amorphous & crystalline regions of polmers polystrene poly(phenylethene) Stereochemical aspects of stereoregular polymers syndiotactic atactic isotactic poly(propene) regular and irregular spatial forms GCE AS A2 organic chemistry revision notes

PART 14 ORGANIC ISOMERISM and Stereochemistry Revision Notes

14.6 Organic stereoisomerism: irregular and stereoregular polymers

Introducing amorphous and crystalline regions of a polymer's molecular structure, and the atactic, isotactic and syndiotactic forms poly(propene) and poly(styrene) as case studies

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All my advanced A level organic chemistry notes

All my advanced A level isomerism and stereochemistry notes

Introduction to stereoregular polymers

The molecular structure of stereoregular poly(propene) and poly(phenylethene) in their atactic, isotactic and syndiotactic forms are described and its influence on the properties of each form of these polymers (once called polypropene and polystyrene). The ideas outlined below can be applied to any thermoplastic polymer

You also need to know what is meant by 'stereoregularity' and the different 'amorphous' and 'crystalline' regions of the internal molecular structure of a polymer (both types of 'local' polymer structure are shown in the diagram below).

You need to know what is meant by the tacticity terms: atactic, isotactic and syndiotactic and how the different molecular structures affect the physical properties of a polymer e.g. strength and heat resistance.

amorphous & crystalline regions in poly(propene) poly(phenylethene) polystyrene stereoregular molecular structure

For the internal molecular structure of a polymer, both amorphous and crystalline regions can exist, in a semi-crystalline state because both regions are present.

The crystalline regions of a polymer are characterized by a highly ordered, repeating pattern of polymer chains packed tightly together - all line up to maximise the intermolecular bonding (* from any type of intermolecular forces)..

In contrast, the amorphous regions have a random, disordered arrangement in which the long polymer molecules are entangled and this reduces the intermolecular bonding. Cooked and served spaghetti is a good analogy!

* The intermolecular bonding may be due to (a) instantaneous dipole - induced dipole, (b) permanent dipole - induced dipole or (c) permanent dipole - permanent dipole (including hydrogen bonding) attractive forces.

Stereoregularity and tacticity

Stereoregularity describes the arrangement of the pendant groups along the polymer chain of repeating units.

In stereoregular polymers, these pendant groups are arranged in a specific, repeating pattern.

The pendant groups are methyl in poly(propene) and phenyl in poly(phenylethene).

There are various types of stereoregularity (or irregularity!) e.g.

(1) Atactic conformation: The pendant groups are arranged randomly on either side of the chain - the least stereoregular structure.

(2) Isotactic conformation: The pendant groups are all on the same side of the chain - the most stereoregular structure.

(3) Syndiotactic conformation: The pendant groups alternate sides of the chain.

The proportion of each type of region significantly impacts the material's properties.

The regularity of a polymer's structure can influence its physical properties, such as its crystallinity and melting point. For example, stereoregular polymers like isotactic poly(propene) are much more crystalline, stronger and have a higher melting point (more thermally stable) than atactic poly(propene), which is rubbery and amorphous at the molecular level.

14.6 Organic stereoisomerism - poly(propene) case studies

Case study 2c.1 Stereoregular polymers

(a) Poly(propene) polypropylene

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.

The pendant side-chain group is methyl -CH₃

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 different pendant 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 be softer and more flexible (than the isotactic form, below) and is used for roofing materials, sealants and other weatherproof coatings.

(2) A more 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 pendant 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.

The isotactic forms are more crystalline, physical stronger and thermally more stable with a higher softening temperature.

(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 polymer tensile strength is isotactic > syndiotactic > atactic

and it is the same order for thermal stability e.g. for poly(propene)

(b) Poly(phenylethene) polystyrene

The pendant side-chain group is phenyl C₆H₅-

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 for poly(phenylethene)

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

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

Of the three stereospecific forms of the polymer, 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 in a more crystalline form.

See other notes on polyalkenes

Nylon - a polyamide thermoplastic e.g. nylon-6,6

Stereoregular nylon polymers have the pendant groups (side chains) on the backbone arranged in a regular, ordered way (e.g. all on same side of chain), rather than randomly distributed on either side of the chain. This regularity (tacticity), as mentioned above, affects the polymer's properties, particularly its crystallizability, which in turn affects the polymer's strength and flexibility.

While nylon is generally known for its toughness and versatility, specific nylon polymers can be made with varying degrees of stereoregularity.

Chiral nylon analogues: research has focused on synthesizing stereoregular, chiral analogues of nylon, which can exhibit specific optical properties (but the study of these particular nylon analogues are beyond pre-university level chemistry).

Structural formula and skeletal formula of Nylon-6,6 where n = very large number

structural formula of stereoregular nylon-6,6 molecular structure

Nylon fibres can be produced to maximise the stereoregularity of the polymer chain to maximise the intermolecular bonding, hence to maximise the strength of the nylon fibre.

Stereoregular nylon-6,6 refers to nylon-6,6 with a controlled manufactured ordered arrangement of its repeating units e.g. highly isotactic and so highly crystalline. Therefore, unlike regular nylon-6,6, where the polymer chain has a random mix of monomer pendant orientations, this stereoregular nylon-6,6 exhibits a specific and repeating pattern. This type of nylon-6,6 can lead to improved properties like greater mechanical strength and heat resistance (higher softening temperature).

Learning objectives for the molecular structure of stereoregular polymers

Know what is meant by a stereoregular polymer.

Be able to describe the amorphous regions and crystalline regions of a polymer and their effect on the physical properties of a thermoplastic.

Be able to describe and interpret molecular structure diagrams of the tacticity of polymer structure in terms of isotactic, syndiotactic and atactic conformations.

Know that isotactic stereoregular polymers are the more crystalline conformation giving greater physical strength and thermal stability.

Know that the regularity (or irregularity) of the conformation of pendant groups affects the properties of the polymer e.g. its physical strength of heat resistance (softening temperature).

Know that the order of polymer increase in tensile strength and thermal stability (softening temperature) is isotactic > syndiotactic > atactic

Know that stereoregular polymers of poly(alkenes) can be manufactured using specialised catalysts e.g. Ziegler-Natta catalysts.

Be able to sketch, describe and explain the structures of isotactic, syndiotactic and atactic conformations of polymers such as poly(propene) polypropylene and poly(phenylethene) polystyrene

Know that many other thermoplastic polymers can be manufactured in stereoregular forms e.g. nylon polymers.

All My synthetic polymer-plastics revision notes pages

Introduction to addition polymers: poly(ethene), poly(propene), polystyrene, PVC, PTFE - their structure, formation and 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

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