IB Chemistry HLTopic 2 — From Models to MaterialsPaper 1 & 2Organic~13 min read
Condensation Polymers (HL)
Addition polymerisation was tidy: everything went in, everything came out. Condensation polymerisation is messier and far more interesting. Every time two monomers join, a small molecule is thrown out — and the link left behind is one that water can later attack. That single difference is why your jumper is nylon and why your body can digest a protein.
📘 What you need to know
In condensation polymerisation, monomers join and a small molecule is released each time a link forms — usually water, sometimes hydrogen chloride.
Each monomer must have two reactive functional groups, one at each end, or the chain cannot keep growing.
An ester link, –COO–, forms between an –OH group and a –COOH group. The polymer is a polyester.
An amide link, –CONH–, forms between an –NH2 group and a –COOH group. The polymer is a polyamide. In proteins it is called a peptide link.
Monomer combinations: diol + dicarboxylic acid, diamine + dicarboxylic acid, or a single monomer carrying both groups (a hydroxycarboxylic acid or an amino acid).
Ester and amide links can be hydrolysed, so condensation polymers can be broken back down. Addition polymers cannot.
Examples: PET (a polyester), nylon-6,6 (a polyamide), and every protein in your body.
What “condensation” means here
The word is borrowed from the idea of water condensing out. In a condensation reaction, two molecules join and something small drops out of the join — almost always water. Take an –OH group on one molecule and a –COOH on another: the H from the –OH and the whole –OH from the acid leave together as H2O, and the two fragments left behind bond to each other.
Do that once and you have made an ester. The trick that turns it into a polymer is giving each monomer two functional groups, one at each end. Then, as soon as one end has reacted, the other end is still free and the chain keeps going in both directions.
Think of each monomer as a person with two hands. One hand shakes, the chain grows by one. Two hands means the chain can extend indefinitely. A monomer with only one functional group is a chain-stopper — it joins on and the growth ends there.
Making a polyester
The commonest route is a diol (two –OH groups) plus a dicarboxylic acid (two –COOH groups). Each join releases one molecule of water and leaves an ester link, –COO–, in the backbone.
The best known polyester made this way is PET, used for drinks bottles and for the fabric sold as polyester. The oxygen atoms sitting in the backbone are exactly what water can later attack.
Making a polyamide
Swap the diol for a diamine and you get a polyamide instead. The –NH2 group loses a hydrogen, the –COOH group loses its –OH, water leaves, and an amide link –CONH– is left in the chain. Nylon-6,6 is made exactly like this.
Nylon-6,6 gets its name from the monomers: six carbons in the diamine and six in the diacid. Knowing that is not required, but it explains the odd-looking name.
A faster route: industry often uses a diacyl chloride in place of the dicarboxylic acid. The –COCl groups are far more reactive, so the reaction is quicker, but the small molecule released is hydrogen chloride rather than water, and the starting material costs more.
When one monomer is enough
You do not always need two different monomers. If a single molecule carries both functional groups, it can react with copies of itself.
A hydroxycarboxylic acid has an –OH group and a –COOH group, so it polymerises into a polyester on its own. Poly(lactic acid), the compostable plastic used in food packaging, is made this way.
An amino acid has an –NH2 group and a –COOH group, so it polymerises into a polyamide. Chain up twenty different amino acids in a specific order and you have a protein.
The two links to know
–OH + –COOH → ester link –COO– + H2O
–NH2 + –COOH → amide link –CONH– + H2O
Why these polymers can break down
Condensation polymerisation removes water. Run it backwards, put water back in, and the link comes apart — that is hydrolysis. Ester and amide links are both vulnerable to it, especially with acid, alkali or an enzyme to help.
This is the deep difference between the two families. An addition polymer’s backbone is nothing but C–C and C–H bonds, which are strong and non-polar with nothing for water to attack. A condensation polymer’s backbone has a polar carbon–oxygen or carbon–nitrogen group built into it at regular intervals, and each one is a place where the chain can be cut.
Your digestive system does exactly this every day: enzymes hydrolyse the peptide links in the protein you eat, turning one long chain back into individual amino acids. Chemistry lessons and biology lessons are describing the same reaction.
Feature
Addition polymerisation
Condensation polymerisation
Monomer needs
A C=C double bond
Two functional groups per monomer
By-product
None — one product only
A small molecule, usually water
Link formed
C–C single bond
Ester –COO– or amide –CONH–
Atom economy
100%
Below 100%, because the small molecule is lost
Can it be hydrolysed?
No
Yes, which is why some are biodegradable
Examples
Poly(ethene), PVC, PTFE
PET, nylon-6,6, proteins
Worked examples
WORKED EXAMPLE
A section of a polymer is —CO—(CH2)4—CO—O—(CH2)2—O—. Identify the two monomers.
Step 1: find the link
—CO—O— is an ester link, so this is a polyester made from a diol and a diacid.
Step 2: cut at each ester link and add back what left
Put an —OH on each carbonyl carbon, and an H on each oxygen.
Step 3: write the two monomersHOOC—(CH₂)₄—COOHHO—(CH₂)₂—OHHexanedioic acid and ethane-1,2-diolcount the carbons in the acid: four in the chain plus the two carbonyls makes six
WORKED EXAMPLE
500 diol molecules and 500 dicarboxylic acid molecules join into one alternating chain. How many water molecules are released?
Step 1: count the monomers in the chain500 + 500 = 1000 monomer unitsStep 2: count the links between them
A chain of 1000 beads has 999 joins.
1000 − 1 = 999 ester linksStep 3: one water per link999 molecules of waterfor a long chain this is effectively one water per monomer, which is the approximation usually used
WORKED EXAMPLE
Explain why a nylon rope slowly weakens in hot alkaline water but a poly(ethene) rope does not.
Step 1: identify the links in each
Nylon has amide links, —CONH—, in the backbone. Poly(ethene) has only C—C and C—H bonds.
Step 2: say what alkali can do
Amide links are hydrolysed by hot alkali, cutting the chains into shorter pieces.
Step 3: link chain length to strength
Shorter chains tangle less and have weaker forces between them, so the fibre loses strength.
Nylon has hydrolysable links; poly(ethene) has none, so it is unaffectedthe same reasoning explains why some condensation polymers are biodegradable
💡 Exam tip
Always state the small molecule released by name. “Water is eliminated” is often worth a mark on its own.
Draw the link clearly: –COO– has the double bond O on the carbon, and –CONH– has an H on the nitrogen. Missing the H is a common loss.
When asked to deduce monomers, cut at the link and add back what left — an H on one side, an –OH on the other.
Check both monomers you write down have two functional groups. If one has only one, you have cut in the wrong place.
If a question mentions a diacyl chloride, the by-product is HCl, not water.
For “compare addition and condensation”, structure your answer around three things: what the monomer needs, what is released, and whether the product can be hydrolysed.
⚠ Common mix-up
Forgetting the by-product. If nothing leaves, it was not a condensation reaction.
Using a monomer with only one functional group. The chain would stop after one join.
Mixing up ester and amide links. Oxygen in the link means ester; nitrogen means amide.
Saying all condensation polymers are biodegradable. They can be hydrolysed; how fast that happens in practice varies enormously.
Assuming a polyester must come from two monomers. A hydroxycarboxylic acid does the job alone.
Calling the peptide link a different kind of bond. It is an amide link — the biologists just use another name for it.
Up next: Features of Dynamic Equilibrium — you have spent this topic asking what substances are made of. The next one asks how far a reaction actually goes, and why so many of them stop before finishing.
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