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National 5 Chemistry Notes

Topic 11 Properties of Plastics

In Topic 7 we found out that crude oil could be used to make a huge range of

useful products. In this topic we look more closely at how we can make plastics

from oil.

Properties of Plastics

Everyday we use plastics in many forms, from carrier bags to furniture. Plastics

are used for so many jobs because they have so many useful properties

e.g. light, flexible, waterproof, tough, electrical insulators

Types of Plastics

Plastics can be divided up into two groups:

1. Thermoplastics 2. Thermosetting Plastics

Problems with Plastics

Most plastics are durable and last a very long time. This is a useful property but

can also cause problems. Since plastics are NON BIODEGRADABLE (do not

decay naturally in the environment) this causes litter pollution.

Plastics also burn easily and produce huge amounts of toxic fumes.

THERMOPLASTICS soften on heating and can be moulded into new shapes.

THERMOSETTING PLASTICS do not soften on heating and cannot be reshaped.

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Making Plastics

The molecules which make up plastics are extremely large. Plastics are

made by the joining together of many small units called MONOMERS.

When the small monomer units join together a POLYMER is formed.

The process of making a polymer by joining many monomers together is called

POLYMERISATION.

There are two types of polymerisation :

1. ADDITION POLYMERISATION

2. CONDENSATION POLYMERISATION

We shall look first at addition polymerisation.

Addition Polymerisation

Addition polymerisation is an important type of polymerisation. Poly(ethene) and

polystyrene are both examples of addition polymers.

The polymerisation is a complicated process but starts with the breaking of the

C=C double bonds. A catalyst is required for this to take place.

ADDITION POLYMERISATION involves the joining together of monomers

containing a C=C double bond.

Note: all the ALKENES contain this type of bond.

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All addition polymers are formed in this way. If we use the letter n to represent

the number of monomer molecules joining together then the process can be

written using molecular formulae as:

nC2H4 (C2H4)n

+ +

e.g. the addition polymerisation of ethene

Stage 1:

many ethene

monomers

Stage 2:

C=C bonds break

Stage 3:

monomers join

to form the

polymer

part of a poly(ethene) polymer

+

Where n = a big number (more than a thousand)

When addition polymerisation takes place the polymer formed always

has a BACKBONE OF CARBON ATOMS.

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Drawing Monomers in the ‘H’ Shape

When showing the addition polymerisation of alkenes it is helpful to draw the

monomers in the special H shape. This focuses attention on the most important

part of the molecule, the double bond.

e.g . drawn in H shape

propene

butene

pentene

Drawing Addition Polymers

Complete the next two pages to show the polymerisation of propene then butene

Stage 1:

propene monomers

Stage 2:

C=C bonds break

Stage 3:

monomers join

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Stage 1:

butene monomers

Stage 2:

C=C bonds break

Stage 3:

monomers join

Naming Polymers

To name the polymer we simply add 'poly' to the name of the monomer.

Complete the table to show the names of some common addition polymers.

Monomer Name Polymer Name

ethene

propene

tetrafluoroethene

phenylethene

chloroethene

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Repeating Units

One thing all polymer molecules have in common is that they are all repetitive.

Big polymer molecules are made from thousands of monomers combined

together so the long polymer chains are made of identical sections that repeat

themselves over and over again. These sections are called the REPEATING

UNITS.

For example the repeating unit in a poly(ethene) molecule is:

Identifying the Monomer from the Polymer

Shown below is a small section of the polymer PVC (polyvinyl chloride ).

The structure of the monomer used to make this polymer is:

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Note: the repeating unit should

be shown inside a bracket with

the bonds at each side left

open.

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Natural Polymers

RUBBER is the only naturally occurring addition polymer. It is obtained

from trees as a white liquid (latex).

All other natural polymers are examples of CONDENSATION POLYMERS.

SILK, HAIR,FUR and DNA are natural polymers and belong to a family called

polyamides.

STARCH (found in potatoes and bread) and CELLULOSE (the main constituent

of wood and paper) belong to a family of natural polymers called

polysaccharides.

Condensation Polymers

In Topic 8 we learned that esters were compounds that could be made by the

reaction of an alcohol with a carboxylic acid

alcohol + carboxylic acid ester + water

The alcohol and carboxylic acid join together through their functional groups

and are held together by a group of atoms known as an ESTER LINK. The two

molecules that have combined only have one functional group each so only one

ester link can form.

In the formation of an ester the hydrogen atom from the alcohol and the

hydroxyl group from the acid combine to form a water molecule.

During CONDENSATION POLYMERISATION monomers join together

by LOSS OF WATER i.e. water is a product in the reaction.

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All esters contain an ester link :

Polyesters

In this unit we shall look at one particular type of condensation polymer called

POLYESTERS.

The monomers used in industry to make polyesters are DIALCOHOLS (diols)

and DICARBOXYLIC ACIDS.

To make a POLYESTER each monomer molecule must have TWO

FUNCTIONAL GROUPS so that the chain can continue to grow

alcohol

carboxylic acid

e.g. ester formation

ester

water

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Some monomers can be very complicated, so to simplify matters we often just

show the functional groups then use boxes to represent the rest of the

molecule.

e.g. formation of a polyester

A POLYESTER contains many ester links (-COO ) along the polymer chain

When diols and a diacids react together the hydrogen atom from the

alcohol and the hydroxyl group (-OH) from the acid combine to form

water molecules.

condensation

polymerisation

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Novel Materials

Since the nineteenth century, scientists have discovered thousands of novel

materials which have changed our lives in so many ways.

Some of these new materials exhibit ‘smart behaviour’. A smart material is one

that changes shape in response to changes in the environment. The change in

shape may or may not be visible!

Examples of novel materials include:

Disposable nappies

Hair gels

Soluble polymers

Conducting polymers

A NOVEL material is a NEW material i.e. one that does not

resemble anything that was previously known or used.

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Learning Intention

Success Criteria

1. I will find out about uses, properties and types

of plastics.

I can:

State the uses of some plastics

Give examples of properties of plastics

State that plastics can be thermosetting or thermoplastic

State that plastics cause litter pollution since they are non-

biodegradable and burn to produce toxic gases.

2. I will find out about two different types of

polymerisation.

I can:

Define the words: monomer, polymer, addition polymerisation

State the two different types of polymerisation as addition and

condensation polymerisation.

3. I will find out how plastics are made by addition

polymerisation.

I can:

State that alkenes are the monomers used in addition polymerisation

Describe how 3 monomers would join in a 3 step process to produce

part of a polymer

State that addition polymers have a backbone of carbon atoms

4. I will find out how to draw monomers in the H

shape.

I can:

Draw monomers in the H shape given full structure

5. I will practice how to show the formation of a

polymer

I can:

Show the formation of a polymer by drawing the 3 step process

6. I will find out how to name a polymer. I can:

Name a polymer by adding ‘poly’ to the name of the monomer

7. I will find out about polymer repeating units I can:

State that a repeating unit is the section of the polymer which

repeats over and over again

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Draw the repeating unit from part of a polymer structure

8. I will practice some examples of drawing the

monomer from the polymer structure.

I can:

Draw the monomer from the given polymer structure

9. I will find out about natural polymers. I can:

Give some examples of natural polymers

State that most natural polymers are condensation polymers.

10. I will find out how monomers join by

condensation polymerisation

I can:

State that condensation polymerisation takes place when monomers

join by loss of water.

State that polyesters are condensation polymers

State that diols and dicarboxylic acids are the monomers used to

make polyesters

State that polyesters contain many ester links

State that when a polyester forms the OH is lost from acid group and

an H from the alcohol to produce water molecules.

11. I will find out about novel materials. I can:

State that a novel material is a new material

State that some novel materials exhibit smart behaviour

Give examples of some novel materials