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Polymerisation

Worksheet Overview

QUESTION 1 of 10

Think of the most important things in your life – your phone, your music player, your vacuum cleaner (if you like having a clean bedroom). They will all be made with plastics in them. Plastics really became common in the 1960's. They where a revolution in chemistry – suddenly it was cheap and easy to make basically everything. Plastics are generally strong and light, because they are based on the strong C-C covalent bond. It's easy to add dyes to them, so they can make very colourful objects. Unlike wood and paper, they do not rot when they get wet.

Unfortunately, the fact that plastics do not rot means that they cause a lot of pollution when we throw them away carelessly. We're now more aware of the pollution problems caused by plastics, and need to work hard to solve them by making plastics which are biodegradable. Biodegradable plastics will break down in nature. In the meantime, plastics are still an incredibly useful group of materials.

The manufacture of plastics comes from oil. Polymers are very big molecules with long chains and they are made from many small molecules called monomersPoly- means many, mono- means one. When you stick together lots of monomers, you get a polymer. Living things make a lot of polymers naturally; proteins, starch and DNA are all examples of natural polymers.

Plastics have names like polythene, polycarbonate, polyvinyl chloride (PVC). The "poly" part tells you that there are lots of repeating molecules joined together. The second part of the name tells you what the monomer is. So in polythene, we have lots of ethene monomers joined in a chain, like this;

Depending on how carefully we prepare the polythene, a polymer chain might have between 4000 and 40 000 monomers. It will often have branches in the chain. These are useful, because they help to strengthen the plastic produced.

Making polymers

The simplest sort of polymer to understand is called an addition polymer, because we just add more and more monomers in a chain, one after the other. 

If we want to make polythene, we need to start with an ethene molecule to make this work. In the diagram below, on the left-hand side, you can see an ethene monomer. If you look in between the two carbons, you can see that there is a double bond (two lines). This is what makes an alkene. If it has a carbon-carbon double bond then it is ripe for being made into a plastic.

 

All we do is a chemical reaction that breaks apart one of the double bonds in the alkene. When this is broken, it is freed up to bond with another alkene. This process means that one alkene can bond to make a chain of two, and then three and then four and you get the idea until the chains are hundreds of bonds long. The final version of the diagram (the one with brackets) is called the displayed formula. The part in brackets repeats, the bonds reaching out link this monomer to its neighbours, and the n at the bottom-right of the brackets tells us that there are many repeats of the monomer.

What controls the properties of the plastic?

Good question – there are two things to think about. One is the monomer used; different monomers make plastics with different properties;

Polymer Properties Uses
Polythene flexible, cheap, electrical insulator plastic bags, insulation for electric wires
Polypropene flexible, strong, tough crates, ropes, plastic furniture
Polyvinyl chloride tough, cheap window frames, water pipes
Polytetrafluoroethene tough, chemically unreactive waterproof clothing, non-stick pans

The other factor is the length and patterning of the polymer chains. Longer chains mean that there are more intermolecular forces . More intermolecular forces mean that the resulting plastic is stronger, less flexible and has a higher melting point. Branched polymers tend to be stronger (because the branches can tangle each other up). Some polymers can also be treated to have cross-links between chains. These also increase the strength (and melting temperature) of polymers.

Despite their problems, plastics are still a very useful group of materials. They are made by joining together alkene molecules, which come from crude oil. That's another reason why we need to be careful how we use the planet's fossil fuel reserves.

Which of these are common properties of plastics?

Waterproof

Strong

Transparent

Light

Conduct electricity

Biodegradable

Match the names of these monomers with the resulting polymers.

Column A

Column B

polythene
ethene
polystyrene
vinyl acetate
polyvinylacetate
styrene

Complete this paragraph about the formation of polypropene.

Column A

Column B

polythene
ethene
polystyrene
vinyl acetate
polyvinylacetate
styrene

Which of these compounds is an alkene?

methane

ethene

polythene

methene, CH2

C15H30

acetone

Which of these factors would make a plastic stronger?

Shorten the polymer chains

Lengthen the polymer chains

Increase the number of branches in the chains

Increase the number of chains

Reduce the number of cross-links

The plastic around electrical wires is often made of polythene. Select the properties which make this a good choice.

electrical insulator

flexible

cheap

strong

can be easily coloured

Why would biodegradable plastics be useful?

They would be stronger

They would be less polluting

They would last longer

What do we call polymers made by adding polymers one after another in a chain?

They would be stronger

They would be less polluting

They would last longer

Which of these diagrams shows the displayed formula for polyethene?

 




Fill the gaps in this paragraph, using the words increases or decreases for each gap.




  • Question 1

Which of these are common properties of plastics?

CORRECT ANSWER
Waterproof
Strong
Light
EDDIE SAYS
The key thing plastics have in common is being based on C-C chains. This makes them light, strong, waterproof, and insulating. Most plastics aren't biodegradable (which is a problem), but scientists are working to make more biodegradable plastics.
  • Question 2

Match the names of these monomers with the resulting polymers.

CORRECT ANSWER

Column A

Column B

polythene
ethene
polystyrene
styrene
polyvinylacetate
vinyl acetate
EDDIE SAYS
Polymer names start with "poly", then have the name of the monomer- so polypropene is lots of propenes. Some of these monomers are a bit more complicated than simple alkenes, but the naming convention still works.
  • Question 3

Complete this paragraph about the formation of polypropene.

CORRECT ANSWER
EDDIE SAYS
The spy name of alkenes is 0014; they have double Bonds (sorry...) The double bond is critical, since one of the bonds can break to allow links between monomers.
  • Question 4

Which of these compounds is an alkene?

CORRECT ANSWER
ethene
C15H30
EDDIE SAYS
Ethene is an alkene, and so is C15H30 (check the C:H ratio). Methene looks like it should be an alkene, but it can't exist (if you only have one carbon atom, you can't make a carbon-carbon bond).
  • Question 5

Which of these factors would make a plastic stronger?

CORRECT ANSWER
Lengthen the polymer chains
Increase the number of branches in the chains
EDDIE SAYS
If we want a stronger plastic, we would want long chains, with lots of branches and cross-links. These factors depend on how we process the plastic; the more slowly and carefully we make the plastic, the stronger it tends to be.
  • Question 6

The plastic around electrical wires is often made of polythene. Select the properties which make this a good choice.

CORRECT ANSWER
electrical insulator
flexible
can be easily coloured
EDDIE SAYS
We definitely need the plastic around an electrical cable to be a good insulator, and it needs to be flexible (since the wire may bend). Price isn't that important (since we're not using much), and the plastic isn't taking any load, so strength isn't important. Colouring the plastic is useful, so we can tell wires apart.
  • Question 7

Why would biodegradable plastics be useful?

CORRECT ANSWER
They would be less polluting
EDDIE SAYS
Until very recently, we didn\'t think much about the problems caused by plastics not rotting; now we have seen the harm caused by small pieces of plastic in the ocean. Scientists are working on biodegradable plastics, but we all need to be careful how we dispose of plastic things.
  • Question 8

What do we call polymers made by adding polymers one after another in a chain?

CORRECT ANSWER
EDDIE SAYS
Addition polymers are the simpler ones to imagine, so start by understanding them. The other type you may need to find out about are called condensation polymers.
  • Question 9

Which of these diagrams shows the displayed formula for polyethene?

 

CORRECT ANSWER
EDDIE SAYS
The displayed formula shows the monomer in brackets, with bonds reaching out either side (the lines showing the bonds go across the brackets). The n at the bottom-right shows there is an unknown number of monomers in the polymer. If we knew, we could put a specific number there.
  • Question 10

Fill the gaps in this paragraph, using the words increases or decreases for each gap.

CORRECT ANSWER
EDDIE SAYS
Both the melting temperature and the overall strength for plastics depends on the strength of the intermolecular forces. As the individual polymer chains get longer, these forces strengthen.
---- OR ----

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