Thursday 12 March 2015

Hydrocarbons (6) Free Radical Addition Polymerisation

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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