How are polymers formed from alkenes?

Polymers are formed from alkenes through a process called polymerisation, specifically addition polymerisation.

In more detail, polymerisation is a chemical reaction in which monomers (small molecules) combine to form a larger molecule known as a polymer. Alkenes, which are hydrocarbons with a carbon-carbon double bond, can undergo a type of polymerisation called addition polymerisation. This process involves the breaking of the double bond, allowing the carbon atoms to form new bonds with other monomers, creating a long chain molecule - the polymer.

The process begins with the initiation step, where a catalyst or an initiator starts the reaction by creating a reactive intermediate. In the case of alkenes, this is often a free radical, a molecule with an unpaired electron. This free radical attacks the double bond of the alkene, breaking it and attaching to one of the carbon atoms, leaving the other carbon atom with an unpaired electron. This creates a new free radical, which can then attack another alkene molecule. This process is known as propagation.

The propagation step can repeat many times, with each new free radical attacking another alkene molecule, breaking its double bond and adding it to the growing polymer chain. This can continue until thousands or even millions of monomers have been added to the chain.

Finally, the termination step occurs when two free radicals combine, forming a stable molecule and ending the chain growth. The result is a long chain polymer, with each repeating unit in the chain being derived from an alkene monomer.

The properties of the resulting polymer can be influenced by the choice of alkene monomer and the conditions under which the polymerisation is carried out. For example, polyethylene, one of the most common plastics, is produced by the polymerisation of ethene, while polypropylene is produced from propene. The degree of polymerisation, which refers to the number of monomer units in the polymer chain, can also affect the properties of the polymer, such as its strength, flexibility, and melting point.