Polymerization disadvantages

There were two different methods for the preparation of site-specific polymers covalent and self-assembly (noncovalent) approaches. The first method involved the formation of more homogeneous chiral cavities with respect to the second approach, because monomer and template were held together by covalent bonds during polymerization. Disadvantages of this procedure depended on the chance to obtain very elongated or tailing spots in particular for the more-retained optical isomer. 

Phosphates are the principal catalysts used in polymerization units the commercially used catalysts are Hquid phosphoric acid, phosphoric acid on kieselguhr, copper pyrophosphate pellets, and phosphoric acid film on quartz. The last is the least active and has the disadvantage that carbonaceous deposits must occasionally be burned off the support. Compared to other processes, the one using Hquid phosphoric acid catalyst is far more responsive to attempts to raise production by increasing temperature. 

A semi-batch reactor has the same disadvantages as the batch reactor. However, it has the advantages of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low concentration of one of the reactants. Semi-batch reactors are also of value when parallel reactions of different orders occur, where it may be more profitable to use semi-batch rather than batch operations. In many applications semi-batch reactors involve a substantial increase in the volume of reaction mixture during a processing cycle (i.e., emulsion polymerization). 

Certain starting materials may give rise to the non-selective formation of regioisomeric enolates, leading to a mixture of isomeric products. Furthermore a ,/3-unsaturated carbonyl compounds tend to polymerize. The classical Michael procedure (i.e. polar solvent, catalytic amount of base) thus has some disadvantages, some of which can be avoided by use of preformed enolates. The CH-acidic carbonyl compound is converted to the corresponding enolate by treatment with an equimolar amount of a strong base, and in a second step the a ,/3-unsaturated carbonyl compound is added—often at low temperature. A similar procedure is applied for variants of the aldol reaction. 

New elastic polymeric materials (resistance to higher stroke or air) can be obtained by using physical modification methods, but using this method, two phases (PS and rubber) in the mixture were formed. Small rubber particles spread as a PS layer and, after awhile, the relationship between the layers decreases and rubber particles gather in the upper layer of the materials. This can be the cause of the loss of resistance of the materials. These material disadvantages have stimulated the polymer synthesis to increase the PS resistance to higher physico-mechanical properties, such as higher temperature and stroke for the chemical modification of PS with various functional modifiers. 

For chromatographic sorbents it is necessary that the polymeric cover be uniformly distributed over the silica surface and chemically coupled to it. The appropriate introduction of the initiator is one of the decisive steps of this task. The first method (binding to the surface) increases the yield of grafted polymer. However in this case a large amount of homopolymer is formed. This disadvantage could be prevented by the application of hydroperoxide initiators in combination with the proper redox-agents [78-81],... 

The ring-opening polymerization of ketene acetals (45, X=0) provides a novel route to polyesters and many examples have now been reported (Scheme 4.27). " "7 A disadvantage of these systems is the marked acid sensitivity of the monomers which makes them relatively difficult to handle and complicates characterization. This area is covered by a series of reviews by Bailey ct a/.177 228 ... 

In contrast to bulk polymerization, solution polymerization provided soluble polymers with high molecular weights using low FeCl3 concentration at 120-140 C.31 A major disadvantage of the above approaches is that all the metal-halide catalysts need to be removed, since the catalyst residue will deteriorate die thermal stability and electrical and other properties. 

Points 1, 3 and 4 are especially important for a theoretical approach of polymerizations. On the other hand, however, there are also some disadvantages which must be taken into account while using quantum chemical methods ... 

The fact that the cationic polymerization could not be experimentally registered for R = —CN, —COOCHj, could be explained in theory with the high n-energy use for the start reaction in contrast to the energy use of R = —Ph—CH3. The vinyl acetate (R = OCOCH3) does not polymerize cationically. This can be explained by the fact that the propagation reaction is so disadvantageous that the formed ions cannot start the polymerization. 

The polymerization rate is controlled by the slowest process. Thus it is important to establish the rate controlling steps. The starting material for the (SPP) can be the dry nylonsalt Z 4) but mostly a low or middle molecular weight polymer is used. The polyamide-salts have the disadvantage of high amine losses 3 4). 

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