Chemguide: Support for CIE A level Chemistry
Learning outcome 21.3(e)
This statement is about less familiar polymers designed for specific uses.
Before you go on, you should find and read the statement in your copy of the syllabus.
Non-solvent based adhesives
In many adhesives, the glue is dissolved in a solvent which evaporates in the air, allowing the glue to set. This statement is about adhesives which set involving the chemical formation of a polymer.
There are a lot of different variants of these, but they involve the formation of a cross-linked, thermosetting, polymer when you mix together the contents of two separate tubes.
You will find a reasonably well explained description of this on this page from the Polymer Science Learning Centre.
Note: It is always a bit risky giving links to other sites, because they can change or disappear over time. If you have any problems with this link, please contact me via the address on the about this site page.
There is no suggestion that you should try to remember the structure of the epoxy resin, or even of the substances mixed to produce it. This is new for the 2016 exam, and at the moment it isn't clear what CIE is likely to ask about this.
You should, however, recognise that this is the formation of what is essentially one huge molecule which sets in whatever shape it was when the reaction took place. Because of the cross-linking, its shape can't be altered by, for example, heating.
This is also new for 2016. You almost certainly won't have to learn the detail of what follows, but it strikes me as possibly a rich source of exam questions if they give you some of this detail and then ask you to explain things about it.
The monomer for superglue is methyl 2-cyanoprop-2-enate, usually called methyl 2-cyanoacrylate. Its full structure is:
What is most interesting about this is that the whole molecule (apart from the CH3 group) is planar, and there are alternating multiple and single bonds exactly right for delocalisation to happen.
But at the end of the delocalisation on the right-hand side of the molecule are the very electronegative nitrogen and oxygen atoms. That means that the delocalised electrons will be pulled strongly towards that side of the molecule.
That leaves the CH2 carbon fairly positively charged, and open to attack by nucleophiles.
That is quite different from what happens in normal carbon-carbon double bonds, which you expect to undergo attack by electrophiles.
In fact, it is so open to attack, that even water vapour in the atmosphere is a strong enough nucleophile.
In showing what happens, I am going to simplify the structure to
A lone pair on the water's oxygen is used to form a new bond to the CH2 carbon, and one of the electron pairs between the carbons is forced on to the other carbon.
By doing this, we have generated a new nucleophile which can attack another methyl 2-cyanoprop-2-enate molecule in the same way that the original water did.
And that produces another longer nucleophile, and so the chain can grow and grow. And what you have made is superglue.
So superglue sets because it is polymerising during the setting process, triggered by water vapour in the air or traces of water or any other nucleophile on whatever you happen to be using the glue on.
Polymers which conduct electricity
There was a part question in June 2012 (papers 41 and 43 Q8(d)) in which you were given a structure for such a polymer, and asked straightforward questions about it.
I will just give you enough information so that you can recognise the possibility that a given polymer can conduct electricity. Anything more than that is way beyond A level chemistry.
If you polymerise ethyne (acetylene), this happens:
What you need to notice are the alternating single and double bonds in the polymer. That continues the whole way along the chain.
You should remember that when carbon forms a double bond, one of its p orbitals is at right angles to the plane of the molecule, and that these overlap sideways to give the pi bond in, say, ethene.
That is happening to every carbon atom in this chain, and all of the p orbitals can overlap sideways to give a delocalised system of electrons the whole length of the chain.
The presence of the delocalised electrons over the whole chain is what enables it to conduct electricity. If you are given any polymer structure where you can trace alternating single and double bonds down the whole chain, then it could have the ability to conduct.
In truth, it isn't so obvious why delocalisation along a single chain can allow the material in bulk to conduct, but as I said before, that is way beyond the sort of knowledge you need for this exam.
The June 2012 question had alternating benzene rings and C=C double bonds. You had to recognise the delocalisation of the electrons, and you were also asked about the geometry of the molecule which enabled it to conduct. The point here was that it had to be planar, because otherwise the p orbitals on the benzene rings wouldn't line up with those in the C=C bonds.
© Jim Clark 2011 (last modified July 2014)