State that plastics are made from polymers

Organic Chemistry – Polymers (Cambridge IGCSE 0620)

Learning objective (AO1)

• State that **plastics are made from polymers** – large macromolecules formed from many repeating monomer units.

What is a polymer?

A **polymer** is a macromolecule consisting of a long chain of repeating structural fragments called the repeat unit. The repeat unit is derived from a smaller molecule known as a **monomer**. The chemical reaction that joins monomers together is **polymerisation**.

Key terminology (AO1)

• Monomer – a simple molecule that can react with identical or complementary molecules to give a polymer.
• Repeat unit – the structural fragment that repeats along the polymer chain; it appears in the IUPAC name of the polymer.
• Polymerisation – the reaction that links monomers. Two main types are addition (chain‑growth) and condensation (step‑growth).
• Plastic – a material whose bulk is mainly a synthetic polymer, usually mixed with additives (plasticisers, stabilisers, fillers) to give the required properties.

Polymerisation types (core requirement)

1. Addition (chain‑growth) polymerisation

• Monomers contain a carbon–carbon double bond (or another unsaturated group).
• The double bond opens and adds to a growing chain one monomer at a time.
• Typical conditions: heat or pressure, a radical or coordination catalyst (e.g. Ziegler–Natta or metallocene for PE and PP).
• By‑product: none – the whole monomer becomes part of the polymer chain.

2. Condensation (step‑growth) polymerisation

• Two different monomers (or a bifunctional monomer) react; each step eliminates a small molecule, most often water.
• Polymer chains grow by the stepwise linking of any two reactive ends.
• Typical conditions: high temperature, an acid or metal‑oxide catalyst (e.g. antimony trioxide for PET).
• By‑product examples:
  • Poly(ethylene terephthalate) (PET) – water is removed continuously.
  • Nylon‑6,6 – water is removed continuously.

Catalysts and why they are needed (AO2)

• Ziegler–Natta / metallocene catalysts – give control over chain growth and stereochemistry, producing isotactic or atactic polymers (important for PE and PP).
• Free‑radical initiators (e.g. peroxides, benzoyl peroxide) – generate radicals that start the chain reaction for PVC and polystyrene.
• Antimony trioxide (Sb₂O₃) or titanium catalysts – accelerate the esterification step in PET and help remove water efficiently.
• Acid catalysts (e.g. phosphoric acid) – protonate carbonyl groups in nylon synthesis, increasing the rate of condensation.

Core synthetic addition polymers (required)

Supplementary condensation polymers (optional)

Natural polymers (supplementary)

• Proteins – polymers of amino‑acid monomers linked by peptide bonds (e.g., wool, silk).
• Cellulose – polymer of glucose units joined by β‑1,4‑glycosidic bonds; main component of plant cell walls.
• These are listed under the *supplementary* section of the Cambridge syllabus and are not part of the core requirement for plastics.

Properties of polymers and their structural origins (core)

• **Molecular weight / chain length** – longer chains give higher tensile strength and higher melting points.
• **Degree of branching** – highly branched polymers (e.g., low‑density PE) are more flexible, whereas linear polymers are more crystalline and rigid.
• **Presence of polar groups** – polymers containing –Cl, –COO– or –NH– groups (e.g., PVC, PET, nylon) are more water‑resistant and have higher boiling points.
• **Additives** – plasticisers increase flexibility; fillers increase stiffness; stabilisers improve resistance to heat or UV.

Environmental impact of plastics (core syllabus wording)

• Disposal methods – land‑fill, recycling and incineration are the three main ways plastics are dealt with.
• Challenges – most plastics are not biodegradable, persisting for centuries in landfill; incineration releases CO₂ and, for PVC, toxic HCl gas unless properly filtered.
• Marine pollution – lightweight fragments become micro‑plastics that accumulate in oceans and harm wildlife.
• Mitigation strategies – recycling, reduction of single‑use items, and development of biodegradable polymers are encouraged in the syllabus.

Practical investigation (AO3 example)

**Determine the density of a polymer sample** – By measuring mass and volume (via water displacement), students can infer the degree of crystallinity: a higher density usually indicates a more crystalline (and therefore more rigid) polymer. This experiment links structure to property and satisfies the experimental component of AO3.

Why plastics are useful (core)

• Properties can be tailored by altering chain length, branching, and by adding fillers or plasticisers.
• Resulting materials may be:
  • Flexible (PE)
  • Rigid (PVC)
  • Transparent (PS)
  • Strong and wear‑resistant (nylon)
  • Chemically resistant (PET)
• They can be shaped easily by moulding, extrusion, blow‑moulding or casting.

Link to Cambridge IGCSE assessment objectives

• AO1 – Knowledge & understanding: define polymer, monomer, repeat unit; state that plastics are made from polymers.
• AO2 – Application: identify the type of polymerisation from a given reaction; choose appropriate catalysts, conditions and recognise by‑products.
• AO3 – Analysis: explain why a particular polymer is suited to a specific use; discuss environmental implications; carry out a simple practical (e.g., density determination) and interpret the results.

Summary

Plastics are primarily synthetic polymers – large macromolecules built from many repeating monomer units. Through addition or condensation polymerisation, chemists produce a wide range of polymers whose physical properties can be engineered for specific applications. Understanding the monomers, repeat units, catalysts, reaction conditions, properties and environmental considerations is essential for the Cambridge IGCSE Chemistry syllabus.