Predicting the type of polymerisation

Chemistry A2 · Polymerisation. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

There are two types of polymerisation, and the monomer tells you which one happens. Addition polymerisation needs a C=C double bond. The double bonds open and the monomers join, with no other product. The backbone of the chain is made only of carbon atoms, and the repeat unit has the same atoms as the monomer.

Condensation polymerisation needs two reactive groups on each monomer, such as –OH, –NH₂, –COOH or –COCl. A small molecule is lost, and the backbone contains ester links (–CO–O–) or amide links (–CO–NH–).

To decide from a section of polymer, look only at the backbone, not at the side groups.

Rules to remember

C=C in the monomer → addition polymer; nothing else is formed
Two functional groups per monomer (two –OH, two –NH₂, two –COOH, two –COCl, or one of each kind that react together) → condensation polymer plus H₂O or HCl
Backbone of carbon atoms only → addition
–CO–O– or –CO–NH– in the backbone → condensation (polyester or polyamide)
An ester or amide group in a side chain does not make a condensation polymer
A monomer with only one reactive group, such as ethanol or ethanoic acid, cannot form a condensation polymer

Common mistake

Students see an ester or acid group and answer condensation. If that group hangs from the chain as a side group and the backbone is all carbon, the polymer was made by addition from a monomer with C=C.

Worked example

Propenamide, CH₂=CHCONH₂, forms a polymer. Which type of polymerisation occurs, and what is the repeat unit?

The monomer has a C=C double bond, so it can polymerise by addition.
It has only one –CONH₂ group and no –COOH or –COCl group to react with it, so it cannot form amide links between monomers.
The C=C bonds open to make a carbon backbone; the –CONH₂ group stays as a side group.

Answer: Addition polymerisation; repeat unit –CH₂–CH(CONH₂)–, with no other product

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Watch: 17 | Esters | Amides | Polymers | Peptides

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