Chemguide: Support for CIE A level Chemistry


Learning outcomes 15.2(d) and 15.2(g)

These statements are about addition polymerisation.

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


Statement 15.2(d)

General

You will already have come across this as a part of statement 15.2(a)(iv). I suggest you start by re-reading my CIE statement 15.2(a). You just need to revisit part (iv) of this statement towards the bottom of the page. I don't really need to add anything to the advice given on that page.

The current statement asks for poly(ethene) and PVC, so make sure that you really understand how their monomers join to make the polymer. But don't restrict yourself to these examples. CIE could ask you about the relationship between any monomer and its addition polymer, however unfamiliar.

The best way of working out what you need to be able to do is to look through as many past papers as you can find.


Drawing the structures of polymers

Pay particular attention to the way that this is done in simple equations on the page about polymerisation of alkenes. For example:

For exam purposes, when you are drawing these:

  • Don't forget to change the double bond in the monomer into a single one in the polymer.

  • Draw the bonds on each end of your bit of polymer so that they extend just outside the brackets.

  • Look carefully at what the question is asking. If it asks you for at least two repeating units, you won't get any marks if you only draw one as in the equation above. Enclose both repeating units inside the brackets.

  • If you are asked for the equation, it must balance. If you are asked for, say, two repeat units, the easiest way to write this would be to start with "2n" monomers, and "n" after the brackets. If in doubt, count the carbon atoms (in terms of "n") on each side of the equation.


Statement 15.2(g)

This is about problems with the disposal of poly(alkenes).

The syllabus mentions two problems. The first is that poly(alkenes) are non-biodegradable. In other words they aren't broken down in the environment, or else are only broken down very slowly.

The teacher support material mentions that the problem is worse for branched chain poly(alkenes) - those with hydrocarbon side groups.

The other problem concerns the disposal of chlorinated polymers like PVC. If these are burnt at too low a temperature, poisonous products such as phosgene (COCl2) and dioxins are formed.

Phosgene was a First World War poisonous gas, and dioxins are extremely toxic.

For interest only, dioxins are a family of related compounds with varying numbers of chlorines. The "m" and "n" in the diagram (taken from Wikipedia) can range from 0 to 4, and can be attached in various positions on the left and right-hand rings.


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© Jim Clark 2011 (last modified June 2014)