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Part 2.
The chemistry of
ALKENES - unsaturated hydrocarbons
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
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 where appropriate for advanced level
organic chemistry students of alkene based addition polymers.
INDEX of ALKENE revision notes
Sub-index
for this page
Introduction
(links) *
Individual addition polymer notes on poly(alkenes)
The thermal stability of poly(alkene) plastics
(this page)
AND
on other pages
More on the uses of polymers - plastics
Problems with using, recycling,
disposing of polymer plastics and pollution problems
INDEX of ALKENE revision notes
All Advanced A Level Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Notes
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Introduction to the formation and
structure of addition polymers formed from alkenes
General equation for the addition
polymerisation of alkenes.
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 for alkenes and very resistant to chemical or biological attack.
They are synthetic material not found in
nature, so no enzymes have evolved to break them down.
Advantages
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
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.
Examples
of poly(alkene) addition polymers - their structure and uses follow ...
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1.
Poly(ethene)
Simplified abbreviated structural
formulae equation:
n CH2=CH2 ===>
–(–CH2–CH2–)–n
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)
Simplified abbreviated structural
formulae equation:
n CH2=CH–CH3
===> –{–CH2–CH(CH3)–}–n
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)
Simplified abbreviated structural
formulae equation:
n CH2=CH–Cl ===>
–(–CH2–CHCl–)–n
Poly(chloroethene) - formation, structure, properties and
uses
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4.
Poly(tetrafluoroethene)
Simplified abbreviated structural
formulae equation:
n CF2=CF2
===> –(–CF2–CF2–)–n
Poly(tetrafluoroethene)
or PTFE - formation, structure, properties and uses
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5.
Polyacrylonitrile
Simplified abbreviated structural
formulae equation:
n CH2=CH–C≡N
===> –{–CH2–CH(C≡N)–}–n
 Poly(acrylonitrile) fibres
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.
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6.
Poly(phenylethene)
Simplified abbreviated structural
formulae equation:
n CH2=CH–C6H5
===> –{–CH2–CH(C6H5)–}–n
Polystyrene - formation, structure, properties and uses
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7.
Poly(methyl methacrylate)
PMMA
Simplified abbreviated structural
formulae equation:
n CH2=C(CH3)COOCH3
===> –{–CH2–C(CH3)(COOCH3)–}–n
The monomer is called methyl prop-2-enoic acid {methyl
ester of prop-2-enoic acid (acrylic acid)}
PMMA is a transparent rigid plastic often
encountered under the trade name Perspex.
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8.
Poly(ethenol)
Simplified abbreviated structural
formulae equation:
n CH2=CH–OH ===>
–{–CH2–CH(OH)–}–n
Sometimes incorrectly written as
polyethanol
Used for 'disposable' laundry bags -
because the polymer dissolves away leaving the laundry clothing to be washed.
Used for detergent capsules in washing machines,
the pod/pouch dissolves away releasing the detergent for washing clothes.
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 steps: R-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:
http://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
x-ref with dfalkanes12
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What
next?
Index of all the ALKENE revision notes
All Advanced Level Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
[SEARCH
BOX] ignore the adverts at the top
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