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Polymers are substances made up of very large molecules formed from a long chain of smaller molecules called monomers joined together end to end.

Polymers can occur naturally, and they may also be made by chemical processes in industry when they are referred to as synthetic polymers. Examples of natural polymers are proteins and starches. Synthetic polymers of wide variety are now produced by the industry depending on their uses. The most common example is plastic.

The process of combining many small molecules into a polymer is called polymerization. During the process, some atoms may be lost from each monomer; the monomer residue incorporated into the polymer is called a repeat unit.

For example, in the polymerization of polyethylene terephthylate (PET), two water molecules are lost from the monomers--terephthalic acid and ethylene glycol--to form the repeat unit. PET is an important polymer resin of the polyester family and it is used in the manufacture of food and liquid containers and fibre for clothing.

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