11.7 - Addition Polymers

Polymers and monomers

• A polymer is a very large molecule made of many small units (monomers) joined by covalent bonds into a long chain. Plastics are made of polymers.
• 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.

Monomers and polymerisation

• A polymer is made by joining together many small molecules, each becoming one short section of the finished chain.
• Monomer - A small molecule that joins up in this way.
• Polymerisation - The reaction in which many monomers join to form a polymer.
• Repeat unit - The shortest length of chain which, repeated, gives the whole polymer molecule (apart from the two ends).

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. Its repeat unit is two of those 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.

Addition polymerisation

• The monomers that make addition polymers are alkenes - molecules with a carbon-carbon double bond.
• That double bond makes an alkene unsaturated, and it's what lets the molecule join up with others.

How the double bond opens

• Under the right conditions the double bond opens: one of the two bonds between the carbon atoms gives way, leaving a single bond, and each carbon atom now has a bond free.
• The free bond on one monomer joins to a carbon atom of the next monomer along, and so on, so many monomers join end to end into one long chain.
• This reaction is called addition polymerisation.
• Nothing is lost and nothing else is made - The polymer is the only product. So the repeat unit has exactly the same kinds of atom, in the same numbers, as the monomer.
• What changes is the bonding: the double bond becomes a single bond, and two new bonds run out of the repeat unit to its neighbours.

Examples

Ethene, C₂H₄ - The simplest alkene: two carbon atoms joined by a double bond, two hydrogen atoms on each. It polymerises to poly(ethene), whose repeat unit is two carbon atoms joined by a single bond, still with two hydrogen atoms each.

Propene, C₃H₆ - Three carbon atoms, a double bond between the first two and a single bond to the third. It polymerises to poly(propene).

Naming addition polymers

• A polymer takes its name from its monomer: put the monomer's name in brackets with poly in front - ethene gives poly(ethene), propene gives poly(propene).
• The formula follows the same pattern: put the monomer's formula in brackets with n after it - C₂H₄ gives (C₂H₄)_n and C₃H₆ gives (C₃H₆)_n.
• Read backwards, the name tells you the monomer: poly(ethene) was made from ethene.