Chemistry_Grade_10-12 (1).pdf

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(a) C C H H H H (b) C C C C H H H H H H H H Figure 10.1: (a) Ethene monomer and (b) polyethene polymer 185
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10.2 CHAPTER 10. ORGANIC MACROMOLECULES - GRADE 12 A polymer may be a chain of thousands of monomers, and so it is impossible to draw the entire polymer. Rather, the structure of a polymer can be condensed and represented as shown in figure 10.2. The monomer is enclosed in brackets and the ’n’ represents the number of ethene molecules in the polymer, where ’n’ is any whole number. What this shows is that the ethene monomer is repeated an indefinite number of times in a molecule of polyethene. C H H C H H n Figure 10.2: A simplified representation of a polyethene molecule 2. Polypropene Another example of a polymer is polypropene (fig 10.3). Polypropene is also a plastic, but is stronger than polyethene and is used to make crates, fibres and ropes. In this polymer, the monomer is the alkene called propene . (a) C C H H CH 3 H (b) C C C C CH 3 H CH 3 H H H H H or n C CH 3 H C H H Figure 10.3: (a) Propene monomer and (b) polypropene polymer 10.2 How do polymers form? Polymers are formed through a process called polymerisation , where monomer molecules re- act together to form a polymer chain. Two types of polymerisation reactions are addition polymerisation and condensation polymerisation . Definition: Polymerisation In chemistry, polymerisation is a process of bonding monomers, or single units together through a variety of reaction mechanisms to form longer chains called polymers. 10.2.1 Addition polymerisation In this type of reaction, monomer molecules are added to a growing polymer chain one at a time. No small molecules are eliminated in the process. An example of this type of reaction is the formation of polyethene from ethene (fig 10.1). When molecules of ethene are joined to each other, the only thing that changes is that the double bond between the carbon atoms in each ethene monomer is replaced by a single bond so that a new carbon-carbon bond can be formed with the next monomer in the chain. In other words, the monomer is an unsaturated compound which, after an addition reaction, becomes a saturated compound. 186
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CHAPTER 10. ORGANIC MACROMOLECULES - GRADE 12 10.2 Extension: 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.
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