Ionic polymerization disadvantages

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

However, from a kinetic and modeling point of view, this site-based nature of ionic polymerization also has some disadvantages. Because the reactivity of the active center strongly depends on the nature of the initiator and on all the other factors in the polymerizing system, the kinetic scheme and parameters of every system are different, and so in general they must be determined again for every change in the system. 

In solution polymerization, the monomer is dissolved in a solvent prior to polymerization. This technique is commonly employed for the ionic polymerization of gaseous vinyl monomers. The solvent facilitates contact of monomer and initiator (which may or may not be soluble in the solvent) and assists dissipation of exothermic heat of reaction. A limitation of this technique is the possibility of chain transfer to the solvent with consequent formation of low molecular weight polymer. An added disadvantage is the need to remove the solvent in order to isolate the solid polymer. In this respect, it is common practice to use a solvent in which the monomer but not the polymer is soluble the polymer is then obtained directly as a slurry and little further purification is necessary. 

The disadvantage of curing fluoroelastomers by ionic mechanism is that dehy-drofluorination required for this reaction produces considerably more double bonds than required for the cross-linking itself. This excess of unsaturation represents weak points in the polymeric chain, which can be attacked by basic substances contained in a contact fluid. This has been actually found when parts cured by this method were exposed to new oil and fuels containing basic additives.19 20... 

As for any other functional polymer, polymeric betaines are accessible by two different synthetic routes (1) the polymerization of zwitterionic monomers or (2) the zwitterionic functionalization of reactive precursor polymers. Both routes have inherent advantages and disadvantages. The polymerization of the zwitterionic monomers leads to polymers with 100% betaine functionality, but their molecular characterization is difficult for several reasons. For instance, the conformation of the polymers in aqueous solution is very sensitive, not only to the ionic strength but also to the type of added salt, and in the case of polycarbobetaines also to the pH. Furthermore, polymeric zwitterions often exhibit strong interactions with other matter, e.g., chromatographic colmnns. Hence, reliable GPC or HPLC measurements are very difficult to perform, if at all. 

Polymers can be prepared by many different processes. Free radical polymerization can be accomplished in bulk, suspension, solution, or emulsion. Ionic and other nonradical polymerizations are usually produced in solution polymerizations. Each technique has characteristic advantages and disadvantages. 

Among the disadvantages, the fouling and degradation of membrane surfaces of the polymeric membrane systems under adverse chemical and thermal conditions are often cited. However, these problems may be partly overcome by proper pretreatment of the effluents, optimizing the process variables and selecting suitable membrane materials. The radioactive wastes most suited for membrane separation are characterized by chemically insignificant amounts of radionuclides and small amounts (few hundred parts per rnilhon) of inactive ionic species. 

The disadvantage of this design is that it uses a liquid electrolyte, for example, acetonitrile, which means that the cell must be hermetically sealed. More recent Gratzel cell designs have used a nonvolatile electrolyte such as a polymer electrolyte or ionic liquid [5]. The possibility of using a polymeric photosensitizer has also been explored, and examples will be discussed later in this chapter. 

If possible from the point of view of efficiency, polymerization of the reactants without any use of a dispersing medium is desired. This method is called Bulk Polymerization. However, in order to absorb the heat released in the reaction, other methods like suspension or emulsion polymerization are frequently used. In this case, the water absorbs heat and provides good control of temperature. Another (less popular) method is the solution (homogeneous or heterogeneous) polymerization, which is mostly utilized in ionic initiation. We compare advantages and disadvantages of each method. (Table 2-4 describes the most useful polymerization methods for some commercial polymers). 

Addition polymerizations proceed either by free-radical or by ionic mechanisms and can be carried out either in bulk solution, i.e., on the neat monomer, or in suspension or emulsion. Each method has its own advantages and disadvantages. The choice of method of polymerization also depends to a very great extent both on the nature of the monomer and on the product desired. Polycondensations or step-reactions proceed according to the mechanism demanded by the reactive functional groups. Some common step-reactions are esterification, amidification, and urethane formation, as well as ring-opening or transesterification. 

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