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. 

  • 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.

  • -


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!)


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