Polymers are very long molecules made by joining thousands of small repeating units called monomers. The process of joining monomers together is called polymerisation. Most plastics are synthetic polymers — human-made materials with properties that can be engineered for specific purposes, from flexible carrier bags to rigid aircraft components.
What is a monomer and what is a polymer?
Before we think about reactions, let's build the mental picture. A monomer is a small molecule that can join to others of the same type — or different types — to form a long chain. A polymer is the very long molecule that results: "poly" means many, and "mer" means part, so a polymer is literally a "many-part" molecule.
A helpful analogy: monomers are like individual beads, and a polymer is the long bead necklace formed by linking thousands of them together. Because the chain can contain thousands to millions of monomer units, polymers behave very differently from their individual monomers — they are far less volatile, often stronger, and can form films, fibres, or rigid shapes.
Polymers are not just synthetic inventions. Natural polymers include:
- Starch and cellulose — both polymers of glucose monomers (differing only in how the glucose units are linked)
- Proteins — polymers of amino acid monomers
- DNA — a polymer of nucleotide monomers
- Natural rubber — a polymer of isoprene monomers
Synthetic polymers include poly(ethene), poly(propene), PVC, nylon, and polyester.
How does addition polymerisation work?
Addition polymerisation is the reaction in which monomers containing a carbon–carbon double bond (C=C) join together. The double bond "opens up" and links one monomer to the next. Because the only product is the long polymer chain — nothing else is released — it is called addition polymerisation.
Numbered worked example — making poly(ethene):
- Start with the ethene monomer: H₂C=CH₂. It has a C=C double bond, making it an alkene.
- Under high pressure (~200 atm) and with a catalyst, the double bond begins to open.
- Each ethene unit bonds to the next: …–CH₂–CH₂–CH₂–CH₂–…
- Thousands of units join to form poly(ethene): –(CH₂CH₂)ₙ– where n can be thousands.
- The resulting polymer contains no double bonds — it is a saturated, long-chain alkane. It is flexible, lightweight, and does not react with most chemicals.
A second example: propene (CH₃–CH=CH₂) polymerises to form poly(propene) — represented as –(CH₃–CH–CH₂)ₙ–. The conditions required are a catalyst, often with heat and pressure. Like poly(ethene), the product has no remaining double bonds.
What are the properties of common synthetic polymers?
The properties of a polymer depend on the monomer it was made from, the length of the chains, and any additional treatment (such as adding plasticisers to PVC to make it flexible).
| Polymer | Monomer | Key properties | Common uses |
|---|---|---|---|
| Poly(ethene) — LDPE | Ethene (C₂H₄) | Flexible, lightweight, low melting point | Plastic bags, cling film |
| Poly(ethene) — HDPE | Ethene (C₂H₄) | Stiffer, harder, higher melting point | Milk bottles, drainpipes |
| Poly(propene) | Propene (C₃H₆) | Tough, flexible, good chemical resistance | Food containers, ropes, carpets |
| PVC (poly(chloroethene)) | Chloroethene (vinyl chloride) | Rigid or flexible (with plasticiser), durable | Pipes, window frames, electrical insulation |
| Poly(styrene) | Styrene (C₈H₈) | Rigid; can be expanded into foam | Packaging, disposable cups, insulation |
| Nylon | Two monomers (condensation) | Strong, elastic, low friction | Tights, rope, clothing, toothbrush bristles |
Note that nylon is produced by condensation polymerisation (not addition), but it is included here because it is a widely encountered synthetic polymer at KS3.
What are the advantages of plastics?
Synthetic polymers became ubiquitous in the twentieth century for good reason. They offer a combination of properties that natural materials rarely match:
- Lightweight: less energy to transport than metals or glass
- Durable: many plastics last decades, making them ideal for pipes, medical equipment, and building materials
- Waterproof and chemically resistant: they do not corrode or rust
- Versatile appearance: can be made transparent or opaque, rigid or flexible, in any colour
- Electrical insulators: used in wire coatings, plugs, and circuit boards
- Cheap to manufacture at large scale from crude oil feedstocks
- Mouldable: can be pressed, blown, or extruded into virtually any shape
These properties explain why plastics replaced wood, metal, and glass in countless applications — from packaging to aircraft panels.
What are the environmental problems with plastics?
The same durability that makes plastics so useful becomes a serious problem at end of life. Most synthetic polymers are non-biodegradable: soil bacteria cannot break them down because the C–C backbone of poly(ethene) does not occur in nature, and no widespread microbial enzyme evolved to attack it. A plastic bag in landfill can persist for 400–500 years.
Key environmental concerns:
- Microplastics: plastics do not biodegrade, but UV light and physical abrasion cause them to fragment into particles smaller than 5 mm. These enter waterways, soils, and food chains, accumulating in fish, birds, and — via seafood — humans.
- Ocean pollution: billions of tonnes of plastic waste enter the oceans; marine animals ingest fragments or become entangled in packaging, fishing line, and bags.
- Burning plastics: incineration releases CO₂, and certain plastics (especially PVC) release toxic chlorine compounds when burned.
- Fossil fuel dependency: most synthetic polymers are made from naphtha, a fraction of crude oil — a non-renewable resource.
What are the solutions to plastic pollution?
The classic framework is the Three Rs — Reduce, Reuse, Recycle — but chemistry adds further options:
- Reduce: use less single-use plastic; choose alternatives where available
- Reuse: use durable containers many times rather than discarding after one use
- Recycle: thermoplastics (polymers whose chains are not cross-linked) soften on heating and can be melted, re-formed, and used again. HDPE, PET, and poly(propene) are widely collected and recycled in the UK. Thermosetting plastics (with cross-links between chains) do not soften — they char or burn — so they cannot be recycled by melting.
- Biodegradable plastics: poly(lactic acid) (PLA), made from corn starch, uses natural monomers that soil bacteria can break down. These decompose more quickly in composting conditions.
- Chemical recycling: breaking polymer chains back to their monomers, which can then be re-polymerised — a promising but still-emerging industrial process.
Understanding the distinction between thermoplastics and thermosetting plastics is essential here: only thermoplastics are recyclable by conventional melting.
Frequently asked questions
What is the difference between a thermoplastic and a thermosetting plastic?
Thermoplastics have long polymer chains that are not cross-linked to each other. When heated, the chains can slide past one another, so the material softens and can be remoulded; when cooled, it hardens again. This reversibility makes thermoplastics recyclable. Examples include poly(ethene), poly(propene), and PVC. Thermosetting plastics have polymer chains held together by strong covalent cross-links between chains. These bonds prevent the chains from moving, so the material does not soften on heating — it chars and eventually burns. Examples are Bakelite and epoxy resin. Because the cross-linked network cannot be undone by heat, thermosetting plastics cannot be recycled by melting.
Are natural rubbers and proteins also polymers?
Yes — polymers are not exclusive to synthetic plastics. Natural rubber is a polymer of the monomer isoprene (a five-carbon hydrocarbon). Proteins are biological polymers made of amino acid monomers linked by peptide bonds, which form via condensation polymerisation. Starch and cellulose are both polymers of glucose monomers (linked differently, which is why starch is digestible but cellulose is not). DNA is a polymer of nucleotide monomers. In each case, a long-chain molecule is built from repeating smaller units — the defining feature of any polymer, whether made in a factory or inside a living cell.
Why do plastics not biodegrade easily?
Most bacteria decompose organic material by producing enzymes that break specific chemical bonds. Soil and water bacteria evolved these enzymes to attack natural molecules — sugars, proteins, fats, and natural polymers such as cellulose. The long, regular carbon–carbon backbone of synthetic polymers like poly(ethene) does not occur in nature, so no widespread microbial enzyme evolved to attack it. Some newer biodegradable plastics are made from naturally occurring monomers (such as lactic acid derived from plant starch). Because lactic acid does occur in nature, existing microbial enzymes can break the bonds in poly(lactic acid), allowing it to decompose in composting conditions far more quickly than conventional plastics.
What is the difference between addition and condensation polymerisation?
In addition polymerisation, monomers must contain a C=C double bond; they link by opening that bond, and the only product is the polymer — no small molecule is released. Poly(ethene) and poly(propene) are examples. In condensation polymerisation, two different types of monomer react, and each time two units join, a small molecule — usually water (H₂O) — is released as a byproduct. Nylon and polyester are made by condensation polymerisation. Biological polymers such as proteins and DNA are also formed by condensation (the peptide bond formation releases water). At KS3, students primarily need to understand addition polymerisation; condensation polymerisation is explored in greater depth at GCSE.
Study polymers, plastics, and all KS3 chemistry with Professor Curie, your AI chemistry tutor — visit aitutors.me.