Polymerisation 

Types of polymerisation

There are two types of polymerisation you need to know:

1. Addition polymerisation
2. Condensation polymerisation. 

Addition polymerisation 

Addition polymerisation is a very important reaction in the plastic industry. Monomers containing at least one double bond form long chains of polymers as the only product. This is why it is known as addition polymerisation. In this reaction the pi (π) bond breaks and the monomers link together to form new carbon-carbon single bonds.

The polymerisation of ethene and chloroethene are shown below

Condensation polymerisation

A different type of polymerisation is condensation polymerisation. In this reaction, a small molecule is eliminated (this could be water, hydrogen chloride or ammonia). Examples include polyesters and polyamides

How are polyesters formed?

A polyester contains an ester link which can be formed from the following bonding together: 

• One monomer which contains two carboxylic acid (dicarboxylic acid) groups and a second monomer which contains two alcohol groups (diol)
• A single monomer which contains both an alcohol and carboxylic acid group

Note: A monomer which contains two acyl chloride groups (dioyl dichloride) can be used instead of a carboxylic acid group

An example of a polyester is Terylene or PET. This is formed by the following reaction.

How are polyamides formed?

Polyamides are also formed by condensation polymerisation and contain an amide link. The monomers required are:

• One monomer containing 2 carboxylic acid groups and a second monomer containing 2 amine groups (diamine)
• A single monomer containing both a carboxylic acid and an amine group (amino acid)

Note: A monomer which contains two acyl chloride groups (dioyl dichloride) can be used instead of a carboxylic acid group

Kevlar is an example of a polyamide and the monomers required are 1,4-diamino benzene and benzene-1,4-dicarboxylic acid (or benzene-1,4-dioyl chloride as an alternative)

How are proteins formed?

Proteins are formed from amino acids which contain both carboxylic acid and amine groups. Therefore, they can form react to form condensation polymers

Depending on the sequence of amino acids, different polymers can be formed. A dipeptide contains two amino acids, they are bonded together with an amide link. A polypeptide is a chain of many amino acids bonded together by amide links.  

Repeating Units

This is the smallest group of atoms which when bonded in succession they make up the polymer chain. This is represented by using square brackets in both the displayed and general formulas. The monomer and repeating unit will be the same with the exception of the carbon-carbon double bond. 

Disposal of addition and condensation polymers

Polymers formed from addition polymerisation are very unreactive or inert as they are non-polar and saturated. This makes them difficult to break down and biodegrade. They can be recycled to form new polymers for a different purpose. Burning plastics is one way of disposing of them. The energy released can generate electricity however this process releases carbon dioxide and water which will contribute to global warming.

Polymers formed from condensation polymerisation, i.e polyesters and polyamides are biodegradable. This is because they contain a carbonyl group, -C=O group which is able to absorb ultraviolet light. Absorbing this both weakens and breaks the molecule into smaller molecules. If condensation polymers are piled up in landfill, ultraviolet light can not reach the polymer to break it down naturally.

Polymer hydrolysis

Condensation polymers can be broken down using hydrolysis reactions. This can be done using either acidic or alkaline conditions.

For polyesters the products of hydrolysis are

• Acid hydrolysis (using hydrochloric acid catalyst) - diol and dicarboxylic acid
• Alkaline hydrolysis (heated with sodium hydroxide) - diol and carboxylic acid salt 

For polyamides the products of hydrolysis are

• Acid hydrolysis -  dicarboxylic acid and ammonium ions
• Alkaline hydrolysis - sodium salts of its monomers (dicarboxylic acid salt and diamines)

When proteins are hydrolysed using acid, amino acids are formed, however the amine group accepts a proton to become -NH3+ .

What keyword definitions do I need to know for polymerisation? 

Some keyword definitions you need to know are:

• Monomer - a small, reactive molecule that reacts to form a polymer
• Polymer - a long-chain molecule that is made up of many repeating units
• Repeating unit - the smallest group of atoms that when connected one after the other make up the polymer chain
• Addition polymers - a polymer that is formed by the reaction of monomers that contain a carbon-carbon double bond
• Condensation polymers - a polymer that is formed by the condensation reaction of two monomers releasing a small molecule

This is a quick summary of some key concepts on polymerisation - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!