Polymerisation
Alkenes react with lots of other substances to form useful products, but a
particularly important reaction occurs when they react with themselves in a
process called POLYMERISATION.
Polymerisation is the joining together of a large number of small molecules, called
monomers, to form one very large molecule, called a polymer.
All the common plastics are polymers.
When unsaturated monomers join together to form a polymer with no other
substance being produced in the reaction, the process is called ADDITION
POLYMERISATION.
You have already met addition polymerisation. The following is a reminder:
1. Poly(propene)
H
n
CH3
C
H
(
C
H
propene
(monomer, unsaturated)
H
CH3
C
C
H
H
)n
poly(propene)
(polymer, saturated)
n is a very large number, often greater than 10,000
You will be familiar with many of the uses of addition polymers already. These
types of materials are used for plastic bags, packaging, food and drink
containers, non-stick coatings, flooring, replacement windows, ropes, crates,
clothing, disposable gloves, insulation etc.
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Properties of Plastics
Polymers (plastics) consist of a tangled mass of molecules in which the atoms are
joined by strong covalent bonds in very long chains. The properties of the polymer
formed depend mainly on which monomer has been used to produce them. However
the same monomer can be processed in different ways to produce polymers with
different properties. Low density polyethene (LDPE) and high density polyethene
(LDPE) can be made from the same monomer but the long chains join together
differently.
The problem with plastics
Although polymers are very useful materials, there are problems with the
disposal of unwanted articles. Many polymers made from fossil fuels are not
biodegradable. This means that decomposers (bacteria and fungi) will not break
them down into simpler substances. If unwanted polymers are put in landfill
(buried in the ground) they simply remain as polymers in the ground.
Products from plant material (wood, paper, cotton etc.) are biodegradable.
When buried, bacteria and fungi break them down into useful nutrients for
further plant growth. Nature recycles its own products!
Many polymers can not be incinerated (burned) to dispose of them either. People
often die from the smoke produced by burning polymers in house fires, long
before the fire itself reaches them. Polymers produce toxic materials (poisons)
when they are burnt, in addition to the expected products of combustion of
hydrocarbons, which are water, carbon dioxide, carbon monoxide and carbon
(soot).
Those polymers that contain chlorine (PVC for example) also produce hydrogen
chloride on burning. Those polymers that contain nitrogen (nylon for example)
produce hydrogen cyanide when they are burnt. Hydrogen cyanide is extremely
poisonous.
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So what are the Solutions to the disposal of unwanted polymers?
Recycling.
Polymers are increasingly being recycled. Recycling polymers is not as cost
effective as recycling metals, but we don't want to live with piles of (unrotting)
plastic and recycling is a better use of the finite raw materials (fossil fuels)
that make polymers.
Making Biodegradable Polymers.
Research into producing biodegradable polymers (making polymers from
cornstarch for example) will increasingly provide useful replacements for the
main polymers of today.
Bio-Polymers
Scientists are currently working on growing polymers inside plants or bacteria.
This is not a natural process but involves the plants being genetically modified.
Condensation polymerisation
Condensation polymerisation involves linking lots of small monomer molecules
together by eliminating a small molecule. Unlike addition polymerisation, there
are two different monomers involved. The monomers are molecules with a
functional group at either end.
The product is therefore smaller than if the two molecules had added together
(hence the term condensation). This is often water from two different
monomers, an H from one monomer, and an OH from the other, the 'spare
bonds' then link up to form the polymer chain.
Nylon (a polyamide) is formed by condensation polymerisation, the structure
of nylon represented below where the rectangles represent the rest of the
carbon chains in each unit.
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This is the same linkage (-CO-NH-) that is found in linked amino acids in
naturally occurring macromolecules called proteins, where it is called the
'peptide' linkage.
Terylene (a polyester) is formed by condensation polymerisation and the
structure of Terylene represented as
This is the same kind of 'ester linkage' (-COOC-) found in fats which are
combination of long chain fatty carboxylic acids and glycerol (alcohol with 3 -OH
groups, a 'triol').
Condensation polymers such as these, are biodegradable since the reactions by
which they were formed can be undone and so the very long chains fall apart.
Natural polymers
These polymers are all man made but nature produces many polymers of its own.
Starch is a polymer made by plants. Plants join glucose molecules together into
very long chains to make starch. Potatoes and bread are rich in starch.
Many of your body parts are made of natural polymers called proteins. Your
body makes these by joining together small molecules called amino acids. Your
hair, skin, muscles etc. are made this way.
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The macromolecules in food
Much of the food we eat contains very large molecules (macromolecules).
Examples are complex hydrocarbons, proteins and fats.
Starch and cellulose are complex carbohydrates formed by joining glucose
molecules (made during photosynthesis) into long chains. The structure below
shows the sugar molecules (rings) joined together via condensation
polymerisation.
Sugar molecules are often simplified to
Starch can then simply be represented as
These bonds can be broken down inside the body by hydrolysis (reaction with
water) to produce the sugars which your body needs for energy.
Proteins are formed by joining amino acid into long chains. The structure below
shows the amino acid molecules joined together via condensation polymerisation
in much the same way as the synthetic polymer ‘nylon’. The amide bond found in
nylon is exactly the same as the peptide bond in a protein. In the diagram ‘R’
represents different groups of atoms.
These peptide bonds can be broken down inside the body by hydrolysis to
produce amino acids which your body needs for growth and repair.
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Topic 18 Polymers
Summary questions
1
Which of the following molecules can be used to make an addition polymer?
2
Which of the following molecules can be used to make an addition polymer?
A
2 only
B
4 only
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6
2, 3 and 4 only
D 3 and 4 only
3
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7
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4
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9
5
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