Chemically polymeric

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

In all cases of electrochemicaHy or chemically polymerized unsubstituted polypyrrole, the final polymer is intractable in both the conducting and insulating forms. In contrast, a broad number of substituted polythiophenes have been found to be processible both from solution and in the melt. The most studied of these systems ate the poly(3-alkylthiophenes) (P3AT). 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

The final conclusion of this short discussion is that electropolymerization is a fast method (a film of about 5 //mean be obtained by polarization in 1 rnin) that uses a complex mechanism (Fig. 12) in which electropolymerization, cross linking, degradation, and chemical polymerization can coexist to produce a mixed material with a cross-linked and electroactive part and a passive fraction.67-71 However, ifwe control the variables acting on the kinetics of the different simultaneous reactions, the complexity also provides flexibility, allowing us to obtain materials tailored for specific applications. 

Two electrons per monomer molecnle go for chemical polymerization (abstraction of two hydrogen atoms from two chain-hnking sites for the first step, dimerization with one hydrogen abstracted per molecnle see Fig. 26.5) ... 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

Among various enzyme immobilization protocols, entrapment in polymer membranes is a general one for a variety of transducers. Formation of a membrane from a solution of already synthesized polymer is simpler and reproducible compared to chemical polymerization. The simplicity of this immobilization procedure should provide reproducibility for the resulting biosensors the latter is strongly required for mass production. 

One of the extensively used synthetic polymers used as a support for immobilization of biocatalysts is polyacrylamide (PAAm) [287,288], The major advantage is that it can be polymerized either chemically or by using radiation. Advantages of y-ray polymerization against chemical polymerization is that the polymerization can be carried out even under frozen conditions thus allowing the matrix to be molded to any form such as beads or membranes [289-291], However one of the major drawbacks of this polymer especially in a membranous form is its brittleness. 

In a related study by Contractor and coworkers [47] similar structures were produced but without the chemical polymerization step. The production of the structure used a polycarbonate membrane, where first a thin layer of Au was deposited over the top and bottom surfaces of the membrane (Figure 1.10a). Upon... 

Last but not least, enzymatic polymerization is more chemoselective than chemical polymerization as witnessed, for instance, by the successful polymerization of functionalized lactones bearing unsaturations and epoxides (Fig. 31) [150]. 

Enzymatic polymerization has been combined with various chemical polymerizations for the synthesis of block copolymers. The choice of chemical polymerization generally depends on the applied strategy for the block copolymer synthesis. These can be divided into three main approaches, as shown in Fig. 4 for the example of enzymatic ROP. It has to be noted that some of these strategies have also been applied for enzymatic polycondensations. 

The second strategy for the chemoenzymatic synthesis of block copolymers from enzymatic macroinitiators employs an individual modification step of the enzymatic block with an initiator for the chemical polymerization (route B in Fig. 4). This strategy has the advantage that it does not depend on a high incorporation rate of the dual initiator. On the other hand, quantitative end-functionalization becomes more... 

Graft copolymers were prepared by both classical strategies, i.e., from enzymatically obtained macromonomers using subsequent chemical polymerization and by enzymatic grafting from hydroxy-functional polymers. 

Lipase-catalyzed transesterification to prepare polyesters (replacing the traditional chemical polymerization at >200 °C) has received considerable attention in recent years. CaLB was found to mediate polyester synthesis in the ionic liquids [BMIm][BF4], [BMIm][PF6], and [BMIm][ Tf2N] at 60°C [110, 111, 112], but the molecular weight of the product was rather low compared with that in a solventless system [113], perhaps owing to the high viscosity of ionic liquid media. 

To prepare hexaaluminates for ceramic applications a slightly different sol-gel process was proposed by Debsikbar.19 Ba-hexaaluminates were prepared via hydrolysis of Al di(isopropoxide) acetoacetic ester chelate and anhydrous Ba acetate obtained by reaction between BaC03 and glacial acetic acid. The substitution of Al(i-OC3H7)3 with the alkoxy ester was intended to control the chemical polymerization during gel formation. The reaction was performed in 1-butanol. The formation of the gel slowly occurred at room temperature in about 10 h. To obtain the final phase the gel precursor was dried at 70 °C for about 2 weeks, ground and calcined at 1200°C for 2 h. However no data on the morphology of the final materials were reported by the author. 

Chemicals. Polymerization Catalysts are used in the production of polymers, such as linear and low-density polyethylene (LLDPE). An example of these catalysts are Ziegler Natta catalysts, which are combinations of titanium halides with aluminium and magnesium alkyls. 

Lucovyl H 4010 is a copolymer obtained by chemical polymerization of a mixture of (principally) butadiene and acrylonitrile on a PVC latex. The mean particle diameter of the resin obtained is very small (less than 0.1 pm). Its refractive index is close to that of PVC, and its compatibility with PVC is excellent. This polymer disperses well in the PVC matrix to give a two-phase system as shown on the electron micrograph in Fig-... 

Cooling water makeup should contain less than 5 ppm SS. If the level of suspended solids is considerably higher, say over 20 ppm SS, pretreatment by the use of polyelectrolytes or filtration is recommended. If the SS level is perhaps 5 to 10 ppm or if there is air-blown dust entering a cooling system, a sidestream sand filter or self-cleaning filter will be of benefit. In cases of high SS, chemical polymeric dispersants may be suitable as a total replacement for the sidestream filter but preferably will be used in tandem with a filter. 

As outlined earlier, three methods of polymerization have been established for the preparation of thiophenes, viz. electrochemical polymerization [189, 190], oxidative chemical polymerization using Lewis acid catalysts such as FeCl3 [191,192], and step-growth condensation polymerization using transition metal-catalyzed coupling reactions [lj]. 

Electropolymerization in acidic media affords free-standing films that are believed to contain varying degrees of cross-linking [267,292,304]. The miscibility of aniline with water allows for a variety of aqueous oxidants, such as ammonium peroxydisulfate, to be used [305]. Chemical polymerization of aniline can also be performed in chloroform through the use of tetrabutyl ammonium periodate [306]. Accordingly, a number of alkyl [301] and alkoxy-substituted [307] aniline derivatives have been chemically polymerized. Unfortunately, functionalization of the aniline nucleus often leads to a decrease in performance in the resulting polymers [308,309]. 

The synthesis of the j>. newington O-antigenic polysaccharide by the chemical polymerization pathway will be discussed as an example (J 6 J 7) The first and the most difficult task consists in the preparation of the monomer itself, i. e. , the properly functionalized, oligosaccharide repeating unit. The most usual approach is to introduce the required functions into the already-existing oligosaccharide molecule. 

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