Our world is full of polymers, from plastic bags and clothing to proteins and cellulose. In organic chemistry, these polymers are formed through a process called polymerisation. This article will help you understand polymerisation reactions better. We'll explain the definition of polymers and take a closer look at two types of polymerisation reactions: addition and condensation polymerisation. Throughout this article, we'll also provide numerous examples of polymerisation reactions. So, if you're interested in learning more about the fascinating world of polymerisation reactions, keep reading!

What is a polymer?

Polymers are large molecules that are made up of repeating units called monomers. Some common examples of polymers include proteins, DNA, and plastics like PVC and polystyrene. In this article, we will focus specifically on plastics such as polyamides, polyesters, and polymers made from alkenes. These plastics are formed through a series of reactions called polymerisation reactions.

If you're interested in learning more about protein and DNA polymers, check out our articles on Proteins Biochemistry and DNA structure. For a more biology-focused view of these structures, we recommend reading our articles on 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:

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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. 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.

The table below shows some examples of monomers and their polymers.

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Free radical polymerisation is a type of addition polymerisation that involves the joining of non-radical monomers in a chain to form a polymer. A free radical is a particle with an unpaired electron in its outer shell. In contrast, non-radicals are particles that lack unpaired electrons. To initiate the reaction, the free radical is formed by heating or exposing the monomers to radiation. This breaks down a bond homolytically, resulting in the formation of two free radicals. The free radical then joins with a monomer to form a larger radical, as shown in the equation below, where R is a free radical and M is a monomer:

The larger radical then adds to another monomer, and the process continues until termination. Termination occurs when two radicals react with each other to form a stable compound. Free-radical polymerisation can be represented by the general equation:

What is condensation polymerisation?

Condensation polymerisation is a type of reaction in which monomers join together to form a large polymer, releasing a small molecule in the process. This small molecule is often referred to as the condensate. Condensation polymers are formed through two different functional groups. These functional groups could be from two different monomers or from a single monomer containing two different functional groups. For the monomers to form a continuous chain, each monomer must have two functional groups. Examples of condensation polymers include polyamides and polyesters. Polyamides are formed through the reaction between a dicarboxylic acid and a diamine. These polymers are commonly used in the production of nylon. On the other hand, polyesters are formed through the reaction between a diol and a dicarboxylic acid. These polymers are used in the production of textiles and packaging materials.

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Polyamides

Polyamides are formed through a reaction between an amine and a carboxylic acid. The monomers used in this process are often diaminoalkanes and dicarboxylic acids, with each monomer having two functional groups to allow for polymerisation. During the reaction, water is released as a condensate, resulting in the formation of the amide functional group -NCO- repeated throughout the polymer.

For instance, Nylon-6,6 is a type of polyamide that is produced by reacting 1,6-diaminohexane with hexane-1,6-dicarboxylic acid, as shown in the image. Nylon-6,6 is a strong and durable material, making it a popular choice for a range of applications, including textiles, engineering parts, and packaging materials.

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.

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.

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?

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.

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. 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 are a crucial process in creating polymers, which are large molecules composed of repeating units called monomers. There are two types of polymerisation reactions: addition polymerisation and condensation polymerisation.

Addition polymerisation involves joining monomers with a C=C double bond, and no additional by-products are formed during the process. Free radical polymerisation is a type of addition polymerisation that involves using radicals to join monomers into a polymer chain.

On the other hand, condensation polymerisation joins together monomers with two different functional groups, releasing a small molecule in the process. This small molecule can be water, alcohol, or another molecule depending on the specific monomers used.

Understanding the differences between addition and condensation polymerisation is important for creating specific types of polymers with desired properties. For further reading on polymers and condensation polymers in particular, check out the articles with the same name.

Polymerisation Reactions

What is polymerisation?

Polymerisation is a chemical reaction in which small molecules called monomers join together to make a large chain molecule called a polymer.

Why is polymerisation a type of addition reaction?

Not all types of polymerisation are addition reactions. However, polymerisation reactions involving alkenes are addition reactions, because they combine two smaller molecules to make a larger one.

What are the two types of polymerisation reaction?

The two types of polymerisation reaction are addition polymerisation and condensation polymerisation.

What happens in a polymerisation reaction?

In a polymerisation reaction, small molecules called monomers join together to form a polymer. In addition polymerisation, they do this by adding across a C=C double bond. In condensation polymerisation, they do this by reacting together and releasing a small molecule known as a condensate. Often this molecule is water.

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