IB Chemistry SLTopic 2 — From Models to MaterialsPaper 1 & 2Materials~11 min read
Polymers
A polymer is not a complicated idea. It is one molecule, made by joining thousands of identical small ones end to end, like beads threaded on a string. Everything odd about plastics — why they are light, why they float, why they last for centuries in a landfill — comes from that single fact about their size.
📘 What you need to know
A polymer is a very large molecule made by covalently joining many small molecules called monomers.
Polymers are also called macromolecules, and normally contain at least 50 repeating units.
The repeat unit is the smallest section that repeats along the chain. Its formula is written in brackets with a subscript n.
A homopolymer is built from one type of monomer; a copolymer from two or more.
Natural polymers include starch, cellulose, proteins and DNA. Synthetic polymers include poly(ethene), PVC, nylon and PET.
Their properties — low density, unreactive, waterproof, electrically insulating, strong — all follow from long covalent chains with only weak forces between them.
Most addition polymers do not break down naturally, which is why plastic waste accumulates.
What makes a molecule a polymer
Ethene, C2H4, is a gas you could hold in a syringe. Join ten thousand ethene units together and you get poly(ethene): a waxy solid you can make a shopping bag from. Nothing new was added and nothing was taken away — the atoms simply became part of one enormous molecule instead of ten thousand tiny ones.
That change in size is what changes the properties. Small molecules are easy to pull apart from each other, so they are gases or runny liquids. A chain thousands of atoms long tangles with its neighbours and has a huge surface in contact with them, so the forces between chains add up to something substantial. The result is a solid.
The chain is drawn with open ends on purpose. A polymer sample contains chains of many different lengths, which is why a polymer has no single fixed relative molecular mass.
Monomer, repeat unit, and that little n
Three words get confused constantly, so pin them down now.
The monomer is the small molecule you start with, before any reaction.
The repeat unit is the section of the finished chain that repeats. It is what goes inside the square brackets.
n is simply how many repeat units there are — often thousands.
How a polymer is writtenn × monomer → [ repeat unit ]n
The monomer and the repeat unit are usually made of the same atoms but joined slightly differently. Getting from one to the other is a standard exam skill, and it is covered properly on the next two pages.
One monomer or several
If every monomer in the chain is the same, the polymer is a homopolymer. Poly(ethene) and PVC are homopolymers — one ingredient, repeated.
If two or more different monomers alternate along the chain, it is a copolymer. Nylon is the classic synthetic example, and proteins are the natural one: your body strings together twenty different amino acid monomers in whatever order the gene specifies.
Proteins are the reason copolymers matter. Change the order of the monomers and you change the whole molecule’s shape and job. Same trick, wildly different outcome — which is worth remembering when biology comes up in a chemistry paper.
Natural and synthetic polymers
Polymer
Natural or synthetic
Monomer
Where you meet it
Starch and cellulose
Natural
Glucose
Food storage in plants; plant cell walls
Proteins
Natural
Amino acids
Enzymes, muscle, hair, antibodies
DNA
Natural
Nucleotides
Genetic information in every cell
Poly(ethene)
Synthetic
Ethene
Bags, bottles, packaging film
Nylon
Synthetic
A diamine and a dicarboxylic acid
Ropes, clothing, engineering parts
Where the properties come from
Every property in the list below comes from the same two facts: the bonds along the chain are strong covalent bonds, and the forces between chains are weak.
The dashed lines are the weak forces. There are a lot of them along a chain thousands of atoms long, which is how something held together by weak forces can still make a solid object.
Property
Reason
Low density
Long tangled chains cannot pack tightly, so a lot of the volume is empty space
Unreactive
The chain is mostly strong C–C and C–H bonds with no polar sites for reagents to attack
Waterproof
Non-polar chains do not attract water molecules, so water is not absorbed
Strong and flexible
Strong covalent bonds along the chain, plus chains that can slide and uncoil past each other
Electrical insulator
No delocalised electrons and no free ions anywhere in the structure
The problem with lasting forever
Being unreactive is exactly what makes a plastic useful and exactly what makes it a problem. Most addition polymers have no bonds that water, air or enzymes can attack, so they are non-biodegradable: they break into smaller and smaller pieces without ever chemically breaking down. A bag made this year can still be recognisable in a century.
A hint of what is coming: polymers that contain ester or amide links in the chain can be attacked by water, so some of them do break down. That is one of the big practical differences between addition and condensation polymers.
Worked examples
WORKED EXAMPLE
A poly(ethene) chain contains 10 000 repeat units. Estimate its relative molecular mass.
The repeat unit is C2H4. Use Ar(C) = 12.01 and Ar(H) = 1.01.
Step 1: mass of one repeat unit(2 × 12.01) + (4 × 1.01) = 24.02 + 4.04 = 28.06Step 2: multiply by the number of units28.06 × 10 000 = 280 600Mr ≈ 2.81 × 105the two end groups add a couple of extra atoms, which is nothing next to 280 000 — ignore them
WORKED EXAMPLE
A sample of poly(propene) has an average Mr of 1.40 × 105. How many repeat units are in an average chain?
The repeat unit is C3H6.
Step 1: mass of one repeat unit(3 × 12.01) + (6 × 1.01) = 36.03 + 6.06 = 42.09Step 2: dividen = 140 000 ÷ 42.09 = 3326about 3.3 × 103 repeat unitsthe word “average” matters — a real sample has chains of many different lengths
WORKED EXAMPLE
Classify each of these as natural or synthetic, and as a homopolymer or a copolymer: PVC, silk, poly(propene).
PVC
Made in industry from chloroethene only.
synthetic homopolymerSilk
A protein, made by an organism from a mixture of different amino acids.
natural copolymerPoly(propene)
Made in industry from propene only.
synthetic homopolymerCount the different monomers, then ask who made it
💡 Exam tip
When explaining any polymer property, name both kinds of force: strong covalent along the chain, weak forces between chains.
“Made of long chain molecules” is not an explanation on its own. Say what the chains do — tangle, slide, pack badly.
Always put brackets in the name of an addition polymer: poly(ethene), not polyethene, in an IB answer.
For mass calculations, work with the repeat unit and ignore the end groups. Say that you are ignoring them.
If asked why a polymer has no single Mr, the answer is that chain lengths vary within a sample.
Link “unreactive” to “non-biodegradable”. Examiners like seeing that the useful property and the environmental problem have the same cause.
⚠ Common mix-up
Confusing monomer with repeat unit. They contain the same atoms but are bonded differently — they are not interchangeable words.
Saying covalent bonds break when a plastic melts. Melting only separates the chains from one another.
Thinking a polymer is a giant covalent structure like diamond. It is not: it is a lot of separate very long molecules, with weak forces between them.
Calling every polymer a plastic. DNA, silk and cellulose are polymers and none of them is a plastic.
Writing a formula without the n. Without it you have drawn a small molecule, not a polymer.
Saying plastics are “too strong” to break down. The reason is lack of reactive sites, not strength.
Up next: Addition Polymerisation — the simplest way of making a polymer, where a double bond opens up, the monomers link, and absolutely nothing is left over.
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