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{"number":"2","post_status":"publish","post_type":"unit","status":"approve","type":"public","user_id":0}
27.1. Formation of Plastics.
Plastics consist of macromolecule in the form of long chains. The long chains are made by joining together many small molecules. The small molecules are called monomers. The big molecule is also called a polymer. The process of joining the monomers together to form a polymer is called polymerisation.
A polymer is a long-chain macromolecule made by joining together many small molecules.
Polymerisation is a reaction in which many small molecules (monomers) join together to make one large molecule.
Polymers are classified into two groups. These are addition polymers and condensation polymers.
27.2. Addition Polymers.
Addition polymerisation is a reaction in which small molecules join together to for one molecule as the only product.
One of the simplest addition polymers is poly(ethene), commonly known as ‘polythene’. It is a synthetic polymer made from ethene.
The ethene molecules has a C = C double bond. During the polymerisation, the double bond opens as follows:
These open units join together to form a molecule of poly(ethene). The overall change is shown in the equation below:
If n ethene molecules join together, the equation for polymerisation can be shown as:
The structure on the right is the repeat unit for poly(ethene). It is the part that repeats many times to give the complete polymer structure.
Poly(ethene) is the most widely used plastic today. The uses include plastic bags, cling film for wrapping food, flexible water pipes, plastic bottles, plastic chairs and insulation around electrical wires.
27.3. Condensation Polymers.
Condensation polymerisation is a reaction in which monomers join together to form polymers with the elimination of small molecules. Two common examples of condensation polymers are nylon and terylene. Both nylon and terylene are synthetic (man-made) polymers.
Nylon.
Nylon is made into fibres to make strong ropes and fishing lines, or woven into cloth to make nylon tents, parachutes and raincoats. Most zippers are made of nylon. Nylon is strong yet light, and can be stretched without breaking. Nylon is made from two monomers. These can be simply written as:
The monomers combine to form the polymers. As they do, water molecules are formed from each pair of monomers. This is shown below:
The repeat unit for nylon can be written as:
The units of a nylon molecule are joined together by this group of atoms. This is called the amide linkage. This type of polymer is called a polyamide. This means that it has many amide linkages.
Terylene.
Terylene is another polymer made from two types of monomer. One is an alcohol and the other is an organic acid. The two monomers can be written simply as:
The monomers combine to form the polymer. At the same time, water molecules are eliminated as shown below:
The repeat unit for terylene can be written as:
The monomer molecules in terylene are joined together by ester linkages. This type of polymer is called a polyester. This means that it has many ester linkages.
Terylene is used to make clothes and sails.
27.4. Pollution Problems Caused by the Disposal of Plastics.
Plastics are often disposed off by burning. However, all plastics produce carbon monoxide gas when they burn. In addition, PVC produces chlorine and nylon produces extremely poisonous hydrogen cyanide gas when burnt.
Plastics are resistant to corrosion and are described as being non-biodegradable as bacteria do not break them down naturally. Hence, discarded plastic objects pollute the environment.
Plastic waste can also kill animals. For example, some sea turtles mistake plastic bags as jellyfish and choke on them.
To make plastic foam, chloroflurocarbons (CFCs) are used. These gases later escape from the plastic foam and enter the atmosphere. CFCs are greenhouse gases and contribute towards global warming.
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