Structure and Bonding of Polymers

Size, Structure and Properties

Although individual intermolecular forces are weak, polymer molecules are so large that the total, cumulative intermolecular force is relatively strong. This is why polymers are typically solid at room temperature. The specific arrangement of the chains determines the plastic's bulk properties:

• Chain length: Longer chains have more points of contact, creating stronger total intermolecular forces. This means more energy is needed to separate them, resulting in higher melting and boiling points.
• Crystallinity: If chains are highly ordered, they pack closer together. This increases the strength of the intermolecular forces and raises the melting point.
• Thermosoftening polymers: These consist of individual chains held together only by weak intermolecular forces. When heated, these forces are overcome, allowing the chains to slide over each other so the plastic melts and can be reshaped.
• Thermosetting polymers: These have strong covalent cross-links bonding the separate chains together. Because these cross-links are rigid, the chains cannot slide. The polymer does not melt when heated.

Representing Polymers

Because it is impossible to draw thousands of atoms in a chain, chemists use a shorthand formula based on the repeating unit. This is the smallest part of the polymer chain that repeats over and over again. Polymers are named by placing the monomer name in brackets after the prefix 'poly-'. For example, ethene becomes poly(ethene).

Here is how you draw the repeating unit for poly(ethene):

1. Start with the monomer, ethene ($C_2H_4$), and change its $C=C$ double bond to a $C-C$ single bond.
2. Draw single bonds extending out from both carbon atoms. These are called continuation bonds (or extension bonds).
3. Draw square brackets around the unit, making sure the continuation bonds pass directly through the brackets.
4. Add a small subscript '$n$' at the bottom right outside the brackets to represent a large, variable number of repeating units.

The structural representation of the poly(ethene) repeating unit looks like this:

$\text{--[--CH}_2\text{--CH}_2\text{--]--}_n$

Worked Example

Explain why poly(ethene) has a much higher melting point than ethene.

Step 1: State the difference in molecular size.

• Poly(ethene) molecules are macromolecules, which are much larger than small ethene molecules.

Step 2: Relate size to the forces between molecules.

• Because the molecules are much bigger, the intermolecular forces between the poly(ethene) chains are larger and more numerous.

Step 3: Relate forces to energy.

• Therefore, significantly more energy is required to overcome the sum of the intermolecular forces in poly(ethene), giving it a higher melting point.