4.1.3 (j,k,l) Polymers from Alkenes, polymer waste and alternatives

Syllabus

(j) addition polymerisation of Alkenes and substituted Alkenes, including:

(i) the repeat unit of an addition polymer deduced from a given monomer

(ii) identification of the monomer that would produce a given section of an addition polymer

(k) the benefits for sustainability of processing waste polymers by:

(i) combustion for energy production

(ii) use as an organic feedstock for the production of plastics and other organic chemicals

(iii) removal of toxic waste products, e.g. removal of HCl formed during disposal by combustion of halogenated plastics (e.g. PVC)

{Benefits of cheap oil-derived plastics counteracted by problems for environment of landfill; the move to re-using waste, improving use of resources.}

(l) the benefits to the environment of development of biodegradable and photodegradable polymers.

{Benefits of reduced dependency on finite resources and alleviating problems from disposal of persistent plastic waste.}

What does this mean?

What is an Addition Polymer?

If you studied GCSE Sciences then you'll be familiar with the idea of polymerisation as making a very long molecule (a polymer) from many small molecules (monomers).

Depending on which GCSE you took you may also be aware that this is Addition Polymerisation and that Condensation Polmerisation works in a different way.

Polymerisation of Alkenes is Addition Polymerisation - if you're asked to explain what this phrase means you should explain what polymerisation means in general (making long molecules from many small ones) and then explain that addition polymers have only one type of monomer which must contain a C=C bond (be Alkenes)

Repeat Units

Since we're not in control of how many monomers join together we can never draw the entire Polyethene molecule.

And every Polyethene molecule will be a different length to its neighbours.

So to represent the reaction we say that there are n ethene molecules joining together.

And we draw the simplest repeating unit of the polymer (although it must contain 2 Carbon atoms because there are 2 in the monomer).

You should be able to draw the repeat unit from the monomer :-

Remove double bond.
Add brackets
Draw single bonds to the left and right of the repeat unit.
Add the n to show that we don't know how many joined up.

You should also be able to draw the monomer from the repeat unit:-

Remove single bonds to the left and right of the repeat unit.
Remove brackets
Draw in double bond
Remove the n.

As you can see, it just doesn't make any difference what is substituted onto the Carbon atoms of Ethene - all these polymerisations proceed just as for Ethene.

Problems with Polymers

The problem with standard Addition Polymers is that they do not biodegrade (rot).

They persist in the environment and this can cause problems such as the famous issue with turtles eating plastic bags thinking they are jellyfish.

Addition polymers may photo-degrade - that is they may break into smaller fragments as a result of sunlight but each fragment is still the same addition polymer and this can then cause different problems - such as ingestion by filter-feeding organisms.

Biodegradable polymers have been and are being developed and in theory these eventually break up into Carbon Dioxide and water but the remaining non-degradable polymers will be with us indefinitely.

Land-filling (burying) them simply delays the issue and wastes land.

Incineration (burning them) removes the polymers and can generate power as an additional benefit - thereby saving fossil fuels.

However, there is worry about toxic products of some polymers and those polymers that contain Chlorine (such as PVC) would create HCl as a waste gas.

This adds expense as it would be necessary to neutralise the waste gases to avoid Acid Rain

Recycling usually means melting down and re-forming into new products.

This is expensive because there are many different polymers in domestic waste and they must be separated by hand before melting.

Inevitably this leads to contamination - so recycled polymers are always lower quality than freshly manufactured ones.

And some polymers are thermoset - they don't melt when heated (although they could still burn).

Thermoset polymers clearly can't be re-used in this way.

But they may be useful as feed-stock.

Chemical feed-stock recycling breaks down polymers without separating them, forming simple gases that can then be used to manufacture pure, fresh polymers.

This system is yet to be shown to work.

So the ideal is to develop a fully biodegradable polymer that can be manufactured from a renewable, non-fossil fuel source.

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