Polymers, Bonding and Materials
From Monomers to Polymers
A polymer is a very large molecule formed by linking many small molecules called monomers together. The properties of a polymer depend on the type of monomer, how the chains are arranged, and how strongly they interact with each other. Choosing the right polymer for a job requires understanding these structure–property relationships.
Monomer: small molecule with reactive sites. Polymer: very large molecule formed by linking many monomers. Two types of polymerisation: addition (monomers with C=C, no by-product) and, at Higher tier only, condensation (monomers with two functional groups, releases water as a by-product).
Addition Polymerisation
Addition polymerisation occurs when alkene monomers react together. The \(C=C\) double bond in each monomer opens up, and the monomers join in a long chain. No atoms are lost — the polymer contains all the atoms of the original monomers.
Important examples of addition polymers and their uses:
- Poly(ethene) — from ethene (\(CH_2=CH_2\)): flexible, low density → carrier bags, packaging film.
- Poly(propene) — from propene (\(CH_3CH=CH_2\)): tougher, higher density → crates, ropes, car parts.
- Poly(chloroethene) / PVC — from chloroethene: rigid form for pipes; plasticised form for cable insulation.
- Poly(tetrafluoroethene) / PTFE — from tetrafluoroethene: very low friction, non-stick → cookware coatings.
The equation for addition polymerisation is written using a bracket with the repeating unit and the letter \(n\) to indicate that the unit repeats many times. For poly(ethene):
$$n\,CH_2=CH_2 \rightarrow [-CH_2-CH_2-]_n$$
The lines extending beyond the brackets show that the chain continues. To go from the monomer to the repeating unit: remove the double bond and show single bonds extending left and right. To go from the repeating unit to the monomer: put the double bond back in.
Condensation Polymerisation
Condensation polymerisation is different from addition polymerisation in two important ways: the monomers must have two reactive groups (one at each end), and a small molecule — typically water — is released at each bond formed. The polymer chain and the by-product are produced simultaneously.
Key examples:
- Polyesters — formed from a diol (two –OH groups) and a dicarboxylic acid (two –COOH groups). The linkage formed is an ester bond (–COO–). Water is released at each bond.
- Polyamides (nylons) — formed from a diamine (two –NH₂ groups) and a dicarboxylic acid (two –COOH groups). The linkage formed is an amide bond (–NHCO–). Water is released at each bond.
PET (polyethylene terephthalate) is formed from ethylene glycol and terephthalic acid, releasing water at each ester bond. It is strong, transparent, and suitable for food contact — which is why it is used for fizzy drink bottles. PET is also spun into polyester fibres for clothing and carpets. It can be recycled — old bottles can be reprocessed into new ones or into fibre fill for jackets.
The choice of PET over glass or aluminium for drinks bottles illustrates a real materials decision: PET is lighter (reducing transport energy), cheaper to produce, shatterproof, and recyclable. However, its persistence in the environment if not properly recycled is a significant disadvantage — a key consideration in a life-cycle assessment.
Thermosoftening and Thermosetting Polymers
The relationship between a polymer's structure and its thermal properties determines how it can be used and how it is processed.
Thermosoftening polymers (thermoplastics) have long polymer chains held together only by weak intermolecular forces. When heated, these forces are overcome and the chains can slide past each other — the polymer softens and can be remoulded. When cooled, the intermolecular forces reform and the shape is retained. This makes thermoplastics recyclable — they can be melted and reshaped repeatedly. Examples: poly(ethene), poly(propene), PET, PVC.
Thermosetting polymers have polymer chains that are joined by strong covalent cross-links. These cross-links form during the initial moulding and are permanent. Heating does not soften the polymer — instead, enough heat will cause it to char or burn. This makes thermosets non-recyclable but gives them excellent heat resistance and structural rigidity. Examples: epoxy resin, Bakelite, melamine (used in electrical sockets, saucepan handles, worktops).
Thermosoftening: chains held by weak intermolecular forces → softens on heating → recyclable. Examples: poly(ethene), PET. Thermosetting: chains joined by covalent cross-links → does not soften → non-recyclable. Examples: epoxy resin, Bakelite (electrical plugs, saucepan handles).
Choosing Materials — Structure Governs Properties
The key principle in materials science is that structure determines properties. When a designer or engineer selects a material, they are really choosing the molecular or atomic arrangement that will deliver the required physical and chemical properties. For polymers, the key questions are: is it addition or condensation? Thermosoftening or thermosetting? How tightly are the chains packed? Are there cross-links?
When scientists and engineers evaluate materials, they do not simply look for the 'best' material — they weigh up competing factors: cost, availability, durability, environmental impact, safety, and recyclability. This is the kind of risk–benefit thinking (IaS4) that is central to materials science and to this specification. PET bottles are lighter and shatterproof but create plastic waste if not collected. Thermosetting polymers resist heat but cannot be recovered after use. No material is ideal in all respects.
Life-cycle assessment (LCA) is a tool scientists use to compare the total environmental impact of different material choices across their whole life cycle — from raw material extraction and manufacturing, through use, to disposal or recycling. An LCA of a PET bottle versus a glass bottle, for instance, would consider energy use at every stage, not just the end-of-life scenario.