Chemical structures polymeric supports

Some studies relative to the influence of Lewis bases,124-127 1,3-diethers in particular,125-127 and the MgCl2 support have also been recently reported. The dependence of the industrially relevant isotactic indexes on the chemical structure of the 1,3-diether donor has been rationalized in the assumption that donor coordination competes with Ti catalytic species formation and xylene-insoluble (highly isotactic) and xylene-soluble (poorly isotacUc) fractions are mainly obtained by polymerization on (100) and (110) cuts, respectively.127... 

Modification of polymers is a topic in polymer science, because new highly valued or improved applications often require sophisticated chemical structures along the polymer chains. One of such timely domains of interest comprises the development of modified polymers as catalysts for chemical processes. Of course, we do not have in mind catalysts, wherein polymers function as inert supports for the active centers and nomore. In fact, our aim is to develop polymeric catalysts, which combine advantages of the other type of catalysts, viz. 

Seki and Tirrell [436] studied the pH-dependent complexation of poly(acrylic acid) derivatives with phospholipid vesicle membranes. These authors found that polyfacrylic acid), poly(methacrylic arid) and poly(ethacrylic acid) modify the properties of a phospholipid vesicle membrane. At or below a critical pH the polymers complex with the membrane, resulting in broadening of the melting transition. The value of the critical pH depends on the chemical structure and tacticity of the polymer and increases with polymer hydro-phobicity from approximately 4.6 for poly(acrylic acid) to approximately 8 for poly(ethacrylic acid). Subsequent photophysical and calorimetric experiments [437] and kinetic studies [398] support the hypothesis that these transitions are caused by pH dependent adsorption of hydrophobic polymeric carboxylic acids... 

Besides the classical polymer introduced by Merrifield (1%-crosslinked chloromethylated polystyrene), a broad variety of polymeric supports is available for SPPS and some of the most popular resins are summarized in Table 1. The chemical structures of some selected resins are presented in Figure 1 and electron micrographs of several examples are displayed in Figure 2. In addition to the solid supports listed in Table 1, there are several other carriers used in peptide synthesis such as the gel-type and macroporous poly(meth-acrylates), coated surfaces like polystyrene films on polyethylene (PEt) sheets, polystyrene-coated polyethylene or polytetrafluoroethylene, and modified glass surfaces. (For recent reviews on polymeric carriers see refs . )... 

Figme 1 Chemical Structures of Selected Polymeric Supports... 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

In a similar vein, a series of papers published between 2002 and 2008 contains spectacular claims of highly enantioselective asymmetric additions of water to styrenes, unsaturated carboxylic acids, or simple terminal alkenes [34-Al]. The catalysts used are of the heterogeneous type and based on chiral biopolymers such as wool, gelatin, or chitosan as solid supports (sometimes in combination with silica or ion-exchange resins) that are doped with transition metal salts. This series of papers contains spectacular claims, insufficient experimental data, and erroneous chemical structures for the biopolymers used. As earlier work from the same group of authors on asymmetric catalysis on bio-polymeric supports is irreproducible [42], one is well advised to await independent confirmation of those results. 

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