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Ingredients
Condensation polymers are often made by joining together molecules that have alcohol or OH groups with molecules that have carboxyl or COOH groups. Alternatively, they may be formed by combining molecules that have amine or NH2 groups with molecules that have carboxyl groups. PET is an example of the first kind; the ingredients used in its manufacture are terephthalic acid and ethylene glycol. Ethylene glycol is the same liquid you use for antifreeze in your car; it has two alcohol groups.
Reaction
In the polymerization reaction, the alcohol combines with the carboxyl group to form a type of linkage called an ester. In the process, the OH from the carboxyl group and the H from the alcohol combines and is lost as a molecule of water. Condensation reactions between amines and carboxylic acids are similar in that they, too, generate a molecule of water as a side product of the reaction.
Continuation
The molecules used for condensation polymerization typically have two groups that can participate in the polymerization reaction, one at each end. Terephthalic acid, for example, has two carboxyl groups, one on either end, and ethylene glycol has two alcohol groups. Consequently, each molecule of terephthalic acid ends up with two molecules of ethylene glycol attached to it on either side, and each molecule of ethylene glycol ends up with two molecules of the terephthalic acid attached to it on either side, so the continuing reaction creates long long chains called polymers.
Characteristics
Unlike other kinds of polymers, condensation polymers tend to have far less cross-linking between adjacent chains, so they tend to make very good fibers --- one of the reasons polyesters are used in fabrics for clothing. They are often spun into fibers by forcing the heated polymer through small holes, a little like the way a spider forms silk from spinnerets. Condensation polymers also tend to grow more slowly and require heat for the reaction to take place.