10.3.1 Addition polymerisation of alkenes

A polymer is built from many small repeating units

1. A polymer has a very high average relative molecular mass.
2. It is built from many small units joined into a long chain.
3. The small molecules that join together are the monomers.
4. The section that repeats along the chain is the repeating unit.
5. The chains in a sample are not all the same length, which is why the mass is an average.

Ethene molecules join to form poly(ethene)

1. Many ethene molecules are brought together under pressure.
2. The double bond in each molecule opens.
3. The opened molecules join to one another end to end, forming a long chain.
4. The polymer formed is poly(ethene).
5. Nothing else is produced, which is what makes this an addition polymerisation.

Other monomers give other addition polymers

1. Any monomer containing a carbon to carbon double bond can polymerise this way.
2. Propene gives poly(propene).
3. Chloroethene gives poly(chloroethene), known as PVC.
4. Tetrafluoroethene gives poly(tetrafluoroethene), known as PTFE.
5. The polymer is named by putting the monomer's name in brackets after poly.

Deducing a monomer from a polymer

1. Take one repeating unit from the chain shown.
2. Remove the bonds that extend out of each end of that unit.
3. Join the two end carbons with a double bond instead.
4. The molecule that results is the monomer.
5. Working the other way, open the monomer's double bond and draw bonds extending from each end.

10.3.2 Uses and properties of polymers

The properties of a polymer decide its uses

1. A polymer is chosen for a job because of what it does, not what it is made from.
2. Flexibility, strength, rigidity and resistance to chemicals all matter.
3. Whether it is a good electrical insulator decides some uses on its own.
4. Behaviour when heated matters wherever the object gets warm.
5. Explaining a use means naming the property and the demand it meets.

Poly(ethene) and poly(propene)

1. Poly(ethene) is flexible, cheap and easily moulded.
2. It is used for plastic bags, bottles and food wrapping.
3. Poly(propene) is tougher and keeps its shape better.
4. It is used for crates, ropes and containers that must take a knock.
5. Both are unreactive, so neither contaminates the food it holds.

PVC and PTFE

1. PVC is rigid and durable, and it resists weather well.
2. It is used for drainpipes and window frames.
3. PVC is also a good electrical insulator, so it covers electrical cable.
4. PTFE is very unreactive and has a very slippery surface.
5. It is used as a non-stick coating on pans and as a tape for sealing joints.

Matching a polymer to a new job

1. Start from the demands the job makes on the material.
2. A job needing flexibility and low cost points to poly(ethene).
3. A job needing rigidity outdoors points to PVC.
4. A job needing a surface nothing sticks to points to PTFE.
5. A sound answer names the polymer and the property that makes it suitable.

10.3.3 Condensation polymers and polyesters

A condensation polymer releases a small molecule

1. The monomers join together to form a long chain, as in any polymerisation.
2. Each time two monomers join, a small molecule is released.
3. That small molecule is usually water.
4. This is what distinguishes condensation from addition polymerisation.
5. An addition polymerisation releases nothing, so all the atoms end up in the polymer.

Each monomer needs two functional groups

1. A monomer must be able to join at both ends to build a chain.
2. It therefore carries a functional group at each end.
3. One monomer has two carboxylic acid groups, $-COOH$.
4. The other has two alcohol groups, $-OH$.
5. A monomer with only one group would stop the chain after one join.

An ester link forms and water is lost

1. A carboxylic acid group on one monomer meets an alcohol group on the other.
2. The two react and join the monomers together.
3. The join formed is called an ester link.
4. A molecule of water is released each time a link forms: $$-COOH+HO-\to -COO-+H_{2}O$$
5. The water comes from the $-OH$ of the acid and a hydrogen from the alcohol.

The polymer formed is a polyester

1. A chain held together by ester links is a polyester.
2. The two monomers alternate along the chain, acid then alcohol.
3. Each monomer's second group is free to react with the next monomer along.
4. The chain grows as long as monomers remain to be added.
5. Polyesters are used as fibres for clothing and as plastic bottles.

10.3.4 Problems with polymers and recycling

The starting materials come from crude oil

1. Most polymers are made from alkenes such as ethene and propene.
2. Those alkenes come from cracking the heavier fractions of crude oil.
3. Crude oil is finite, so the supply of starting material is limited.
4. Demand for polymers competes with demand for fuels from the same oil.
5. As oil becomes scarcer, the cost of making polymers rises.

Polymers persist in landfill

1. Most polymers are not biodegradable, so microorganisms cannot break them down.
2. A polymer buried in landfill therefore persists for a very long time.
3. Landfill sites fill up, and new sites take up land.
4. Polymers that reach rivers and the sea remain there, harming wildlife.
5. The unreactivity that makes a polymer useful is what makes it persist.

Burning polymers releases harmful gases

1. Burning a polymer releases carbon dioxide, which is a greenhouse gas.
2. Incomplete combustion releases carbon monoxide, which is toxic.
3. Burning PVC releases hydrogen chloride, which is acidic and corrosive.
4. Those gases must be removed from the exhaust before it reaches the air.
5. Burning does at least release energy that can be used, and it takes up no land.

Recycling requires sorting

1. Different polymers must be melted separately to be reused.
2. A mixture of polymers gives a material of poor quality.
3. Sorting is therefore needed before anything can be reformed.
4. Sorting by hand is slow and costly, and automatic sorting needs expensive equipment.
5. Items are often labelled with a code to make sorting possible.

Weighing the case for recycling

1. Recycling saves crude oil, since less new polymer has to be made.
2. It uses less energy than making the polymer from oil.
3. It reduces landfill and the litter that escapes from it.
4. Against that, collecting and sorting cost money and energy of their own.
5. A recycled polymer is often of lower quality, so it cannot always replace new material.

10.3.5 Natural polymers: DNA, starch and proteins

Natural polymers are built the same way

1. A natural polymer is made by a living organism rather than in a factory.
2. It is still a long chain built from many small monomers.
3. The chains are still long enough to give a high relative molecular mass.
4. Three natural polymers matter here: DNA, starch and proteins.
5. Each is named by the kind of monomer it is built from.

DNA is made from nucleotides

1. DNA is the polymer that carries genetic information.
2. Its monomers are called nucleotides.
3. There are four different nucleotides in DNA.
4. The order in which they are joined is what stores the information.
5. The names of the individual nucleotides are not needed here.

Starch is a polymer of sugars

1. Starch is made by plants as a store of energy.
2. Its monomers are sugars.
3. Many sugar molecules join into long chains to make the starch.
4. Breaking starch back down releases the sugar for the organism to use.
5. Starch is a natural polymer, so it is biodegradable, unlike most synthetic ones.

Proteins are polymers of amino acids

1. A protein is built from monomers called amino acids.
2. Many amino acids join into a chain to make the protein.
3. Different proteins contain different amino acids in a different order.
4. That order is what gives each protein its own job in the organism.
5. Enzymes are proteins, which is how a catalyst can be built this way.