Polymers are substances whose molecules have high molar masses and are composed of a large number of
repeating units. There are both naturally occurring and synthetic polymers. Among naturally occurring polymers are
proteins, starches, cellulose, and latex. Synthetic polymers are produced commercially on a very large scale and have
a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers.
Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined
sequentially, forming a chain. In many polymers, only one monomer is used. In others, two or three different
monomers may be combined. Polymers are classified by the characteristics of the reactions by which they are formed.
If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer. If some
of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation
polymer. Most addition polymers are made from monomers containing a double bond between carbon atoms. Such
monomers are called olefins, and most commercial addition polymers are polyolefins. Condensation polymers are
made from monomers that have two different groups of atoms which can join together to form, for example, ester
or amide links. Polyesters are an important class of commercial polymers, as are polyamides (nylon).