What are the catalysts used in addition polymerisation?
A typical catalyst is di-benzoyl peroxide (C6H5CO2)2.
Its mode of action in the high pressure conditions of
addition polymerisation is to cleave at the O-O bond into two free radicals.
The breaking of a covalent bond in this way is called
homolytic fission.
This is the initiation
step in the polymerisation process
The resulting particles are very reactive and soon break up
again into carbon dioxide and a benzyl free radical C6H5•
The outline equations below picture this initiation process:
The benzyl free radical is also very reactive and when it
collides with an alkene molecule it has sufficient energy to break the π bond and attach itself to the alkene
creating a larger free radical like this:
In this example R• stands for the phenyl radical and it
attacks the styrene (or phenylethene ) molecule at its double bond.
A bond forms between the styrene and phenyl species from two
electrons: the odd electron in the radical and a π bond electron.
The other π bond electron is available as the odd electron to form a new bond with
another styrene molecule so:
This is the propagation
stage.
Clearly, if two growing chains each with a free radical end
approach each other the chains will terminate.
You ought to satisfy yourself that you can construct
equations of this type for simple alkenes like propene or ethane using benzoyl
peroxide as the free radical initiator.
Pages on the "Mole" and "Using the Mole" in chemical calculations are here
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