= CH – CH3
Cl – CH·
This radical attacks the molecules that are present in good amounts: the other molecules of propene:
The main reaction can be stopped if a radical (the unpaired electron) meets another radical, also with an unpaired electron
- C3H6Cl· + C3H6
- C6H12· + C3H6
This is a chain reaction causing enormous chains. Every time a bigger radical is made that every time again can catch a monomere.
In this way macromolecules are formed.
Those two electrons immediately form a normal covalent bond; the action is over and the polymerisation comes to an end.
There are different ways to close such a chain reaction:
The example uses 'radicals': neutral particles with an unpaired electron.
Such a structure is rather unstable, and such an unpaired electron we indicate with a dot.
In general, the products of poly addition are: plastics in many variations, depending on the choice of the monomeres.
You can limit the reaction to one kind of monomeres, but also apply different monomeres in one process.
Then the so called co-polymeres are produced.
An example is ‘TEFLON’ or TEFAL = poly-tetra fluor ethene, that is applied in the industry of cooking pans.
This is a badly absorbing polymere. In practice, in such a pan will food not easily burn.
Another example is PVC (PolyVinylChloride) = polychloroethaan.
This is applied in huge amounts, for example in plumbing, water tubes, isolation materials, etc.
In another module we come back on this item.
The technique of polymerisation
In factories, polymeres are mostly made in grain form. These grains then are treated in the more specialised industries to produce final products.
Often the grains are heated, and the substance becomes then softer to make it suitable for moulding.
Here we distinguish two different processes:
- The (macro)molecules do not make in between bondings, no side chains appear. No threedimensional network is formed.
The product remains a certain flexibility. The material can be recycled: you kan grain it again, heat it up and again put into the moulds.
- The molecules form a threedimensional network, making intermolecular bonds.
For example if extra double bonds are/were available.
These products will be hard, with no flexibility. Recycling is more difficult in this case.
Alkadienes and alkatrienes can also suffer polymerisation with the result: a substance with macromolecules that still remain unsaturated.
Such an unsaturated polymere still has properties of unsaturated substances, or: a certain flexibility (not liquid of course, the molecules are too big). You get something like rubber.
We can reduce the elasticity of such a substance by (partial) addition.
This is applied, for example, in the vulcanisation process of car tires.
Those tires would be too elastic and unfit to serve any car.
But at vulcanising (part of) the double bonds in the rubber are added, if possible with Oxygen atoms, but often and more effective with sulphur atoms.
The material remains enough elasticity, but becomes harder.
Another special aspect at the formation of polymeres is the mixing of different monomeres, creating 'co-polymeres'.
The molecules connect alternately.
Macromolecules of polymeres are so big, that the substance alway will be a solid.
They might sometimes dissolve more or less in water, like proteins or polysaccharides.
Then you will observe a certain cloudy mixture, not transparant, just because these molecules are so big.
The macromolecules are dissolved, but so big that you can 'see' them.
There is a special 'polymere chemistry' where the capacity exists to produce extremely specialised polymeres with very special applications.
The two most important types of polymerisation are the polyaddition and the polycondensation.
Benzene does not contain real double bonds. The 6 C-atoms of the ring are connected in another way (see module 4).
Addition of benzene is therefor not easy. The six bondings are very stable and do not allow being opened for an addition process.