Frank N. Kelley, dean of the College of Polymer Science and Polymer Engineering at the University of Akron, provides the following explanation.
Synthetic polymers are produced by chemical reactions, termed "polymerizations." Polymerizations occur in varied forms--far too many to examine here--but such reactions consist of the repetitive chemical bonding of individual molecules, or monomers. Assorted combinations of heat, pressure and catalysis alter the chemical bonds that hold monomers together, causing them to bond with one another. Most often, they do so in a linear fashion, creating chains of monomers called polymers.
Some polymerizations join entire monomers together, whereas others join only portions of monomers and create "leftover" materials, or by-products. Co-polymers can be formed using two or more different monomers. And two or more polymers can be combined to produce an alloy, or blend, that displays characteristics of each component.
For an example, let's consider the common plastic polyethylene, which is found in such items as grocery bags, toys and bottles. The monomer ethylene is composed of two carbon atoms, each bonded to two hydrogen atoms and sharing a double bond with one another. Polyethylene consists of a chain of single-bonded carbon atoms, each still carrying its two hydrogen atoms.
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 radical is now happy, but this initiating reaction creates another free radical associated with the ethylene molecule's other carbon atom. 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.
Growing chains may also attach themselves to one another. Most commonly, chains join end to end, but sometimes they join end to backbone, making branched polyethylene molecules.
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; the ethylene molecules are simply added together. 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. Condensation reactions result in "condensed" polymers that have less total mass than the monomers used to create them and the by-products, or "condensates," combined.
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.