Origin of plastics

Crude oil is used to make most plastics and synthetic fibres.

Long chain hydrocarbons can be cracked or broken into a mixture of short chain hydrocarbons, some of which are saturated and others unsaturated.

 
Synthetic fibres

Synthetic fibres are man-made (made by the chemical industry) and do not occur naturally. Plastics are examples of synthetic materials.

Many items of clothing contain materials such as polyester, polyamide, terylene, rayon, dralon etc. which are all man-made fibres, and are called synthetic fibres.

 
Properties of plastics in relation to their uses

 
Advantages and disadvantages of plastics

 
Problems when plastics burn

Some plastics burn or smoulder and give off toxic fumes. They can produce thick black smoke, make toxic fumes and use up large amounts of oxygen.

As plastics contain carbon, they all produce carbon dioxide, carbon monoxide (as air runs out), and smoke.

Biodegradable Plastics

Plastic packaging does not rot away and causes major litter problems. Many plastics are non-biodegradable, meaning they will not rot away in nature.

Plastics are now being introduced that are biodegradable e.g. Biopol (biodegradable polymer).

 
Thermoplastic and thermosetting polymers

Thermoplastic plastics are those which can be resoftened on heating e.g. poly(ethene), poly(amide).

Thermosetting plastics are those which cannot be resoftened on heating e.g. bakelite, melamine.

Both type of plastics consist of long, tangled chains but in thermosetting polymers, there are links between the chains which gives a much more rigid structure.
You can quickly test your knowledge of the above information.

 
Plastics are polymers

A polymer is a very big molecule made from many small molecules (called monomers) which repeat through the structure.

The process in which the monomers join to make a polymer is called polymerisation.

 
Structure of the monomers

Many plastics or polymers are made from unsaturated monomers obtained by cracking fractions of crude oil.

The simplest monomer is ethene, C₂H₄, an alkene (which contains a carbon to carbon double bond).

Ethene monomers join together to give a polymer called poly(ethene), which is often called polythene.

This process is called polymerisation

 
How do monomers join to make a polymer?

1. Addition Polymerisation

   A chemical is added which breaks the double bond between the two carbon atoms of ethene to make a very reactive unit.

   

   
   Reactive units join together, end to end and a big molecule or polymer is made.

   

   This molecule consists of many small units joined together and repeating along the length of the polymer.

   The process is called addition polymerisation because the monomer units join (or add) together to give one product, by a series of reactions in which the double bond breaks.

   Many plastics are made from alkenes, or from unsaturated molecules made from alkenes.

   Modern Name of Monomer	Original Name of Monomer	Modern Polymer Name	Original Polymer Name
   Ethene	Ethene	Poly(ethene)	Polythene
   Chloroethene	Vinyl chloride	Poly(chloroethene)	Polyvinyl chloride (P.V.C.)
   Propene	Propene	Poly(propene)	Polypropene
   Phenylethene	Styrene	Poly(phenylethene)	Polystyrene

   
   You can quickly test your knowledge of the above information.

    
   

2. Condensation polymerisation
   • When monomer units join together, they can do so not only by breaking double bonds, but also by reacting across functional groups between the monomers.

   • When monomers have two functional groups in each molecule, a long polymer chain can be built up by reacting the functional groups at either end of the molecule.

   • When these functional groups react together, a new bond is formed between the monomer units and a small molecule (often water) is released. When this happens, the reaction is described as condensation polymerisation.

   

   An example of a condensation reaction was covered in the Nomeclature topic and the Reaction of carbon compounds topic during the formation of an ester.

   An example of a condensation polymerisation reaction is shown to the right.

   When the polymer forms between monomers that contain two functional groups (the -OH and the -COOH groups), a water molecule is released from the compound, allowing a new bond to form between the monomer groups.

 
Polyesters

Polyesters are condensation polymers in which the monomers units are linked together by ester groups (-COO-).

This happens when monomer units combine to form an ester linking group (as in the above reaction).

In the reaction shown on the right, an alcohol monomer with two -OH groups (one at either end of the molecule) reacts with a carboxylic acid monomer that has two -COOH groups (again, one at either end of the molecule).

Again, when the acid group (-COOH) reacts with the alcohol group (-OH), a new bond is formed and a molecule of water is released in the process. This means that when we form a polyester, the reaction is called a condensation polymerisation.

 
Amines

The amines belong to a homologous series based on the alkanes in which a hydrogen has been replaced with the amine functional group -NH₂.

The names of the members of this series are derived by the prefix which tells you the number of carbon atoms present and all have the ending -amine. Some members of this family are shown below:

Structure	Name	Structural formula	Shortened structural formula
	Methylamine	CH3NH2	CH5N
	Ethylamine	CH3CH2NH2	C2H7N
	Propylamine	CH3CH2CH2NH2	C3H9N
	Butylamine	CH3CH2CH2CH2NH2	C4H11N

 
Polyamides

Polyamides are polymers that are formed in a reaction between an amine and a carboxylic acid group in the monomer units. The monomer units join together and form an amide link (-CONH-).

Either the monomers have both a carboxylic acid group and an amine group present, or two monomers are required: One with two carboxylic acid groups and another with two amine groups present.

New words and their meanings

Plastics - a wide variety of large molecules made from products from crude oil distillation

Synthetic - man-made

Fibre - a large molecule which is made into long threads

Natural - occurring in nature

Biodegradable - broken down into smaller pieces by living organisms

Toxic - harmful

Thermoplastic - a plastic that can be resoftened by heating e.g. poly(ethene)

Thermosetting - a plastic that cannot be resoftened by heating e.g. bakelite

Monomers - small units that join together to give a very big molecule

Polymers - very big units made when many monomer molecules join together

Polymerisation - the process in which monomers join to give a polymer

Cracking - a reaction in which long-chain hydrocarbons are converted into unsaturated hydrocarbons

Addition Polymerisation - the making of a polymer by a series of addition reactions

Functional groups - a groups of atoms in a compound that give the compound certain properties. e.g. the -OH group in alkanols and the -COOH group in alkanoic acids

Condensation Polymerisation - Polymerisation that occurs between two functional groups and a small molecule is released (this is often water, but can also be another small molecule).

Polyesters - a condensation polymer in which the monomers are linked by ester groups (-COO-).

Amine - a homologous series with the functional group -NH₂.

Polyamide - a condensation polymer which involves the reaction between an amine and a carboxylic acid group in the monomer units. The monomers then link using an amide link (-CONH-).