Chemguide: Support for CIE A level Chemistry

Learning outcome 35: Polymerisation

35.3: Degradable polymers

Learning outcomes 35.3.1, 35.3.2 and 35.3.3

These statements are about the degradability of polymers.

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

Statement 35.3.1

Polyalkenes contain only carbon-carbon single bonds and carbon-hydrogen bonds, both of which are relatively unreactive. Most organisms don't have the biochemical pathways necessary to break them down either.

Polyalkenes are therefore difficult to biodegrade, and remain in the environment for a considerable time. There isn't much more you can say about that!

Statement 35.3.2

Many common polymers can be degraded by the action of light - particularly UV light. It leads to discolouring, and breaking of the polymer chains, which causes cracking or crumbling of the material.

This was new in the previous syllabus, and the questions asked basically just wanted you to be aware that this is possible. The mechanisms involved are complicated, and vary from polymer to polymer.

As an example, low density polythene (LDPE), which is used in plastic bags amongst other things, becomes brittle on long exposure to UV light.

LDPE contains lots of branches off the polymer chains, and reactions involving UV light produce free radicals at these junctions. Further reactions involving oxygen can convert these into carbonyl groups (C=O). More interactions with UV can split the chains around the carbonyl groups.

This is all very complicated, and I am not the least bit confident about it! Nobody is going to expect you to learn it. Just be aware that some polymers can be degraded by light, and perhaps remember that LDPE is one example of this.

Statement 35.3.3

I'm not certain what CIE want here. The statement talks about polyesters and polyamides being biodegradable, which implies that they can be broken down by organisms involving enzyme-controlled hydrolysis of the ester or amide links.

But the details of that are way beyond an A level chemistry course. I am giving you links to straightforward chemical reactions involved in hydrolysing these compounds.

Questions asked simply want you to know that polyesters and polyamides can be hydrolysed by acids or alkalis. One question I came across would also accept the simple term enzyme hydrolysis as an answer.

You will find the hydrolysis of polyesters at the bottom of the page about polyesters. You might want to follow the link to the hydrolysis of esters to remind yourself about the basic chemistry.

You will find the hydrolysis of polyamides at the bottom of the page about polyamides. Again, you might want to follow the link to the hydrolysis of amides to remind yourself about the basic chemistry.

Beware, though! You have to know about the acidic and alkaline hydrolysis of simple esters and amides. There is no reason why CIE couldn't ask a more detailed question about the hydrolysis of polyesters and polyamides in the future. This statement gives them permission to do that.

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© Jim Clark 2020