7.5 - Addition Polymers

What a polymer is

• A polymer is a very large molecule made of many small units (monomers) joined by covalent bonds into a long chain.
• A single polymer molecule can run to thousands of carbon atoms - far larger than the molecules of most substances, which are small molecules of a few atoms each.
• Each carbon atom uses two of its four bonds to join the carbon atoms on either side of it in the chain, leaving two bonds for the atoms attached to its side.

Poly(ethene) and the repeat unit

Poly(ethene) is a chain of carbon atoms joined by single covalent bonds, with two hydrogen atoms on every carbon atom.

The chain is the same short section repeated over and over. This section is the repeat unit - in poly(ethene) it's two carbon atoms with their four hydrogen atoms.

A chain is too long to draw, so a polymer is drawn as its repeat unit:

• The repeat unit is drawn out with a bracket on each side.
• A bond runs out through each bracket (left and right) to show the chain carries on in both directions.
• A letter n goes outside the bottom right of the bracket, standing for the (very large) number of repeat units.

Bonding and why polymers are solids

• Within a polymer molecule the atoms are held by strong covalent bonds.
• Between one molecule and the next there are only intermolecular forces, which are weak for small molecules.
• But a polymer molecule is so long that there are a great many of these forces along its length, and together they add up to a relatively strong attraction between chains.
• That's why polymers are solids at room temperature, where most small-molecule covalent substances are gases or liquids.

Spotting a polymer in a diagram

• Polymer - A very long chain of atoms (usually carbon) with small groups along it, or that chain drawn once as a repeat unit in brackets with n.
• Small molecule - A handful of atoms on their own.
• Giant covalent structure - Bonds carry on in every direction.