Polymerisation of Alkenes
All of the alkenes can be made to perform an reaction called polymerisation.
It usually requires a catalyst and pressure and some heat.
If we take the alkene propene as an example. The single molecules (known
as “the monomer”) look like this
H
H
C
H
H
H
C
C
C
H
H
H
H
H
C
C
H
C
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
C
C
C
C
C
H
But the polymer “polypropene” looks like this:
H H
H
H H
H H
etc
H
C
H
C
H
H
H
H
H C H H C H H C H H C H
H
H
H
H
etc
H
H
Polymerisation of Alkenes
How does this happen?
Look at the monomer and polymer, how are they different? Note which carbon
atoms are forming the continuous chain in the polymer.
H
H
C
H
C
H
H
CH3
H
C
C
H
H
CH3
H
C
C
H
H
CH3
C
H
C
CH3
One part of the carbon to carbon double bond breaks in each molecule.
H
H
C
H
C
H
H
H
C
C
H
H
H
C
C
H
H
C
H
C
CH3
CH3
CH3
CH3
And this “spare” bond links up to a neighboring molecule, forming a chain
with thousands of “units” in it. Is this reaction one of addition?
Polymerisation of Alkenes
We can represent the polymerisation as follows:
H
CH3 H
C
C
H
CH3 H
C
C
H
CH3 H
C
H
H
H
C
C
H
Many propene monomer molecules
etc.
C
CH3
H
H
polymerised by catalyst,
heat and pressure
H
CH3 H
CH3 H
CH3 H
CH3
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
etc.
polypropene
This is called the “repeating unit”
Or more simply as an equation:
H
n
CH3
C
H
C
H
polymerise
H
CH3
C
C
H
H
n
Frequently Asked Questions about Polymers
1. Why are polyalkene polymers unreactive, compared to their monomers?
These polymers are saturated molecules, whereas the monomers are
unsaturated.
2. Are there any natural polymers?
Certainly. In biological systems, polymers are common. Starch is a polymer of
glucose (a “polysaccharide”), as is cellulose. Proteins are polymers of amino
acids and DNA is a polymer of subunits called nucleotides.
3. These natural polymers can be digested by living things. Can any living
thing like bacteria digest synthetic polymers, such as polyethene or polyvinyl
chloride (PVC)?
Non. These synthetic polymers such as polystyrene etc., are not susceptible to
biological decay. They are said to be non biodegradable. This makes them an
environmental hazard, since they will never decompose.
4. Are all polymers alike and what are they used for?
Polymers vary considerably in their properties, though all are chemically
unreactive. Polyethene is quite soft while PVC is very rigid. Polyethene is
used to make carrier bags while PVC is used to make window frames and
CD boxes. Polypropene is used to make milk crates.