Doc Brown's Advanced A Level
Organic Chemistry Revision Notes - Help in Revising Advanced Organic Chemistry
PART 14 ORGANIC ISOMERISM
and Stereochemistry Revision Notes
14.6 Organic stereoisomerism - stereoregular
poly(propene) and poly(styrene) case studies
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isomerism and stereochemistry notes
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The molecular structure of stereoregular
poly(propene) and poly(styrene) in their atactic, isotactic and syndiotactic forms is
described and its influence on the properties of each form of polypropene
All My synthetic
polymer-plastics revision notes pages
Introduction to addition polymers: poly(ethene), poly(propene), polystyrene, PVC,
PTFE - structure, uses
More on the
uses of plastics, issues with using plastics, solutions and recycling
Introducing condensation polymers: Nylon, Terylene/PET, comparing
thermoplastics, fibres, thermosets
for more advanced level organic chemistry students
Polymerisation of alkenes to addition polymers - structure, properties, uses of
The manufacture, molecular structure, properties and uses of
chemistry - a mention of
The structure, properties and uses of
polyesters and polyamides involving aromatic monomers
chemistry of amides including Nylon formation, structure, properties and uses
Stereoregular polymers -
isotactic/atactic/syndiotactic poly(propene) - use of Ziegler-Natta
14.6 Organic stereoisomerism - poly(propene) case studies
study 2c.1 Stereoregular
poly(propene) using alkylaluminium (the German Ziegler) catalysts
and the Italian
produced a mixture of two forms of poly(propene).
Using Ziegler-Natta catalyst you can get very
stereospecific addition polymerisation to particular regular orientation
of the side-chains off the backbone carbon chain of the polymer.
An amorphous (non-crystalline) form called atactic
poly(propene) which has an irregular structure due to the random
arrangement of the methyl groups attached to the main carbon-carbon chain.
at random about 50% of hydrogen/methyl groups in front/back of C-C-C chain viewing
The two different side groups are
randomised on either side of the polymer carbon atom chain.
Atactic poly(propene) is formed
by a non-stereospecific catalyst.
It tends to softer and more flexible (than the isotactic form, below) and is
used for roofing materials, sealants and other weatherproof coatings.
A crystalline regular structure form called isotactic
poly(propene) in which all the side chain methyl groups have the same regular
orientation along the carbon-carbon polymer chain.
100% of methyl groups in front of C-C-C chain viewing plane, all H's at
Isotactic poly(propene) is formed
by a stereospecific catalyst.
Each side group is always on the
same side of the polymer carbon atom chain.
structure maximises the molecule-molecule contact and so increasing the
intermolecular forces compared to the atactic form.
This regular structure
is much stronger (than the atactic form above) and is used in sheet and
film form for packaging and carpet fibres.
Ziegler managed to produce 100% yield of the isotactic form which is an
example of a stereoregular polymer (shown below).
catalysts allows chemists to make polymers with properties tailor-made for
a specific purpose.
In syndiotactic polypropene, the methyl groups
regularly from side to side down the -C-C-C- chain.
regular alternation of 50% of hydrogen/methyl groups in front/back of C-C-C chain viewing
Each side group is alternately on
the same side of the polymer carbon atom chain.
is formed by a stereospecific catalyst.
Its properties are
closer to those of isotactic poly(propene) rather than the atactic form i.e. the
regular polymer structure produces stronger intermolecular forces and a
more crystalline form than the atactic poly(propene).
The atactic form of poly(propene) is made
by free radical polymerisation.
The isotactic and syndiotactic
stereospecific forms are made using Ziegler-Natta polymerisation catalysts.
all have the same simplified structural polymer formula of poly(propene), i.e.
, but the spatial orientation of the (CH3) groups
allows the different stereoisomeric forms to exist with different physical
The order of polymer tensile strength is isotactic
> syndiotactic > atactic poly(propene)
The structure of poly(phenylethene)
The three stereochemical forms of
The comments for poly(propene) also
apply here to poly(phenylethene),
so, again, the order of strength is
isotactic > syndiotactic > atactic
The atactic form of polystyrene is
made by free radical polymerisation.
The isotactic and syndiotactic
stereospecific forms are made using Ziegler-Natta polymerisation
Isotactic poly(phenylethene) is the most
regular structure and the strongest form where the intermolecular forces of
attraction are maximised because the molecules can pack the closest together
of the three stereospecific forms of the polymer.
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