Crown ethers polymeric

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

Interestingly, soluble polymeric crown ethers can have extraction coefficients up to 250 times larger than those of the corresponding monomers. Macromolecules such as polyvinyl [15]crown-5 and polyvinyl [18]crown-6 are readily soluble in organic solvents and many extraction systems involving their use have been developed (B-81MI52103). 

Paraquat, an ionic molecule, is more rigid than polymeric crown ethers. In addition, die crown ether has to adopt a restricted conformation for complexation to occur. The total increase of the rigidity for derived polyrotaxanes 84 depended on the min value [118, 119]. The higher the value of min, the higher Tg of the poly-rotaxane. Paraquat is a crystalline material but the polyrotaxanes are amorphous. 

A novel approach to the production of chiral, polymeric, crown ethers incorporating isomannide was developed by a Japanese group. The optically active divinyl ether 103 was polymerized with cationic catalysts to afford 104, consisting of only cyclic constitutional units.191 In addition, another crown ether (105), containing five ethylenedioxy moieties, was prepared. 

Until now, i.e., in 25 years of research, only two reviews on liquid crystalline crown ethers have been published [8, 9]. As both reviews cover the field only partially and, e.g., the fascinating polymeric crown ethers as well as taper-shaped liquid crystalline crown ethers are not discussed, we decided to give the first comprehensive review. The present chapter will be structured according to the molecular structure of the mesogens. The discussion of each type of crown ether mesogen will start with small molecules, continue with polymeric compounds, and conclude with possible applications (where applicable). 

In addition to post-functionalizing polymers by bonding the macrocycle to the preformed polymer backbone, macrocycles can be incorporated into polymer matrices by direct polymerization of the macrocycle, either by a step-growth mechanism or a chain-growth mechanism. [46] Polymeric crown ether stationary phases were pioneered by Blasius et al. [34, 59-62] These resins were used to separate both cations (including protonated amines) with a common anion, and anions with a common cation in high... 

Figure 6. Recent examples of polymerized crown ethers (see ref. 46)...

A covalent pore required for bilayers must be approximately 40 A long and contain a polar core for ion transit and a non-polar exterior for membrane interaction. Oligopeptides with high helical content, polymeric crown ether and bouquet -shaped crown ethers have been reported. The only such... 

Polymeric crown ethers are prepared by condensation, polymerization, or substitution [37]. Above all, condensation polymers based on monobenzo- and dibenzocrown ethers are immobilized on the surface of a solid substrate, and are used in chromatographic applications. They are distinguished by chemical and thermal stability and exhibit a relatively high capacity. Modified and non-modified silicas are used preferentially as support materials. 

The ratio of 0-alkylation/(0 + C alkylation) in alkylation of phenoxide anions by allylic halides can be controlled by the presence of poly(vinylmonobenzo-18-crown-6) or benzo-18-crown-6. (61). The results of these studies shown that the percentage of 0-alkylated product was consistently higher when a soluble polymeric crown ether was employed. 

The complexation of metal salts by neutral macrocyclic ligands is well known. Polymeric crown ethers are an expanding group of fimctionat ion exchangers capable of selective sorption of alkali metals such as K, Cs, Na, and Li. The crown ether may be derived from a conventional chlormethylated hydrocarbon backbone which is converted to a polybenzylated catechol. Crown ethers are highly reversible and possess rapid reaction kinetics, thus allowing for an interesting thermal elution procedure whereby a species is sorbed at 20°C and eluted at fiO C. 

Copolymerization of polymeric crown ether with silica gel or other support materials is another way to incorporate crown ether into stationary phases. Blasius and coworkers thoroughly studied methods for polymerizing cyclic polyethers with various polymeric matrices and applied... 

Figure 9 (a) A polymeric crown ether stationary phase. (Repro-dnced from Ref. 56. lUPAC, 1982.) (b) structure of the polyether modified silica stationary phase. (Reproduced from Ref. 57. American Chemical Society, 1983.)... 

Unfortunately, the approaches described above have some inherent disadvantages. First, for columns of the type in Figure 9(a), the flow rate is usually low (0.05-0.1 mlmin ) because of the low mechanical rigidity, needed to withstand high pressure, of polymeric crown ether resins. This results in long analysis times. Second, columns like those based on 12 suffer from poor hydrolytic stability due to the potential for acid or base hydrolysis furthermore, the underlying silica structure can be ionized and thus tends to concentrate eluent as well as analyte ions, resulting in a loss of selectivity. Third, the separation efficiencies of both types of column material have not been... 

Polymeric crown ether phases have been thoroughly investigated for their applications in chromatography by Blasius et al. [70-75]. They are characterized by chemical and thermal stability, are compatible with a variety of organic solvents, and are of high capacity. Thus, Blasius et al. separated anions with pure methanol as the mobile phase. Because the stability constants of the cation crown ether complexes depend on the type of solvent, the solvent content in the mobile phase can be used as an additional parameter for optimizing the chromatographic separation. 

Polymeric crown ether resins can be prepared in two different ways ... 

Polymeric crown ether resins are mechanically unstable and are, therefore, operated with low flow rates between 0.05 and 0.1 mL/min resulting in long... 

Polymeric crown ether resins are mechanically unstable and are, therefore, operated with low flow rates between 0.05 and 0.1 mL/min resulting in long analysis times. Typical for all polymeric crown ether phases is the relatively low chromatographic efficiency, which does not meet today s requirements. However, this problem can be overcome by immobilizing crown ether polymers on the surface of a solid support. Modified and non-modified silica are predominantly used as support materials. Igawa et al., for instance, coated silica particles with the above-mentioned polyamide crown ether resin and obtained significantly better separations then with the rmcoated resin. 

Polymeric crown ethers may also be bonded chemically to the silica surface. For that purpose, the support material is modified with 3-(N-methacryloylam-ino)-propyl groups, and subsequently co-polymerized with (4-methacryloylam-ino)-benzo-15-crown-5 [55]. Stationary phases, in which the crown ether molecules are chemically bonded to silica via Si-O-C linkages, are obtained with silica treated with thionyl chloride via reaction with 4-hydroxymethylbenzo-18-crown-6 (see Fig. 3-56, upper right). Since Si-O-C-bonds are prone to hydrolysis, water-free methanol is used as eluant for such stationary phases. Much more stable are silicas modified via Si-C-bonds (see Fig. 3-56, bottom left and right) that are prepared by using 4-vinylbenzo-18-crown-6 or 4-butene-18-crown-6 and... 

Crown polyethers have also been synthesized (Kopolow et al., 1971, 1973) by polymerization of the corresponding vinyl monomer (4 -vinylmonobenzo-15-crown-5) (Scheme 13-4). The polymeric crown ethers were obtained as amorphous solids, softening at 122-128°C and with an average molecular weight of 11,600. The polymeric polyethers, just like the monomeric crown ethers, had the ability to bind alkali metal, particularly potassium, ions. Hence, these have been used for extraction of potassium. The crown polyethers form a complex with KMn04, which is... 

Acetophenones substituted with methoxy or phenoxy groups in the para position had been shown to undergo successful activated Ru catalyzed copolymerization with 1,3-divinyltetramethyldisiloxane [18]. Activated Ru catalyzed copolymerization of 4-acetylbenzo crown ethers with 1,3-divinyltetramethyldisiloxane provides a synthetic route to a/r(carbosilane/siloxane) copolymers which incorporate crown ethers, as shown in Figure 11. Polymeric crown ether/lithium perchlorate complexes have been prepared. Of particular note, the sterically congested ortho C-H bond, which is between the acetyl group and the crown ether ring, is still reactive [19,20]... 

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