Reactive Polymers, Ion Exchangers, Sorbents最新文献

A critical overview is presented on the use of anion-exchange-type resins, conventional and modified, including also polymeric crown ethers and related moieties,as catalysts in organic synthesis. The analysis of the available material has been organized to highlight the effects of specific structural and physical parameters of the polymer matrix, such as loading extent and topology of active sites, degree of crosslinking, porosity, surface area and particle size. Semi-quantitative evaluation of the above parameters has been carried out by comparing the specific activity and selectivity of polymer catalysts with their low molar mass analogues in standard nucleophilic substitution, addition and elimination reactions. Examples have been included of polymer catalysts that are more active and sometimes more selective than the corresponding low molecular weightanalogues. Cooperative effects of polymer-constrained structural units play a key role in determining the reported behaviour.

The effect of the strong anionic oxidant persulfate, S₂O₈²⁻, on the ion exchange capacity of Bio-Rad AG1-X4 100–200 mesh anion exchange resin was investigated when present as the counterion, i.e., with the exchanger in the reactive persulfate form. The decrease of ion exchange capacity as a function of time and the increase in swelling due to scission of polymer chains by the oxidant were determined. It was found that very little (dry) capacity loss occurred during storage of air-dried S₂O₈²⁻-form resins at ambient temperature. However, a progressive decrease in capacity was noted when the resin samples were kept fully water swollen. The potential effect of initial pH was briefly examined. Some experiments with chromate- and vanadate-form resins were also carried out.

During chloromethylation of macroporous styrene-divinylbenzene copolymers, postreticulation of unreacted vinyl groups takes place as side reaction. Local molecular analysis by Roman microspectrometry showed that the distribution of pendent vinyl groups inside the copolymers heads after chloromethylation was heterogeneous. The relationship between the distribution of chlorine-containing groups and vinyl groups was studied, and their inhomogeneous distribution explained by the textural heterogeneity of the heads and by postreticulation affecting the centre more than the periphery of the heads.

This article establishes the importance of the active site spacing in the resin matrix on the relative selectivity for monovalent and divalent ions. The selectivity trends of dicarboxylate/chloride exchange on trialkylamine resins provide experimental verification of the hypothesis that resins with closely spaced functionalities have an inherent preference for divalent ions over monovalent ions. For a typical trimethylamine resin, the isotherm data show a monotonic decrease in selectivity with chain length whereas for the triethyl and tributylamine resins, an increase followed by a decrease in selectivity is observed. A theoretical analysis of ion exchange equilibria is developed to quantify the hydrophobic and electrostatic components of the overall dianion selectivity. A phenomenological model is used to estimate the site spacing distance for each resin and the results suggest a nonhomogeneous distribution of ionogenic groups in the gel phase. The charge separation concept, which is applicable to both anion and cation exchangers, is validated with supportive evidence from other research studies.

The outer coat (exine) of plant pollen grains and many related microspores derived from algae or fungi is composed of the material sporopollenin, probably produced biochemically by an enzyme-controlled oxidative polymerisation of carotenoids. Sporopollenin is exceptionally resistant to chemical and physical attack. The material from the commercially available Lycopodium clavatum retains the full morphology of the original spore grain after processing. It may also be functionalised without any change in microscopic appearance by direct amination with ethylene or propylene diamine, sulphonation with chlorosulphonic acid and carboxyalkylation of the former aminoethylsporopollenin which produces a carboxymethylsporopollenin. These materials have been used as column materials for the successful separation of a selection of nucleosides, nucleotides, α-amino acids and transition metals. The advantages of sporopollenin over conventional ion-exchange materials are discussed.