Chemguide: Support for CIE A level Chemistry

Learning outcome 21.3(a)

This statement is about the properties and structure of polymers - addition and condensation.

Before you go on, you should find and read the statement in your copy of the syllabus.

It is hard to see what this statement is asking which isn't asked elsewhere in this section. It was in the previous syllabus in a "Design and materials" section.

Questions on this were often about polymers which you might not have come across before, and which you would have to work out during the exam from things you knew from other parts of the syllabus.

I suspect that is similar to what is going on here - otherwise I can't see the point of it.

The best way of understanding this is to look at what has been asked in previous exams, and you can perfectly well use questions from the old syllabus because the statement is the same.

To start with, have a look at the following:

  • June 2013 paper 41 Q8

  • June 2013 paper 42 Q8

There is no way you can possibly practise every possible question that might arise, but you can see the sort of things which might come up by looking at these and similar questions from papers between 2007 and 2015.

You should concentrate on paper 4 (or 41, 42, 43) and questions from Part B which is where this statement was tested.

There is one other point which isn't specifically mentioned by the syllabus, but you need to be aware of:

Thermoplastic and thermosetting polymers

In November 2011 papers 41 and 42, CIE asked a three mark question about thermoplastic polymers, despite the fact that the term isn't mentioned in the syllabus or in any of the support material. I can't find it in the Coursebook either. If you hadn't come across the term, the question is impossible to answer.

A clearer term to use than "thermoplastic" is "thermosoftening". If you talk about thermosoftening plastics and thermosetting plastics, it suddenly becomes much clearer what you are talking about.

Remember: thermoplastic = thermosoftening

Thermosoftening plastics are ones which soften, and eventually melt, on heating. Typical examples include poly(ethene), PET (poly(ethylene terephthalate) - a polyester) and nylon.

These plastics can be re-melted after use and shaped into new forms. For example, PET is used for drinks bottles to hold mineral water, soft drinks, and so on. After recycling, the plastic can be remelted and spun into fibres to make clothing when it would now be called polyester.

The reason they soften and melt on heating is that the eventually enough energy is supplied to overcome the intermolecular attractions between the long chain molecules, and they become free to move around to some extent.

Thermosetting plastics are quite different. In the production of thermosetting plastics, covalent bonds are made between the original polymer chains to give a cross-linked structure which is essentially one huge molecule.

Because covalent bonds have to be broken to allow anything to move, much more heat has to be supplied, and this tends to break down the whole structure into random bits. Instead of softening and then melting on heating, thermosetting plastics tend to char (blacken), but remain solid.

Examples of this include Bakelite (one of the first plastics) and epoxy resins.

You couldn't recycle one of these materials to use for something else.

A further comment on the November 2011 question

The question asked you to say which type of polymerisation produces thermoplastic polymers, and the mark scheme gave the answer "addition polymerisation". This implies that only addition polymers are thermoplastic (thermosoftening). That is simply untrue. PET (mentioned above) is a condensation polymer, as is nylon, which also melts on heating.

The question is not only unfair because it isn't mentioned by CIE anywhere in their syllabus or support material, but is also a question that can't meaningfully be asked. Both types of polymerisation (addition and condensation) can produce thermoplastic polymers.

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© Jim Clark 2011 (last modified July 2014)