We immediately recognize that these monomers can make an ester , so that we expect our polymer to be linked by ester functional groups. The resulting polymer is called polyethylene terephthalate and is the common polyester of plastic pop bottles and polyester fabrics. The term polyester also puts our focus on the functional group which is made in the polymerization reaction. Step-growth polymerization involves normal functional group reactions. Polymers result because the monomers have two functional groups per molecule.
Polymers resulting from additions to alkenes are chain-growth polymers.
In these processes each addition step results in a longer chain which ends in a reactive site. The mechanism of each addition step is the same, and each addition step adds another monomer to extend the chain by one repeating unit. Each step in this polymer formation is an addition to an alkene. The mechanism is in most cases a free radical addition. In free radical reactions the pi pair of electrons separates.