Polymers are long chain-like molecules which are formed from several repeating smaller molecules known as monomers. Monomers are those small molecules which may be linked chemically to form the long chained molecules with high molar masses called polymers.
So, polymers are like chains, and monomers are the individual chain-links which we connect chemically to form the chained polymers.
There are two types of monomers:
Unsaturated small molecules (for example, ethene and chloroethene)
Molecules containing two functional groups (such as propane-1,3-diol and hexane-1,6-diamine)
These types of monomers each form polymers by a different form of polymerization.
If you can remember, unsaturated compounds, like the alkenes, undergo addition reactions typically. Thus, unsaturated monomers form polymers via addition polymerization. The product formed is an addition polymer.
Monomers with two functional groups (bi-functional groups) often tend to have two identical functional groups. These monomers are unlikely to bond with one another, and, as a result, two different bi-functional monomers are usually used to make up polymers. These monomers undergo condensation polymerization to form polymers, and the product formed is a condensation polymer.
This polymerization is usually accomplished by subjecting and unsaturated compound to heat and pressure while in the presence of the requisite catalyst. When ethene is subjected to high temperature, pressure and a suitable catalyst in addition polymerization:
The double bond in the ethene is broken, leaving one electron on each carbon atom unpaired, and therefore, subject to reaction. This reactive species can react with other ethene molecules to form a polymer chain:
As you can see, no loss of material occurs in addition polymerization. The polymer (polyethene) has the same empirical formula as the monomer (ethene), CH2.
In condensation polymerization, two (usually different) bi-functional monomers are joined by eliminating a small molecule like water between them. Unlike addition polymerization, where the only product is the polymer, condensation polymerization produces the polymer and a small molecule (like water).
The best example of condensation reactions is seen in the formation of polyesters, polymers containing several ester linkages. These are formed in condensation reactions between di-alcohols (or diols) and diacids. A di-alcohol has two -OH (hydroxyl/alcohol) groups while a di-acid has two -COOH (carboxyl/organic acid) groups.
Remember how esters are formed, with a reaction between an alkanol/alcohol and an alkanoic acid? Polyesters use the same concept, except the two functional groups allow the chain to be continued on either end. Let;s look at the reaction in greater detail:
(Let the shapes represent an arbitrary number of carbon atoms)
Above, you can see that a simple ester is formed with an uncombined alcohol group on one side and an uncombined acid group on the other side. This means that each side can continue to condense with another alcohol or acid group to form longer and longer polyester chains, losing a water molecule each time.