Advanced Level Organic Chemistry: ISOMERISM - stereoisomerism - poly(propene)
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) case study
The molecular structure of stereoregular poly(propene) in its atactic, isotactic and syndiotactic forms is described and its influence on the properties of each form of polypropene
14.6 Organic stereoisomerism - poly(propene) case studies
Case study 2c.1 Stereoregular polymers e.g. poly(propene)
Making poly(propene) using alkylaluminium (the German Ziegler) catalysts, the Italian chemist Natta, produced a mixture of two forms of poly(propene).
(1) 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. It tends to softer and more flexible (than the isotactic form, below) and is used for roofing materials, sealants and other weatherproof coatings.
Atactic poly(propene): at random about 50% of hydrogen/methyl groups in front/back of C-C-C chain viewing plane.
(2) 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. The stereoregular 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. Using Ziegler-Natta catalysts, Ziegler managed to produce 100% yield of the isotactic form which is an example of a stereoregular polymer (shown below). Using these catalysts allows chemists to make polymers with properties tailor-made for a specific purpose.
Isotactic poly(propene): 100% of methyl groups in front of C-C-C chain viewing plane, all H's at back.
(3) In syndiotactic polypropene, the methyl groups alternate regularly from side to side down the -C-C-C- chain. 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).
Syndiotactic poly(propene): regular alternation of 50% of hydrogen/methyl groups in front/back of C-C-C chain viewing plane.
They all have the same simplified structural polymer formula of poly(propene), i.e. -[-CH2-CH(CH3)-]-n , but the spatial orientation of the (CH3) groups allows the different stereoisomeric forms to exist with different physical properties.
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