Polymers

Polymers are very long molecules made by joining many small molecules, called monomers. Addition polymers are made from alkenes: the C=C double bond opens and the monomers join in a chain, with nothing else made. Condensation polymers are made from monomers with two functional groups; a small molecule such as water is given off each time two join. Many natural molecules are polymers: proteins, starch, cellulose and DNA.

Key facts

  • Addition polymerisation: many alkene monomers join; the double bond opens. Ethene → poly(ethene), propene → poly(propene), chloroethene → poly(chloroethene) (PVC).
  • The repeating unit is drawn in brackets with an n, and with bonds through the brackets.
  • The Mᵣ of an addition polymer is n × the Mᵣ of the monomer: no atoms are lost.
  • Condensation polymerisation: two kinds of functional group react and water (or another small molecule) is given off. A diol and a dicarboxylic acid make a polyester.
  • Natural polymers: amino acids → proteins; glucose → starch and cellulose; nucleotides → DNA.
  • Most addition polymers are not biodegradable, so reusing and recycling them matters.

Worked examples

Example 1

Draw the repeating unit of poly(propene) from its monomer, propene.

  1. Draw the monomer with its C=C double bond and the four groups around it.
  2. Change the double bond to a single bond and draw a bond out from each carbon.
  3. Put brackets across the outer bonds and write n after them. The groups (H, H, H, CH₃) stay where they were.

Answer: two carbons in brackets, with CH₃ on one, and n

Example 2

A molecule of poly(ethene) has a relative formula mass of 56,000. How many ethene monomers (Mᵣ 28) joined to make it?

  1. In addition polymerisation nothing is lost, so Mᵣ(polymer) = n × Mᵣ(monomer). Mᵣ of C2H4 = 2 × 12 + 4 × 1 = 28
  2. Divide the polymer's Mᵣ by the monomer's. 56,000 ÷ 28 = 2000

Answer: 2000

Example 3

Compare how a polyester and poly(ethene) are made.

  1. Poly(ethene): one kind of monomer, with a C=C bond. The bonds open and the polymer is the only product (addition).
  2. Polyester: two kinds of monomer, a diol (two –OH groups) and a dicarboxylic acid (two –COOH groups).
  3. They join alternately with ester links, and a water molecule is given off for every link (condensation).

Answer: polyester: condensation, two monomers, water given off; poly(ethene): addition, one monomer, nothing else made