Macroporous particles

Only particles of linear or very slightly cross-linked <0.6%) polymers may be produced by dispersion polymerization. Obviously, dispersion polymerization may be used for the production of monosized seed particles, which, after transfer to aqueous conditions, are used for the production of different cross-linked and macroporous particles by the activated swelling and polymerization method. 

The advantages of monosized chromatographic supports are as follows a uniform column packing, uniform flow velocity profile, low back pressure, high resolution, and high-speed separation compared with the materials of broad size distribution. Optical micrographs of 20-p,m monosized macroporous particles and a commercial chromatography resin of size 12-28 p,m are shown in Fig. 1.4. There is a clear difference in the size distribution between the monodispersed particles and the traditional column material (87). 

The physical characteristics of uniform macroporous particles produced in the existence of different diluents are exemplified in Table 13. Some important results obtained in this study are summarized as follows ... 

The macroporous particles prepared by using only linear polystyrene as diluent yielded lower pore volume and specific surface area values. 

The pore volume and the specific surface area of the uniform macroporous particles increased and the average pore size decreased with the increasing divinylbenzene concentration within the monomer phase. 

Lipases catalyse reactions at interfaces, and to obtain a high rate of interesterification the reaction systems should have a large area of interface between the water immiscible reactant phase and the more hydrophilic phase which contains the lipase. This can be achieved by supporting the lipase on the surface of macroporous particles. 

Mixtures of triglycerides, triglycerides plus free fatty adds or triglycerides plus fatty acid alkyl esters are used as reactants in fat modification processes. These mixtures are exposed to lipases supported on macroporous particles in the presence of a small amount of water. Liquid substrates (oils) can be reacted without use of a solvent, but with solid reactants (fats) it is necessary to add a solvent to ensure that the reactants and products are completely dissolved in the organic phase. Various water immisdble solvents can be used, but hexane is preferred for commercial operation because this solvent is already used industrially for the processing of oils and fats. 

Macroporous cation exchangers, 14 387 Macroporous gels, 13 738 Macroporous molecular sieves, 16 849 Macroporous particles, apparent effective diffusivity and, 15 730 Macroporous resins, 14 393, 397 Macroreticular sulfonated styrene-... 

Steroids (neutral or conjugated) and their dansylhydrazine derivatives Polyacrylic gel-based macroporous particles Acetonitrile-water-240 mM ammonium formate buffer, pH 3 (55 40 5) (also gradient elution) 350 mm x 100 pm i.d. 250 mm active length, laser induced fluorescence detection or coupling with electrospray-ion-trap mass spectrometry... 

Since electroosmotic flow can exist in both the interparticle and intraparticle spaces, numerous studies have focused on the existence of intraparticle flow in CEC. Several groups have investigated the existence of electroosmotic flow in wide-pore materials [41-44], A model was developed to estimate the extent of perfusive flow in CEC packed with macroporous particles [41] by employing the Rice and Whitehead relationship. Results showed the presence of intraparticle EOF in large-pore packings (> 1000 A) at buffer concentrations as low as 1.0 mM. Additional parameters had been investigated [43,44] to control intraparticle flow by the application of pressure to electro-driven flow. Enhancement in mass transfer processes was obtained at low pore flow velocities under the application of pressure. The authors pointed out that macroporous particles could be used as an alternative to very small particles, as smaller particles were difficult to pack uniformly into capillary columns. 

Later work on macroporous particles in CEC indicated a much stronger effect of pore flow on the separation efficiency [16], In this study it was shown that at moderate ionic strengths in the range of 0.1-10 mM, fully perfusive behavior could be created with particles having pore sizes of 50-400 nm. Plate... 

Figure 11 CEC with macroporous particles. (A) Column length 8.3 cm N= 15,000. (B) Column length 72 cm N= 230,000. Stationary phase, Nucleosil 4000-7 C18 mobile phase, ACN/water (70/30) solutes, PAHs.

A1 Rifai et al. studied the chromatographic behavior of macroporous particles in reversed-phase electrochromatography and compared the results with pressure-driven LC with capillary columns as well as with standard-bore 4.6-mm-i.d. columns [23]. Using 400-nm-pore particles they obtained 650,000 plates/m in the CEC mode, corresponding to a reduced plate height of only 0.2. These high efficiencies were obtained for low retained solutes and provided a five-fold improvement of the optimal efficiency obtained in pressure-driven LC on the same column, and a 10-fold improvement compared to the 4.6-mm-i.d. column. 

Macroporous particles have also been used in CEC to accomplish separation of enantiomers. Wikstrom et al. examined the use of vancomycine bonded... 

N ew opportunities and future directions in the area of microchannel emulsification are most likely in the areas of scale-up [140,141], encapsulation/polymeriza-tion [123, 125, 158, 164—169], rapid quenching of droplets [135], and the use of emulsions as templates for uniform macroporous particle structure formation [172]. MicroChannel emulsification is also likely to open up new opportunities with systems that are highly shear-sensitive [120, 135, 173]. The ability to scale up the process will spur new markets that require high production rates and the production of monodisperse capsules and polymer particles. Such developments will be useful in drug delivery applications and will contribute to the further quantification of micro-particle properties. Additionally, the use of monodisperse emulsions as particle templates is likely to enhance the utility of highly functional nanoparticles in need of a deployment mechanism [172]. 

Relative to small molecules, the effects of mass transfer on conversion and catalytic constants will be greater for macromolecular substrates. Since diffusion of substrates and products to and from the catalyst will be largely determined by the size of macroporous particles, this variable is particularly important when assessing catalytic supports for enzyme-catalyzed polymer synthesis and modification reactions. To our knowledge, no systematic studies have been reported on how size of macroporous resins influences enzyme activity. 

A further issue concerns the possible intrusion of heat effects. This issue has been addressed both theoretically and experimentally [52,57], and a simple criterion for the vaUdity of the isothermal approximation was developed. The isothermal assumption is normally vahd for measurements with zeoHte crystals (< 150 pm), but for large macroporous particles some intrusion of heat-transfer effects can be anticipated when the heat of adsorption is large. 

One can view th e monoliths as a single big porous particle. Thus, some of the preparation procedures use similar ingredients as the procedures used to make tnacroporous particles by suspension polymerization. Consequently, the structures of the monoliths are similar to the pore structure of macroporous particles, as can easily be seen by electron microscopy. Also similar chemistries are available, including styrene- ivinylbenzene and methacrylates, which have been proven to form sufBciently rigid structures to be useful in HPLC. But the tedmology of the formation of the monoliths is less constrained than the suspension polymerization used to form particles, and thus a broader range of chemistries is available. The classic monoliths were based on polyurethanes (20). Recently, silica-based monoliths were formed in a capillary (24). 

Continuous process for air cleaning using macroporous particles as... 

Data obtained from Wang and Wu 30 mL of solvent, 0.80 meq of catalyst (1 g), 40-80 mesh of macroporous particle, 20°C... 

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