Polyesters ester linkages forming

Polyesters (short for pOi meric esters ) are condensation polymers that are formed when dihydric alcohols react with dicarhoxylic acids. An ester linkage is formed at each end of each monomer molecule to build up large molecules. A useful polyester is prepared from terephthalic acid and ethylene glycol. 

When the new ester linkage is formed, a secondary alcohol group forms as well. The new secondary alcohol may also react with polyester main-chain linkages to form copolymer through a deg-radative transesterification process. Consequently, two very different types of copolymer may be formed in the blends of this section. If some frac-... 

Polyesters are eneountered in many forms. They are important as laminating resins, moulding compositions, fibres, films, surface coating resins, rubbers and plasticisers. The common factor in these widely different materials is that they all contain a number of ester linkages in the main chain. (There are also a number of polymers such as poly(vinyl acetate) which contain a number of ester groups in side chains but these are not generally considered within the term polyester resins.)... 

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

Polyesters form via a condensation reaction between a dicarboxylic acid and a dialcohol to create an ester linkage, as shown in Fig. 24.1. By far, the two most common polyesters are polyethylene terephthalate and polybutylene terephthalate, the chemical structures of which are shown in Fig. 24.2. These two polymers differ from one another by the length... 

Figure 12.6.1 Ester linkages that connect monomers to form polymeric molecules known as polyesters.

Table 2.2 shows two common polymers that are formed by condensation. Notice that DacronT", a polyester, contains ester linkages between monomers. Nylon-6, a polyamide, contains amide linkages between monomers. 

Although we will not be discussing the mechanism of each type of step growth polymer because these reactions are very similar to the corresponding monomer chemistry, we should be aware of this analogy. For instance, an acid reacts with an alcohol under acid-catalyzed conditions by a certain well-studied and proven mechanism. This same mechanism is followed each time an ester linkage of a polyester is formed. One such transformation is outlined in Fig. 14.8. The equilibrium is shifted in the direction of the product by distillation of the water from the reaction mixture (and condensing it in a separate container—hence the name condensation polymers for this type). 

We have so far given the impression that all polymers are formed fully armed, as it were, from monomers already having the correct functionality. This is, indeed, often the case because it can be very difficult to persuade polymers to carry out any reactions—reagents cannot penetrate their interiors, Polyester fabrics can be washed without any of the ester linkages being hydrolysed in the washing machine because the water cannot penetrate the fibres. However, some useful reactions, including ester hydrolysis, can be carried out on complete polymers,... 

In addition to the polyester cutin, plant cuticles may contain minor amounts of a highly aliphatic polymer, lacking ester linkages, formed predominantly from chains of CH2 units. This polymethylenic material may be cross-linked by ether bonds and has been termed cutan.The equivalent counterpart of suberin is termed suberan. The related chemically resistant material in algae is called algaenan. All these polymethylenic materials are discussed in more detail in Section 4.4. la. 

The technique is also applicable to cross-linked polyesters made from unsaturated polyesters and styrene, as in the sheet-molding compounds used in cars and boats.165 The ester linkages were broken by treatment with ethanolic potassium hydroxide or by ethanolamine. The latter was preferred, because no waste salts were formed by neutralization, as they were with the potassium hydroxide. The filler and glass fibers could be removed at this stage, if desired. The alcohol-soluble product was suitable for use in new bulk-molding compounds. A method that allows the cross-linked polyester to recycled back into the same use... 

Polyester Imide Enamels. The chemistry of the polyester Imide enamels differs from that of the polyesters in that methylene dianiline is one of the basic building blocks. Reaction with anhydrides such as trimellitic anhydride provides for the imide content, and the remaining acid group of the trimellitic anhydride is available to form ester linkages. 

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