Hydrocarbons (5) Making Addition Polymers

We've seen how hydrocarbons split into three groups: alkanes, alkenes and alkynes.

These groups are formally called homologous series.

They each have a general formula, a gradation in physical properties and similar chemical properties.

For example the general formulae for these homologous series are:

Alkanes     Cn H2n+2
Alkenes     Cn H2n
Alkynes     Cn H2n-2

We've also seen that all alkanes are saturated molecules.

All alkanes have single covalent bonds between their atoms of hydrogen and carbon.

Alkenes by contrast have at least one double carbon carbon bond.

The genius of this double bond is however that the two bonds are not identical, one is weaker than the other.

Why is this genius?

The weaker bond allows the alkenes to be much more reactive than the alkanes.

The alkenes also allow chemists to be much more creative in their chemistry.

So alkenes decolorise orange or brown bromine water whereas alkanes do not.

The bromine water reaction is a test for the double bond:

The reaction with bromine is an addition reaction because two reactants add together to make one product.

You can catch a video of this test here

Now if the molecule of bromine can add across an alkene double bond, what's to stop other molecules adding across the double bond - all they need is a weak covalent bond.

Alkenes themselves have a weak covalent bond so couldn't they add to each other?

When this happens many thousands of times the product is an addition polymer

Polymer is this peculiar word for a molecule composed of many other molecules poly - many and mer - molecule.  

How can we represent this on paper?

Here is the example of polythene:  

n is a very large number say 1500 and there are four monomer units shown in the polythene molecule.

We can simplify the structure of a polymer like polythene in this way:

Here the polymer repeating unit is placed in brackets with the n outside; note also that the bonds, joining the polymer repeat units together, extend outside the brackets.

The reaction takes place using a catalyst and under high pressure.  

Under high pressure the monomer molecules of ethene are more likely to collide with each other.   

Here are a few other example polymerisation reactions:

Other groups of atoms can replace the chlorine atom in chloroethene and give rise to other addition polymers.  

This is polystyrene:

This is polypropene:

n

propene                                             polypropene  

You ought to be able to draw the displayed formula of an addition polymer from the displayed formula of its monomer.

You also ought to be able to draw the displayed formula of the monomer from the displayed formula of its polymer.  

In a subsequent post, I will discuss free radical polymerisation, the reason why the catalyst is used in polymerisation.

Pages on the "Mole" and "Using the Mole" in chemical calculations are here