Polymer sheet

Their studies involved the partial polymerization of NCAs of mixtures of specific amino adds having known e.e.s, followed by determination of the e.e.s of the amino adds in both the resulting polypeptides and in the residual unreacted NCA monomers. [94] In a typical experiment it was found that when an optically impure leucine NCA monomer having an l > d e.e. of 31.2% was polymerized to the extent of 52 % to the helical polyleucine peptide, the e.e. of the polymer was enhanced to 45.4 %, an increase of 14.2 %. In the same experiment the e.e. of the unreacted leucine NCA monomer was depleted to a similar extent. Analogous experiments with valine NCAs of known e.e.s, however, led to a reverse effect, namely, the preferential incorporation of the racemate rather than one enantiomer into the growing polyvaline peptide. This finding was interpreted to be the result of the fact that polyvaline consists of (3-sheets rather than a-helices, emphasizing that the Wald mechanism applies only to a-helix polymers. At about the same time Brach and Spach [95] showed that, under proper conditions, (3-sheet polymers could also be implicated in the amplification of amino add e.e.s. 

Microtubule polymers form spontaneously from isolated tubulin in buffer solution with GTP and Mg++ at a temperature of 30-37 °C. Sheet polymers form spontaneously from tubulin in buffer with GTP, Mg++ and Zn [7], The crystals must be exquisitely flat in order to yield good diffraction when tilted to high angles. This can be a limiting problem, since they are so thin and flexible. Careful attention to all of the preparation parameters and properties of the supporting film, though, produces specimens that can diffract remarkably well. 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Examples of Flat Sheet Polymer Membranes Used in the Referenced Works and Their Properties... 

Pairwise Cu-Cu distances of 242 pm are found in the [Cu(CN)(NH3)] sheet polymer formed from Cu(CN) power in aqueous NH3. Phosphine adducts of [RCsCCu] yield yellow crystals of [RC=CCuPR3] with n = 4 for R = Ph, R = Me, and a zigzag CU4 chain displaying Cu-Cu distances of 245 and 269 pm with bridging acetylides . A square-planar Cu4 tetramer (Cu-Cu 242 pm) with bridging trimethyl-silylmethyl groups is prepared from Cu(I) iodide and alkyllithium in hexane-ether °. 

Lomas DA, Elliott PR> Sidhar SK, Foreman RC> Finch JT, Cox DW, et al. al-Antitrypsin Mmalton (Phe52-deleted) forms loop-sheet polymers in vivo Evidence for the C sheet mechanism of polymerization. J Biol Chem 1995 270 16864-70. 

Polymers have a wide range of properties and uses. Mylar sheet polymer (another polyethylene terephthalate) is used to protect documents, such as this photocopy of a later version of the Declaration of Independence (top). Another kind of polymer coating is used to make shatterproof covers on glass items such as fluorescent light bulbs (bottom). 

Table 5.5 Examples of flat-sheet polymer membranes, used In the referenced works, and their properties... 

A common route to phthalocyanine sheet polymers involves the condensation of 1,2,4,5-tetra-cyanobenzene with a metal salt, Scheme 4.1. Unfortunately, the sheet polymer is contaminated with polyisoindolenine and triazine linkages, which reduce its planarity and conductivity. Since these polymers are generally insoluble and not sublimable, it is difficult to remove the defects or impurities. Wohrle has developed an improved route to 2 (M = Cu11) by employing lithium pentanolate to reduce the density of defects in the resulting polymers.23 Improved approaches for thin-film preparation were also explored. 

Scheme 4.1 Synthesis of phthalocyanine-based sheet polymer 2.

Achar has synthesized cobalt, nickel, and copper phthalocyanine sheet polymers from the condensation of the metal phthalocyanine tetracarboxylic acids at 400°C (Scheme 4.2).24 These... 

The next step Ifom Silicon-based sheet polymers to tubular structures has been performed so far only on the computer. In Chapter 17, Th. Frauenheim et al. describe the structure and the electronic properties of silicide, silane and siloxane nanotubes. The structures can be understood in terms of conventional graphitic carbon nanotubes by replacing the flat hexagons by puckered rings. The electronic properties depend on the tube diameter. Potential applications are discussed. 

Silicon- and Germanium-Based Sheet Polymers and Zintl Phases... 

Figure 15.1. Structures of the trigonal-rhombohedral tr6 modifications of the Zintl phases CaSi2 and CaGe2, of the sheet polymers polygermyne (GeH) and siloxene (Si2HOH) and of the polygermyne calcium dihydroxide intercalation compound (Ca(OH)2(GeH)2) . In each case, a full unit cell in the c direction is shown.

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