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Advanced A Level Organic Chemistry: ISOMERISM - stereoisomerism - poly(propene)

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Doc Brown's Advanced A Level Organic Chemistry Revision Notes - Help in Revising Advanced Organic Chemistry

PART 14 ORGANIC ISOMERISM and Stereochemistry Revision Notes

ATACTIC poly(propene) amorphous irregular structure (polypropene/polypropylene) (c) doc b

14.6 Organic stereoisomerism - stereoregular polymers

poly(propene) and poly(styrene) case studies

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All my advanced A level organic chemistry notes

All my advanced A level 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 methods

Introducing condensation polymers: Nylon, Terylene/PET, comparing thermoplastics, fibres, thermosets

Extra notes for more advanced level organic chemistry students

Polymerisation of alkenes to addition polymers - structure, properties, uses of poly(alkene) polymers

The manufacture, molecular structure, properties and uses of polyesters

Amides chemistry - a mention of polyamides

The structure, properties and uses of polyesters and polyamides involving aromatic monomers

The chemistry of amides including Nylon formation, structure, properties and uses

Stereoregular polymers -  isotactic/atactic/syndiotactic poly(propene) - use of Ziegler-Natta catalysts

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.

skeletal formula of poly(propene) polypropene polypropylene polymer advanced A level organic chemistry doc brown's revision notes 

(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) amorphous irregular structure (polypropene/polypropylene) (c) doc b

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) the most regular crystalline structure (polypropene/polypropylene) (c) doc b

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 crystalline structure (polypropene/polypropylene) (c) doc b

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 strength is isotactic  >  syndiotactic  >  atactic  poly(propene)

 


Poly(phenylethene) polystyrene

skeletal formula of poly(phenylethene) polystyrene polyphenylethene poly(styrene) polymer advanced A level organic chemistry doc brown's revision notes

The structure of poly(phenylethene)  or  polystyrene

stereospecfic forms of poly(phenylethene) atactic isotactic syndiotact poly(styrene) polystyrene polymer molecular structure

The three stereochemical forms of poly(phenylethene), poly(styrene).

The comments for poly(propene) also apply here to poly(phenylethene),

so, again, the order of strength is isotactic  >  syndiotactic  >  atactic  poly(phenylethene)

The atactic form of polystyrene is made by free radical polymerisation.

The isotactic and syndiotactic stereospecific forms are made using Ziegler-Natta polymerisation catalysts.

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.


See other notes on polyalkenes


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