Additional reactions – This takes place when many small monomers join with each other. So Ethene is able to link with another Ethene and another and another. This is what happens:
This is mainly represented as:
n represents the numbers of Ethenes there are to make the polymer.
This happens many times and a long chain of Ethenes is formed which then creates the polymer, Poly(Ethene) or commonly known as Polythene.
In addition polymerisation, all monomers have one kind of reactive group: a carbon – carbon double bond. The active centre reacts and then adds another monomer molecule, then another, then another, regenerating the active centre each time. There are four common types of an active centre:
- The free–radical type which is an atom with an unpaired electron.
- The carbanion type with a carbon–centred negative ion.
- The carbocation type with a carbon–centred positive ion.
- The metal complex type.
This is a standard example of a chain reaction and continues until two active centres encounters another, which then allows it to deactivate. Addition reactions have only the polymer made as an end product. Since there are no double bonds in the polymer, it is now an Alkane.
The addition polymerisation is also known as the chain–growth polymerisation.
Addition reactions have these particular features:
- Molecular species can react only with active centres.
- Chains are long at all times.
- Some monomers remain until the end of the reaction.
- Reactions are relatively fast, generating lots of heat.
Condensation reactions – When this occurs, there is always another end product other than the polymer. The reactive groups in condensation polymerisation are at the end of each monomer, which then connects to another end of an extra monomer.
A and B are monomers. The extra part in the end product is seen with the overlaps in the shape of A and B. With a growing chain, it has a reactive group on the ends, which means it can be joined with another long chain and makes it suddenly get larger.
Two types of different reactive groups are required for condensation polymerisation. Reactions can be done with one monomer with two kinds of reactive group (A–B) or two different monomers (A–A and B–B)
The key features of condensation polymerisation are:
- Any two molecular types can react.
- Chains start short and steadily grow
- Most monomers are gone early in the reaction
- Reactions are relatively slow and generate little heat
This table shows the differences in Addition and Condensation reaction in time.
The table below shows the name of the monomer, the formula, what it is as a polymer and the polymer’s formula.
Uses of Polymers
Ethene (Polyethene) – detergents, bottles, bags, garden furniture, buckets, signs, trash cans.
Chloroethene (polyvinylethene or PVC) – plastic sheets, artificial leather, cooking oil bottles, drainage and sewer pipes, tile, credit cards.
Phenylthene (polystyrene) – containers, foam, signs, toys, disposable cups and utensils, packaging.
Tetra Fluoroethene (polytetrafluoroethene) – non-stick coating in frying pans.
Other uses of polymers – snacks and other food containers, hand cream, toothpaste, cosmetic containers.
Recycling
Although polymers are extremely useful, there are problems dealing with unwanted polymers. So how do we recycle them?
Problems:
- Polymers are not biodegradable and so bacteria cannot break them down. If we bury the polymers, it will remain there.
- Polymers produce toxic materials (poison) when burnt. Many people die from the fumes of polymers in house fires, before the fire has time to reach them. Burning is not a good way of disposing polymers.
Recently, we have been trying to recycle more and more polymers because polymers have an average of 34 years of life-span; we are beginning to see the waste from polymers created in the 1960’s. It is less efficient and more costly than recycling other materials such as metals, but it is better than living among piles of polymers which cannot decay. Currently, only 3.5% of all plastics made are recycled compared to 34% of paper, 22% of glass and 30% of metals. Paper, class and metal are easy to recycle more than once and can be reused, but plastics are more complicated.
Solutions:
To recycle polymers, it firstly needs to be separated into groups, depending on what polymer it was made from. Some polymers are then melted, depending on whether or not they have a lot of synthetics in them – the ones with the least synthetics are able to be recycled. The polymers cross-linked with synthetics are difficult to stretch and less flexible. The melted polymers can then be moulded into a new product. There are two areas polymers can be categorised into: polyethelyne, which are High Density Polyethene (HDPE) and Low Density Polyethene (LDPE). The polymers are sorted into codes which help consumers separate their plastics before disposing of them.
Conclusion
From this study, I have learnt the differences between Addition and Condensation reactions in polymerisation. I understand the importance of polymers in daily life and know how it is recycled and the problems to do with it.
