Polymerisation: Building Big Molecules from Small Ones

students, in this lesson you will learn how chemists turn small molecules called monomers into giant molecules called polymers. This is a key idea in organic chemistry and in the IB Chemistry HL topic Reactivity 3 — What Are the Mechanisms of Chemical Change?. Polymerisation helps explain how the structure of a molecule controls its reactivity, how reaction mechanisms work, and why chemistry can create materials with very different properties from their starting substances 🌟.

What you will learn

Explain what polymerisation means and use the correct terms: monomer, polymer, repeat unit, and chain growth.
Describe the two main types of polymerisation: addition polymerisation and condensation polymerisation.
Use reaction ideas to predict the products formed from common monomers.
Connect polymerisation to organic mechanisms and to the wider idea of making useful materials.
Use real examples such as plastics, nylon, and proteins to understand why polymerisation matters in everyday life.

What is polymerisation?

Polymerisation is the chemical process in which many small molecules, called monomers, join together to form a very large molecule called a polymer. The word comes from “poly,” meaning many, and “mer,” meaning parts. A polymer can contain hundreds or thousands of repeating units linked in a chain or network.

A key idea for students is that polymerisation is not just “making a bigger molecule.” It is about a reaction mechanism that creates a new substance with different properties. For example, the monomer ethene is a gas, but poly(ethene) is a solid plastic used in bags and packaging. The chemical change alters melting point, strength, flexibility, and durability.

In IB Chemistry HL, polymerisation fits into Reactivity 3 because it shows how molecules react through specific pathways. The mechanism matters: some polymerisations happen by opening double bonds, while others happen by joining two functional groups and releasing a small molecule such as water.

Addition polymerisation: joining alkene monomers

Addition polymerisation happens when unsaturated monomers, usually alkenes, join together. The $\mathrm{C=C}$ double bond is important because it can open up and allow monomers to connect into a long chain.

A simple example is the polymerisation of ethene to poly(ethene):

$$n\,\mathrm{CH_2=CH_2} \rightarrow [-\mathrm{CH_2-CH_2}-]_n$$

Here, $n$ means a large number of monomer molecules. The bracketed part is the repeat unit, and the subscript $n$ shows that it repeats many times.

How the mechanism works

Addition polymerisation often follows a chain reaction mechanism with three stages:

Initiation – a reactive species is formed, often a free radical.
Propagation – the reactive species adds to monomers again and again.
Termination – the chain stops growing when radicals combine or are otherwise removed.

A common industrial example uses peroxide initiators. The peroxide breaks to form radicals, which attack the alkene double bond. The double bond opens, creating a new radical at the end of the chain. That radical then attacks another monomer, and the chain grows.

This mechanism is useful because it explains why many addition polymers can be made quickly and efficiently. It also helps explain why the polymer structure is often similar to the starting monomer, apart from the loss of the $\mathrm{C=C}$ bond.

Real-world examples

Poly(ethene) is used in plastic bags, bottles, and film.
Poly(propene) is used in ropes, containers, and fibres.
Poly(chloroethene), also called PVC, is used in pipes and insulation.

These materials are chosen because they are lightweight, resistant to water, and easy to shape. However, many are not biodegradable, so their persistence in the environment is a major issue 🌍.

Condensation polymerisation: making polymers with small molecule loss

Condensation polymerisation is different from addition polymerisation because each link formed between monomers usually produces a small molecule as a by-product, often water or hydrogen chloride.

This type of polymerisation usually involves monomers with two functional groups. The functional groups react with each other, forming a new covalent bond while eliminating a small molecule.

Example: polyesters

A common polyester is made from a diol and a dicarboxylic acid. One example is ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

When these monomers react, ester links form. The repeated linking and loss of water molecules produces a polymer chain.

Example: polyamides

Polyamides such as nylon are formed from a diamine and a dicarboxylic acid, or from a single monomer containing both functional groups. During the reaction, amide links form and water is eliminated.

A simplified idea of the bond formation is:

$$\mathrm{-COOH + H_2N- \rightarrow -CONH- + H_2O}$$

This is a useful way to remember that condensation polymerisation depends on functional group chemistry. In IB terms, it connects directly to understanding organic mechanisms and reaction patterns.

Why condensation polymers matter

Polyesters are used in clothing fibres and bottles.
Polyamides are used in textiles, fishing line, and engineering materials.
Some condensation polymers are strong because of hydrogen bonding between chains, which increases tensile strength.

This links to the idea that molecular structure affects macroscopic properties. Small changes in functional groups can create a material that is tougher, more elastic, or more heat-resistant.

Structure, properties, and mechanisms

Polymerisation is a great example of how chemical structure controls properties. students, remember that the same atoms can behave very differently depending on how they are connected.

Why polymers have special properties

Polymers are large, long-chain molecules, so their properties depend on:

chain length
branching
intermolecular forces
cross-linking
presence of polar groups

For example, poly(ethene) is non-polar, so its chains are held together mainly by London dispersion forces. That makes it flexible and relatively low-melting compared with ionic solids. In contrast, polyamides can form hydrogen bonds between chains, making them stronger and less easily stretched.

Mechanistic thinking in IB Chemistry HL

The IB course expects you to explain reactions using mechanisms, not just memorise products. For polymerisation, that means being able to say:

what kind of monomer is used
what bond or functional group reacts
whether a small molecule is eliminated
how the chain grows
why the product has the observed properties

For addition polymerisation, the double bond is the reactive site. For condensation polymerisation, the functional groups are the reactive sites. This is the same kind of thinking used across organic reaction pathways in Reactivity 3.

Comparing addition and condensation polymerisation

It is very helpful to compare the two main types.

| Feature | Addition polymerisation | Condensation polymerisation |

|---|---|---|

| Monomers | Usually alkenes | Usually monomers with two functional groups |

| Small molecule by-product | None | Yes, often water |

| Key reacting site | $\mathrm{C=C}$ bond | Functional groups such as $\mathrm{-OH}$, $\mathrm{-COOH}$, $\mathrm{-NH_2}$ |

| Type of mechanism | Chain growth | Step growth |

| Examples | Poly(ethene), PVC, poly(propene) | Nylon, polyesters, proteins |

This comparison helps you organise the topic clearly. Addition polymerisation often involves radicals and chain growth. Condensation polymerisation is often stepwise and depends on functional group reactions.

Links to biology, industry, and the environment

Polymerisation is not just a plastics topic. Nature also uses polymerisation. Proteins are polymers made from amino acids, and DNA is a polymer made from nucleotides. These biological polymers are made through condensation reactions, which shows that the same chemical principles apply in living systems too 🧬.

In industry, polymerisation allows the production of materials with specific properties for packaging, clothing, medical devices, and construction. Chemists can design monomers and reaction conditions to change flexibility, strength, or resistance to chemicals.

There are also environmental questions. Many synthetic polymers are very stable, so they do not break down easily. Recycling, reuse, and alternative materials are important because large-scale polymer use creates waste and pollution. Understanding polymerisation helps explain why these materials are persistent and why their disposal is challenging.

Conclusion

Polymerisation is the process of making large molecules from many smaller monomers. In addition polymerisation, alkene double bonds open and link together without forming by-products. In condensation polymerisation, functional groups react and a small molecule such as water is eliminated. students, the important IB Chemistry HL idea is that polymerisation is a mechanism-based process: the way atoms and bonds change determines the properties of the final material. This makes polymerisation a central example of Reactivity 3 because it shows how chemical change can be explained, predicted, and applied in the real world.

Study Notes

Polymerisation is the formation of a polymer from many monomers.
A monomer is a small molecule that can react to form a chain.
A polymer is a very large molecule made of repeating units.
In addition polymerisation, alkene double bonds open and link monomers with no small molecule by-product.
In condensation polymerisation, monomers with two functional groups form bonds and eliminate a small molecule, often water.
The repeat unit is the part of the polymer structure that repeats many times.
Addition polymerisation often follows a chain mechanism with initiation, propagation, and termination.
Condensation polymerisation is often a step-growth process.
Polymer properties depend on chain length, branching, intermolecular forces, and cross-linking.
Poly(ethene), PVC, nylon, and polyesters are important examples.
Polymerisation connects to Reactivity 3 because it shows how reaction mechanisms explain chemical change and material properties.
Natural polymers such as proteins and DNA show that polymerisation is also important in biology.