Polymers
Addition polymers
Addition polymers are formed when molecules add together to form a product without
making any side products. All polymers made from alkenes are addition polymers. The
process is exothermic because stronger sigma bonds between the monomers replace weak
pi bonds within the monomer.
Mechanisms of addition polymerisation
There are two mechanisms, homolytic (free radical) and heterolytic (ionic).
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Homolytic addition polymerisation
This is the mechanism used to make low density polyethene. A free radical initiator is
used, at high pressure (1000 atm) and moderately high temperature (200oC).
Free radicals formed by breakdown of the initiator attack ethene molecules:
The product of the above reaction reacts with more ethene to extend the chain:
Individual chains can terminate by abstracting hydrogen from other chains, but this causes
branching.
Note that the readiness of free radicals to attack C-H bonds is responsible for the
branching process.
•
Heterolytic addition polymerisation
Heterolytic addition polymerisation can be started by cations or anions. The mechanism
used to make high density polyethene is cationic. A mixture of triethylaluminium and
titanium(IV)chloride (Zeigler-Natta catalyst) is heated at 60oC under 2-6 atm in an inert
solvent.
The Zeigler-Natta complex is attacked by an ethene molecule:
The product of the above reaction reacts with more ethene to extend the chain:
Chains can terminate by proton loss:
Note that there is no scope for branching to occur in this mechanism.
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Polymers
In addition to what you know about high and low density polyethene from module two,
in module four you need to know about the following addition polymers:
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Polypropene
By looking at the general formula of polypropene, you can see that every other carbon
atom has a methyl group attached to it. If you draw out the zigzag backbone of the
polymer, you can see that these groups can be orientated above or below the plane of the
zigzag backbone.
Show the methyl groups all oriented on the same side of the zigzag:
The most useful form of polypropene is the one where the methyl groups are arranged on
the same side. This packs most efficiently in the solid state, is the most dense, most
resistant to wear and has the highest tensile strength. Zeigler-Natta catalysts are able to
polymerise propene such that the methyl groups are oriented all on the same side, which
is known as iso-tactic, which is an example of stereo-regular polymerisation.
Polypropene is used for carpet tiles, sacking, ropes, fishing nets, as well as being used for
kitchenware and plastic bottles, like high density polythene.
Give an equation to show the polymerisation of propene:
•
Polychloroethene (PVC)
Polychloroethene exists in two forms, though for different reasons than above. The rigid
form is used for pipes and guttering and window frames, whereas the flexible form is
used for waterproof clothing (in sheets, not fibres), synthetic leather and the insulating
cover of electrical cables.
The difference between these polymers is achieved using plasticisers, molecules that
resemble fats and oils in structure, which disrupt the packing between the polymer chains.
PVC with plasticisers is flexible, without plasticisers it is rigid, indeed the U in UPVC
windows, stands for unplasticised.
As plasticiser molecules resemble fats, they can be leached out of the plastic if it is in
contact with fats, such as when PVC Clingfilm is used to wrap meat. These molecules are
hazardous to health, so having PVC Clingfilm is contact with fatty foods is to be avoided.
Give an equation to show the polymerisation of chloroethene:
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Polymers
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Polytetrafluoroethene (TEFLON, PTFE)
Give an equation to show the polymerisation of tetrafluoroethene:
Teflon has the unusual property of being the slipperiest synthetic substance. In addition,
the replacement of C-H bonds in ethene with the C-F bonds in tetrafluoroethene results in
a polymer that is resistant to chemical attack. Applications that these properties lend
Teflon to are plumbers tape (dry-lubricates joints enabling them to be sealed without
making them un-screwable), lab taps, non-stick coatings for clothing and pots and pans.
Polyphenylethene (polystyrene)
Give an equation to show the polymerisation of phenylethene:
Polystyrene also exists in two forms, but again for different reasons than already discussed
for other polymers. Polystyrene exists as a high-density polymer, and a low density
expanded polymer. The expanded polymer is formed by injecting a gas into the liquid
polymer, and as a result is full of pockets of gas, which make it lightweight, rigid,
crushable and a very good insulator. These properties lend expanded polystyrene to its
main application as packaging. An impact will cause damage to the polymer rather than
the object that is packaged, and being lightweight it does not add to transport costs. As an
insulator, it is used for hot drink cups and refrigerator insulation.
Non-expanded polystyrene is also used for drinks cups, but these are much thinner and
not particular good at heat insulation. It is also used for making screwdriver handles.
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Polymers
Condensation polymers
Condensation polymers form when molecules join together and the elements of a small
molecule, such as HCl or H2O are released.
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Polyesters
Polyesters are formed when diacid chlorides and diols polymerise. This reaction occurs
readily in the lab at room temperature because diacid chlorides are very reactive. In
industry, the cheaper, less reactive dicarboxylic acid is used, the lesser reactivity of the
dicarboxylic acid being compensated for by more forcing industrial conditions.
Show benzene-1,4-dioic acid and ethan-1,2-diol making a polyester:
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Polyamides (nylons)
Nylons are formed when diacid chlorides and diamines polymerise, dioic acids are used
in industry for the same reasons as above.
Show hexandioic acid and 1,2-diaminohexane making a nylon 6:6:
Polyamides and polyesters can form from a single monomer, if one of each functional
group is present at each end of the molecule. For example, nylon 6:6 can also form from 6amino hexanoic acid:
Polyamides and polyesters both have strong intermolecular forces between their chains,
permanent dipoles and hydrogen bonds respectively. This means they can tolerate being
spun into fibres, enabling synthetic fabrics to be made. However, unlike wool, they do not
allow water vapour to pass through them easily, so they do not ‘breathe’ well.
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Polymers
Polyamides and polyesters have relatively reactive functional groups present compared to
the very inert polyalkenes. Amide and ester linkages are polar and can be hydrolysed in
acidic or alkaline conditions, consequently they pose a lesser threat to the environment as
waste than do polyalkenes such as polyethene.
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Hydrolysis of polyesters gives the dioic acid and the diol from which the polyester was
formed. If the conditions are alkaline, you get the dioc acid salt.
Show the products of acidic hydrolysis of PET.
Show the products of alkaline hydrolysis of PET
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Hydrolysis of polyamides gives the dioic acid and the diamine from which the
polyamide was formed. If the conditions are acidic, you get the diamine salt, if they are
alkaline you get the doic acid salt.
Show the products of acidic hydrolysis of Nylon-6,6.
Show the products of alkaline hydrolysis of Nylon-6,6.
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