Resin microporous

Microporous beads are weakly crosslinked resins obtained by suspension polymerisation of styrene and divinylbenzene in the absence of any porogen agent. This process leads to the formation of a homogeneous network evidenced by a glassy and transparent appearance. The most commonly used supports for solid-phase organic synthesis and catalysis are styrene-divinylbenzene copolymers crosslinked with only 1-2% DVB. Many of their derivatives are commercially available [20]. 

Styryl-terminated Frechet-type dendrimers have been introduced as novel polymer crosslinkers by Seebach et al. [43-45]. They are constituted of four to 16 peripheral styryl units attached to aryl end branches of dendritic TADDOL, BINOL or Salen ligands and were copolymerised with styrene by suspension polymerisation. The catalytic performance of the polymer-bound catalyst was identical to that of the homogeneous analogues however, the supported catalysts could be used in many consecutive catalytic runs with only small loss in catalytic activity. A major drawback of fixing the catalytic unit in the core of the crosslinker is the poor loading capacity of the final polymer (0.13-0.20 mmol g 0 especially when high amounts of catalysts (10-20 mol%) are needed. 

In contrast to macroporous resins, microporous beads have a low internal surface area in the dry state of less than 10 m /g (determined by N2 BET) [22], due 

Concerning the chemical stability of polystyrene resins and their derivatives, it has been shown that they are relatively stable towards weak oxidants, strong bases and acids. In fact, reactions that are known to proceed on alkyl-snbstituted aromatic compoimds, especially electrophilic snbstitutions, wiU also occur on crosshnked polystyrene [lOj. Strong oxidants at elevated temperatures and electrophUic reagents should therefore be avoided [ 10,23 ]. 

Due to the easy handhng of polystyrene microbeads a large number of recent reviews have appeared to highhght the tremendous developments in this area [3-8]. Some more detailed examples are presented in the chapter of Uozumi et al. Despite the great efforts made in this area, it should be noted that many catalytic reactions using transition metal catalysts in combination with sohd phase supports often require larger amoimts (1-100 mol%) of catalyst due to 

20 mmol g 0 especially when high amounts of catalysts (10-20 mol%) are needed. 

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Continuous processes for copolymer production were developed initially for the microporous resins. The system generally involves injecting the monomer mix into the aqueous phase through orifice plates. Droplet size is controUed by the diameter of the holes in the plate and the rate at which the monomer is injected into the aqueous phase. The continuous process produces copolymer beads which have greater uniformity in size than those produced in batches. 

A problem associated with this procedure is the difficulty in removing excess reagents from the microporous resin. The chloride content was fairly high (0.25 irmol/g., ca 15% of original) in 0CH2CpH 2 as no chlorcmethyl absorbance was seen in the IR, this implied that NaCl was trapped in the resin. Elemental analysis (C, 88.90% H, 7.47% Cl, 0.90% total, 97.27%) suggested the presence of other impurities, which appeared to persist even after extensive extraction with solvent (THF-ethanol). 

Some industries practice ion exchange in nonaqueous systems. These solvents may cause resin particles to shrink or swell. Shrinkage has a negative effect on the kinetics, whereas swelling opens up the structure and improves migration of those constituents to be adsorbed. Microporous resins usually do not work well in nonaqueous systems because of the disappearance of porosity. Macroporous resins, however, are more satisfactory in these systems since porosity is retained even if the resins are dried completely. More functional groups on outer and inner surfaces are available for exchange as a result of the... 

P4-18b Ethyl acettde is an extensively used solvent and can be formed by the vapor-phase esterification of acetic acid and ethanol. The reaction was studied using a microporous resin as a catalyst in packed-bed reactor [Ind. Eng. Chetn. Res., 26(2), 19S(I987)]. The reaction is first-order in ethanol and pseudo-zero-order in acetic acid. Fw tm equal molar feed rate of acetic acid and ethanol the sped fic reaction rate is 1.2 dm /g cat 10111. The total molar feed rate is 10 mol/min, the initial pressure is 10 atm, the temperature is U8°C, and the pressure drop parameter, a, equals O.Ol g K... 

Macroporous resins (sometimes called macroreticular resins) are prepared by a special suspension polymerization process. Again, as with microporous resins, the polymerization is performed while the monomers are kept as a suspension of a polar solvent. However, the suspended monomer droplets also contain an inert diluent that is a good solvent for the monomers, but not for the material that is already polymerized. Thus, resin beads are formed that contain pools of diluent distributed throughout the bead matrix. After polymerization is complete, the diluent is washed out of the beads to form the macroporous structure. The result is rigid, spherical resin beads that have a high surface area. 

Reaction of polymeric alcohols 3 with Ti(OPr )2 Cb in CH2CI2 at room temperature gave Ti catalysts 4. Ti content of the polymers was determined through plasma analysis and revealed that conversion of the -OH groups had been complete. An important difference between macroporous and microporous resins is the fact that for gel-type polymers both isopropoxy... 

The porosity of an ion exchange resin determines the size of the molecules or ions that may enter an ion exchange particle and determines their rate ofdiffusion and exchange. Porosity is inversely related to the cross-linking of the resin. However, for gel-type or microporous resins, the ion exchange particle has no appreciable porosity until it is swollen in a solvating medium such as water. 

The pore size of a microporous resin can be determined using water soluble standards, such as those listed in Table Ifthe resin is made with... 

Gel-type, microporous, resins must swell to expose their catalytically active sites, whereas macroreticular resins have a permanent pore structure (inside these pores, catalytically active sites reside). Pores of the macroreticular resins can be described acceptably in terms of the conventional cylindrical pore model (pore diameter and volume). Pore structure, size, pore volume, and so on have been studied intensively in recent years. Examples of analytical techniques include X-ray microprobe analysis, ESR spectroscopy, NMR, and inverse steric exclusion chromatography (ISEC) the latter yields the best quantitative assessment of the nanomorphology of swollen resins. 

Even in microporous resins, one can expect some heterogeneities due to the different reactivities of the monomers and crosslinking mixtures involved in the suspension polymerisation process [22, 26]. However, it has been demonstrated experimentally that the distribution of reactive sites is homogenous throughout the whole bead [27-30]. Based on geometrical considerations, one should also be aware of the fact that in microbeads with a diameter of 100 pm, 50% of the active sites are within the first 10 pm of the outer shell [27]. 

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