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One way to produce polyethylene is called "free radical polymerization." As in other polymerizations, the process has three stages, known as initiation, propagation and termination. To begin, we need to add a catalyst to our supply of ethylene. A common catalyst is benzoyl peroxide, which when heated has the habit of splitting into two fragments, each with one unpaired electron, or free radical. These fragments are known as initiator fragments.

The unpaired electron naturally seeks another and finds a convenient target in the double bond between the carbon atoms in the ethylene molecule. Taking an electron from the carbon bond, the initiator fragment bonds itself to one of the monomer's carbon atoms.

The new radical also seeks a partner. And so ethylene monomers begin attaching themselves in a chain, creating new radicals each time and lengthening the chain. This stage is called propagation.

Eventually, free radical polymerization stops due to what are called termination reactions. For example, instead of stealing an electron from double-bonded carbons or a nearby propagating chain, the carbon atom with the free radical sometimes steals an entire hydrogen atom from another chain end. The polymer end--robbed of its hydrogen--easily forms a double bond with its adjacent carbon atom, and polymerization stops.

Because every part of the ethylene monomer is included in the finished polymer, the free radical polymerization of polyethylene is referred to as an addition polymerization

Polymerizations that use only portions of a monomer, however, are known as condensation polymerizations. The monomers that condense with each other must contain at least two reactive groups in order to form a chain.

For example, poly(ethyleneterepthalate), a polyester known as PET that is commonly found in soda bottles, forms from a reaction of two monomers: ethylene glycol and terephthoyl chloride. At the reaction's end, an atom of hydrogen and an atom of chlorine are left out of each PET molecular junction, resulting in a by-product of hydrogen chloride (HCl) gas.