What is a polymer?
A polymer is simply a plastic. Polymers (plastics) are used a lot in our modern lifestyle and is made from one main raw material crude oil.
How polymers are made?
Polymers are simply large molecules in a chain made from many small molecules called monomers (Poly - Many, Mono - One). These monomers make the polymers by joining together in a process called polymerisation. Many of the polymers that we use in our everyday life are made from alkenes.
Why Alkenes?
Alkenes are used to make polymers because the double bond in alkenes can be broken and that bond can be used to join the molecule to another alkene molecule.
When the alkene (In this example ethene) is heated at high pressures and heat in the presence of a catalyst, the molecules in ethene react with each other to form poly(ethene).
High density and low density polymers?
Depending on how the alkene was heated the plastic made will be high density (Strong plastic) or low density (flexible plastic). If ethene is
Heated: 200°C
Pressure: 2000 atm
Catalyst:trace of oxygen
A low density poly(ethene) is formed. It is low density because the molecules are loosely packed because the molecules in the chains are more branched.
If ethene is
Heated: 60°C
Pressure: 2 atm
Catalyst:Ziegler-Natta
A high density poly(ethene) is formed. It is high density because the molecules are tightly packed together and the polymer branches are more compact making the plastic more rigid.
The uses of the low density poly(ethene) is to make plastic bags and cling film as the polymer made is flexible and soft.
The uses of the high density poly(ethene) is to make buckets, bins and bottles(anything upright and strong) because the properties of the high density poly(ethene) is that it is harder and stiffer than low density poly(ethene)
source: http://www.scienceray.com/Chemistry/Explanation-of-Polymers.695573
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