Useful polymers formed by radical chain reactions

Many household polymeric materials with which you are probably familiar are made with a radical chain reaction process. Polyethylene (PET), the plastic material used to make soft drink bottles and many other kinds of packaging, is produced by the radical polymerization of ethylene (ethene in IUPAC nomenclature). A radical initiator such as benzoyl peroxide undergoes homolytic cleavage when subjected to high temperatures.

In the propagation phase, the benzoyl radical (X• in the figure below) adds to the double bond of ethylene, generating a new organic radical.  Successive ethylene molecules add to the growing polymer, until termination occurs when two radicals happen to collide. In the figure below, the growing PET polymer is terminated by a benzoyl radical, but in an alternative termination step two growing PET radicals could condense.

Other small substituted alkene monomers polymerize in a similar fashion to form familiar polymer materials. Two examples are given below.