7.3.2 Condensation polymerisation
Condensation polymerisation links monomers that have two functional groups
Definition
Condensation polymerisation
A type of polymerisation in which monomers with two functional groups join repeatedly, usually releasing a small molecule such as water when each new link forms.
- Monomers are the small starting molecules used to build a polymer.
- In condensation polymerisation, each monomer has two functional groups, so it can react at two places.
- The simplest condensation polymers form from two different monomers, each with two of the same functional group.
- The two monomers join repeatedly, so sections from each appear along the chain.
- Because each monomer links at both ends, the chain keeps growing.
Key Idea
- Two functional groups on each monomer allow repeated joining into a long chain.
- In the simplest pattern, sections from the two different monomers alternate along the chain.
Every new link forms by a condensation reaction that releases water
Definition
Monomer
A small molecule that can join with many other monomer molecules to form a polymer.
- Functional groups on neighbouring monomers react to make a new covalent link.
- As each link forms, a small molecule is removed, usually water.
- A condensation reaction is one where molecules join while a small molecule is lost.
- Repeating the condensation reaction builds the chain and releases many water molecules.
Common Mistake
- Do not put the water inside the polymer repeating unit.
- The water is a separate product, made each time a new link joins two monomers.
Ethanediol and hexanedioic acid join to make a polyester
Definition
Functional group
An atom or group of atoms that determines the characteristic chemical reactions of an organic compound.
- Ethanediol, HO-CH2-CH2-OH\text{HO-CH}_2\text{-CH}_2\text{-OH}HO-CH2-CH2-OH, has two hydroxyl groups, one at each end.
- Hexanedioic acid, HOOC-(CH2)4-COOH\text{HOOC-(CH}_2\text{)}_4\text{-COOH}HOOC-(CH2)4-COOH, has two carboxyl groups, one at each end.
- At each join, an -OH\text{-OH}-OH from a carboxyl group and an -H\text{-H}-H from a hydroxyl group combine to make H2O\text{H}_2\text{O}H2O.
- The remaining parts form an ester link, -COO-\text{-COO-}-COO-.
- Reacting at both ends builds a chain of many ester links, so the product is a polyester.
- One representation of the polyester is shown here.
- [-O-CH2-CH2-O-C(=O)-(CH2)4-C(=O)-]n\left[\text{-O-CH}_2\text{-CH}_2\text{-O-C(=O)-(CH}_2\text{)}_4\text{-C(=O)-}\right]_n[-O-CH2-CH2-O-C(=O)-(CH2)4-C(=O)-]n
Exam technique
- For full marks, name the two functional groups on each monomer, say a small molecule (water) is lost at each link, and connect the monomer structures to the repeating unit.
- Show the ester link as -COO-\text{-COO-}-COO- and keep every other atom of the monomers in the chain.
The repeating unit shows how the monomers joined
Definition
Polymer
A substance made from very large molecules in which many atoms are linked by strong covalent bonds.
- A repeating unit is the smallest section of the chain that repeats.
- The repeating unit here contains a section from one ethanediol and one hexanedioic acid molecule.
- The atoms lost as water do not appear in the repeating unit.
- The ester links show where the hydroxyl and carboxyl groups reacted.
- Bonds passing through the square brackets show the chain continues in both directions.
- The nnn outside the brackets shows the repeating unit occurs many times.
Self review
- Why must each monomer have two functional groups?
- Why is this process called condensation polymerisation?
- Which functional groups are in ethanediol and hexanedioic acid?
- What type of link joins the units in the polyester?
- What do the brackets and nnn show in a polymer structure?
