4hSynthetic polymers

A polymer is one very long molecule made by joining up many small ones. Those small starting molecules are monomers — mono meaning one, poly meaning many.

What happens

In addition polymerisation:

1. The carbon-carbon double bond in each monomer opens.
2. That frees a bonding position at each end of every molecule.
3. The molecules join end to end into one long chain.
4. No other product is formed.

That last point is the defining feature. Everything that goes in ends up in the polymer.

Only unsaturated monomers can do this

Without a double bond there is nothing to open, so alkanes cannot polymerise. This is why cracking matters — it is the source of the alkenes the plastics industry runs on.

Naming

Put poly in front of the monomer's name, with the monomer in brackets:

The brackets are part of the accepted form.

Monomer and repeat unit are not the same

This is the distinction that costs the most marks.

The repeat unit is written inside brackets with a line passing through each side, and n outside to show it repeats many times.

Every atom of the monomer appears in the repeat unit. Only the bond changes.

Working between them

Repeat unit → monomer: change the single bond between the two carbons back to a double bond, and remove the extending bonds. Everything attached to the carbons stays exactly as it is.

Monomer → repeat unit: the reverse. Open the double bond to a single, and add a bond extending from each carbon.

No atoms are added or removed in either direction.

Examiners report two errors here repeatedly: leaving the double bond in the repeat unit, and drawing both the monomer and the repeat unit without saying which is the final answer.

Evidence that nothing is lost

Poly(ethene) made from 1000 ethene molecules has exactly 1000 times ethene's relative molecular mass.

That exact multiple is the proof that no small molecule is released — which is precisely what separates addition from condensation polymerisation.

Separate Chemistry only.

A different way to build a polymer

Condensation polymerisation joins monomers by losing a small molecule each time a link forms. For a polyester, that small molecule is water.

dicarboxylic acid + diol → polyester + water

The two monomers

The prefix di- is the essential part. Each monomer needs two functional groups so the chain can grow at both ends.

A monomer with only one functional group can form only one link, so the chain stops after two units. That is exactly why ethanol and ethanoic acid make a single ester and not a polymer — one functional group each, and a dead end.

The linkage

An −OH from the diol reacts with a −COOH from the acid, water is lost, and an ester linkage −COO− forms between them.

The polymer is called a polyester because it is held together by many ester links.

Its repeat unit contains one section from the acid and one from the diol, joined by that linkage, with bonds extending from each end and n outside the brackets.

Telling the two types apart

The clearest test is the second product. If water is released, it is condensation.

Biopolyesters

Most polyesters, like most plastics, persist for a very long time. Some — known as biopolyesters — are biodegradable, meaning microorganisms can break them down. That is the specification's definition (4.50) and the one to give.

(Outside the course, biopolyester is often used for a polyester made from plant sources, which is not the same thing as being biodegradable.)

That matters because it addresses the central disposal problem directly: the material breaks down naturally after use, rather than accumulating in landfill.

The property that makes addition polymers useful is the same one that makes them a problem.

Inert, and therefore permanent

Addition polymers are chemically inert — they do not react with much of anything. That is why they are ideal for packaging, pipes and containers.

It is also why they are not biodegradable: microorganisms cannot break them down. Buried in landfill, they stay largely unchanged for decades or longer.

So landfill sites fill up, and land is used that could be used for something else.

When answering, that dual role is the point worth making: the inertness is simultaneously the advantage and the problem.

Burning them

Burning waste plastic recovers energy, which can generate electricity. But there are real problems, and they need stating precisely.

It depends on the polymer. Complete combustion of poly(ethene) produces only carbon dioxide and water. It is not automatically toxic.

The genuine hazards are:

• Chlorine-containing polymers. Poly(chloroethene) releases hydrogen chloride, which is toxic and acidic, contributing to acid rain.
• Incomplete combustion. Any polymer burned with insufficient oxygen can produce poisonous carbon monoxide.
• Carbon dioxide. Burning any polymer releases it, and it is a greenhouse gas.

A blanket claim that burning plastics produces toxic gases is too broad to earn full marks. Name the polymer or the condition.

Recycling

Recycling avoids both landfill and burning, though sorting mixed plastics is difficult and not every polymer can be recycled economically.

What a good answer looks like

These questions ask you to explain problems, so link cause to consequence:

"Addition polymers are inert, so microorganisms cannot break them down, so they remain in landfill for a very long time and take up land."

That chain — property, then mechanism, then consequence — is what the marks follow.