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Simply put, addition polymerisation is the reaction where alkenes (unsaturated molecules) are reacted to produce long chain molecules (macromolecules or polymers). The mechanism for addition polymerisation is extremely complex. As far as A level studies are concerned it is a basic free radical addition process. In reality both anionic and cationic polymerisation mechanisms are also possible.
As with the AS reaction of alkanes and halogens with UV light, the addition polymerisation of alkenes has the three stages of initiation, propagation and termination.
The initiation stage involves an initiator molecule breaking apart into free radical molecules. Examples of initiators include benzoylperoxide, (C6H5COO)2, di-t-butylperoxide, (C(CH3)3O)2, and azo-bis(iso-butyronitrile)(AIBN), (NCC(CH3)2N)2 :
e.g. for benzoylperoxide
e.g. for azo-bis(iso-butyronitrile)
The molecules split apart at particular temperatures, enabling a degree of control in the polymerisation reaction. Even oxygen can initiate free radical polymerisation and so alkenes frequently need to be kept in an airtight container.
The initiator radical then reacts with an alkene monomer unit, forming the beginnings of a polymer chain,
where I = the initiator molecule and R = any organic group, e.g. H, CH3, Cl, C6H5.
The propagation step is the reaction of this free radical chain with alkene molecules, generating a new free radical which reacts with further alkene units creating larger chains,
where R = any organic group, e.g. H, CH3, Cl, C6H5.
As can be seen the alkene unit is unsymmetrical and therefore has two ends a head (i.e. the carbon atom with the R group attached) and a tail (i.e. the carbon atom without the R group attached). The reaction above shows tails adding to heads all the way along the chain. Of course, other combinations are possible, e.g. head to head and tail to tail, leading to different structures and properties for the polymers. This addition can be largely controlled by the choice of alkene.
Chain transfer to polymer can also occur as a propagation step in polymerisation. This is the process where a growing radical is transferred from the end of a chain to the middle of another polymer chain,
Growth of a chain from this point forms branches on the polymer chains, which can lead to reduced melting point and mechanical strength for the polymer.
Chain transfer to monomer, solvent and special chain transfer agents are also possible.
The termination step involves the reaction of any two free radicals together, called combination termination, yielding a new bond and stopping chain growth, for example,
Other, more complex termination steps are possible, including disproportionation :
For any polymer other than poly(ethene), the carbon atom attached to the R group in the alkene unit becomes a chiral centre when a polymer chain is formed :
As you can see the carbon atoms marked (*) all have four different groups attached to them because the left hand part of the chain is different to the right hand part of the change (providing the carbon atom is not precisely in the middle of a chain)
Once the alkene units have formed a polymer chain the orientation of the groups around the marked carbon atoms cannot be changed. This leads to what is called the tacticity of a polymer.
When all the R groups lie on one side of the carbon chain the polymer is described as isotactic :
When all the R groups lie on alternating sides of the carbon chain the polymer is described as syndiotactic :
If the R groups show no regular pattern along the polymer chain then the polymer is called atactic :back to top
Polyesters contain the ester group, -COO-, and just like their smaller relatives are formed the reaction between a carboxylic acid and an alcohol. Both these molecules must have two reactive ends though, so that chain growth is possible.
An example of an often used man-made polyester is poly(ethylene terephthalate) (PET) made from terephthalic acid (benzene-1,4-dicarboxylic acid) and ethylene glycol (ethan-1,2-diol) :
Polyamides contain the amide (or peptide) group, -CONH-, and just like their smaller relatives are formed the reaction between a carboxylic acid and an amine. Both these molecules must have two reactive ends though, so that chain growth is possible.
Two examples of common man-made polyamides are Kevlar, made from terephthalic acid (benzene-1,4-dicarboxylic acid) and 1,4-diaminobenzene,
and nylon made from a variety of dicarboxylic acids and diamines and even a cyclic amide,back to top
Low-density poly(ethene) (i.e. low melting point and mechanical strength) is used for carrier bags and food wrap.
High-density poly(ethene) (i.e. higher melting point and mechanical strength) is used for containers, e.g. for camera film.
Poly(propene) is used for containers that require more wearing, and isotactic poly(propene) is used to construct climbing ropes, because of its great lateral strength.
Poly(chloroethene), also known as poly(vinyl chloride) or pvc, is used to make vinyl records, double glazing window frames and specialty clothing.
Poly(phenylethene), also know as poly(styrene), is used to make containers, and when the molten polymer is blown up with carbon dioxide or nitrogen gas, the resulting material is used for packaging.
Poly(methyl methacrylate) is one of the components used to make Perspex, along with other poly(acrylates).
Poly(methacrylic acid) is one of the components used in making superglue.back to top
written by Dr Richard Clarkson : © Saturday, 1 November 1997
Updated : Monday 26th March, 2012
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