From plastic bags and proteins to clothing and cellulose, our world is full of polymers. They are formed through polymerisation.
- This article is about polymerisation reactions in organic chemistry.
- We'll define polymer before looking more closely at different types of polymerisation reactions.
- This will include exploring addition and condensation polymerisation.
- Within this article, you'll find plenty of examples of polymerisation reactions.
What is a polymer?
A polymer is a large molecule made up of repeating units called monomers.
Examples of polymers include proteins, DNA, and plastics such as polyvinyl chloride (PVC) and polystyrene. We will focus here on these plastics, including polyamides, polyesters, and polymers made from alkenes, and the reactions that form them. These reactions are collectively known as polymerisation reactions.
Check out Proteins Biochemistry and DNA structure for a look at protein and DNA polymers. If you want a more biology-focused view of these structures, you can find out more in the articles Proteins and DNA.
Types of polymerisation reactions
Polymers are formed in two different types of reactions. These depend on the functional groups of the monomer or monomers used:
- Addition polymers are made from monomers with a C=C double bond. These monomers are based on ethene and there is no by-product.
- Condensation polymers are made from monomers with two different functional groups. A small molecule is released in the reaction, usually water.
Let's explore these types more closely.
What is addition polymerisation?
Addition polymerisation is the joining together of monomers with the C=C double bond to form a large molecule known as a polymer. This process doesn't produce any other by-products.
Alkenes can undergo addition polymerisation to form long hydrocarbon chain polymers known as polyalkenes. The monomers used can all be the same alkene, or of multiple different types. The C=C double bond in each monomer opens up and binds to the adjacent monomer to form a C-C backbone. This is shown below with the example of ethene:
Addition polymerisation with ethene. Vaia Originals
Representing addition polymers
Addition polymerisation can be represented by the following equation. We use -R to represent any varying alkyl or aryl group. The letter 'n' represents the number of alkene monomers used, which tends to be very large:
Ethene polymerises to form poly(ethene), also known as polythene. Vaia Originals
This is called a polymerisation equation.
Naming
Addition polymers are named using the prefix poly- and the name of their alkene monomer, in brackets. For example, chloroethene polymerises to form poly(chloroethene). However, many of these polymers have different trade names, and poly(chloroethene) is also known as polyvinyl chloride, or PVC.
From polymer to monomer (part 1)
When asked to find the repeating unit of a given addition polymer, you should remember that each monomer is based around a C=C double bond. Therefore, every pair of carbons in the C-C backbone of the polymer will belong to a different monomer. The monomers can also be worked by identifying the polymer’s repeating pattern, such as in the example below:
From polymer to monomer. Vaia Originals
The table below shows some examples of monomers and their polymers.
A table showing different monomers and their polymers. Vaia Originals
Free radical polymerisation
Free radical polymerisation is a type of addition polymerisation in which a free radical joins non-radical monomers in a chain, forming a polymer.
A free radical is a species with an unpaired outer shell electron. Non radicals are therefore species without an unpaired outer shell electron.
The radical is initiated by heating or radiation. This destroys a bond homolytically, meaning one electron goes to each of the two new molecules, creating two free radicals. The radical then adds to a monomer, forming a larger radical. This is shown by the equation below, where R is a free radical and M is a monomer:
The larger radical then adds to another monomer:
The process continues until termination. Termination involves two radicals reacting together to form a stable compound:
Free-radical polymerisation can be represented by the following general equation:
What is condensation polymerisation?
Condensation polymerisation is a type of condensation reaction in which monomers join together to form a large polymer, releasing a small molecule in the process. This small molecule is often called the condensate.
Condensation polymers are based around two different functional groups. These could be from two unique monomers, or from one monomer containing two different functional groups. In order for the monomers to form a continuous chain, there must be two functional groups on each monomer.
Examples of condensation polymers include:
Polyamides
Polyamides are formed in the reaction between an amine and a carboxylic acid. As each monomer must have two functional groups in order to form a polymer, the monomers are often diaminoalkanes and dicarboxylic acids.
A dicarboxylic acid, left, and a diamine, right. Vaia Originals
The condensate released is water, resulting in the amide functional group -NCO- repeated throughout the molecule. This can be represented by the following equation, in which the molecules lost are shown in red.
A condensation reaction between a diamine and a dicarboxylic acid forms a polyamide. Vaia Originals
For example, Nylon-6,6 is made from 1,6-diaminohexane and hexane-1,6-dicarboxylic acid:
Nylon-6,6. Vaia Originals
Polyesters
Polyesters are formed from alcohols and carboxylic acids. Often the monomers are diols and dicarboxylic acids.
A diol, left, and a dicarboxylic acid, right. Vaia Originals
Again, the condensate is water, resulting in the ester functional group -COO- repeated throughout the molecule. The general equation is shown below:
A condensation reaction between a diol and a dicarboxylic acid forms a polyester. Vaia Originals
For more information on alcohols, amines, or carboxylic acids, see Alcohols, Amines, and Carboxylic Acids and Esters respectively.
From polymer to monomer (part 2)
To identify the monomers used from a polymer chain, it can help to locate the repeating -COO- or -NCO- functional group. You can then divide the molecule into its repeating units. For example, this polymer is made up of 1,2-ethanediol and butane-1,4-dioic acid:
From polymer to monomer. Can you spot the repeating -COO- group? Vaia Originals
Sometimes, only one monomer is required for a condensation reaction. This occurs if the molecule contains two different suitable functional groups. For example, 2-hydroxyethanoic acid can form a polymer with the following repeated unit:
Sometimes, only one monomer is required for a condensation reaction. This occurs if the molecule contains two different suitable functional groups. For example, 2-hydroxyethanoic acid can form a polymer with the following repeated unit:
Because this species contains both the -OH and -COOH functional groups, it doesn't require another type of molecule in order to form a polymer. Vaia Originals
How do addition and condensation polymerisation differ?
The following table shows the similarities and differences between addition and condensation polymerisation.
A table comparing addition and condensation polymerisation. Vaia Originals
If you want to learn more about polymers, visit the article with the same name (Polymers) for further reading. You can also check out Condensation Polymers to find out more about condensation polymers in particular.
Polymerisation Reactions - Key takeaways
- Polymers are large molecules made up of repeating units called monomers. They are formed in polymerisation reactions.
- Addition polymerisation joins together monomers with a C=C double bond. It has no additional by-products.
- Condensation polymerisation joins together monomers with two different functional groups, releasing a small molecule in the process.
- Free radical polymerisation is a type of addition polymerisation. It involves using radicals to join monomers into a polymer chain.
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