Polymers and Monomers
In 1933, ICI chemists Eric Fawcett and Reginald Gibson accidentally polymerised ethene at 1,400 atmospheres, creating the first 8 grams of polyethylene, a material now produced at 100 million tonnes per year.
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Q1 · Think about plastic bottles, rubber gloves, nylon rope, and DNA, what could such different materials possibly have in common at the molecular level?
Q2 · Why do you think joining thousands of small identical molecules together into one giant chain would produce a material with very different properties from the original small molecules?
● Know
- The relationship between monomers and polymers
- The difference between addition and condensation polymerisation
- Common examples of polymers and their monomers (polyethylene from ethene, nylon, DNA)
● Understand
- Why addition polymerisation requires unsaturated monomers (double bonds)
- Why condensation polymerisation produces a byproduct
- How chain length and monomer type affect polymer properties
● Can do
- Describe how addition polymerisation works using ethene as an example
- Distinguish addition from condensation polymerisation
- Identify the repeat unit and monomer from a polymer structure
Squeeze a plastic bottle: it flexes under your hand and springs back to shape without snapping, a behaviour impossible for the ethene gas molecules it was built from, because the individual gas molecules have been chained into a structure thousands of times longer, changing every mechanical property. A polymer is a very large molecule built from many smaller repeating units called monomers. In addition polymerisation, alkene monomers react with each other through the opening of their C=C double bonds. Each double bond opens, and the reactive ends chain together with adjacent monomers, building a continuous polymer chain. The overall equation for polyethylene: $n\text{CH}_2\text{=CH}_2 \rightarrow \text{(-CH}_2\text{-CH}_2\text{-)}_n$. The subscript 'n' (the degree of polymerisation) can be anywhere from a few hundred to several million, depending on the reaction conditions.
The polymer has dramatically different properties from the monomer. Ethene (C₂H₄) is a gas at room temperature, flammable, and has a boiling point of −104 °C. Polyethylene is a solid, flexible, non-flammable, chemically inert material with a melting point of 120–130 °C. The transformation is entirely chemical, the same atoms are present, but their arrangement into a giant chain changes every macroscopic property. This is why materials scientists say "the polymer's properties are determined by its structure", and in addition polymerisation, the structure is controlled by choosing the monomer and the chain length.
Polymerisation of vinyl chloride (CH₂=CHCl): n × CH₂=CHCl → (–CH₂–CHCl–)ₙ. This is polyvinyl chloride (PVC). The chlorine atom on every second carbon gives PVC its flame resistance and rigidity, used in Australian drainpipes, electrical conduit, and building cladding. One small atom difference in the monomer creates a completely different polymer.
Iplex Pipelines (NSW and Victoria) manufactures PVC and polyethylene pipes for Sydney Water and major infrastructure projects. The polymer granules arrive from overseas, are melted and extruded into pipes at Iplex's Ingleburn factory, and installed in NSW water and sewer networks, the entire water supply system relies on polymer chemistry from addition polymerisation.
Different alkene monomers produce polymers with very different properties. Polyethylene (PE): from ethene (CH₂=CH₂). Low-density PE (LDPE), branched chains, flexible, transparent film for plastic bags. High-density PE (HDPE), straight chains, rigid, used for milk bottles, pipes, and cutting boards. Polypropylene (PP): from propene (CH₂=CHCH₃). Stiffer than PE, higher melting point (~165 °C), used for car bumper bars, food containers, and carpet fibres. Polyvinyl chloride (PVC): from vinyl chloride (CH₂=CHCl). Rigid when unplasticised (pipes, profiles); flexible when plasticised (hoses, vinyl flooring). Polystyrene (PS): from styrene; rigid and brittle as solid plastic; expanded into foam (EPS) for insulation and packaging.
The monomer structure determines the polymer's properties because every side group on the chain creates steric effects, polar interactions, or chain stiffness. Polypropylene's methyl (–CH₃) group makes its chains stiffer than polyethylene's. PVC's chlorine makes it flame-retardant and polar. Polystyrene's large benzene ring stiffens the chain. Understanding this structure–property relationship is the core intellectual skill in polymer science, if you can see the monomer, you can predict the polymer's behaviour.
HDPE (high-density polyethylene) milk bottles: recycling code 2. 3 billion litres of milk are sold in Australia per year, almost all in HDPE bottles. HDPE's density is 0.95 g/cm³, it floats on water, making it easy to separate in recycling streams. Recycled HDPE is remoulded into garden furniture, playground equipment, and wheelie bins across Australia.
Pact Group's Australian manufacturing plants convert recycled PE, PP, and PVC granules from Australia's container deposit and kerbside schemes back into new packaging products, closing the loop from petroleum-sourced polymer through consumer use, recycling, and remanufacture, all using polymers from addition polymerisation.
The length of a polymer chain, the degree of polymerisationdirectly controls its mechanical properties. Longer chains → higher tensile strength (stronger, harder to pull apart) because chains are more entangled, requiring more force to separate them. Longer chains also → higher melting point (more intermolecular forces to overcome) and lower flexibility (longer chains are less free to move). This is why ultra-high-molecular-weight polyethylene (UHMWPE, n ≈ 100,000–200,000) is used for bulletproof vests, while low-molecular-weight PE (n ≈ 1,000) is a wax used in polishes.
Cross-linkingforming covalent bonds between adjacent polymer chains, increases hardness and reduces elasticity dramatically. Natural rubber is too soft and sticky to be useful; vulcanisation (heating with sulfur) introduces cross-links between adjacent rubber chains, making the polymer harder and more elastic. Branching disrupts the regular packing of chains, reducing crystallinity and lowering the melting point. LDPE (branched) melts at ~110 °C; HDPE (unbranched) melts at ~135 °C, the same monomer, but the chain architecture creates different properties.
UHMWPE fibres (trade name Dyneema/Spectra) have tensile strength 40× that of steel by weight, they are used in cut-resistant gloves worn by workers at Australian meat processing and glass cutting facilities. The same polyethylene chain, just 100,000× longer than in a plastic bag, gives a material harder to cut than a steel wire.
Ansell's Kalgoorlie and Sydney distribution centres supply UHMWPE cut-resistant gloves to Australian mining and manufacturing workers. The polymer in each glove is the same addition polymerisation product of ethene, but with a degree of polymerisation approximately 1 million times greater than a supermarket bag, giving it entirely different mechanical properties.
Longer polymer chains give higher tensile because the chains are more entangled. Longer chains also raise the point and reduce flexibility. Ultra-high-molecular-weight polyethylene is strong enough to be used in vests. Cross-linking forms covalent bonds between adjacent polymer . Vulcanising rubber introduces these cross-, making it harder and more durable.
At the start of this lesson, you heard that a single molecule of polyethylene in a plastic bag can contain over 100,000 carbon atoms all bonded in a single chain, and that this chain was built by 50,000 ethene molecules, each contributing its double bond to the next. Polymerisation is chemistry at industrial scale.
Now that you've worked through the lesson, can you explain the difference between a monomer and a polymer, and describe how addition polymerisation links ethene molecules together? How does the length of the chain affect the properties of the final polymer?
Q1. Explain the difference between a monomer and a polymer. Use polyethylene and ethene as your example.
Q2. Describe the process of addition polymerisation using ethene as the monomer. Include what happens to the double bond and draw or describe the repeat unit of polyethylene.
Q3. Compare addition and condensation polymerisation. For each, describe: the type of monomers required, whether a byproduct is produced, and one example of a polymer made this way.