12.1 Polymers

Polymers are large molecules built from many smaller repeating units called monomers. These monomers bond covalently to form long chains giving polymers their unique properties. Polymerisation is the joining up of many monomers to form a big molecule. Polymers can be classified into two categories based on their synthesis: addition polymers and condensation polymers.

12.1.1 Addition polymers

Addition polymers are formed by adding monomer units without losing the small molecules. Monomers usually contain double or triple bonds that open and combine to form a polymer. The number of carbon atoms in these chains can vary from 4000 to 40,000. For example; Ethene is the smallest hydrocarbon containing a carbon- carbon double bond. Ethene molecules can join together to produce very long chains. Part of the double bond is broken, and the electrons in it are used to join neighbouring molecules. This process is called addition polymerisation.

At high temperatures and high pressures, the monomer ethene joins up to give a polymer called polyethylene or polyethene.

Repeating units: For a polymer, a repeating unit is the same as a monomer, except that the double bond is replaced by a single bond after polymerization.

Repeating unit: $-{\mathrm{C}\mathrm{H}}_2-{\mathrm{C}\mathrm{H}}_2-$$- \mathrm{CH}_{2} - \mathrm{CH}_{2} -$ Polyethylene (PE) consists of ethylene monomer $(\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2)$$\mathrm{(CH_{2} = CH_{2})}$

Monomer: ${\mathrm{C}\mathrm{H}}_2={\mathrm{C}\mathrm{H}}_2$$\mathrm{CH}_{2} = \mathrm{CH}_{2}$

For example, Polypropylene (PP) consists of propylene monomer $(\mathrm{C}{\mathrm{H}}_2=\mathrm{C}\mathrm{H}-\mathrm{C}{\mathrm{H}}_3)$$\mathrm{(CH_{2} = CH - CH_{3})}$

Monomer: ${\mathrm{C}\mathrm{H}}_2=\mathrm{C}\mathrm{H}-{\mathrm{C}\mathrm{H}}_3$$\mathrm{CH}_{2} = \mathrm{CH} - \mathrm{CH}_{3}$

Repeating unit: $-{\mathrm{C}\mathrm{H}}_2-\mathrm{C}\mathrm{H}({\mathrm{C}\mathrm{H}}_3)-$$- \mathrm{CH}_{2} - \mathrm{CH}(\mathrm{CH}_{3})-$ or

Example: Polyvinylchloride consists of vinyl chloride monomer $(\mathrm{C}{\mathrm{H}}_2=\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{l})$$\mathrm{(CH_{2} = CHCl)}$

Chloroethene is an ethene molecule in which one of the hydrogen atoms is replaced by a chlorine atom. It is called vinyl chloride. Ploymerising vinyl chloride gives polychloroethene. Its commercial name is polyvinylchloride(PVC).