Polymers are synthesized from alkenes through a chemical process known as polymerization, specifically a type called addition polymerization.

Polymerization involves a chemical reaction in which small molecules, known as monomers, combine to form larger molecules called polymers. Alkenes, which are hydrocarbons characterized by a carbon-carbon double bond, can undergo addition polymerization. In this process, the double bond in the alkene is broken, enabling the carbon atoms to form new bonds with other monomers, thereby creating a long-chain molecule—the polymer.

The process begins with the initiation step, where a catalyst or initiator triggers the reaction by generating a reactive intermediate. For alkenes, this reactive species is often a free radical, which is a molecule possessing an unpaired electron. This free radical attacks the double bond of the alkene, breaking it and attaching to one of the carbon atoms. The other carbon atom is left with an unpaired electron, resulting in the formation of a new free radical, which can then react with another alkene molecule. This sequence of events is referred to as propagation.

During the propagation step, the process can repeat numerous times. Each newly formed free radical can attack another alkene molecule, breaking its double bond and incorporating it into the growing polymer chain. This can continue until thousands or even millions of monomer units are added to the chain.

Eventually, the termination step occurs when two free radicals combine, resulting in the formation of a stable molecule and halting the growth of the polymer chain. The outcome is a long-chain polymer, with each repeating unit in the chain derived from an alkene monomer.

The properties of the resulting polymer can be influenced by the choice of alkene monomer and the specific conditions under which the polymerization is conducted. For instance, polyethylene, one of the most widely used plastics, is produced through the polymerization of ethene, while polypropylene is synthesized from propene. Additionally, the degree of polymerization, which indicates the number of monomer units in the polymer chain, can significantly affect the polymer’s properties, such as its strength, flexibility, and melting point.