Addition Polymerisation

Polymers are large molecules with high relative formula masses. They form when smaller molecules, called monomers, are linked together through covalent bonds. Each monomer in a polymer is a repeat unit.

Polymers that are manufactured are called synthetic polymers. Some examples of synthetic polymers are nylon, plastics and polyester. This process requires the use of a catalyst and high pressures. Polymers that are produced naturally are called biological polymers.

  • Synthetic polymers are manufactured. To produce synthetic polymers, a catalyst and high pressures are needed. Some examples of synthetic polymers include nylon, plastics and polyester.
  • Biological polymers are produced naturally.

As alkenes are unsaturated, they can be used to make polymers through a process called polymerisation. In this type of reaction, the double bond between the carbons in the monomer opens up, allowing it to link with other monomers. The polymer formed is called an addition polymer.

For example, ethene molecules can link together in a polymerisation reaction. In this case, the addition polymer formed is poly(ethene).

A chemical reaction demonstrating the polymerisation of ethene. On the left is the molecular structure of ethene, shown as two carbon atoms double-bonded, each bonded to two hydrogen atoms. Next to this molecule, there's a blue arrow pointing right, indicating the conversion process. On the right is the molecular structure of poly(ethene), represented as a repeating chain of carbon atoms singly bonded to each other and to two hydrogen atoms. The repeating unit is enclosed in brackets with an 'n' outside, indicating many repetitions of the unit.

In this polymerisation reaction:

  • Ethene is the monomer
  • Poly(ethene) is the polymer

Other addition polymers can also be produced using monomers containing C=C bonds, including poly(propene), poly(chloroethene) (PVC), and poly(tetrafluoroethene) (PTFE).

For example, poly(chloroethene) or PVC is formed by linking together chloroethene monomers through polymerisation reactions. The image below illustrates this process:

The polymerisation process of vinyl chloride into polyvinyl chloride. On the top left, labelled "Monomer", the chemical structure of vinyl chloride is presented as CH2=CH-Cl. Directly below, a more detailed skeletal structure shows a carbon-carbon double bond with hydrogen atoms and a chloride atom attached. On the bottom left, a colourful molecular model represents vinyl chloride with yellow spheres for carbon atoms, red for hydrogen, and blue for chloride; a double bond between the carbon atoms and single bonds to hydrogen and chloride are indicated.To the right, under the label "Polymer", is the representation of polyvinyl chloride. The chemical structure is shown as a repeating unit of -CH2-CH-Cl-. Directly below, a detailed skeletal structure depicts the carbon atoms connected by single bonds, with attached hydrogen and chloride atoms. On the bottom right, a molecular model of the polymer showcases a chain of yellow carbon spheres, red hydrogen spheres, and blue chloride spheres, all connected by single bonds. A purple arrow labelled "Polymerisation" connects the monomer and polymer sections, indicating the transformation process.

Representing Addition Polymers

When representing addition polymers, it can be difficult to show the entire molecule due to its large size. Instead, repeat units are drawn to represent their structure and formula. To write a repeat unit, follow these steps:

1. Change the double bond in the monomer to a single bond in the repeat unit (C-C instead of C=C).

2. Add a bond to each end of the repeat unit and extend them beyond the bracket.

3. On the right of the bracket, indicate the number of repeating units with a subscript n.

Diagram showing the transformation of a general alkene monomer to a polymer. The monomer has a double bond between two carbon atoms, labelled with substituents W, X, Y, and Z. After polymerisation, indicated by a blue arrow, this becomes a repeat unit in the polymer with single bonds connecting the carbon atoms and the same substituents W, X, Y, and Z displayed.

Properties of Polymers

The table below shows some examples of polymers, and their properties and uses.

Poly(ethene)Flexible, inexpensive and can be made into thin films.Carrier bags, shampoo bottles and food wrap.
Poly(propene)Flexible, strong and shatter-resistantBuckets, bowls, crates, ropes and carpets
Poly(chloroethene)Tough electrical insulator and can be made hard/flexible.Insulation for electrical wires, windows, gutters and pipes.
Poly(tetrafluoroethene)Slippery and chemically unreactive.Non-stick coatings for pans and containers for laboratory substances.

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