Synthetic and naturally occurring polymers

Chemistry · Organic chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Chemistry topics

The idea

A polymer is a very long molecule made by joining many small molecules called monomers.

In addition polymerisation the monomers are alkenes. The C=C double bond of each monomer opens and the molecules join into a chain with single bonds only. The polymer is the only product, so the repeating unit has the same atoms as the monomer.

In condensation polymerisation (Higher Tier) each monomer has two functional groups, so it can link at both ends. A small molecule, usually water, is lost every time a link forms. Polyesters and polypeptides are made this way. Living things make polymers too: proteins, DNA, starch and cellulose.

Rules to remember

To draw a repeating unit: change C=C to C–C, keep all the other atoms, and draw a bond out of each end through the brackets, with n after the bracket.
Naming: the polymer of ethene is poly(ethene), of propene is poly(propene), of chloroethene is poly(chloroethene).
Addition polymerisation gives one product; condensation polymerisation gives the polymer and a small molecule such as water.
Polyester: a diol (two –OH groups) + a dicarboxylic acid (two –COOH groups) → polyester + water.
Amino acids have an amine group, –NH₂, and a carboxylic acid group, –COOH. They join by condensation to make polypeptides; proteins are made from different amino acids in one chain.
DNA is two polymer chains, made from four different nucleotides, twisted into a double helix.
Starch and cellulose are polymers of glucose; proteins are polymers of amino acids.

Common mistake

Students leave the double bond in the repeating unit of an addition polymer. The double bond is used up when the monomers join, so the chain has only single bonds between the carbon atoms.

Worked example

A non-stick coating is an addition polymer with the repeating unit –CF₂–CF₂–. What is the formula of its monomer, and is any other product made?

An addition polymer is made from an alkene-type monomer, so the monomer must contain C=C.
Put the double bond back between the two carbon atoms of the repeating unit and remove the bonds at the ends: CF₂=CF₂.
In addition polymerisation the monomers simply add together, so no small molecule is lost.

Answer: The monomer is CF₂=CF₂ (C₂F₄), and the polymer is the only product.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Alkenes, alcohols and carboxylic acids Purity, formulations and chromatography →Take a Chemistry test