mechanism 32 - the peroxide catalysed free radical polymerisation of ethene

• Step (1) For peroxide catalysed polymerization, the initial free radicals are formed by the homolytic bond fission of the O-O bond in a peroxide molecule.

  • The O-O bond (bond enthalpy 146 kJmol^-1) is the weakest and will break before any others e.g. the C-C bond enthalpy is 348 kJmol^-1 and the C-H bond enthalpy is even stronger at 412 kJmol^-1. (see similar arguments in alkane chlorination)

    • Step (1) illustrates how to use half-arrows to show a homolytic bond fission step.

• Step (2) is a chain propagation step, in which the 'alkoxy radical' adds onto an alkene molecule.

  • If you split open half the alkene double bond (i.e. one bond pair), one electron pairs up with the unpaired electron of the 'alkoxy radical' to form a C-O bond. The other electron remains unpaired at the end of the radical, so that one active free radical is replaced by another.

    • Step (2) illustrates how to use half-arrows in a polymerisation chain propagation step, where the radical adds onto the polymer molecule to make an even longer radical to continue the chain reaction.

• Step (3) illustrates all the further subsequent chain propagation steps in which the polymer free radical adds on another monomer alkene molecule to make the chain longer and longer.

• Step (4) represents one possible chain termination, were the electrons of two free radicals pair up to form a covalent C-C bond. 

  • See the chlorination of alkanes for how to use half-arrows in a termination step.

• FURTHER COMMENTS

  • At GCSE level the presence of a very tiny % of oxygen atom in poly(ethene) is unlikely to have been mentioned, but has no influence on the properties of the polymer, since the extremely long chain of -CH₂-CH₂-'s predominates!

  • The use of Ziegler-Natta catalysts to make more stereo-specific high density poly(ethene) and high density poly(propene) involves an ionic mechanism.

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APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)