Addition polymerisation

Previous Page: Polymers

Sometimes a single type of unit, or monomer, forms the chains of a polymer. Ethylene (ethene) breaks its double bond, leaving it with two free ends, becoming a free radical. Under the right conditions, they can disrupt the double bonds in other ethene molecules and start making links.

 

CH2=CH2 + CH2=CH2

-CH2-CH2-CH2-CH2-CH2-CH2-

ethene

ethene

polyethene segment

This radical can then attack another ethene molecule and add it on the end, and so on until all the ethenes are added on, or a radical is added to the other end, leaving no free carbon bonding sites. In the real world these chains tend to be of the order of thousands or millions of monomers (single units) long, and have molar masses measured in kilograms instead of grams. Thus is created polyethylene (also known as polyethene and polythene).

 

Another example of this sort of reaction is natural rubber, from monomers of 2-methyl-buta-1,3-diene, also called isoprene.

  CH3
   |
CH2 =C.- CH.= CH2
1 2 3 4

isoprene

 CH3
   |
-CH2 -C.= CH.- CH2
1 2 3 4
 CH3
   |
-CH2 -C.= CH.- CH2
1 2 3 4
 CH3
   |
-CH2 -C.= CH.- CH2-
1 2 3 4

natural rubber

This process is referred to as addition or chain-reaction polymerisation because of the way each monomer is added to the chain. The chains get very long very quickly. With several chains growing at the same time a typical reaction is completed in seconds or less.

 

Compare this to buta-1,3-diene and styrene (phenylethene) combining to form a synthetic rubber, such as you’ll find in your car tyres:

(The hexagons with circles inside represent benzene.)

Again, the open bond at each end indicates this is just a small fragment of the total chain. The only difference here is that there are two different monomers involved, rather than the single monomer in the previous examples. This is still an addition reaction.

There is no particular reason that addition polymers with multiple monomers must have their monomers alternate. They are usually shown this way for simplicity, but it isn't a requirement.

 

Other common addition polymers are:

 

Next Page: Condensation Polymerisation

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