Polymers polyHIPEs

The work was also extended to the application of functionalised emulsion-templated polymers (polyHIPEs) with CTAB as a suitable medium and also a source of in situ release of the surfactant for a wide range of fundamental free radical reactions.18... 

If a high internal phase emulsion is prepared in which the continuous phase contains one or more monomeric species, and polymerisation is initiated, a novel type of highly porous material is produced. Polymers of this type are referred to as PolyHIPE, using the nomenclature devised by Unilever scientists [128],... 

The range of monomers which can be employed is largely dictated by the physical chemistry of the emulsion system. For instance, monomers must be sufficiently hydrophobic to allow the formation of stable w/o HIPEs. In addition, most systems which have been studied have used polymerisation methods which require either an initiation step, or addition of a catalyst. This is due to the fact that the first step in the preparation of the polymer is the preparation of HIPE this can only proceed satisfactorily in the absence of any significant degree of polymerisation. Thus, it can be seen that radical addition polymerisation is suitable for the synthesis of PolyHIPE polymers, whereas condensation polymerisation can be more problematical. Also, the latter reactions often generate water as the by-product, hence the aqueous component of the HIPE is inhibiting to the polycondensation. 

By far the most studied PolyHIPE system is the styrene/divinylbenzene (DVB) material. This was the main subject of Barby and Haq s patent to Unilever in 1982 [128], HIPEs of an aqueous phase in a mixture of styrene, DVB and nonionic surfactant were prepared. Both water-soluble (e.g. potassium persulphate) and oil-soluble (2,2 -azo-bis-isobutyronitrile, AIBN) initiators were employed, and polymerisation was carried out by heating the emulsion in a sealed plastic container, typically for 24 hours at 50°C. This yielded a solid, crosslinked, monolithic polymer material, with the aqueous dispersed phase retained inside the porous microstructure. On exhaustive extraction of the material in a Soxhlet with a lower alcohol, followed by drying in vacuo, a low-density polystyrene foam was produced, with a permanent, macroporous, open-cellular structure of very high porosity (Fig. 11). 

The concentration of the surfactant in the monomer phase was found to be critical to the formation of a stable polymer foam [129,130]. At least 4% surfactant, relative to the total oil phase, was required for PolyHIPE formation, whereas formulations containing above 80% resulted in the formation of an unconnected or closed-cell material. Surfactant levels between 20 and 50% were deemed to be optimum at all internal phase volumes. Additionally, Litt et aL [131] demonstrated that block copolymer surfactants can be used to prepare water-in-styrene HIPEs. From these, highly porous uncrosslinked polystyrene PolyHIPE materials were synthesised. 

PolyHIPE materials possess many peculiar properties as a result of their unique cellular structure. Referring specifically to open-cell polymers (which have received the most attention in the literature), they are characterised by a very low dry bulk density, typically less than 0.1 gem-3, due to their highly porous, interconnected structure. 

PolyHIPE polymer HIPE composition Surface area (m2g-1)a... 

PolyHIPE, in granular form, has recently been employed as a support for bicatalyst systems [136]. Styrene/DVB porous polymers were prepared with free vinyl groups (acryloyl, allyl and vinylbenzyl) on the surfaces of the cavities. Impregnation with solutions of quaternary onium monomers, with subsequent polymerisation, resulted in grafting onto the pendant double bonds, to give a surface-quaternised material (Fig. 19). 

Fig. 19. Preparation of quaternary onium-containing PolyHIPE polymers...

The ability of PolyHIPE materials to absorb liquids has been exploited in experiments on their potential use as carrier materials for the safe transport of hazardous or flammable liquids [128]. A poly(styrene/DVB) sample was able to absorb twenty times its own weight of liquid paraffin, simply by immersing the material in the liquid. However, the problem in this application is the subsequent removal of the liquid from the polymer. This can only be achieved by vacuum distillation, which is very difficult with high boiling liquids. 

The idea of the preparation of porous polymers from high internal phase emulsions had been reported prior to the publication of the PolyHIPE patent [128]. About twenty years previously, Bartl and von Bonin [148,149] described the polymerisation of water-insoluble vinyl monomers, such as styrene and methyl methacrylate, in w/o HIPEs, stabilised by styrene-ethyleneoxide graft copolymers. In this way, HIPEs of approximately 85% internal phase volume could be prepared. On polymerisation, solid, closed-cell monolithic polymers were obtained. Similarly, Riess and coworkers [150] had described the preparation of closed-cell porous polystyrene from HIPEs of water in styrene, stabilised by poly(styrene-ethyleneoxide) block copolymer surfactants, with internal phase volumes of up to 80%. 

The resorcinol-formaldehyde polymers have been used to prepare highly porous carbon materials, by controlled pyrolysis in an inert atmosphere [144,154], The microstructure of the carbon is an exact copy of the porous polymer precursor. Poly(methacrylonitrile) (PM AN) PolyHIPE polymers have also been used for this purpose. These monolithic, highly porous carbons are potentially useful in electrochemical applications, particularly re-chargeable batteries and super-capacitors. The RF materials, with their very high surface areas, are particularly attractive for the latter systems. 

Open-cell PolyHIPE materials have also been prepared from hydrophilic methacrylates which, on hydrolysis, yield hydrophilic polymethacrylic acid-based species [155]. Stable HIPEs containing high levels of glycidyl methacrylate can also be formed, from which porous polymers can be made. These have considerable potential for further exploitation due to the reactive epoxide group [156],... 

A wide range of polymeric materials can be prepared from HIPEs. Polymerisation of the continuous phase yields highly porous cellular polymers with a monolithic structure. These are known as PolyHIPE polymers, and possess a number of unique properties including, in most cases, an interconnected cellular structure and a very low dry-bulk density. Their very high porosity favours their use as supports for catalytic species, precursors for porous carbons and inert matrices for the immobilisation of enzymes and micro-organisms. 

Similarly, a monolithic polymer of PolyHIPE functionalized with tris(aminoethyl)amine captures acid chlorides in solution with high efficiency (entry 36).42 Contrary to suspensions of polymer beads, the porous polymer monolith is used in a flow-through reaction format. 

The functionalisation by thiols was performed as follows Small cubes of (vinyl)polystyrene polyHIPE [3.00 mmol C=C/g (1 g, 3 mmol C=C)] were impregnated with toluene by freeze-drying under vacuum and suspended in toluene (20 mL). The thiol (8 mmol), AIBN (5 mg, l%mol. / C=C) were then added. The suspension was heated at 70°C for 48 h. The polymer was filtered off, extracted with acetone overnight on a Soxlet apparatus and dried in vaccum at 60°C, overnight. 

Functionalized microporous polymers (with a superimposed nanostructure in the form of nanopores within the walls of the micropores) have been prepared through a high internal phase emulsion (HIPE) polymerization route.These polymers (known as PolyHIPE Polymers, PHP) were subsequently metallized by solution deposition followed by heat treatment. 

Akay, G. Birch, M.A. Bokhari, M.A. Microcellular polyhipe polymer (PHP) supports osteoblastic growth and bone formation in vitro. Biomaterials 2004, 25, 3991 000. 

Bokhari, M. Birch, M. Akay, G. Polyhipe polymer a novel scaffold for in vitro bone tissue engineering. Adv. Exp. Med. Biol. 2003, 534, 247-254. 

A PolyHIPE was employed in the form of a disk as support for the immobilization of flavine for further catalysis of the oxidation of benzyldi-hydronicotinamide in a flow system, demonstrating very good results [369]. PolyHIPE polymers also proved to be a usefiil support for solid-phase peptide synthesis [372]. In this case, large cells of the monolith were filled... 

In this study, after a brief introduction to PI we provide the bases of a technique for the preparation of polymeric micro-porous materials, known as polyHIPE polymers (PHPs) which are now used extensively in PIM, and micro-reactor technology. These polymers are prepared through the high internal phase emulsion (HIPE) polymerization route. In order to control the pore size, the flow-induced phase inversion phenomenon is applied to the emulsification technique. The metalization of these polymers and formation of nano-structured micro-porous metals for intensified catalysis are also discussed. Finally, we illustrate the applications of these materials in chemical- and bioprocess intensifications and tissue engineering while examining the existence of several size-dependent phenomena. 

Figure 7.6 Basic polyHIPE polymer structures (a) primary pores with large interconnecting holes (b) primary pores with nano-sized interconnecting holes (c) large coalescence pores (three such pores are partially shown) dispersed into the primary pores in the process of coalescence and (d) detail of the coalescence pores. Note that these pore structures can be prepared over a wide size range...

The effect of pore size on cell concentration in the polyHIPE support at different times was determined from DNA analysis of the cell/polymer constructs. The results are shown in Figure 7.15 which indicates that DNA content per construct increases with increasing pore size. Since the pore size span in the experiments is not large (2.5-fold) the differences between the DNA contents are not as marked as the case for chondrocytes where the pore size span was 10-fold. Nevertheless, the present results also indicate that for a given cell type, an optimum pore size is present for cell proliferation as indicated by DNA content in... 

Figure 7,15 Effect of pore size on DNA concentration as a function of time in culture of rat osteoblast cells in hydroxyapatite-coated polyHIPE polymer support, illustrating the dependence of cell proliferation rate on support pore size...

Bokhari M.A., Akay G., Birch M.A., Zhang S. 2005. The enhancement of osteoblast growth and differentiation in vitro on a peptide hydrogel - PolyHIPE Polymer hybrid support material. Biomaterials, 26, 5198-5208 (in press). 

The catalytic properties of these various crosslinked polymer-supported ephedrines have been tested in the asymmetric addition reaction of diethylzinc to benzaldehyde and the results are smnmarized in Table 10. The reactions were carried out either in toluene or hexane at 0°C or 20-23 °C. As it can be noticed in Table 10, the enantioselectivities were higher in toluene. Compared to linear polymers 153-155, lightiy crosslinked chiral polymers led to the best results in terms of yield and ee. The use of Amberlite XAD-4 and Polyhipe as support proved to be not efficient. It was assumed that probably for both supports the catalytic sites are more in the inner surfaces of the pores and that result in a high local concentration of aminoalcohol residue which lead to a significant site-site interaction. Chiral polymer 156a was also used in the asymmetric ethylation of 2-methoxybenzaldehyde and 4-chlorobenzaldehyde imder similar conditions and ees reached 90-93% when the reaction was performed at 0°C. 

Figure 6.19 Electron micrograph of a polystyrene Polyhipe polymer, cell size typically...

Silverstein MS (2014) PolyHIPEs recent advances in emulsion-templated porous polymers. Prog Polym Sci 39(l) 199-234... 

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