Polymers are chains of monomers linked together.  Natural polymers such as starch and rubber have existed since the beginning of time.  Man-made polymers have existed since about 1832.  Propylene and ethylene are two of the most common polymers produced today. 

Polymerization

Polymerization occurs when monomers are joined together to form polymers.  This is achieved through polymer synthesis.  Addition polymerization describes the situation when monomers are added one by one to a polymer chain.  There are three significant reactions that take place in addition polymerization: initiation (birth), propagation (growth), and termination (death).

Initiation

When a free radical joins to a monomer and breaks its double bond, transferring its electron this is called initiation.   The instability of carbon-carbon double bonds in the monomer makes them susceptible to reaction with the unpaired electrons in the radical.  In this reaction, the active center of the radical "steals" one of the electrons from the double bond of the monomer, leaving an unpaired electron at the end of the chain.  Addition can occur at either end of the monomer. 

Propagation

In the propagation stage, the electron transfer process and consequent shift of the active center proceeds down the chain.  The entire propagation reaction usually takes place within a fraction of a second.  During this time, thousands of monomers are added to the chain.  The entire process stops when the termination reaction occurs.

Termination

Typically the growth of a polymer chain ends by the termination reaction.  In theory however, the propagation reaction could continue until the supply of monomers runs out.  Instead, termination usually happens either by combination or disproportionation.  Combination occurs when two growing chains join and form a single chain.  Disproportionation stops the propagation reaction when a carbon-carbon double bond takes the place of the missing hydrogen.