IB Chemistry SLTopic 2 — From Bonding Models to MaterialsPaper 1 & 2Materials~10 min read
Polymers
Take a small molecule and join thousands of copies of it end to end, and you get a polymer — a molecule so large it behaves like a material rather than a chemical. Plastics, proteins, starch and DNA are all built this way.
📚 What you need to know
A polymer is a large molecule made by chemically joining many small molecules called monomers.
A polymer contains at least 50 repeating units, joined by strong covalent bonds.
Polymers are also called macromolecules because of their size.
One type of monomer gives a homopolymer; two or more types give a copolymer (nylon, proteins).
Typical properties: low density, unreactive, water-resistant, strong and durable.
Most synthetic polymers are non-biodegradable, which is why plastic pollution lasts.
Monomers and polymers
The two words are all you need to keep straight:
A monomer is the small starting molecule. “Mono” = one.
A polymer is the long chain built from many of them. “Poly” = many.
The units are held together by covalent bonds, which is why polymer chains are so tough — breaking the chain means breaking real bonds, not just intermolecular forces.
Thousands of separate monomer molecules become one enormous chain. Real polymers have at least 50 repeating units and often many thousands.
The 50 units figure is worth remembering. Below that we would just call it a large molecule. It is the sheer number of repeats that gives a polymer its material properties — the same chemistry, scaled up until it behaves like plastic.
One monomer or several
Homopolymers are made from a single type of monomer. Poly(ethene) is just ethene repeated; poly(chloroethene), better known as PVC, is just chloroethene repeated.
Copolymers are made from two or more different monomers. Nylon is a copolymer, and so are proteins — which are built from twenty different amino acids in whatever order the organism needs.
Why polymers are so useful
Every one of these properties comes back to the structure — long chains of covalently bonded atoms, loosely packed together:
Property
Why
Where it matters
Low density
The chains are loosely packed, with plenty of space between them
Lightweight packaging, car parts
Unreactive
C–C and C–H bonds are strong and chemically stable
Food containers, chemical storage
Water-resistant
The chains are mostly nonpolar, so water is not attracted to them
Bottles, waterproof clothing
Strong and durable
Strong covalent bonds run the length of every chain
Pipes, ropes, construction materials
Natural and synthetic polymers
Polymers were not invented by chemists — living things have been making them for a very long time.
Natural polymers are built by organisms from biological monomers. Proteins come from amino acids, starch from glucose, and DNA from nucleotides. DNA is a spectacular example: millions of nucleotides linked into a double helix, using just four different monomer units arranged in different orders.
Synthetic polymers are man-made, mostly from crude oil. Poly(ethene), PVC and nylon are everywhere precisely because they are durable, unreactive and water-resistant.
DNA as a polymer. It is worth pausing on this one. Four monomers — adenine, thymine, guanine and cytosine — joined in different sequences along a sugar–phosphate backbone. The order of the monomers is the information. That is the same principle as poly(ethene), just used to store a genome instead of making a carrier bag.
The environmental problem
The very thing that makes synthetic polymers useful is what makes them a problem. Because they are chemically stable and unreactive, they are also non-biodegradable — there is nothing in the environment that easily breaks those C–C bonds.
Plastics do not rot away naturally, so they accumulate in landfill and in the oceans.
The pollution is long-lasting — centuries, in many cases.
This has made plastic waste one of the major global environmental concerns.
There is a real tension here that exam questions like to probe. Durability is the property we want from a plastic pipe and the property we regret once the pipe is thrown away. Same chemistry, opposite verdict.
WORKED EXAMPLE
Explain why poly(ethene) is used for food packaging, and why this same choice creates an environmental problem.
Why it is usefulUnreactive, so it will not contaminate the food.Water-resistant and low density, so it keeps food dry and adds almost no weight.Why it is a problemThe same chemical stability means nothing in nature breaks it down.it is non-biodegradable, so it accumulates as long-lasting pollution
💡 Exam tip
Always link a property back to structure: long chains, strong covalent bonds, loose packing, nonpolar.
Know at least one natural and one synthetic example. DNA and poly(ethene) will cover most questions.
For environmental questions, name the reason: chemically stable → non-biodegradable.
You do not need to memorise full polymer structures — but you do need to recognise a repeating unit.
⚠️ Common mix-up
Monomer and polymer are not interchangeable. Ethene is the monomer; poly(ethene) is the polymer.
Polymers are molecules, not giant covalent structures. Each chain is a separate molecule, held to its neighbours by intermolecular forces.
“Plastic” is not a synonym for “polymer”. Plastics are one family of synthetic polymers; DNA is a polymer and definitely not a plastic.
Non-biodegradable does not mean indestructible. It means organisms cannot break it down.
Up next: Addition Polymerisation — the actual reaction that turns alkene monomers into polymer chains, and how to work backwards from a polymer to find its monomer.
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