Polymers have many applications and new uses are being developed. However, they can be difficult to dispose of.

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.

In general: lots of monomer molecules → a polymer molecule.                                                               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.                                      Displayed formulas of polymers

Polymer molecules are very large compared with most other molecules, so the idea of a repeating unit is used when drawing a displayed formula. When drawing one, starting with the monomer:

·         change the double bond in the monomer to a single bond in the repeating unit

·         add a bond to each end of the repeating unit.

Uses of polymers

Different polymers have different properties, so they have different uses. The table below gives some examples.                                                                                                                          Examples of polymers and their uses                                                                                    polythene: plastic bags and bottles                                                                                      polypropene: crates and ropes                                                                                            polychloroethene: water pipes and insulation on electricity cables

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 clothing)

·         fillings for teeth

·         dressings for cuts

·         hydrogels (for example for soft contact lenses and disposable nappy liners)                                        smart materials (for example shape memory polymers for shrink-wrap packaging).                                           Problems with polymers

One of the useful properties of polymers is that they are unreactive, so they are suitable for storing food and chemicals safely. Unfortunately, this property makes it difficult to dispose of polymers. They can cause litter and are usually sent to landfill sites.

Biodegradable plastics:                                                                                                                                                                              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.        Recycling:                                                                                                                                                                                                        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.

Ethanol

Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It's also useful as a fuel. For use in cars and other vehicles, it is usually mixed with petrol.                         Structure of ethanol

Ethanol molecules contain carbon, hydrogen and oxygen atoms.                                   Structure of ethanol:            C₂H₅OH

Making ethanol from ethene and steam

Ethanol can be made by reacting ethene (from cracking crude oil fractions) with steam. A catalyst of phosphoric acid is used to ensure a fast reaction.

ethene + steam → ethanol

C₂H₄ + H₂O → C₂H₅OH

Notice that ethanol is the only product. The process is continuous – as long as ethene and steam are fed into one end of the reaction vessel, ethanol will be produced. These features make it an efficient process, but there is a problem. 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.                                                                                                                                                                                                 Fermentation

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 + 2CO₂

Unlike ethene, sugar from plant material is a renewable resource.