Doc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

10.3 Reaction mechanisms of ALKENES

Free radical polymerisation

doc b oil notes


10.3.7 The free radical addition polymerisation of an alkene

The organic synthesis of poly(alkenes) like poly(ethene) from alkenes

  • What is the mechanism of polymerising ethene to form poly(ethene)?

  • e.g. nCH2=CH2 ==> -(-CH2-CH2-)-n      [see mechanism 32 below]

  • Polyalkenes such as low density poly(ethene) can be made by polymerising the alkene monomer with oxygen or peroxide catalysts.

organic reaction mechanisms

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 onto 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. 


    • 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.

  • -

keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms steps free radical mechanism of the polymerisation of alkenes, ethene polymerised to poly(ethene)

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

GCSE grade 9-1 & IGCSE CHEMISTRY Doc Brown's Travel Pictures & Notes
GCSE 9-1 Physics Revision Notes GCSE 9-1 Biology Revision Notes
All website content Dr Phil Brown 2000 onwards. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial. Email doc b:

 Doc Brown's Chemistry 


 For latest updates see

 Have your say about doc b's website