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Initiation, propagation and termination

There are three stages in the process of addition polymerisation. Initiation refers to a chemical reaction that triggers off another reaction. In other words, initiation is the starting point of the polymerisation reaction. Chain propagation is the part where monomers are continually added to form a longer and longer polymer chain. During chain propagation, it is the reactive end groups of the polymer chain that react in each propagation step, to add a new monomer to the chain. Once a monomer has been added, the reactive part of the polymer is now in this last monomer unit so that propagation will continue. Termination refers to a chemical reaction that destroys the reactive part of the polymer chain so that propagation stops.

A polymerisation reaction takes place and the following polymer is formed:

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?
  1. The monomer is:

  2. The monomer has a double bond between two carbon atoms. The monomer must be an alkene.

  3. 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.

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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 (a carboxylic 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 [link] .
    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 resistant 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.

    Interesting fact

    Polyester is not just a textile. Polyethylene terephthalate is in fact a plastic which can also be used to make plastic drink bottles. Many drink bottles are recycled by being reheated and turned into polyester fibres. This type of recycling helps to reduce disposal problems.
  2. Nylon Nylon was the first polymer to be commercially successful. Nylon replaced silk, and was used to make parachutes during World War 2. Nylon is very strong and resistant, and is used in fishing line, shoes, toothbrush bristles, guitar strings and machine parts to name just a few. Nylon is formed from the reaction of an amine (1,6-diaminohexane) and an acid monomer (adipic acid) ( [link] ). The bond that forms between the two monomers is called an amide linkage . An amide linkage forms between a nitrogen atom in the amine monomer and the carbonyl group in the carboxylic acid.
    An amine and an acid monomer (a) combine to form a section of a nylon polymer (b).

Questions & Answers

how do you see a product if it is exothermic
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When ∆H<0
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acids and bases
Newton's law
How does a catalyst affect the rate of chemical reaction
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define Doppler effect
Emihle Reply
It is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
Doppler effect is the change in frequency or pitch detected by a listener because the velocity of the sound source is different from that of a listener relative to the medium of sound propagation
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Simphiwe Reply
Firstly you must know how energy changes during chemical reactions...secondly the collision theory.... Rate and extent of reactions.... And factors that affect the rate of chemical reactions... If u capture all the topic then this chapter will be easy for you the is no way you can fail it...
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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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