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Polymerisation Reactions

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Chemistry

From plastic bags and proteins to clothing and cellulose, our world is full of polymers. They are formed through polymerisation.

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, which are collectively known as polymerisation reactions.

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.

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.

In addition polymerisation, no by-products are produced.

Alkenes can undergo addition polymerisation to form long hydrocarbon chain polymers. 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:

Polymerisation reactions, additional polymerisation ethene, StudySmarterAddition polymerisation with ethene. Anna Brewer, StudySmarter 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:

Polymerisation reactions, additional polymerisation ethene polyethene, StudySmarterEthene polymerises to form poly(ethene), also known as polythene. Anna Brewer, StudySmarter 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:

Polymerisation reactions, additional polymerisation monomers, StudySmarterFrom polymer to monomer. Anna Brewer, StudySmarter Originals

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

Polymerisation Reactions, additional polymerisation monomers polymers comparison, StudySmarterA table showing different monomers and their polymers. Anna Brewer, StudySmarter Originals

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
  • Polyesters

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.

Polymerisation reactions, dicarboxylic acid diamine example, StudySmarterA dicarboxylic acid, left, and a diamine, right. Anna Brewer, StudySmarter 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.

Polymerisation reactions, polyamide condensation reaction, StudySmarterA condensation reaction between a diamine and a dicarboxylic acid forms a polyamide. Anna Brewer, StudySmarter Originals

For example, Nylon-6,6 is made from 1,6-diaminohexane and hexane-1,6-dicarboxylic acid:

Polymerisation reactions, Nylon-6,6, StudySmarterNylon-6,6. Anna Brewer, StudySmarter Originals

Polyesters

Polyesters are formed from alcohols and carboxylic acids. Often the monomers are diols and dicarboxylic acids.

Polmerisation reactions, diol dicarboxylic acid example, StudySmarterA diol, left, and a dicarboxylic acid, right. Anna Brewer, StudySmarter Originals

Again, the condensate is water, resulting in the ester functional group -COO- repeated throughout the molecule. The general equation is shown below:

Polymerisation reactions, polyester condensation reaction, StudySmarterA condensation reaction between a diol and a dicarboxylic acid forms a polyester. Anna Brewer, StudySmarter 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:

Polymerisation reactions, polymer to monomer example, StudySmarterFrom polymer to monomer. Can you spot the repeating -COO- group? Anna Brewer, StudySmarter 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:

Polymerisation reactions, monomer with two functional groups, StudySmarterBecause this species contains both the -OH and -COOH functional groups, it doesn't require another type of molecule in order to form a polymer. Anna Brewer, StudySmarter Originals

Proteins are a type of polymer that is also formed in a condensation reaction. Proteins are made up of long chains of amino acids, which each contain an amine group and a carboxyl group. These react together to form a polyamide. See Biological Organic for more on this topic.

How do addition and condensation polymerisation differ?

The following table shows the similarities and differences between addition and condensation polymerisation.

Polmerisation reactions, addition vs condensation polymerisation, StudySmarterA table comparing addition and condensation polymerisation. Anna Brewer, StudySmarter Originals

Free radical polymerisation

Free radical polymerisation is a type of 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:

Polymerisation Reactions - Key takeaways

  • Polymers are large molecules made up of repeating units called monomers.
  • Addition polymers are made from alkene monomers with a C=C double bond.
  • Condensation polymers are made from monomers with two different functional groups, such as carboxylic acids and alcohols.
  • Free radical polymerisation involves using radicals to join monomers into a chain.

Polymerisation Reactions

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

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.

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

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.

Final Polymerisation Reactions Quiz

Question

What is a polymer?

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Answer

 A polymer is a large molecule made up of repeating units called monomers.


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Question

How many products are formed in addition polymerisation?


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Answer

1

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Question

How many different types of functional groups are required for addition polymerisation?


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Answer

1

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Question

What are the monomers used in addition polymerisation based on?


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Answer

Alkenes with a C=C double bond.

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Question

How many products are formed in condensation polymerisation?


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Answer

2

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Question

How many different types of functional groups are required for condensation polymerisation?


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Answer

1

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Question

What is a condensate?


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Answer

A condensate is the small molecule by-product released in a condensation reaction.

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Question

What is the generic name for the polymer formed in the reaction between an alcohol and a carboxylic acid, and what is the condensate?


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Answer

The generic name is polyester. The small molecule released is water.

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Question

Compare and contrast addition and condensation polymers.


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Answer

  • Both polymers are long chains made up of repeating units called monomers. 
  • Both polymers can be made up of just one type of monomer, or various different ones. 
  • However, whilst addition polymers only require one type of functional group, the alkene C=C double bond, condensation polymers require two different functional groups. These could be alcohols and carboxylic acids, or amines and carboxylic acids, for example.
  • Addition polymerisation does not produce a small molecule byproduct whereas condensation polymerisation does.

Show question

Question

What sort of polymerisation does propene undergo?


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Answer

Addition

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Question

Give an example of a molecule that reacts with 1,2-diaminoethane to form a polymer.


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Answer

A carboxylic acid, for example ethanoic acid.

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