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.
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.
You may wish to view this BBC News item (2002) about degradable carrier bags.
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.
You can read the entire article from the BBC website Polymers
For a more in-depth look at how are Polymers formed or made you can check out this resource
Polymerization requires chemical reactions, and chemical reactions happen as a consequence of collisions of molecules; if monomers never encounter each other, they can never react! Therefore, we would expect that the rate of a reaction has a dependence on collision frequency. This is an especially important consideration for polymers because polymers can be really big which means they diffuse slowly, leading to a lower rate of collision, providing fewer opportunities for reactions. However, because the polymer is large and diffuses more slowly, each encounter between reactants actually has a longer duration, which favors a reaction. These two effects — less overall collisions but longer collision duration — are assumed to balance each other out. Analyses of polymer reaction kinetics suggest that this is a reasonable assumption in most cases. We are therefore going to make this very key assumption called the principle of equal reactivity, which is that reactivity does not vary as a function of polymer size. This assumption underlies all of our future analysis of polymerisation and is important to keep in mind.
Now to actually make a polymer in the first place, you have to have the correct degree of functionality. Clearly, for a polymer to form and grow, 1) there has to be an initial reaction between monomers, and 2) there has to be a reaction between monomers and the growing polymer molecule. Consider the two molecules in Figure 2.1, where A and B are able to react together to form a bond. Because each molecule only has one A or B group, once they react, there is no more functionality left to continue the polymerization. Thus, these molecules don’t have enough functionality to form a polymer.
Another good resource is http://www.essentialchemicalindustry.org/polymers/polymers-an-overview.html
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