Monomers and polymers
- Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers.
- Alkenes can act as monomers because they are unsaturated (they have a double bond)
- Ethene can polymerise to form poly(ethene), also called polythene.
- Propene can polymerise to form poly(propene), also called polypropylene.
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Displayed formulas of polymers
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Uses of polymers
- Polymers have properties that depend on the chemicals they are made from, and the conditions in which they are made.
- For example, there are two main types of poly(ethene): LDPE, low-density poly(ethene), is weaker than HDPE, high-density poly(ethene), and becomes softer at lower temperatures.
- Modern polymers have many uses, including:
- new packaging materials, waterproof coatings for fabrics (such as for outdoor clotting), fillings for teeth, dressings for cuts, hydrogels (for example for soft contact lenses), smart materials (for example shape memory polymers for shrink-wrap packaging).
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Problems with polymers
- They are unreactive, so they are suitable for storing food and chemicals safely
- Unfortunately, this property makes it difficult to dispose of polymers.
- Most polymers, including poly(ethene) and poly(propene) are not biodegradable, so they may last for many years in rubbish dumps.
- However, it's possible to include substances such as cornstarch that cause the polymer to break down more quickly. Carrier bags and refuse bags made from such degradable polymers are available now.
- Many polymers can be recycled. This reduces the disposal problems and the amount of crude oil used.
- But the different polymers must be separated from each other first, and this can be difficult and expensive to do.
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- Ethanol is the type of alcohol found in alcoholic drinks and it's also useful as a fuel. For use in cars and other vehicles, it is usually mixed with petrol.
- Ethanol molecules contain carbon, hydrogen and oxygen atoms.
- Ethanol can be made by reacting ethene (from cracking crude oil fractions) with steam. ethene + steam → ethanol
- C2H4 + H2O → C2H5OH
- Ethene is made from crude oil, which is a non-renewable resource. It cannot be replaced once it is used up and it will run out one day.
- Sugar from plant material is converted into ethanol and carbon dioxide by fermentation. The enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this process happen.
C66H12O6 → 2C2H5OH + 2CO2Unlike ethene, sugar from plant material is a renewable resource.
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