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Pre-university Advanced Level Organic Chemistry: The structure, properties and uses of poly(alkenes)

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Part 2. The chemistry of ALKENES - unsaturated hydrocarbons

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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.


Introduction (and links) 

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

1. Poly(ethene) 2. Poly(propene)
3. Poly(chloroethene) PVC 4. Poly(tetrafluoroethene) PTFE
5. Poly(acrylonitrile) 6. Poly(phenylethene) Polystyrene
7. Poly(methyl methacrylate) PMMA 8. Poly(ethenol)
9. Poly(propenamide) 10. Poly(propenoic acid)
11. Polyvinylacetate (PVA) 12. Poly(methyl cyanoacrylate)
13. Poly(but-1-ene) 'polybutylene' 14. An example of a copolymer

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

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

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

doc b oil notes

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 (Mr 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. ~200oC 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 60oC 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

polymerisation equation for ethene monomer to poly(ethene) polythene polymer advanced A level organic chemistry doc brown's revision notes

skeletal formula of poly(ethene) polythene polyethylene polymer advanced A level organic chemistry doc brown's revision notes

Simplified abbreviated structural formulae equation:  n CH2=CH2  ===>   (CH2CH2)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 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 ?

polymerisation equation for propene monomer to poly(propene) polypropylene polymer advanced A level organic chemistry doc brown's revision notes

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

Simplified abbreviated structural formulae equation:  n CH2=CHCH3  ===>   {CH2CH(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

polymerisation equation for chloroethene monomer to poly(chloroethene) polyvinyl chloride PVC polymer polymer advanced A level organic chemistry doc brown's revision notes

skeletal formula of poly(chloroethene) polyvinyl chloride PVC polymer advanced A level organic chemistry doc brown's revision notes

Simplified abbreviated structural formulae equation:  n CH2=CHCl  ===>   (CH2CHCl)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, 500oC 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

polymerisation equation for tetrafluoroethene monomer to poly(tetrafluoroethene) PTFE polymer advanced A level organic chemistry doc brown's revision notes

skeletal formula of poly(tetrafluoroethene) PTFE polytetrafluoethene polymer advanced A level organic chemistry doc brown's revision notes

Simplified abbreviated structural formulae equation:  n CF2=CF2  ===>   (CF2CF2)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 ?

polymerisation equation for propenenitrile acrylonitrile monomer to poly(propenenitrile) polyacrylonitrile polymer advanced A level organic chemistry doc brown's revision notes

skeletal formula of poly(propenenitrile) polyacrylonitrile polymer advanced A level organic chemistry doc brown's revision notes

Simplified abbreviated structural formulae equation:  n CH2=CHCN  ===>   {CH2CH(CN)}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

polymerisation equation for phenylethene styrene to poly(phenylethene) polystyrene polymer advanced A level organic chemistry doc brown's revision notes

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

Simplified abbreviated structural formulae equation:  n CH2=CHC6H5  ===>   {CH2CH(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 ?

polymerisation equation for methyl methacrylate monomer to poly(methyl methacrylate) polymethylmethacrylate PMMA Perspex polymer advanced A level organic chemistry doc brown's revision notes

skeletal formula of poly(methyl methacrylate) polymethylmethacrylate PMMA polymer advanced A level organic chemistry doc brown's revision notes

Simplified abbreviated structural formulae equation: 

n CH2=C(CH3)COOCH3  ===>   {CH2C(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 ?

polymerisation equation for ethenol vinyl alcohol monomer to poly(ethenol) polyvinyl alcohol polymer advanced A level organic chemistry doc brown's revision notes

skeletal formula of poly(ethenol) polyvinyl alcohol polyethanol polyvinylalcohol polymer advanced A level organic chemistry doc brown's revision notes

Simplified abbreviated structural formulae equation:  n CH2=CHOH  ===>   {CH2CH(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.

hydrogen bonding between poly(ethenol) PVA polyvinyl alcohol molecules and water advanced A level organic chemistry doc brown's revision notes

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-Hδ+llllδ-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.

manufacture of poly(ethenol) from polyvinyl acetate using methanol polymerising vinyl acetate transesterification reaction by-product methyl methanoate

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.


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9. Poly(propenamide) and 10. Poly(propenoic acid) dating from ?

These are two more poly(alkenes), but with some extra properties not usually associated with poly(alkenes).

Poly(propenamide) Poly(propenoic acid) when crosslinked swell up as absorb water by hydrogen bonding used in disposable nappies water absorbing gels in soil of pot plants

From the diagrams above, you need to be able to identify the repeating unit of the polymer derived from the monomers i.e -(H2C-CH-CONH2)-  and   -(H2C-CH-COOH)-, but written out as clear structural formulae, AND be able to identify the original monomer molecular structure from a given polymer structure.

When poly(propenamide) and poly(propenoic acid), particularly when cross-linked swell up as they absorb water by hydrogen bonding via the amino NH2, hydroxy OH and carbonyl C=O groups

e.g. N-Hδ+llllδ-O-H2  and  CO-Hδ+llllδ-O-H2.

These two polymers find use in a variety of 'domestic' products e.g.

(i) Absorbent disposable nappies

(ii) To retain water, water absorbing gels are added to the soil of potted plants or hanging baskets.

(iii) One of my daughters bought me a toy dinosaur egg for Christmas - a sort of 'chemistry' present!

I didn't know at the time that it was made of poly(propenamide) or poly(propenoic acid).

The shell is made of a non-cross-linked polymer that is water soluble.

After the egg is placed in water the shell dissolves or expands and breaks, but the yoke is a cross-linked polymer that absorbs water and expands.

This creates the impression the egg hatches and a small dinosaur emerges from the egg!


12. Poly(methyl 2-cyanopropenoate) Poly(methyl cyanoacrylate)

Very similar in structure to PMMA (a -CN group instead of a -CH3 group).

polymerisation of methyl 2-cyanopropenoate to poly(methyl 2-propenoate) molecular structure poly(methyl cyanoacrylate) structural formula equation

The structural formula equation for the polymerisation of the monomer methyl 2-cyanopropenoate to poly(methyl 2-propenoate).

Also known as poly(methyl cyanoacrylate)

skeletal formula of poly(methyl cyanoacrylate) poly(methyl 2-propenoate)

The molecular structure and skeletal formula of methyl 2-cyanopropenoate to poly(methyl 2-propenoate).

Poly(methyl cyanoacrylate) is used in super glue.


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13. Poly(but-1-ene) 'polybutylene'

The alkene but-1-ene can be polymerised with peroxide or Ziegler-Natta to form poly(but-1-ene.

structural formula equation skeletal formula equation for addition polymerisation of but-1-ene (1-butene) to yield poly(but-1-ene) or poly(1-butene) polybutylene

The structural formula equation and skeletal formula equation for the addition polymerisation of but-1-ene (1-butene) to yield poly(but-1-ene) or poly(1-butene).

abbreviated structural formula skeletal formula of poly(but-1-ene) molecular structure poly(1-butene) polybutylene poly(butylene)

The abbreviated structural formula and skeletal formula for poly(but-1-ene).

Although it softens at 135oC, because of its specific properties, poly(but-1-ene) is mainly used in pressure piping, flexible packaging, water heaters, compounding and hot melt adhesives.

The softening point is relatively low because CH2CH3 side-chains prohibit very close packing of the main polymer chains.


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14. Example of a copolymer - copolymerisation off ethene and propene monomers

It is possible for 'polymer design' purposes to copolymerise two different alkenes into polymer chains of alternating monomer units.

For example you can copolymerise ethene and propene to make a copolymer of these two alkenes.

copolymerisation of ethene and propene alkene monomers copolymer polymers molecular structure structural formula example explained diagram

Make sure, if given a copolymer structure, you can recognise what the original monomers were.


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The thermal stability of poly(alkene) plastics

When poly(alkene) polymers are heated, they initially soften and at higher temperatures they melt.

At higher temperatures still, undergo thermal decomposition giving off hydrocarbon molecules of low molecular mass e.g. alkanes and alkenes such as methane, ethene, ethane, propene and pentene, but only a small % of the products are the original monomer - not easy to recycle plastics by this particular mode, which is why a lot of waste plastic is burned to generate electrical power!

Melting temperatures: poly(ethene) 180 to 270oC;  poly(propene) 200 to 280oC;  poly(styrene) 170 to 280oC.

There is no sharp melting point for polymers because they are not pure compounds in the sense the molecules are not identical.

For poly(alkenes) like everyday 'polythene', there is a wide distribution of molecular masses and each size of molecule has its own melting point.

Matters are further complicated by a little bit of cross-linking in them too (you probably didn't know that!).

Therefore thermoplastic polymers soften, and eventually melt, over a wide temperature range - you see the same effect in animal fats, butter (derived from milk fats) or margarine (derived from vegetable oils) where there is a range of glyceride esters.

The activation energies for these thermal decompositions are high because it involves breaking strong carbon-carbon or carbon-hydrogen bonds in the polymer chain.

The activation energies (Ea) for purely thermal degradation in an inert nitrogen atmosphere are quoted below and the lowest temperature at appreciable thermal decomposition begins.

For poly(styrene): Ea ~150-200 kJ/mol for thermal decomposition >400oC,

For poly(ethene): Ea ~150-240 kJ/mol for thermal decomposition >400oC,

For poly(propene): Ea ~250 kJ/mol for thermal decomposition >400oC.

Very reactive free radicals (species with an unpaired electron), are initially formed by the weakest C-C bonds breaking.

After that, a whole series of complex free radical reactions breakdown the plastic to give stable hydrocarbon products with no free electrons.

The activation energies (Ea) for thermooxidative degradation in air are quoted below and the lowest temperature at appreciable thermal decomposition begins. All the values are lower than in nitrogen because the oxygen from air allows the easier formation of the highly reactive free radicals.

For poly(styrene): Ea ~125 kJ/mol for thermal decomposition >250oC,

For poly(ethene): Ea ~80 kJ/mol for thermal decomposition >250oC,

For poly(propene): Ea ~90 kJ/mol for thermal decomposition >200oC.

Some examples of the free radical reactions are quoted below ( = unpaired electron on the reactive free radical):

initiation stepsR-H  ===> R  +  H  (R = alkyl)

or  R-R  ===> 2R  (heterolytic bond fission in the carbon chain to give two free radicals)

R  +  O2  ==> ROO  (peroxide radical formed with oxygen, chain propagation step)

ROO  +  RH  ===>  ROOH  + R  (chain propagation step)

ROOH  ===> RO  +  RO  (further initiation step via heterolytic bond fission of the weak O-O peroxide bond)

After that, a whole series of complex free radical reactions breakdown the plastic to give stable hydrocarbon products with no free electrons, but the free radicals are more easily formed at lower temperatures than in inert nitrogen, hence the lower decomposition temperatures.

 

This sort of research is important when considering the high temperature thermal degradation and combustion (incineration) of large of poly(alkene) polymers e.g. trying to recycle the original monomer or ensuring complete combustion.

The data for this section was obtained from:

https://www.sci.utah.edu/publications/Pet2001a/2001_Peterson_Vyazovkin_Wight_Polystyrene_Polyethylene_and_Polypropylene.pdf

The poly(propene) used in this study had an average molecular mass of 12,000

The poly(styrene) used in this study had an average molecular mass of 128,000

See also Section (3) The vapourisation and thermal decomposition of alkanes


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