Chemistry_Grade_10-12 (1).pdf

Worked example 48 polymerisation reactions question a

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Worked Example 48: Polymerisation reactions Question: A polymerisation reaction takes place and the following polymer is formed: C W Y C X Z n Note: W, X, Y and Z could represent a number of different atoms or combinations of atoms e.g. H, F, Cl or CH 3 . 1. Give the structural formula of the monomer of this polymer. 2. To what group of organic compounds does this monomer belong? 3. What type of polymerisation reaction has taken place to join these monomers to form the polymer? Answer Step 1 : Look at the structure of the repeating unit in the polymer to determine the monomer. The monomer is: C W Y C X Z Step 2 : Look at the atoms and bonds in the monomer to determine which group of organic compounds it belongs to. The monomer has a double bond between two carbon atoms. The monomer must be an alkene. Step 3 : Determine the type of polymerisation reaction. In this example, unsaturated monomers combine to form a saturated polymer. No atoms are lost or gained for the bonds between monomers to form. They are simply added to each other. This is an addition reaction. 187
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10.2 CHAPTER 10. ORGANIC MACROMOLECULES - GRADE 12 10.2.2 Condensation polymerisation In this type of reaction, two monomer molecules form a covalent bond and a small molecule such as water is lost in the bonding process. Nearly all biological reactions are of this type. Polyester and nylon are examples of polymers that form through condensation polymerisation. 1. Polyester Polyesters are a group of polymers that contain the ester functional group in their main chain. Although there are many forms of polyesters, the term polyester usually refers to polyethylene terephthalate (PET). PET is made from ethylene glycol (an alcohol) and terephthalic acid (an acid). In the reaction, a hydrogen atom is lost from the alcohol, and a hydroxyl group is lost from the carboxylic acid. Together these form one water molecule which is lost during condensation reactions. A new bond is formed between an oxygen and a carbon atom. This bond is called an ester linkage . The reaction is shown in figure 10.4. (a) HO CH 2 CH 2 O H + H O C O C O OH H 2 O molecule lost ethylene glycol terephthalic acid (b) HO CH 2 CH 2 O C O C O OH ester linkage Figure 10.4: An acid and an alcohol monomer react (a) to form a molecule of the polyester ’polyethylene terephthalate’ (b). Polyesters have a number of characteristics which make them very useful. They are re- sistant to stretching and shrinking, they are easily washed and dry quickly, and they are resistant to mildew. It is for these reasons that polyesters are being used more and more in textiles . Polyesters are stretched out into fibres and can then be made into fabric and articles of clothing. In the home, polyesters are used to make clothing, carpets, curtains, sheets, pillows and upholstery.
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