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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
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 |
TOP OF PAGE and
sub-index
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 (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
TOP OF PAGE and
sub-index
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 Baekalite
See also
comparison of thermoplastic addition
polymers with thermoplastic synthetic fibres from
condensation polymers and hard rigid thermoset plastics
TOP OF PAGE and
sub-index
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)
TOP OF PAGE and
sub-index
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, 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
TOP OF PAGE and
sub-index
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
TOP OF PAGE and
sub-index
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
TOP OF PAGE and
sub-index
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)
TOP OF PAGE and
sub-index
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.
TOP OF PAGE and
sub-index
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-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.
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.
TOP OF PAGE and
sub-index
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).
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).
The structural formula equation for the polymerisation of
the monomer methyl 2-cyanopropenoate to poly(methyl 2-propenoate).
Also known as poly(methyl cyanoacrylate)
The molecular structure and skeletal formula of methyl
2-cyanopropenoate to poly(methyl 2-propenoate).
Poly(methyl cyanoacrylate) is used in super
glue.
TOP OF PAGE and
sub-index
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.
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).
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.
TOP OF PAGE and
sub-index
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.
Make sure, if given a copolymer structure, you can
recognise what the original monomers were.
TOP OF PAGE and
sub-index
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:
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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