Part 2.8 The chemistry of ALKENES - unsaturated hydrocarbons - addition polymerisation to produce poly(alkene) 'plastics'

 Index of GCSE level Oil - Useful Products Chemistry Revision Notes

2.8 The reaction of alkenes to form addition polymers - structure, properties & uses of poly(alkene) polymers

Please note that I'd already written quite a bit about poly(alkenes) before reaching my Part 2. Alkenes, for my Advanced Level Organic Chemistry Notes, so I'm not repeating much of the material.

Links will point to relevant pages, BUT extra notes are added on this page where appropriate for advanced level organic chemistry students of alkene based addition polymers.

Advanced A level students MUST know all the material on the GCSE level pages, much of which I do NOT repeat, but provide links to the relevant polymer pages.

Introduction (and links) 

Individual addition polymer notes on poly(alkenes) for advanced A level students on this page

The thermal stability of poly(alkene) plastics (this page)

Introduction to the formation and structure of addition polymers formed from alkenes

General equation for the addition polymerisation of alkenes.

The monomer has a C=C double bond (sigma + pi) and the weaker pi bond is broken and two new strong C-C sigma bonds are formed linking the monomer molecules together to form the polymer, a poly(alkene).

Since a weak bond is replaced by a strong bond, the polymerisation is an exothermic process.

Introduction to the formation and structure addition polymers from alkenes

Free radical polymerisation to give poly(alkene) polymers e.g. ethene ==> poly(ethene)

Due to the large size of the molecule (M_r typically ?), strong C-C, C-O, C-F or C-F covalent bonds, addition polymers made from alkenes are very resistant to chemical or biological attack.

They are synthetic materials not found in nature, so no enzymes have evolved to break them down.

Reaction conditions for poly(alkene) manufacture

(a) Free radical polymerisation process

The process is conducted at an elevated temperature e.g. ~200^oC and under high pressure.

The polymerisation can be initiated by any source of free radicals e.g. organic peroxides R-O-O-R which split homolytically R-O-O-R  ===>  2RO• to promote the chain reaction.

Free radicals are very reactive and can attack at any point of a growing polymer chain, and the result is a highly branched poly(alkene) polymer which has quite a mixed composition, particular in the case of poly(ethene) and poly(propene).

Poly(phenylethene) ('polystyrene') is also manufactured in this way, but the polymer chains are not as branched.

Mechanism details Free radical polymerisation to give poly(alkene) polymers e.g. ethene ==> poly(ethene)

 

(b) The Ziegler-Natta catalyst process

The Ziegler-Natta process uses highly specialised catalysts of titanium and aluminium compounds e.g. TiCl₃ and Al(CH₂CH₃)₂Cl. The mechanism is complex and involves the formation of a complex between the catalyst and the monomer molecule.

The process works at temperatures as low as 60^oC and the alkene is passed over the catalyst bed.

The conversion % is quite low, but unreacted monomer molecules are recycled over the catalyst.

The addition is much more stereospecific than free radical polymerisation, far less branching occurs and unbranched poly(ethene is made in this way.

The result is a very regular arrangement of the polymer units derived from the monomer and because the molecules can line up more closely to each other, the intermolecular forces are increased making a stronger polymer.

The Ziegler-Natta process has two advantages over the free radical polymerisation.

(i) Much less random branching in the polymer chain, more controlled stronger polymer.

(ii) Lower temperatures can be used, saving energy.

See also Stereoregular polymers e.g. poly(propene) and poly(phenylethene)

 

Advantages poly(alkene) materials

It is useful to have materials that resist corrosion and chemical attack, that can be used to make a variety of objects for many applications and can have a long useful life - the properties and uses are described eight different polymers.

 

Disadvantages of poly(alkene) materials

Unfortunately, the chemical stability and long life mean that when discarded into the environment they cause pollution effects, litter, bulk plastics in land-fill sites, plastics affect wild-life e.g. animals can be trapped in plastic mesh, plastic objects found in the stomachs of dead animals.

Toxic gases can be produced in incinerators.

 

AND on other pages

More on the uses of polymers - plastics

Problems with using, recycling, disposing of polymer plastics and pollution problems

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Examples of poly(alkene) addition polymers - their structure and uses follow ...

1. Poly(ethene) dating from 1939

Simplified abbreviated structural formulae equation:  n CH₂=CH₂  ===>   –(–CH₂–CH₂–)–_n

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(ethene) derived from the ethene monomer i.e -(-CH₂-CH₂-)-, but written out as a clear structural formula, AND be able to identify the original monomer molecular structure of ethene from a given poly(ethene) polymer structure.

Examples of poly(alkene) polymer molecules e.g. manufacture of poly(ethene)

Summary of the properties and uses of poly(ethene)

A few C-C cross-links put still plastic - pliable and low melting/softening temperature.

BUT NOT a with lots of C-C cross-links to give a rigid 3D network of bonds like the full cross-linked rigid thermoset structure like Melamine or the old brown Bakelite (Baekalite)

See also comparison of thermoplastic addition polymers with thermoplastic synthetic fibres from condensation polymers and hard rigid thermoset plastics

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2. Poly(propene) dating from ?

Simplified abbreviated structural formulae equation:  n CH₂=CH–CH₃  ===>   –{–CH₂–CH(CH₃)–}–_n

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(propene) derived from the propene monomer i.e. -{-CH₂-CH(CH₃)-}-, but written out as a clear structural formula, as identifying the original monomer, AND be able to identify the original propene monomer molecular structure from a given poly(propene) polymer structure.

Poly(propene) - formation, structure, properties and uses

The stereoisomers of poly(propene)

See organic chemistry Part 14.6 Stereoregular polymers -  isotactic/atactic/syndiotactic poly(propene)

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3. Poly(chloroethene) dating from 1938

Simplified abbreviated structural formulae equation:  n CH₂=CH–Cl  ===>   –(–CH₂–CHCl–)–_n

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(chloroethene) derived from the chloroethene monomer i.e -(-CH₂-CHCl-)-, but written out as a clear structural formula, AND be able to identify the original chloroethene monomer molecular structure from a given poly(chloroethene) polymer structure.

Chloroethene, the monomer for producing poly(chloroethene), PVC, is made in two stages from ethene, which originates from cracking oil fractions. In this case the haloalkane is an intermediate compound.

ethene  +  chlorine  == (1) =>  1,2-dichloroethane  == (2) ==>  chloroethene  +  hydrogen chloride

H₂C=CH₂  +  Cl₂  ===>  ClH₂CCH₂Cl  ===> H₂C=CHCl  +  HCl

Stage (1) This addition reaction catalysed by iron(III) chloride (FeCl₃) and is exothermic.

Stage (2) Is a thermal decomposition and elimination reaction, 500^oC and at high pressure (1.5 to 3.0 MPa (15 - 30 atm) and is a very endothermic reaction.

The anhydrous hydrogen chloride formed can be used to make other chloroalkanes or chloroalkenes, or dissolved in water to make hydrochloric acid.

Poly(chloroethene) - formation, structure, properties and uses

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4. Poly(tetrafluoroethene) Teflon dates from 1950

Simplified abbreviated structural formulae equation:  n CF₂=CF₂  ===>   –(–CF₂–CF₂–)–_n

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(tetrafluoroethene) derived from the tetrafluoroethene monomer i.e -(-CF₂-CF₂-)-, but written out as a clear structural formula, AND be able to identify the original monomer molecular structure from a given polymer structure.

Best known for non-stick surfaces and non-lubricated bearings in machinery because PTFE has a very low coefficient of friction.

PTFE has a helical structure with the fluorine atoms on the outside of the coils.

The very strong C-F bonds make PTFE very resistant to chemical attack.

For more see Poly(tetrafluoroethene) or PTFE - formation, structure, properties and uses

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5. Polyacrylonitrile or poly(propenenitrile) dating from ?

Simplified abbreviated structural formulae equation:  n CH₂=CH–C≡N  ===>   –{–CH₂–CH(C≡N)–}–_n

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(propenenitrile) derived from the propenenitrile monomer i.e -{-CH₂-CH(CN)-}-, but written out as a clear structural formula, AND be able to identify the original monomer molecular structure from a given polymer structure.

Poly(acrylonitrile) fibres are used for clothing fabrics. The polymer is stretched and extruded so that the molecules line up alongside each other to maximise the intermolecular forces between them - so increasing and maximising the strength of the fibres.

It has a low density and good thermal stability and chemical stability.

Used to make strong fibres

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6. Poly(phenylethene) 'polystyrene' dating from 1930

Simplified abbreviated structural formulae equation:  n CH₂=CH–C₆H₅  ===>   –{–CH₂–CH(C₆H₅)–}–_n

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(phenylethene) derived from the phenylethene monomer  i.e -{-CH₂-CH(C₆H₅₎-}-, but written out as a clear structural formula, AND be able to identify the original monomer molecular structure from a given polymer structure.

Polystyrene - formation, structure, properties and uses

See organic chemistry Part 14.6 Stereoregular polymers -  isotactic/atactic/syndiotactic poly(phenylethene)

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7. Poly(methyl methacrylate)   PMMA dating from ?

Simplified abbreviated structural formulae equation: 

n CH₂=C(CH₃)COOCH₃  ===>   –{–CH₂–C(CH₃)(COOCH₃)–}–_n

The monomer is called methyl 2-methylpropenoate {methyl ester of prop-2-enoic acid (acrylic acid)}

Therefore the systematic name for PMMA is poly(methyl

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(methyl methacrylate) derived from the monomer, but written out as a clear structural formula, AND be able to identify the original monomer molecular structure from a given polymer structure.

PMMA is a transparent rigid plastic often encountered under the trade name Perspex and used as an alternative to glass and making baths.

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8. Poly(ethenol) dating from ?

Simplified abbreviated structural formulae equation:  n CH₂=CH–OH  ===>   –{–CH₂–CH(OH)–}–_n

Sometimes incorrectly written as polyethanol

On the 2nd diagram, on the left, you need to be able to identify the repeating unit of poly(ethenol) derived from the theoretical ethenol monomer i.e -{-CH₂-CH(OH)-}-, but written out as a clear structural formula, AND be able to identify the original monomer molecular structure from a given polymer structure.

Poly(ethenol) has the unusually poly(alkene) property of dissolving in water - the hydrogen bonding with water allows solvation.

The polymer chain has lots of hydroxy groups to hydrogen bond in water  e.g. CO-H^δ+llll^δ-O-H₂  (see above diagram).

Uses of poly(ethenol)

(i) 'Disposable' laundry bags - because the polymer dissolves away leaving the laundry clothing to be washed.

(ii) Detergent capsules (liquitabs) in washing machines, the pod/pouch dissolves away releasing the detergent for washing clothes.

 

The monomer and poly(alkene) 11. in my notes called  Polyvinylacetate (PVA) !

The systematic name for poly(vinyl acetate) is poly(ethenyl ethanoate).

The 'theoretical' monomer 'ethenol' H₂CH=CHOH is too unstable to be of synthetic use.

Therefore, poly(ethenol) has to be manufactured by an indirect route using polyvinylacetate (PVA), diagram below.

Vinyl acetate (an old name for an unsaturated ester of ethanoic acid) is polymerised to give poly(vinyl acetate).

The polymer desired, poly(ethenol) is effectively made from an ester of ethanoic acid.

Therefore, on heating the poly(vinyl acetate) with the alcohol methanol, a transesterification reaction yields poly(ethenol) and an ester by-product of methyl ethanoate.

Poly(vinyl acetate) is used in paints and adhesives.