

3 Application of metallic nanoparticle immobilized on porous polymers as supported catalysts

Key features of nanoparticles

Nanoparticles have been widely used as catalysts in the past decade, as they offer large surface area and consequently enhanced catalytic activity. Well-documented reviews discussing both synthesis and catalysis aspects can be found in the literature [START_REF] Moreno-Mañas | Formation of Carbon-Carbon Bonds under Catalysis by Transition-Metal Nanoparticles[END_REF][START_REF] Astruc | Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis[END_REF] and, in most cases, these reviews focused on a sole metal type. The reader is kindly referred to the reviews by Takale et al. [START_REF] Takale | Gold nanoparticle (AuNPs) and gold nanopore (AuNPore) catalysts in organic synthesis[END_REF] or Daniel & Astruc [START_REF] Daniel | Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology[END_REF] for critical discussion about gold nanoparticles (Au NPs) and their catalytic uses, while Gawande et al. [START_REF] Gawande | Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis[END_REF] and Ranu et al.

[129] focused on copper nanoparticles (Cu NPs). Chen & Holt-Hindle [START_REF] Chen | Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications[END_REF] and Astruc [START_REF] Astruc | Palladium Nanoparticles as Efficient Green Homogeneous and Heterogeneous Carbon-Carbon Coupling Precatalysts: A Unifying View[END_REF] detailed platinum (Pt NPs), and palladium (Pd NPs) nanoparticles, respectively. Herein, we purposely restricted the discussion to the cases of metal NPs supported on organic porous polymers for catalysis in organic chemistry. Although supported catalysts have been much less discussed, reviews present in the literature focused mainly on inorganic materials as supports [START_REF] Ranu | Copper Nanoparticle-Catalyzed Carbon Carbon and Carbon Heteroatom Bond Formation with a Greener Perspective[END_REF]. More specifically, one may cite the comprehensive survey by Corma and Garcia [START_REF] Chen | Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications[END_REF] who exclusively summarized recent trends in nanogold supported onto inorganic supports as catalysts for organic synthesis as well as that from Campelo and coworkers [START_REF] Astruc | Palladium Nanoparticles as Efficient Green Homogeneous and Heterogeneous Carbon-Carbon Coupling Precatalysts: A Unifying View[END_REF] describing the synthesis and applications of nanoparticles of various metals.

Regarding the immobilization of nanoparticles onto solid supports, two main ways have been reported to date in the literature, hereafter referred to as ex-situ and in-situ ways (Fig. 6). It is crucial to mention that in order to achieve robust surface anchoring of metal nanoparticles (MNPs), the supports may preferably bear accessible chemical moieties able to induce specific interactions with the metal in its ionic or reduced forms. Amines, thiols, cyanos, and carboxylic acids are representative examples ensuring strong interactions with metals, such as gold, copper, palladium. In some cases, the interactions can occur between the chemical units attached on solid surface and the stabilizing agent decorating the NP surface.

Porous polymers with chelating ability can be easily designed using monomers incorporating functional side groups that may act as either chelation sites (for example 2-(dimethylamino)ethyl methacrylate) or reactive sites for post-polymerization functionalization. In the latter case, one may cite GMA, 4-VBC, NAS, or GCMA.

The ex-situ way requires the synthesis of the NPs prior to their immobilization onto the pore surface. Well-established methods were typically applied for the synthesis of metal colloids. In a further step, the NPs are immobilized onto the solid supports. A major advantage of the ex-situ approach is that it offers the possibility to use commercially available nanoparticles. Although it makes easier the whole synthesis process and provides fine control over the size distribution and colloidal stability of the NPs, it may be restrictive in terms of available size and shape. In contrast, the in-situ way implies the generation of the NPs in presence of the solid supports and is thus a deposition precipitation process [START_REF] Munnik | Recent Developments in the Synthesis of Supported Catalysts[END_REF]. First, the support is immerged into a solution containing the salt of the metal of interest for impregnation purpose. Then, the metal is reduced to its zero state through a reduction step. Several reducing agents are routinely used in organic synthesis (Table 5) and can be also envisaged for metal reduction, like NaBH4 [START_REF] Wuithschick | Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on Mechanistic Understanding[END_REF][START_REF] Wuithschick | Illustrating the formation of metal nanoparticles with a growth concept based on colloidal stability[END_REF], citrates [START_REF] Wuithschick | Turkevich in New Robes: Key Questions Answered for the Most Common Gold Nanoparticle Synthesis[END_REF][START_REF] Kettemann | Missing Piece of the Mechanism of the Turkevich Method: The Critical Role of Citrate Protonation[END_REF] (in the so-called Turkevitch process), hydrazine [START_REF] Boutonnet | The preparation of monodisperse colloidal metal particles from microemulsions[END_REF] or even H2 [START_REF] Shim | Preparation of Rh-containing polycarbonate films and the study of their chemical properties in the polymer[END_REF][START_REF] Groppo | Formation and Growth of Pd Nanoparticles Inside a Highly Cross-Linked Polystyrene Support: Role of the Reducing Agent[END_REF]. The choice of the reducing agent will depend on the size and shape desired for the NPs and is crucial to achieve controlled synthesis. Indeed, depending on the metalreducing agent pair, various levels of control on the shape, size and surface distribution will be obtained. Other important parameters are the strength of the interaction between metal ions and the chemical groups at the surface of the support as well as the reducing agent/salt precursor ratio. Although easy to implement, successful surface nanostructuration of porous polymers with nanoparticles implies that the NPs are strongly anchored so that leaching phenomenon does not occur. Indeed, the latter is detrimental to product purity as the presence of metal, even at the trace levels, may induce toxicity to human beings. As such, the selection of the chelating moiety is highly important and must be rationalized with respect to the nature of the metal and stabilizing agents. Generally speaking, the stronger the interaction between the support and the catalyst, the lower the possibility of leaching. One should of course keep in mind that the conditions for the immobilization of the NPs may differ significantly from the conditions for the catalysis applications. Changes such as solvent, pH and temperature may affect the strength of the chelation. Finally, the supported catalyst may be in contact with a variety of chemicals in the course of the catalytic cycles that may pollute the catalyst surface and eventually affect the turnover number and frequency. Thus, regeneration is usually required after several catalytic cycles.

Nanoparticles supported by macroporous polymers

Supported catalysts have not been widely described in the literature and this is particularly true in the case of porous polymer-based supports. Herein, we purposely opted for a discussion on the basis of the type of polymeric support rather than the type of the catalyzed reaction or nature of the metal NP (Table 6). As discussed above, macroporous polymers are essentially generated using a solvent as a porogen [START_REF] Svec | New Designs of Macroporous Polymers and Supports: From Separation to Biocatalysis[END_REF]. Using such a process, many examples focused on the synthesis of monolithic polymers within microchannels. The as-generated monolithic-based microsystems can be used after immobilization of metal nanoparticles either as sorbents for solid phase extraction or separation purposes or as microreactors for flow through catalysis. One may briefly mention papers from teams of Svec, Buchmeiser, Connolly or Carbonnier who have extended the concept of metal nanoparticules decorating monoliths as chromatographic microcolumns to supported catalysts in continuous flow microreactors [START_REF] Xu | Porous Polymer Monolithic Column with Surface-Bound Gold Nanoparticles for the Capture and Separation of Cysteine-Containing Peptides[END_REF][START_REF] Cao | Polymer Monoliths with Exchangeable Chemistries: Use of Gold Nanoparticles As Intermediate Ligands for Capillary Columns with Varying Surface Functionalities[END_REF][START_REF] Connolly | High-capacity gold nanoparticle functionalised polymer monoliths[END_REF][START_REF] Guerrouache | Site-specific immobilisation of gold nanoparticles on a porous monolith surface by using a thiol-yne click photopatterning approach[END_REF]. In their pioneering work, Nikbin, Ladlow & Ley described in 2007 the synthesis of a vinylbenzyl chloride-based column and its subsequent functionalization with triethylamine to generate ammonium groups at the pore surface [START_REF] Nikbin | Continuous Flow Ligand-Free Heck Reactions Using Monolithic Pd [0] Nanoparticles[END_REF]. The porous features of the obtained monolithic column were investigated both by MIP and BET techniques, showing a median pore size of 3.15 µm and a surface area of about 5 m 2 .g -1 . The available surface groups were used to immobilize Pd NPs onto the pore surface through a deposition precipitation process. First, the porous polymer was flushed with an aqueous solution of Na2PdCl4 (20 mM) followed by in-situ reduction of the surface immobilized Pd ions with sodium borohydride. The reactor was used for the Mizoroki-Heck reaction of different iodobenzyls with styrene and acrylate derivatives providing high yields above 80% (Table 7). The authors conclusively demonstrated that flowthrough processes were superior to batch reaction, notably because of the possibility of processes automation.

The Buchmeiser group worked on the preparation of porous materials into chromatographic columns based on Ring Opening Metathesis Polymerization (ROMP), and suggested several potential applications. Restricting the discussion to supported catalysis, they developed monoliths bearing platinum (Fig. 7A) or palladium-based nanoparticles, obtained through the in-situ reduction of the corresponding salts, and proved their efficiency for several catalytic reactions like hydrosilylation [START_REF] Bandari | Polymeric monolith supported Pt-nanoparticles as ligandfree catalysts for olefin hydrosilylation under batch and continuous conditions[END_REF] and carbon dioxide conversion [START_REF] Taori | Selective Reduction of CO2 with Silanes over Platinum Nanoparticles Immobilised on a Polymeric Monolithic Support under Ambient Conditions[END_REF] or Mizoroki-Heck [START_REF] Bandari | Ring-Opening Metathesis Polymerization Based Pore-Size-Selective Functionalization of Glycidyl Methacrylate Based Monolithic Media: Access to Size-Stable Nanoparticles for Ligand-Free Metal Catalysis[END_REF], Suzuki-Miyaura [START_REF] Bandari | Ring-Opening Metathesis Polymerization Based Pore-Size-Selective Functionalization of Glycidyl Methacrylate Based Monolithic Media: Access to Size-Stable Nanoparticles for Ligand-Free Metal Catalysis[END_REF] (Table 7) and Sonogashira [START_REF] Bandari | Formation of Pd-Nanoparticles within the Pores of Ring Opening Metathesis Polymerization-Derived Polymeric Monoliths for Use in Organometallic Catalysis[END_REF] (Table 7). Of particular interest, the authors reported high TurnOver Numbers (TONs, representing the number of moles of reactants that a mole of catalyst can convert before decrease in the catalyst activity).

TONs values higher than 600 000 were reported for the hydrosilylation reaction. Connolly's team used glycidyl methacrylate-and vinyl azlactone-based monoliths to anchor as-prepared gold nanoparticles [START_REF] Floris | Fabrication and characterisation of gold nano-particle modified polymer monoliths for flow-through catalytic reactions and their application in the reduction of hexacyanoferrate[END_REF] as well as bimetallic platinum/palladium nanoflowers [START_REF] Floris | Agglomerated polymer monoliths with bimetallic nano-particles as flow-through micro-reactors[END_REF]. In both cases, the NPs were synthetized ex-situ according to literature protocols, and then flushed directly into polymer-filled capillaries or pipette-tips. Amine moieties were grafted onto the polymeric surface using ethylenediamine and were used to anchor both types of nanoparticles. The as-designed supported catalysts were used to reduce a ferric complex, hexacyanoferrate Fe(CN)6 3-, into Fe(CN)6 4-using NaBH4 as a co-reagent. BET measurements performed on the GMA-and vinyl azlactone-based monoliths bearing gold and bimetallic nanoflowers showed a significant decrease in the surface area of the materials before and after adsorption of preformed nanoobjects from 40 to 12 m 2 .g -1 . The authors explained these rather surprising results by a potential clogging of the porosity by the adsorbed 20-nm sized nanoflowers. surface area lower than 10 m 2 .g -1 but a large porous volume of ca. 1.5 cm 3 .g -1 [START_REF] Guerrouache | Functionalization of Macroporous Organic Polymer Monolith Based on Succinimide Ester Reactivity for Chiral Capillary Chromatography: A Cyclodextrin Click Approach[END_REF]. One of the first examples described the preparation of diacid-decorated porous monolith [START_REF] Poupart | Copper nanoparticles supported on permeable monolith with carboxylic acid surface functionality: Stability and catalytic properties under reductive conditions[END_REF]. Such carboxylic acids were used to anchor copper nanoparticles using two immobilization processes. On the one hand, commercially available copper nanoparticles (with mean diameter in the range 40-60 nm) were percolated into the monolithic structure and the microreactor used directly after a rinsing step. On the other hand, Cu 2+ ions were initially immobilized onto the surface of the functionalized monolith. In a second step, an aqueous NaBH4 solution was injected in the monolith to generating the nanoparticles through reduction. Both microreactors were used to catalyze hydride-mediated reduction of one nitroarene, the o-nitrophenol. The best yield (68.5 % at a flow rate of 0.3 µL.min -1 ) was obtained when preformed NPs were used while the in-situ approach led to slightly lower yields (40 and 55 % for flow rates of 4 and 1.5 µL.min -1 , respectively) (Table 8).

In another implementation, the NAS-based monolith was used as support for gold nanoparticles. As described in the work of Khalil et al. [START_REF] Khalil | Gold-decorated polymeric monoliths: In-situ vs ex-situ immobilization strategies and flow through catalytic applications towards nitrophenols reduction[END_REF], ethylenediamine was grafted on the monolith surface. The resulting primary amines, in their protonated form, were used as ligands to immobilize Au colloids. Both ways of nanoparticles immobilization, namely in-situ and ex-situ pathways were investigated. In the former case, an aqueous solution of HAuCl4 was percolated to immobilize Au 3+ ions followed by reduction using an aqueous NaBH4 solution. For the ex-situ way, commercially available Au NPs with a diameter of 20 nm and a citrate stabilization layer were used. Interaction between the carboxylates of the citrates and the ammonium form of the primary amines at the pore surface provided strong interfacial interaction, leading to robust anchoring of the nanoparticles. The microreactors were used for nitroarenes reduction, using p-nitroaniline, o-nitrophenol, m-nitrophenol and p-nitrophenol as model molecules. Different parameters were investigated to optimize the reaction yields, like the reactants concentration, the column length, the flow rate allowing for complete conversion of the nitroarenes into the corresponding aromatic amines. Of particular interest, it was shown that the in-situ approach provided higher reaction yields as compared to the ex-situ using the same flow conditions.

An extension of this work was provided in the paper by Liu et al. [START_REF] Liu | Gold nanoparticlessupported histamine-grafted monolithic capillaries as efficient microreactors for flow-through reduction of nitro-containing compounds[END_REF] were NAS-based monoliths were functionalized with amine moieties derived from histamine. Commercially available Au NPs with different sizes 5 nm, 20 nm (Fig. 7B) and 100 nm were used. The aggregation behavior of the NPs at the monolith surface as well as the coverage density were found to depend on the chemical nature of the amine ligand and size of the nanoparticles.

While the higher diameters (100 and 20 nm) were the most homogeneously and densely covered columns, difficulties were encountered with the 100 nm Au NPs in the filling and the back pressure obtained from the capillary. The catalytic efficiency of these microreactors was first established for p-nitrophenol and then extended to dinitro derivatives, namely 2,5dinitrophenol, 2,4-dinitroaniline, 2,6-dinitroaniline and 3,5-dinitroaniline. Interestingly, the best results were obtained for the 20 nm nanoparticles, instead of the 5 nm ones, and for a flowrate of 5 µL.min -1 (Table 8).

Table 8. Examples of nitro coupounds reactions achieved using polymer-supported metal nanoparticles. The reactions conditions and yields are also presented. [START_REF] Poupart | Novel in-capillary polymeric monoliths arising from glycerol carbonate methacrylate for flow-through catalytic and chromatographic applications[END_REF]. Such a carbonate ring can be easily converted into a urethane group when reacting with a suitably chosen amine-bearing molecule. A determination of the cycles were tested with an average yield of about 60 %, thus ascertaining the reusability of the supported catalyst.

Nanoparticles supported by nanoporous polymers

As previously discussed, pore sizes of nanoporous materials are appealing for catalysis applications, as they can provide a filtration phenomenon occurring simultaneously to the catalytic activity. Few examples in the literature described the use of nanoporous polymers arising from diblock copolymers. The first example of nanopore decoration with a metal was reported by Ryu et al. [START_REF] Ryu | Highly Ordered Gold Nanotubes Using Thiols at a Cleavable Block Copolymer Interface[END_REF] who generated gold nanorods at the interface of thiolated polymers. Unfortunately, the authors did not mention any application. Recently, Grande's group started to investigate how such pores could be decorated with nanoparticles and considered the use of the resulting composite materials as supported catalysts.

In 2015, the synthesis of a diblock copolymer made of PS and PLA and bearing a disulfide bridge junction between both blocks was first developed [START_REF] Droumaguet | Clickable" thiol-functionalized nanoporous polymers: from their synthesis to further adsorption of gold nanoparticles and subsequent use as efficient catalytic supports[END_REF]. After the synthesis of the dual initiator and both blocks by ATRP and ROP, an orientation step of the resulting functional copolymer was implemented using a channel die process. Cleavage of the disulfide bridge was achieved using triphenylphosphine (TPP), revealing the pores as well as the thiol moieties at the pore interface. Au 3+ ions were then immobilized and further reduced with NaBH4. The as-obtained porous polymer-supported gold nanoparticles were used as efficient catalysts to reduce p-nitrophenol. Reaction yields of ≈ 70 % were calculated after 1 h of reaction for five consecutive catalytic runs.

In another implementation, the same authors used an acetal junction between the two blocks [START_REF] Poupart | Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(d,l-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes[END_REF]. The dual initiator was synthetized via a two-step process, and after the polymerization processes, orientation of the block copolymer structure was performed via solvent vapour annealing of films casted onto Si wafers. The acetal link between both blocks could be easily cleaved with trifluoroacetic acid and then, functionalized with amine molecules after reductive amination reaction. Amine-decorated pores were covered with in-situ generated gold nanoparticles (Fig. 8A). First, boronic homocoupling and nitroarene reduction were considered separately. Finally, the two reactions were coupled in a cascade reaction process involving 3-nitrobenzene boronic acid. After formation of the 3,3'-dinitrobiphenyl through homocoupling reaction, the nitro moieties were successfully reduced using NaBH4-mediated reduction yielding 3,3'-diaminobiphenyl as a major product. Copyright 2017 (Reproduced with permission from the American Chemical Society).

Besides the use of diblock copolymers, several authors have implemented polymer-based membrane materials exhibiting nanoporosity to design supported catalysts. Remigy and Lahitte's group discussed the use of commercially available polyethersulfone membranes with a pore size of 200 nm. The membranes were modified to anchor Pd NPs and further used in several reactions, including nitrophenol reduction [START_REF] Emin | Influence of UV grafting conditions and gel formation on the loading and stabilization of palladium nanoparticles in photografted polyethersulfone membrane for catalytic reactions[END_REF][START_REF] Gu | Hybrid Catalytic Membranes: Tunable and Versatile Materials for Fine Chemistry Applications[END_REF], Suzuki-Miyaura cross coupling [START_REF] Gu | Hybrid Catalytic Membranes: Tunable and Versatile Materials for Fine Chemistry Applications[END_REF][START_REF] Gu | High catalytic efficiency of palladium nanoparticles immobilized in a polymer membrane containing poly(ionic liquid) in Suzuki-Miyaura cross-coupling reaction[END_REF][START_REF] Gu | Catalytic membrane reactor for Suzuki-Miyaura C-C cross-coupling: Explanation for its high efficiency via modeling[END_REF]] (Table 7) or hydrogenation of trans-4-phenyl-3-buten-2-one [START_REF] Gu | Membrane Reactor Based on Hybrid Nanomaterials for Process Intensification of Catalytic Hydrogenation Reaction: an Example of Reduction of the Environmental Footprint of Chemical Synthesis from a Batch to a Continuous Flow Chemistry Process[END_REF].

Interesting comparison was performed considering the use of these membranes in batch mode and under flow-through conditions. The latter conditions proved to be superior providing faster reactions. Indeed, while the reactions could be performed within a 10 s range in flow conditions, the batch mode required 6 h for full conversion. Another interesting result was that no byproducts were observed in the flow-through mode. This was assumedly assigned to a lower kinetic of formation of the side product.

Other research groups focused on using membranes with embedded nanoparticles. One may cite the work from Mora-Tamez et al. [START_REF] Mora-Tamez | Simultaneous AuIII Extraction and In Situ Formation of Polymeric Membrane-Supported Au Nanoparticles: A Sustainable Process with Application in Catalysis[END_REF] who considered the use of Au NPs immobilized within cellulose triacetate-based membranes. The originality of the approach lies in the extraction of Au (III) ions by the membranes and their simultaneous in-situ reduction with a citrate solution (Fig. 8B). Such supports with embedded NPs were used for the reduction of pnitrophenol. The authors mentioned reaction yield as high as 95.4 % after 25 min of reaction (Table 8). Membranes were also characterized using BET and nitrogen adsorption/desorption isotherms. Specific surface area values ranging from 67 to 137 m 2 .g -1 and pore volume values from 0.048 to 0.097 mL.g -1 were found.

Likewise, Clark's group used biobased nanoporous polymers for catalysis purposes. Starchbased porous supports were obtained by solvent exchange between water and ethanol, and subsequently used to anchor palladium nanoparticles [START_REF] Budarin | Palladium nanoparticles on polysaccharide-derived mesoporous materials and their catalytic performance in C-C coupling reactions[END_REF]. Palladium acetate was put in the presence of the starch-based materials acting simultaneously as reducing agent and support for the resulting nanoparticles, seemingly self-reducing the precursory metallic ions. Such polymeric materials, characterized by N2 physisorption, exhibited a specific surface area of 190 m 2 .g -1 and an average pore size of 8.2 nm through the BET equations (Equation 2and

Equation 3) as well as the BJH method, respectively. Mizoroki-Heck (Table 7), Sonogashira (Table 7) and Suzuki-Miyaura reactions were performed under microwave irradiation using the starch-supported Pd NPs. The microwave activation permitted to reduce the reaction time as the reactions could be achieved in less than 10 min. In contrast, the authors provided a comparison with other data published in the literature without the use of microwave and for which the reaction times were in the range of 4-12 h. Although the authors concluded on the superiority of the starch-based materials in terms of improved reaction yields, lower reaction times, and renewability of the catalysts, a reliable comparison with traditional catalysts such as Pd/C or silica-supported NPs is, to our point of view, very difficult because most of the studies that the authors referred to did not mention the use of microwave activation.

Finally, microporous polymers (with pores below 2 nm) were used as catalytic supports.

Although most of examples in the literature mentioned the direct use of a polymer network as the heterogeneous catalyst due to a specific site like a specific chemical moiety [START_REF] Merino | Monofunctionalization of porous aromatic frameworks to use as compatible heterogeneous catalysts in one-pot cascade reactions[END_REF] or a metallo-organic complex [START_REF] Xie | Highly Stable and Porous Cross-Linked Polymers for Efficient Photocatalysis[END_REF], some examples about polymer-supported nanoparticles can also be found. Zhang et al. [START_REF] Zhang | Solution-Dispersible, Colloidal, Conjugated Porous Polymer Networks with Entrapped Palladium Nanocrystals for Heterogeneous Catalysis of the Suzuki-Miyaura Coupling Reaction[END_REF] designed a porous network via a direct Sonogashira coupling of an aromatic trialkyne and 1,4-dibromobenzene. The as-obtained nanoporous polymer was further characterized using N2 sorption. First, BET measurements gave a specific surface area of 421 m 2 .g -1 and a pore volume of 0.27 mL.g -1 . In this case, the pore size was not determined using the BJH theory but was calculated by the nonlocal density functional theory (NLDFT), a computational quantum mechanical modelling that allowed for highlighting the presence of three populations of pores in such a material with sizes centered on 0.6, 1.3, and 3.1 nm. The polymeric material was subsequently immersed into an acetone solution of Pd(OAc)2. After stirring at 90 °C, a Pd NPs-loaded polymer was obtained. Different Suzuki-Miyaura C-C coupling reactions were performed using a large panel of halogenoarenes (iodo and bromo) along with phenylboronic acid. High yields (> 85 %) and short reaction times (less than 4 h) were obtained. Comparison with Pd/C catalysts suggested that such nano Pd-decorated frameworks allowed for a threefold decrease of the reaction times (from 9 h for Pd/C to 3 h) to reach similar reaction yields. Five catalytic cycles were performed and only a limited reduction in catalytic activity was observed as expressed by the decrease of a few percent of the reaction yields, while leaching effect was quantified to be less than 1 % for each cycle.

In their interesting work, Du et al. [START_REF] Du | Heptazine-Based Porous Framework Supported Palladium Nanoparticles for Green Suzuki-Miyaura Reaction[END_REF] prepared polymer networks through a nucleophilic substitution of chlorines pending on the cyameluric chloride monomer by amines of piperazine. The as-obtained heptazine framework was immersed into an acetone solution of palladium acetate under reflux, allowing for the generation of the Pd NPs by self-reduction (Fig. 8c). Similarly to the studies achieved by Zhang et al. about microporous polymeric materials, the surface area of their porous heptazine framework was also determined through BET measurements using nitrogen sorption. Surface area of 106 and 73 cm 2 .g -1 and pore volume of 0.43 and 0.33 mL.g -1 were found by the authors for the materials before and after immobilization of Pd NPs, respectively. Such surface area values are rather unexpected; one would indeed expect higher values for hybrid materials, likely due to the adsorption of metallic nanoparticles at the pore surface of these polymeric frameworks. Pore size distribution was also found in the 2-8 nm range. With such hybrid catalysts, Suzuki-Miyaura couplings were performed using different pairs of bromoarenes derivatives and phenylboronic acids. Yields above 80 % of conversion were obtained except for the 2-bromonaphtalene along with the arylboronic acid as well as for the 4-nitrobenzene boronic acid along with bromobenzene, for which yields remained below 40 %. A tentative explanation for the obtained yields was provided by the authors based on the large steric hindrance of 2-bromonaphtalene as well as the poor solubility of 4-nitrobenzene boronic acid. Here again, five catalytic runs were performed consecutively showing a limited decrease of the catalytic activity and ICP measurements performed before and after the five cycles showed negligible leaching phenomena.

Nanoparticles supported by biporous polymers

Macroporous and nanoporous polymers are very different systems in terms or permeability, porosity, surface area which are key features for catalysis applications.

Macroporous polymers possess large pores providing enhanced permeability for the liquid to penetrate into the pores but a poor specific surface area. In contrast, nanoporous frameworks afford a larger specific surface area, while a lower accessibility to the pores. A high permeability may favor better accessibility of the reactants to the catalysts, while a large specific surface area should allow for higher density of metal nanoparticles on the support surface. If considered simultaneously, these two criteria may provide more efficient catalytic processes. Based on this simple consideration, biporous materials containing both macropores and nanopores may appear as attractive candidates for nanocatalyst supports.

An easy way of making biporous materials relies on the fabrication of polyHIPEs. One of the pioneering groups in the field of polyHIPE preparation and use as catalysts is Deleuze's. In 2005, they described polystyrene- [START_REF] Desforges | Palladium Nanoparticle Generation within Microcellular Polymeric Foam and Size Dependence under Synthetic Conditions[END_REF] and poly(vinylbenzyl chloride)-based [START_REF] Desforges | Generation of Palladium Nanoparticles within Macrocellular Polymeric Supports: Application to Heterogeneous Catalysis of the Suzuki-Miyaura Coupling Reaction[END_REF] polyHIPEs as supports for in-situ generated Pd NPs (Fig. 9A). Both styrene and VBC-based polymeric supports showed a specific surface area of 902 m 2 .g -1 as determined by BET, while pore size distribution in the 10-80 nm range was induced by a porogenic solvent added to the HIPE polymerization feed. The hybrid supports were used for the hydrogenation of an alkene, i.e. allyl alcohol, and for Suzuki-Miyaura cross-coupling reactions (Table 7). The authors reported reaction times of 1 h and 70 h for near-completion hydrogenation and coupling reactions, respectively. The authors discussed their results with respect to other published results obtained with catalysts such as Pd/C using the prepared supported catalysts in powder forms. Polystyrene based polyHIPEs were also used by the same group as supports for gold nanoparticles [START_REF] Féral-Martin | Integrative chemistry toward the first spontaneous generation of gold nanoparticles within macrocellular polyHIPE supports (Au@polyHIPE) and their application to eosin reduction[END_REF]. In this case, HAuCl4 ions solutions were simply deposited and the PS induced self-reduction. Supported Au NPs were then used for the reduction of a dye, Eosin Y, and the reaction was repeated three times. Near-completion reactions were achieved within one hour and under mild conditions (25 °C). Pores in the 200-291 µm range, were found for such monoliths depending on the samples, while a porosity ratio of 82% was found by MIP. Another way to design nanostructured catalysts relies on the use of electrospun materials. The as-obtained fibers possess several interesting properties for catalytic applications, such as a large surface to volume ratio and superior mechanical properties. Therefore, they are also usually used as membrane-like materials, which can be beneficial to catalysis, as discussed above. To date, electrospun materials have been mostly used for environmental catalytic applications, like hexavalent chromium (Cr VI ) or nitro group reduction. Nevertheless, asprepared electrospun supports are not widely used as catalyst supports, as most reports in the literature mentioned the use of the polymer mats as precursors for calcination for, as an example, creating titania fibers. Most examples in the literature describe the use of polymer fibers already containing chelating moieties like carboxylic acids or amines. Shi's group described the use of polyethyleneimine (PEI) blended with poly(vinyl alcohol) (PVA) as mats for the support of gold [START_REF] Fang | Facile immobilization of gold nanoparticles into electrospun polyethyleneimine/polyvinyl alcohol nanofibers for catalytic applications[END_REF] and palladium [START_REF] Huang | Efficient Catalytic Reduction of Hexavalent Chromium Using Palladium Nanoparticle-Immobilized Electrospun Polymer Nanofibers[END_REF] nanoparticles. Au NPs were used for the successful reduction of nitro compounds, while Pd NPs were applied to the generation of Cr III from Cr VI , which is highly carcinogenic. In the same way, Xiao's group used a blend of poly(acrylic acid) (PAA) and PVA to chelate in-situ generated (sodium borohydride as reducing agent) Ag NPs (Fig. 9B) for the catalytic reduction of p-nitrophenol [START_REF] Xiao | Size-tunable Ag nanoparticles immobilized in electrospun nanofibers: synthesis, characterization, and application for catalytic reduction of 4-nitrophenol[END_REF] (Table 8).

Another interesting recent work is that from Pandey's team [START_REF] Chappa | Dual-Functional Grafted Electrospun Polymer Microfiber Scaffold Hosted Palladium Nanoparticles for Catalyzing Redox Reactions[END_REF], who used electrospinning to prepare poly(ether sulfone) (PES) fibers and took advantage of the presence of the ether sulfone moieties to perform photolysis under UV irradiation to initiate the growth of polyGMA chains. The pendant oxirane groups were then opened with hydrazine providing directly attachment of the reducing agents on the support surface. A palladium salt was put in contact with the fibers via an aqueous solution of palladium chloride and self-reduced. Hybrid fibers were applied to the reduction of hexavalent chromium as well as p-nitrophenol (Table 8) but also the less common reduction of hexavalent uranium (U VI ) to U IV .

Last but not least, one may mention the use of the double porogen templating approach allowing for easily combining two levels of porosity and broadening the range of accessible pore shape. Ly et al. recently designed doubly porous PHEMA-based materials as supports for gold nanoparticles [START_REF] Ly | Functionalized Doubly Porous Networks: From Synthesis to Application in Heterogeneous Catalysis[END_REF][START_REF] Ly | Versatile functionalization platform of biporous poly(2-hydroxyethyl methacrylate)-based materials: Application in heterogeneous supported catalysis[END_REF] (Fig. 9C). They used fused NaCl particles as macroporogens and isopropanol as a porogenic solvent for the production of the nanopores. The obtained monoliths, i.e. monoporous with the higher porosity level, monoporous with the lower porosity level and biporous ones, have been thoroughly characterized through mercury intrusion porosimetry by using the Washburn equation (Equation 1). Data gathered in this study showed that average pore sizes of 42 µm, 9 µm, 40 µm, and 8 µm were obtained by MIP for monoporous with the higher porosity level, monoporous with the lower porosity level and biporous HEMA-based polymeric frameworks, respectively. More importantly, the porosity ratio of such a biporous polymer was estimated to be 92 %, which could be of upmost interest for heterogeneous supported catalysis applications. The surface of the biporous polymers was chemically modified in order to have amines or thiols directly on the surface. HAuCl4 solution was used to load gold ions onto the surface and NaBH4 was used as a reducing agent. Reduction of 4-nitrophenol was performed in order to prove the catalytic efficiency of the as-prepared hybrids. Differences in the size and/or distribution of the nanoparticles were observed as a function of the nature of the chelating group (-NH2 vs. -SH), thus leading to differences in the reaction yields. It was shown that thiol functions led to bigger nanoparticles, and also surprisingly to leaching of NPs. Monoporous materials were also synthetized in order to highlight the superiority of such doubly porous materials. While monomodal porous polymers, i.e. with macroporosity or nanoporosity only, showed rather similar efficiency, the doubly porous homologues exhibited higher catalytic activity. The higher density of nanoparticles associated with the latters along with their higher porosity ratios as compared to the nanoporous and macroporous materials were assumed to account for such results (Table 8).

Critical appraisal of the different strategies

This review presented a critically selected overview of the various polymeric materials so far implemented as potential supports for the adsorption of metallic nanoparticles meant for supported heterogeneous catalysis. Each of these systems has inherent advantages/drawbacks depending on their preparation conditions, etc. This section will bring a critical appraisal of the different porous polymeric systems in terms of preparation, main characteristics, catalytic properties, durability, etc. and of the related hybrid systems and their catalytic properties.

Fused silica capillaries filled with polymer monoliths are easy to prepare, dynamic loading of the reactants in such microsystems being an undeniable advantage for the successful functionalization of the pore surface with chemical grafts of interest and successive immobilization of metallic nanoparticles through the in situ or ex situ strategies. In this way, each preparation step is completed in a few hours or even in a few minutes. Supported catalytic reactions operated in flow-through conditions have the major advantage to directly give the desired product, without the need for any further purification step, provided of course that no byproduct(s) is (are) generated during the catalytic reaction. More interestingly, such microsystems are supposed to be easily scaled up and might be used in automation processes, as mentioned by Nikbin et al. [START_REF] Nikbin | Continuous Flow Ligand-Free Heck Reactions Using Monolithic Pd [0] Nanoparticles[END_REF], that is to say that a chromatographic-column sized catalytic reactor would be able to do what a tiny in-capillary microreactor can do. Such a scale-up process would definitely solve the major issue regarding in-capillary monolithic hybrid reactors, such as slow flow rates (about a few µL.min -1 ) due to rather high backpressures and limited quantities of reactants that can be converted, i.e. generally a few milligrams, due to microcolumns size/volume. Too high backpressures dramatically decrease the lifetime of the column, reducing its reusability/durability. Moreover, such important backpressure phenomena might lead to a higher leaching of adsorbed metallic nanoparticles, which would be detrimental to further flow-through supported catalytic processes.

Nanoporous polymers arising from diblock copolymers can lead to different controlled accessible morphologies (from cylinders to gyroids or to lamellae), thus enabling to tune the porosity of the support. Nevertheless, a non-negligible series of not trivial synthetic steps is required to produce them, which could be detrimental for their transfer to industrial processes.

Besides, depending on the alignment procedure and on the quantities of copolymer needed, the orientation procedure time can be dramatically increased. For channel die processing, a few hundreds of milligrams of copolymers are required to determine the best orientation conditions, while for solvent vapour annealing, a diluted copolymer solution is enough for film nanostructuration on silicon wafers. Yet, there is no widespread use of these nanoporous materials for catalysis purposes.

Polymeric membranes seem to be the candidates of choice for efficient supported catalytic reactions. However, some drawbacks could be found [170]. First, they need a specifically designed and optimized reactor. Unfortunately, the production costs for a specific reactor chamber must be added to the efforts for creating catalytic membranes, which are not trivial.

Moreover, as with capillary-based microreactors, a specific adjustment of the flow rate of reactant solution to reach optimized reaction rates is necessary. Finally, one should keep in mind that polymeric membranes are known to have a limited durability, depending especially on their thermal, chemical, and mechanical properties. Indeed, the harsher the catalytic reaction conditions, the quicker they degrade, adding higher costs of renewal, even if some improvements have been achieved regarding the durability of such polymer-based membranes.

needs to be performed to clearly correlate the pore morphology to the mass transfer properties of the resulting materials. Very few of such doubly porous crosslinked polymers have been used so far in the area of heterogeneous supported catalysis, only bringing limited information in the field. Finally, no mechanical characterization data have been published yet on such catalytic supports, which could bring some interesting information on their resistance to different experimental conditions.

Conclusions and prospects

In conclusion, this review critically overviews the main types of polymers used as potentially interesting supports for metallic nanoparticle immobilization. The as-prepared hybrid materials seem to constitute candidates of choice in the area of heterogeneous supported catalysis, as demonstrated by some notable works performed gathered in this review article. Production costs for these porous polymers remain rather low, when compared to their inorganic counterparts, while their preparation is rapid, making them suitable for various applications, including heterogeneous supported catalysis. However, progress for optimizing such polymer-based supports remains a milestone in order to optimize the resulting hybrid materials. For instance, the optimization of specific surface area of such porous polymers is of upmost importance, as it would definitely bring such organic materials a step forward when compared to their inorganic counterparts. It was recently demonstrated that HEMA-based porous polymers arising from reversed HIPEs can be further hypercrosslinked through a two-step synthetic pathway to generate porous polymers with largely enhanced surface area of about 1500 m 2 .g -1 [START_REF] Mezhoud | Novel hypercrosslinking approach toward high surface area functional 2-hydroxyethyl methacrylatebased polyHIPEs[END_REF]. Another very promising research area could rely on the development of porous metallic nanoparticles. Indeed, such nanometals can develop a very large specific surface area, e.g. 8973 m 2 .g -1 for the outer surface area and 58724 m 2 .g -1 for the inner surface area of 80 nm hollow porous gold nanoparticles, and thus constitute promising candidates for the development of adsorbed catalysts at the surface of porous polymers. The Holy Grail of such research area would rely on the preparation of advanced hybrid systems that would be constituted of both high surface area counterparts, i.e.

porous nanometal and porous polymer support. Other morphologies of metallic nanoparticles could also be largely envisioned as they could also lead to enhanced activities of the resulting supported nanocatalysts [START_REF] Zeng | A Comparison Study of the Catalytic Properties of Au-Based Nanocages, Nanoboxes, and Nanoparticles[END_REF].

Based on this overview concerning porous polymers meant for heterogeneous supported catalysis, different key experimental parameters have to be indeed carefully taken into consideration and especially porosity range and morphology of the materials as well as nature of chemical moieties exposed at the pore surface so as to optimize the interactions between the support and the metal (precursor). The catalytic processes involved, namely batch or flowthrough, but also the envisioned catalytic reaction are crucial parameters that matter for the appropriate selection of the supports, notably regarding the stability and durability of the hybrid systems in diverse experimental conditions (temperature, pressure, solvent nature, etc.). Beyond the application of such hybrid systems in supported heterogeneous catalysis, it is essential noticing that they could also be used as sensors or sorbents in analytical sciences, filters for CO2 sorption, or nanoreactors for the capture and release of biomolecules such as cysteine-bearing peptides/proteins, for instance.

Fig. 1 .

 1 Fig. 1. Examples of macroporous polymeric materials derived from the use of porogenic agents. (A) N-acryloylsuccinimide-based monoliths obtained in the presence of a porogenic solvent. [24], Copyright 2007 (Reproduced with permission from Elsevier Ltd). (B) polystyrene macroporous monolith obtained after leaching of a semi-interpenetrated polycaprolactone oligomer. [32], Copyright 2010 (Reproduced with permission from Elsevier Ltd). (C) Poly(D,L-lactic-co-glycolic acid)-based frameworks using NaCl cubic particles as macroporogens [34]. Copyright 2005. Reproduced with permission from Elsevier Ltd. (D) Porous poly(2-hydroxyethyl methacrylate) material obtained upon removal of sintered poly(methyl methacrylate) beads as 3-D macroporogenic template. [35], Copyright 2014 (Reproduced with permission from Elsevier Ltd).

Fig. 2 .

 2 Fig. 2. (A) Morphology diagram of AB diblock copolymers obtained upon orientation of respective BCP domains depending on the segregation regime (χN) and the volume fraction of the minor component (f): L, H, Q 229 , Q 230 , CPS and DIS stand for lamellae, hexagonally packed cylinders, spherical phases with Im3m, 3D gyroid phase, close-packed (fcc or hexagonal) symmetry, or disorder respectively. [42], Copyright 2006 (Reproduced with permission from the American Chemical Society). (B) Schematic representation of the various

Fig. 3 .

 3 Fig. 3. Examples of nanoporous polymeric materials. (A) Nanopores arising from selective hydrolysis of the PLA sacrificial block in a PS-b-PLA diblock copolymer. [51], Copyright 2011 (Reproduced with permission from Elsevier Ltd). (B) Nanopores arising from an intrinsically microporous polytriazine network. [54], Copyright 2008 (Reproduced with permission from Wiley-VCH publishers).

Fig. 4 .

 4 Fig. 4. Examples of biporous polymeric materials observed by SEM. (A) Biporous polymeric material generated by a combination of electrospinning and particle leaching. [92], Copyright 2001 (Reproduced with permission from Wiley-VCH publishers). (B) Biporous polymeric material obtained via high internal phase emulsion. [100], Copyright 2007 (Reproduced with permission from Elsevier Ltd.).

Fig. 5 .

 5 Fig. 5. Examples of doubly porous materials obtained through the double porogen approach. (A) Porous polyacrylonitrile prepared via a combination of electrospinning and CaCO3 particle leaching. [101], Copyright 2013 (Reproduced with permission from Wiley-VCH

Fig. 6 .

 6 Fig. 6. Scheme representing the two main ways, namely ex situ and in situ ways, for the immobilization of nanoparticles onto a porous support.

Fig. 7 .

 7 Fig. 7. Examples of macroporous polymeric materials bearing metallic nanoparticles. (A) insitu generated Pt NPs onto a ROMP-generated-matrix capillary. [158], Copyright 2012 (Reproduced with permission from the Royal Society of Chemistry). (B) ex-situ generated Au NPs onto a NAS-matrix capillary. [146], Copyright 2017 (Reproduced with permission from the Royal Society of Chemistry). (C) ex-situ generated Cu NPs onto a NAS-matrix capillary. [31], Copyright 2015 (Reproduced with permission from Elsevier Ltd.). (D) in-situ generated Au NPs onto a DSDMA bulk monolith. [159], Copyright 2017 (Reproduced with permission from Elsevier Ltd.).

Fig. 8 .

 8 Fig. 8. Examples of nanoporous polymeric materials bearing metallic nanoparticles. A) in-situ generated Au NPs onto a PS arising from diblock copolymers. [63], Copyright 2017 (Reproduced with permission from the American Chemical Society). B) in-situ generated Au NPs within a cellulose membrane. [143], Copyright 2017 (Reproduced with permission from Wiley VCH). C) in-situ generated Pd NPs onto a microporous polyheptazine. [155],

Fig. 9 .

 9 Fig. 9. Examples of biporous polymeric materials bearing metallic nanoparticles. (A) in-situ generated Pd NPs onto a PS polyHIPE. [162], Copyright 2005 (Reproduced with permission from the American Chemical Society). (B) in-situ generated Ag NPs onto poly(acrylic acid) fibers. [142], Copyright 2012 (Reproduced with permission from the Royal Society of Chemistry). (C), (D) in-situ generated Au NPs onto a biporous poly(HEMA-co-EGDMA) bulk monolith. [104], Copyright 2016 (Reproduced with permission from Wiley VCH).

Table 1 .

 1 Irreversibly cleavable junctions used for the preparation of functionalized nanoporous polymers.

	Type of block	Junction chemical	Chemical function
		Cleavage agent/stimulus	References
	junction	structure	remaining after cleavage
		Brønsted or Lewis acid
	Trityl ether		[56, 57]
		(ex: TFA)
			Primary alcohol
	o-nitrobenzyl	UV light
			[58-60, 74]
	ester	(λ = 350 nm)	Carboxylic acid
	o-nitrobenzyl	UV light
			[61]
	carbamate	(λ = 300 nm)	Primary amine
	Hemiacetal
		TFA	[62]
	ester
			Carboxylic acid
	Acetal	TFA	[64]
			Alcohol
	Acetal	TFA	[63]
			Aldehyde

Table 2 .

 2 Reversible junctions used for the preparation of functionalized nanoporous polymers.

	Type

of block junction Junction formula Cleavage agent/stimulus Chemical function remaining after cleavage References

	Hetero Diels-
		110 °C	[69]
	Alder adduct
		Thio-carbonyl-thio RAFT agent
	[4π+4π]
		UV light (λ = 280 nm)
	anthracene		[55, 65]
		130-200 °C
	photodimer	Anthracene
	Disulfide bridge	DTT, TPP or glutathione	[66, 67]
		Thiol
	Oxi-imines	TFA	[68]
		Primary amine

Table 3 .

 3 Supramolecular junctions used for the preparation of functionalized nanoporous 1

	2	polymers.

Table 4 .

 4 Common techniques used for the characterization of pore diameter in porous polymers and their characteristics.

	Technique	Pore diameter range	Principle
	Mercury Intrusion Porosimetry (MIP)	≈ 300 µm -≈ 10 nm	Washburn equation
	Gas sorption	300 nm -< 2 nm	Barrett-Joyner-Halenda (BJH) theory
	DSC thermoporometry	300 nm -5 nm	Gibbs-Thomson equation

Table 5 .

 5 Examples of common reducing agents used for the preparation of nanoparticles.Typical conditions for reduction are also provided.

	Reducing agent	Formula	Conditions	References
	Sodium borohydride	NaBH4	Aqueous solution, r.t.	[133, 134]
			Aqueous
	Sodium Citrate		solution, heat	[135, 136]
			(reflux)
	Dihydrogen	H2	Heat	[138, 139]
			Aqueous
	Hydrazine	H2N-NH2	solution, heat or	[137, 140]
			ultrasound

Table 6 .

 6 Example of catalyzed reactions performed using metal nanoparticles supported on polymers.

	Name of the reaction	Reactant(s)	General conditions	Catalyst	Product(s)	References
	Nitroarenes reduction		H2O, NaBH4, r.t.	Au, Ag, Cu, Pt, Pd		[31, 63, 66, 105, 106, 140-148]
	Mizoroki-Heck coupling		DMF, Base, ≈ 100 °C	Pd		[149-151]
		X = I, Br
	Suzuki-Miyaura coupling		Ethanol or DMF, Base, ≈ 100 °C	Pd		[145, 150-157]
		X = I, Br
	Boronic homocoupling		Ethanol, Base, ≈ 65 °C	Au		[63]
	Sonogashira coupling		H2O and THF, Base, ≈ 50 °C	Pd		[151, 157]
		X = I, Br
	Hydrosilylation		n-hexane, 45 °C	Pt		[158]
	Eosin Y reduction		H2O, NaBH4, r.t.	Au		[100, 159]
	Reduction of hexacyanoferrate(III)	Fe(CN)6 3-	H2O, NaBH4 or thiosulfate, r.t.	Au or Pd/Pt	Fe(CN)6 4-	[20, 160]

Table 7 .

 7 Examples of C-C coupling reactions achieved using polymer-supported metal nanoparticles. The reactions conditions and yields are also presented.

		coupling	Sonogashira						coupling	Mizoroki-Heck					coupling	Suzuki-Miyaura	Reaction
	W, 2 min	2 mmol DABCO, 130 °C, MW 300	tBuOK, 1 eq. NBu4Br, 50 °C, 24 h	THF/H2O (50/50 % v/v), 1.5 eq.		300 W	5mmol trimethylamine, 90 °C, MW		°C, flowrate: 0.05 mL.min -1	DMF, 0.3 mmol trimethylamine, 130	130 °C, 72 h	Dioxane, 0.6 mmol benzonitrile, 80-	EtOH; 2.5 eq. tBuOK, 50 °C, 24 h	h tBuOK, 1 eq. NBu4Br, 50 °C, 24	THF/H2O (50/50 % v/v), 1.5 eq.	Experimental conditions
	Pd	Pd			Pd			Pd	Pd	Pd	Pd	metal catalyst	Supported
				2												54
	>90 % to >99 %	TONs: 90 to 4130	% to 95 % (various haloarenes tested)	35 % to 85 % for t = 10 min	Styrene	50 % to >99 % for t = 5 min	<20 % to 83 % for t = 1 min	Methyl acrylate	reactants)	of <50 % to 87 % (various pairs	77 % to 100 %	96 %	TONs: 63 000 34 400 to	tested) % to 99 % (various haloarenes	Yield

Acknowledgments .................................................................................................................... References ................................................................................................................................

Acknowledgments

Financial support from CNRS and UPEC is gratefully acknowledged. The authors are indebted to UPEC for providing R. Poupart with a Ph.D. grant.

Abbreviations

AM acrylamide pore size of these in-capillary monoliths was achieved using mercury intrusion porosimetry.

The authors found an average pore size centered around 2.2 µm but also another lower porosity level in the 50 nm range that was not visible by SEM because of the detection limit of the apparatus. Carboxylic acids were grafted at the surface via a two-step process and then used to chelate platinum ions. NaBH4 was used as a reducing agent for the generation of Pt NPs directly onto the pore surface. The as-prepared supported catalyst was further used for the total reduction of p-nitrophenol (Table 8).

Beside the design of porous materials for flow-through applications, Poupart et al. also prepared bulk polymeric materials using the porogenic solvent approach. A dimethacrylate monomer bearing a disulfide bridge, namely bis(2-methacryloyl)oxyethyl disulfide (DSDMA), was used with the aim to eventually produce thiol-containing monoliths [START_REF] Poupart | Gold nanoparticles immobilized on porous monoliths obtained from disulfide-based dimethacrylate: Application to supported catalysis[END_REF].

Such an approach was followed because of the difficulty to polymerize thiol-containing monomers with the occurrence of chain transfer reactions. Herein, they used a protected thiol in the form of disulfide. After polymerization with a dimethacrylate as a cross-linker and using toluene as a porogenic agent, thiols were generated using D,L-dithiothreitol (DTT).

Several solvents were used as porogens (methanol, ethanol, a cyclohexanol/dodecanol mixture as well as toluene). Mercury intrusion porosimetry was performed on all samples, even after DTT-mediated cleavage of the disulfide bridge contained in the monoliths. Average pore sizes of 6 and 0.5 µm were determined for methanol and ethanol, respectively. Similarly, average pore sizes of 0.01 and 1 µm were obtained for the cyclohexanol/dodecanol mixture and toluene, respectively. No significant variation of pore size or porosity ratio was found upon selective cleavage of disulfide bridges. Gold ions were subsequently anchored to the thiolated surface and further reduced using sodium borohydride to generate Au nanoparticles (Fig. 7C). Although aggregation trends could be seen onto the SEM pictures, the as-prepared bulk catalysts were used to reduce an organic dye, Eosin Y. Up to six consecutives catalytic Microporous networks have one major advantage, namely their specific surface area. Indeed, the pores consist in voids generated by the monomer assembly, and are in the micropore range. This could logically lead to catalytic supports declined for reactions in the gas phase.

Yet, examples of catalytic reactions in liquid media still exist with such polymer-based microporous supports. However, they have a low permeability, and this is especially true for pure carbon-based networks [START_REF] Cheng | Soluble Conjugated Microporous Polymers[END_REF]. This limitation could notably prevent catalytic reactions from occurring efficiently.

PolyHIPEs display a really well-accessible high porosity with interconnected pores that enable high flow-through processes. However, since the void size is large (cavities are in the tens/hundreds of micrometers range), specific surface areas are quite low [START_REF] Cameron | High internal phase emulsion templating as a route to well-defined porous polymers[END_REF]. Works achieved by Sherrington's group attempted to overcome this issue [START_REF] Hainey | Synthesis and ultrastructural studies of styrene-divinylbenzene Polyhipe polymers[END_REF], notably by using porogenic solvents in addition to the HIPE process. However, resulting porous polymers faced a new limitation, i.e. poor mechanical resistance of the monolith during flow-through processes or even collapse of the porous structure.

Electrospun materials have risen since the mid 1990's, period during which such polymeric fibers could be implemented for nanotechnology applications. As catalytic support, they offer an interconnected porosity resulting from their engineering process. Moreover, they are already used to prepared filtration membranes, which may lead to flow-through catalytic reactors. Nevertheless, some limitations still exist. One may mention the difficulty to produce uniform mats with a fiber diameter lower than 50 nm [START_REF] Dzenis | Spinning Continuous Fibers for Nanotechnology[END_REF]. Indeed, a smaller diameter of polymeric fiber would lead to a smaller volume of these fibers, thus enhancing the surface to volume ratio. Progress is already on the way to overcome this issue by studying the solvent evaporation during the electrospinning process, among others.

Finally, the double porogen approach affords high porosity ratios (> 90%) and interconnected porosities, provided that the porogens are suitably chosen. However, further investigation still