Polyether, structure

The concentration of this hybrid structure (polyether block linked to a polyurea block) is low, but it is enough to assure a good stability of the dispersion. This very reactive system is based on the big difference in reactivity between primary amines and hydroxyl groups and the -NCO groups of the diisocyanate. Thus the -NCO group reacts 3300 times more rapidly with primary amines than with secondary hydroxyl groups and around 1000 times more rapidly than with the primary hydroxyl groups. 

Since 1950 a number of polyether antibiotics have been discovered using fermentation technol ogy They are characterized by the presence of sev eral cyclic ether structural units as illustrated for the case of monensm in Figure 16 3a Monensin and other naturally occurring polyethers are similar to crown ethers in their ability to form stable complexes... 

Polyester (Section 20 17) A polymer in which individual structural units are joined by ester bonds Polyether (Section 16 4) A molecule that contains many ether linkages Polyethers occur naturally in a number of antibi otic substances... 

The polyether antibiotics were first recognized as a separate class with the pubHcation of the structure of monensia ia 1967 (1). Several members of the group have siace found commercial appHcation as anticoccidials ia poultry farming and ia improvement of feed efficiency for mmiaants (see Feeds and FEED additives). 

Polyether ether ketone (PEEK) (structure B of Table 21.5). This material was first prepared in the laboratories of ICI in 1977 and test marketed in 1978. The material is now marketed by Victrex as Victrex PEEK. 

Polyether ketone (PEK) (structure A). In 1986 three European companies (BASF, Hoechst and ICI) announced they were developing the material (to be marketed under the trade names of Ultrapek, Hostatec and Victrex PEK respectively). 

Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

Interesting graft polymers based on silicone polymers are finding use in the manufacture of polyurethane foams, particularly, of the polyether type (see Chapter 27), because of their value as cell structure modifiers. 

Figure 3.10 shows the secondary ion spectra recorded from within the defect and its surroundings. The spectrum from the defective area clearly shows the presence of a polluting perfluorinated polyether structure (the C Fy peaks and the peak at mass 47 amu (CFO" ) are diagnostic of the polyether structure). 

Venustatriol, a marine-derived antiviral agent, as with many polyether structures, is a straightforward problem for retrosynthetic analysis. The major issues, clearance of stereocenters and topologically strategic disconnection, were readily resolved to generate the pathway of synthesis described below. 

Macrocyclic polyethers containing the 2.2-paracyclophane unit are interesting structures and several such compounds have been prepared . Despite the diverse structural possibilities, the syntheses of these molecules have generally been accomplished by straightforward Williamson ether syntheses. The only unusual aspect of the syntheses appears to be a novel approach to certain paracyclophanes developed by Helgeson (see footnote 7a in Ref. 91). The first step of Eq. (3.28) illustrates the formation of the required tetrol, which is then treated with base (KOH or KO-t-Bu) and the appropriate diol dito-sylate to afford the macrocycle. 

From their structures, it appears that the hydrolytic stability of macrocyclic lactones must necessarily be inferior to macrocyclic polyethers. Ease of synthesis of the cyclic esters is therefore one of the aspects which commend them to interest. It is probably for this reason that such lactones have not been made more often by the interesting approach of Kdgel and Schroder . These workers report the ozonolysis of dibenzo-18-crown-6 in a mixture of methanol and dichloromethane at —20°. Reduction of the ozon-ide at —75° using dimethylsulfide followed by warming and addition of acetone led to formation of 6 in 14% yield. The bis-oxalate had mp 164—165° from acetone, very similar to that of the starting crown. The transformation is illustrated below in Eq. (5.9). 

Structure-chemical aspects of complex formation in metal halide-macrocyclic polyether systems 99UK136. 

The advantage of the activated displacement polymerization is the facile incorporation of different and unconventional structural units in the polymer backbone. Most of the heteroarylene activated polyethers prepared by this route are soluble in many organic solvents. The solubility behavior of new polyethers is shown in Table 8. In contrast to many polyphenylenequi-noxalines, poly(aryl ether phenylquinoxalines) prepared by the quionoxaline activated displacement reaction are soluble in NMP. Solubility in NMP is important since it is frequently used for polymer processing in the microelectronics industry [27]. 

Polyetherimides show no crystallinity as evidenced from calorimetry measurements. The heteroarylene like phenylquinoxaline [27], oxadiazole [30], and benzoxa-zole [56] activated polyethers show TgS from DSC thermograms, with no evidence of crystallization, indicating amorphous or glassy morphology. Furthermore, wide angle x-ray scattering measurements show no evidence of crystalline or liquid crystalline type morphologies, consistent with an amorphous structure. F polyether... 

Thermoplastic polyester rubbers are also block copolymers of polyethers and polyesters. The polyester groups are capable of crystallisation and the crystal structures act like cross-links. These materials have good hydrocarbon resistance. Similar thermoplastic polyamide rubbers are also now available. 

Cholanic acid also possesses the ability of transporting cations across a lipophilic membrane but the selectivity is not observed because it contains no recognition sites for specific cations. In the basic region, monensin forms a lipophilic complex with Na+, which is the counter ion of the carboxylate, by taking a pseudo-cyclic structure based on the effective coordination of the polyether moiety. The lipophilic complex taken up in the liquid membrane is transferred to the active region by diffusion. In the acidic region, the sodium cation is released by the neutralization reaction. The cycle is completed by the reverse transport of the free carboxylic ionophore. 

In mimicking this type of function, noncyclic artificial carboxylic ionophores having two terminal groups of hydroxyl and carboxylic acid moieties were synthesized and the selective transport of alkali metal cations were examined by Yamazaki et al. 9 10). Noncyclic polyethers take on a pseudo-cyclic structure when coordinating cations and so it is possible to achieve the desired selectivity for specific cations by adjusting the length of the polyether chain 2). However, they were not able to observe any relationship between the selectivity and the structure of the host molecules in an active transport system using ionophores 1-3 10). (Table 1)... 

The result is explained by considering the stacking structure between the quinoline moiety and the benzene ring linked to the carboxylic acid, which gives the cavity size adequate for Li+. (Fig. 3) Several selective host molecules for Li+ such as [13]crown-4 18), [14]crown-4 19), [16]crown-4 20>, or noncyclic polyether amide derivatives 21) also possess trimethylene moiety, and this is an interesting finding from the point of view of molecular design of new host molecules for Li+. 

In 1967 Agtarap et al. disclosed the isolation and structure of monensic acid (1).3 Compound 1, now known as monensin, is produced by a strain of Streptomyces cinamonensis, and exhibits broad-spectrum anticoccidial activity. Since its introduction on the market in 1971, monensin has been used very successfully to combat coccidial infections in poultry and as an additive in cattle feed.4 In the polyether family, monensin occupies a position of some historical significance. Although monensin was the fifth polyether... 

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

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