Condensation polymerisation

Condensation polymerisation is a reaction in which monomers with two functional groups join into a long chain, and a small molecule, usually water, is released every time two monomers link. Polyesters such as Terylene, polyamides such as nylon, and natural polymers such as proteins are all made this way. It is spelt condensation polymerization in the US.

How condensation polymerisation works

Each monomer has a reactive functional group at both ends. One end of a monomer reacts with an end of the next monomer, forming a link and giving off a small molecule. Because each monomer still has a free group at its other end, the chain can keep growing in both directions.

The simplest condensation polymers use two different monomers, each with two of the same functional group, so the chain alternates between them:

For n molecules of each monomer, about 2n water molecules are released. (Strictly it is one fewer, because the two ends of the chain keep their groups, but in exam equations the ends are usually ignored and 2n is written.)

Addition vs condensation polymerisation

The quickest way to tell them apart in a question: if the monomer has a C=C double bond and nothing else reactive, it is addition. If it has two groups such as -OH, -COOH or -NH₂, it is condensation.

Polyesters

A diol (two -OH groups) reacts with a dicarboxylic acid (two -COOH groups). Each -OH reacts with a -COOH to make an ester link and a water molecule, just as an alcohol and a carboxylic acid make an ester. AQA GCSE Chemistry uses this example:

With formulae, using n of each monomer:

The part in square brackets is the repeating unit: one diol and one diacid, minus the water. At A-level you meet Terylene (PET), made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. It is used for clothing fibres and plastic bottles.

Polyamides

A diamine (two -NH₂ groups) reacts with a dicarboxylic acid. Each -NH₂ reacts with a -COOH to make an amide link, -CONH-, and a water molecule.

• Nylon 6,6 is made from hexane-1,6-diamine and hexanedioic acid. Both monomers have six carbon atoms, which is where the name comes from. It is used for clothing, ropes and carpets.
• Kevlar is made from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid. Its straight chains line up and are held together by hydrogen bonds between the amide groups, which makes it very strong. It is used in bulletproof vests.

In industry, an acyl chloride (-COCl) is sometimes used instead of the acid. Then the small molecule given off is hydrogen chloride, HCl, instead of water.

Amino acids, proteins and other natural polymers

An amino acid has two different functional groups in the same molecule: an amine group, -NH₂, and a carboxylic acid group, -COOH. So a single type of monomer can polymerise by itself. The -NH₂ of one amino acid reacts with the -COOH of the next to form a polypeptide and water. The simplest amino acid, glycine, H₂NCH₂COOH, polymerises like this:

Different amino acids can be combined in the same chain to make proteins. Several other natural polymers are also made by condensation reactions:

In living things the links are called peptide bonds (proteins) and glycosidic bonds (starch and cellulose). The alpha and beta glucose guide explains how glucose links up.

Example: the mass of a dipeptide

Glycine has Mr 75. Two glycine molecules join to make a dipeptide. What is its Mr? One water molecule (Mr 18) is lost, so the dipeptide has Mr 2 × 75 − 18 = 132. Three glycines would lose two water molecules: 3 × 75 − 2 × 18 = 189.

Check your understanding.

Breaking condensation polymers down

Ester and amide links can be broken by hydrolysis, which adds water back across each link. This is why polyesters and polyamides are biodegradable, while addition polymers such as poly(ethene), with only strong C-C and C-H bonds, are inert and are not.

Common mistakes

• Forgetting the water. Condensation polymerisation always gives off a small molecule as well as the polymer.
• Saying condensation monomers have a C=C bond. That is addition polymerisation.
• Drawing the repeating unit with the -OH and -H still on. The atoms that leave as water are not in the polymer.
• Getting the water count wrong. n of each of two monomers gives 2n water; n amino acids give n water.
• Calling a protein's link an ester link. Amino acids form amide links, called peptide links in biology.

Practice questions

1. What is the small molecule usually released in condensation polymerisation?
2. Name the two types of monomer that make a polyamide.
3. Name the monomer of starch and of proteins.
4. An amino acid has Mr 89. Calculate the Mr of a chain made from four of these amino acids.

Answers: 1) Water. 2) A diamine and a dicarboxylic acid. 3) Glucose; amino acids. 4) Three water molecules are lost: 4 × 89 − 3 × 18 = 302.

Practise with a short timed paper

Polymers come up in GCSE Chemistry and A-level Chemistry organic chemistry. The daily GCSE Chemistry Higher and A-level Chemistry: Year 2 papers are short timed papers of 8 questions, with a new one every day.

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