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
10.3.7 The free radical addition polymerisation
of an alkene
organic synthesis of poly(alkenes) like poly(ethene) from alkenes
What is the mechanism
of polymerising ethene to form poly(ethene)?
as low density poly(ethene) can be made by polymerising the
alkene monomer with oxygen or peroxide catalysts.
mechanism 32 -
the peroxide catalysed free radical polymerisation of ethene
For peroxide catalysed
polymerization, the initial free radicals are formed by the
homolytic bond fission of the O-O bond in a peroxide molecule.
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.
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.
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
The use of
Ziegler-Natta catalysts to make more stereo-specific high
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
and Organic Synthesis INDEX