14.6 Organic stereoisomerism - poly(propene) case studies

Case study 2c.1 Stereoregular polymers

(a) Poly(propene)

Making poly(propene) using alkylaluminium (the German Ziegler) catalysts and the Italian chemist Natta, 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.

(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.

Atactic poly(propene): at random about 50% of hydrogen/methyl groups in front/back of C-C-C chain viewing plane.

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.

(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.

Isotactic poly(propene): 100% of methyl groups in front of C-C-C chain viewing plane, all H's at back.

Isotactic poly(propene) is formed by a stereospecific catalyst.

Each side group is always on the same side of the polymer carbon atom 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.

(3) In syndiotactic polypropene, the methyl groups alternate regularly from side to side down the -C-C-C- chain.

Syndiotactic poly(propene): regular alternation of 50% of hydrogen/methyl groups in front/back of C-C-C chain viewing plane.

Each side group is alternately on the same side of the polymer carbon atom chain.

Syndiotactic poly(propene)  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.

 

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.

The order of polymer tensile strength is isotactic  >  syndiotactic  >  atactic  poly(propene)

 

Poly(phenylethene) polystyrene

See other notes on polyalkenes

A LEVEL ADVANCED LEVEL ORGANIC CHEMISTRY Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)