BP Educational Service
Science at Work
Polymers for life?
What is a polymer?
Products made from oil and gas are all around
you. They are the building blocks of modern life.
You will find them in:
– furniture, cars and electronics
– fibres used to make clothes and textiles
– bags, boxes and trays used in packaging.
All these products are made from a family
of chemicals known as polymers.
Polymers get their name from two Ancient
Greek words:
– poly means ‘many’
– mer comes from meros, which means ‘parts’.
Poly-mer = ‘many parts’
Polymers are made from molecules called
monomers. Mono comes from the word
for ‘one’: Mono-mer.
Bottles made from polymers
1. How might these words explain how
monomers become polymers?
Polymers are made from oil and gas. These are
limited resources.
Society will need to change how it makes,
uses and disposes of plastics and other polymers.
Scientists must find ways to re-use polymers
to make new products.
This is a technological challenge. Scientists are
already discovering new solutions, but technology
alone cannot solve the problem: our attitudes
towards using and recycling resources will need to
change too. A new recycling technology will not
benefit anyone unless people save their plastics,
so that recycling can take place.
1
Alkanes and alkenes
Alkenes are also chains of carbon atoms with
hydrogen atoms attached, but they contain at least
one double covalent bond. These double bonds can
open up, allowing new atoms or molecules to join
on. Alkenes are therefore unsaturated.
Two important types of hydrocarbon are alkanes
and alkenes.
Alkanes are made of one or more carbon atoms
joined by single covalent bonds, and surrounded
by hydrogen atoms. They have no spare bonds that
can open up and allow other molecules to join on.
This means that alkanes are saturated.
H
Ethane model
H
H
C
C
H
H
H
Ethane structural
formula
Ethene model
H
H
C
C
H
H
Ethene structural
formula
Alkanes have the formula CnH2n+2. The first four
alkanes are:
All alkenes with one double bond have the formula
CnH2n. The first three alkenes are:
Methane
CH4
Ethene
C2H4
Ethane
C2H6
Propane
C3H8
Butane
C4H10
Propene
C3H6
Butene
C4H8
Alkenes do not burn cleanly; they produce a lot
of soot. They are not useful as fuels. But because
they are unsaturated, alkenes can react with other
molecules to form new substances. Alkenes can
be joined together in long chains to make polymers.
2. Pentane has five carbon atoms.
What is pentane’s chemical formula?
Alkanes burn cleanly to produce carbon dioxide and
water. They are useful as fuels. But because they
are saturated, they cannot be used as monomers
to make plastics and other polymers.
4. Why is there no alkene that contains just
one carbon atom?
3. Why can’t a saturated molecule join onto
other molecules?
2
Alkanes and alkenes
Like other hydrocarbons, alkanes and alkenes have
properties that depend on their size.
Short chain alkanes
and alkenes…
Long chain alkanes
and alkenes…
boil at lower temperatures
boil at higher temperatures
are more flammable
are less flammable
are less viscous
and flow easily
are more viscous
are more volatile
are less volatile
This link between properties and chain length is
very useful. It allows refineries to separate crude
oil into different components. Each component,
or ‘fraction’, contains hydrocarbons with
similar properties.
5. Why do you think BP uses its refineries to
isolate components of crude oil that have
similar properties?
One fraction is ‘naphtha’. Naphtha is processed to
produce a mixture of alkenes. Manufacturers use
these as monomers to make a variety of plastics.
Packaging made from polymers
Another important fraction is the lightest one:
a mixture of gases. This mixture includes ethane,
which can be separated from the other gases.
This ethane is processed to produce ethene,
the most important building block for plastics
and other polymers.
6. Why does ethane need to be changed into
ethene to make polymers?
3
Making polymers
The ethene molecules join in an addition reaction.
This is an example of addition polymerisation.
Polymers are made when many small molecules
join together to form a single, long chain molecule.
The small molecules, or monomers, each have a
double covalent bond. Under certain conditions,
these bonds open up. This allows many
monomers to join together.
7. The polyethene molecules must end at some
point. What atom do you think attaches to
the single covalent bond at the ends of the
polymer molecule?
Propene (C3H6) can polymerise to form
polypropene (also known as polypropylene).
n
...to form a section
of polymer
Many monomers join...
H
H
H
H
C
C
C
C
H CH3
propene
n
H CH3
polypropene
Monomers joining to form a polymer
Polymers are extremely long. The longest
hydrocarbons in crude oil can contain up to 60
carbon atoms. A polymer molecule may be
thousands of carbon atoms long.
When polymers form, the double bonds between
carbon atoms in the monomers are replaced by
single bonds. This means that simple polymers like
polyethene cannot easily bond with other molecules.
Ethene (C2H4) is the simplest monomer. Ethene
polymerises to form polyethene (sometimes also
known as polyethylene).
n
H
H
H
H
C
C
C
C
H
H
H
H
ethene
8. Polymers are often used to make containers
to store reactive chemicals. Why do you
think this is?
Some polymers form ‘branches’ on their long
chains. Other polymers have ‘cross-links’ that join
chains together. These branches and cross-links
change the properties of the polymer.
n
By using polymers with different properties,
chemists can devise plastics that have many
different uses.
polyethene
4
Using and recycling polymers
Polymers have different physical properties. These
properties depend on the monomer used, and on
the type of bonds between polymer chains.
There are two types of polyethene:
Low density polyethene
(LDPE)
High density polyethene
(HDPE)
Polyethene pellets in the recycling process
LDPE
HDPE
How it is made
High temperature and very
high pressure
Low temperature, and slightly
raised pressure; a catalyst controls
the reaction
Type of polymer chain
Include many branches
Relatively straight and
rarely branched
How molecules pack together
Cannot pack closely together
Can pack closely together
Intermolecular forces
between chains
Weak
Strong
Properties
Soft and pliable
Hard and strong
Uses
Drink bottles and plastic bags
Garden furniture, food storage
containers and pipes
9. Why do the branches in LDPE mean
the intermolecular forces are weak?
10. How does the structure of HDPE make
it harder and stronger than LDPE?
5
Using and recycling polymers
Polymers have many different uses. How they
are used depends on their properties.
11. Match the polymers in the table below
to their uses.
Name
Property
Use
Polystyrene
Can be ‘expanded’ to make a rigid
foam full of trapped air
Low density polyethene
Light and stretchable
Window frames
Polytetrafluoroethene
Extremely slippery, heat resistant
Plastic bags
Polyvinylchloride
Can be made rigid and light resistant
Polypropene
Heat resistant, strong and hard
Non-stick saucepan coatings
Electric kettles
Insulated packaging
Despite their usefulness, polymers present two
challenges for the future:
Recycling waste like plastic packaging has several
advantages:
– How we will make them
– It reduces the amount of oil and gas needed
to make new plastics
– How we should dispose of them
– It means less material goes into landfill
or must be incinerated
Polymers are made from oil and gas. These are
finite resources and one day they will run out.
The price of oil and gas is rising, because more
people around the world need the energy these
fuels can provide.
– It helps reduce carbon emissions because
making recycled plastic generally uses less
energy than producing plastic from ‘new’
crude oil
12. Why will the availability of oil and gas
affect how plastics are made and used
in the future?
Even recycling has its limitations. Not all plastics
can be recycled and some waste plastics are
contaminated with substances that can be harmful.
Polymers are extremely durable. They can take
many hundreds of years to break down in landfill
sites. One alternative to landfill is incineration.
The other is to recycle.
13. How can recycling reduce the amount
of oil and gas needed to make plastics?
6
Glossary
Addition polymerisation – a chemical reaction in
which unsaturated molecules join to form a chain.
Covalent bond – a chemical bond formed by a
shared pair of electrons.
Monomer – a molecule that can be bonded to
other identical molecules to form a polymer.
Polymer – a large chain molecule formed by many
identical smaller molecules bonded together.
Saturated – a hydrocarbon molecule that contains
no carbon–carbon double bonds, and therefore has
the greatest possible number of hydrogen atoms.
Unsaturated – a hydrocarbon molecule that
contains one or more carbon–carbon double
bonds, and therefore has fewer than the
maximum possible number of hydrogen atoms.
BP Educational Service
PO Box 105,
Rochester, Kent, ME2 4BE
Tel: +44 (0) 871 472 3020
Fax: +44 (0) 871 472 3021
Email: [email protected]
Website: www.bp.com/bpes
© BP International Ltd 2008