Properties, Uses, and Historical Context of Addition Polymers Revision Notes | A Level AQA Chemistry

1. Why Addition Polymers Matter

Plastics are everywhere — packaging, pipes, clothing, medical devices. Almost all of them are addition polymers. Understanding why they behave the way they do, and why some are rigid while others are stretchy, comes down to bonding and intermolecular forces.

This lesson covers three tightly linked ideas: the chemical unreactivity of polyalkenes, the intermolecular forces that govern their physical behaviour, and the specific properties and uses of PVC — including how a plasticiser changes everything. The historical context of how this knowledge developed is woven throughout, because AQA expects you to appreciate that polymer science evolved over time rather than appearing fully formed.

2. From Alkene to Polymer — A Quick Structural Recap

When an alkene undergoes addition polymerisation, the $\pi$ bond in each C=C breaks and the monomers link together to form a long chain of C-C single bonds. No atoms are lost — every atom in the monomer ends up in the polymer. The result is a backbone made entirely of C-C $\sigma$ bonds, with hydrogen atoms (and any substituents) attached along the sides.

The structure above represents a short section of a polyalkene chain. Notice there are no double bonds remaining — the $\pi$ bond has been consumed entirely during polymerisation. This structural feature is the key to understanding why addition polymers are so chemically unreactive.

3. Why Addition Polymers Are Unreactive

Alkenes are reactive precisely because of their $\pi$ bond — it is a region of high electron density that attracts electrophiles and allows addition reactions to occur. Once polymerisation is complete, that $\pi$ bond no longer exists. The polymer backbone consists entirely of strong C-C $\sigma$ bonds and C-H bonds.

C-C bonds have a bond enthalpy of approximately 347 kJ mol$^{-1}$ and C-H bonds approximately 413 kJ mol$^{-1}$. Both are strong and require significant energy to break. Neither bond presents a site of high electron density that would attract an electrophile, nor a site of low electron density that would attract a nucleophile.

This unreactivity is enormously useful in practice — a poly(ethene) bag does not corrode, react with food, or degrade in acid or alkali. However, it also creates a significant environmental problem: addition polymers resist biodegradation and persist in the environment for hundreds of years. AQA expects you to appreciate both sides of this.

4. Intermolecular Forces Between Polyalkene Chains

Individual polymer chains do not exist in isolation — in a solid polymer, millions of chains are packed together. The forces between those chains determine whether the material is hard or soft, rigid or flexible, high-melting or low-melting. This is where intermolecular forces become central.

For a simple polyalkene such as poly(ethene), the chains are non-polar. The only intermolecular forces present are London dispersion forces (also called induced dipole-induced dipole forces). These arise from temporary, instantaneous dipoles caused by the movement of electrons within the chain.

London dispersion forces increase with the length of the polymer chain and with the surface area of contact between chains. Longer chains have more electrons and greater surface area, so the cumulative London dispersion forces across the entire chain are substantial — even though each individual interaction is weak. This is why poly(ethene) is a solid at room temperature despite being made from a gas (ethene).

PVC has stronger intermolecular forces than LDPE because the polar C-Cl bonds create permanent dipole-dipole attractions, while LDPE also has weaker London forces due to branching. Comparing PVC with HDPE gives a cleaner contrast in intermolecular-force type.

5. Poly(chloroethene) — PVC: Properties and Uses

Poly(chloroethene) — universally known as PVC (polyvinyl chloride) — is one of the most widely produced plastics in the world. Its monomer is chloroethene ($\text{CH}_2\text{=CHCl}$), and the repeating unit is $\text{-[CH}_2\text{-CHCl]-}$. The presence of the chlorine atom on every other carbon gives PVC a set of properties quite distinct from poly(ethene).

These properties make unplasticised PVC (uPVC) ideal for window frames, guttering, drain pipes, and door frames — applications that demand rigidity, weather resistance, and longevity. The material does not rot, corrode, or react with rainwater or mild acids, and it maintains its shape under load.

5a. How a Plasticiser Modifies PVC

Rigid PVC is not suitable for every application. Electrical cable insulation, flooring, clothing, and medical tubing all require a flexible material. The solution is to add a plasticiser — a small molecule that is mixed into the PVC during processing.

Plasticiser molecules position themselves between the PVC chains. Because they are small and mobile, they increase the average separation between chains. This reduces the strength of the permanent dipole-dipole interactions and London dispersion forces between adjacent PVC chains, allowing the chains to slide past one another more readily.

The macroscopic result is a softer, more flexible material with a lower glass transition temperature. The greater the proportion of plasticiser added, the more flexible the PVC becomes. This is why the PVC used in a garden hose feels completely different from the PVC used in a window frame — the chemistry is the same, but the plasticiser content is very different.

Worked example: Explaining the Effect of a Plasticiser on PVC — Exam Walkthrough

Explain how adding a plasticiser to PVC changes its properties. [4 marks]

1. A plasticiser is a small molecule added to PVC during processing. — [B1] — establish what the plasticiser is before explaining what it does.
2. The plasticiser molecules position themselves between the PVC polymer chains. — [B1] — this is the key structural point. The plasticiser does not break bonds within chains; it acts between chains.
3. This increases the separation between PVC chains, reducing the strength of the intermolecular forces (permanent dipole-dipole interactions and London dispersion forces) between them. — [B1] — name the specific intermolecular forces for full credit at A-Level.
4. The chains can now slide past one another more easily, making the PVC softer and more flexible. — [B1] — link the molecular explanation to the observable property change.

Answer: A plasticiser is a small molecule that sits between PVC chains, increasing chain separation and reducing the strength of intermolecular forces (dipole-dipole and London dispersion forces). This allows chains to slide past one another more readily, making the PVC softer and more flexible.

6. The Historical Development of Polymer Knowledge

AQA's specification asks you to appreciate that knowledge of polymer production and properties has developed over time. This is not a straightforward point — it reflects how science actually works: through incremental discovery, revision of ideas, and the interplay between industrial need and academic research.

PVC was first synthesised accidentally in the 19th century, but its commercial potential was not recognised until the 1920s and 1930s. Early chemists did not fully understand the macromolecular nature of polymers — the idea that plastics were made of very long chain molecules was controversial until Hermann Staudinger's work in the 1920s established the macromolecular hypothesis. Staudinger was awarded the Nobel Prize in Chemistry in 1953 for this contribution.

The development of Ziegler-Natta catalysts in the 1950s (also Nobel Prize-winning work) allowed chemists to control the stereochemistry of polymer chains, producing materials with very different properties from the same monomer. This demonstrated that understanding the relationship between molecular structure and bulk properties was not static — each advance in synthesis or characterisation technique opened new possibilities.

More recently, concerns about the environmental persistence of addition polymers have driven research into biodegradable alternatives and improved recycling technologies. The understanding that unreactivity — once seen as purely advantageous — creates long-term environmental problems is itself a development in scientific knowledge. This evolution of understanding is exactly what AQA means when it asks you to 'appreciate' the historical context.

7. Comparing Addition Polymers — Properties and Applications

Now that the underlying principles are established, it is worth comparing several common addition polymers directly. The differences in properties arise from differences in chain structure, substituents, and the resulting intermolecular forces — exactly the reasoning you need to apply in an exam.

Notice the pattern: wherever a substituent introduces polarity (as with Cl in PVC), the intermolecular forces are stronger, the melting point is higher, and the material is harder. Wherever chains are long and pack closely together (as in HDPE), London dispersion forces accumulate to give a rigid material even without polar groups. Branching in LDPE prevents close packing, so the London dispersion forces are weaker and the material is softer and more flexible.

8. Exam Technique and Common Mistakes

Questions on addition polymers appear regularly in AQA papers, often as 'explain' or 'evaluate' questions worth 3-6 marks. The marking schemes reward precise chemical reasoning — vague answers about polymers being 'strong' or 'stable' will not score well. Here is how to approach the most common question types.

The structural reason matters: once the alkene pi bond has opened, the polymer backbone is made of C-C and C-H sigma bonds, and the side groups then control many of the material properties.