Addition Polymers & Plastic Pollution (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about addition polymers and the problem of plastic pollution.

The two go together because the unreactivity that makes these polymers useful is exactly what makes them so hard to dispose of.

Addition polymerisation joins alkene monomers into long chains made of saturated, non-polar carbon-carbon bonds. Those bonds are what make the polymer unreactive, and that unreactivity cuts both ways: it is why poly(alkenes) are so useful as plastics, and it is why they are non-biodegradable and cause long-term pollution when they are thrown away.

We'll take the polymerisation first, and the repeating unit it produces. Then the properties of the chains and how plasticisers change them. Then the disposal problem, and the recycling methods that address it.

Addition polymerisation is one of the most important addition reactions of alkenes, and it forms the basis of the plastics industry. It is the reaction in which many monomers, each containing at least one carbon-carbon double bond, form long chains of polymers as the only product. Just as in the other addition reactions of alkenes, the pi bond in each monomer breaks and the monomers link together to form new carbon-carbon single bonds.

The repeating unit is the smallest group of atoms that, connected one after another, makes up the polymer chain, and it is shown by square brackets. For poly(alkenes) and substituted poly(alkenes) made from one type of monomer, the repeating unit is the same as the monomer, except that the carbon-carbon double bond has become a carbon-carbon single bond. That single bond is what the rest of this topic follows from.

Addition polymers are unreactive. The properties of PVC, poly(chloroethene), can be altered by adding a plasticiser: PVC is rigid enough to be used for making drainpipes, but with a plasticiser it can be made flexible enough to make pool liners.

A plasticiser makes a polymer more flexible by preventing the polymer chains from being close to one another. That disrupts the van der Waals forces between the chains, making them weaker, and as a result the chains slide over each other more easily. The forces between the chains can be changed; the unreactive bonds along them cannot.

Poly(alkenes) are extremely important in everyday life as plastics, but the disposal of these polymers is problematic. They are very large alkane molecules which are unreactive and therefore do not undergo any chemical reactions; they are resistant to chemical attack. The chain is full of saturated, non-polar carbon-carbon bonds, and that is why they cannot be attacked and broken down by biological agents such as enzymes.

Because of that unreactivity, polymers are non-biodegradable and take up to hundreds of years to decompose when dumped in landfill sites, so throwing them away causes long-term pollution of the environment. Burning them instead results in harmful combustion products, which again pollute the environment. This is the same unreactivity that made them useful in the first place.

Possibly the easiest method to reduce the amount of plastic is to reuse it rather than throwing it away. The alternative is to recycle it, and there are two routes.

Feedstock recycling involves heating the plastic to a high enough temperature that the polymer bonds break and monomers are formed; those monomers are then used to produce new plastics. Mechanical recycling is more straightforward: the different types of plastic are separated and washed thoroughly, ground up into small pellets, then heated until they melt and remoulded so that they can be used again.

Neither is a perfect solution. Some plastics, like poly(propene), can only be heated and reused a number of times, because each time the plastic is heated some of the chains break, and over time the properties of the plastic are degraded.

The section of the polymer chain shown inside the square brackets is the repeating unit, and not the monomer. The monomer is the same as the repeating unit except that it has carbon-carbon double bonds instead of carbon-carbon single bonds.

Addition polymerisation joins monomers containing a carbon-carbon double bond into long chains, with the polymer as the only product, and the repeating unit differs from the monomer only in having a carbon-carbon single bond in place of the carbon-carbon double bond. The chains are held together by van der Waals forces, which a plasticiser can weaken to make the polymer more flexible.

The bonds along the chain are saturated and non-polar, so the polymer is unreactive and non-biodegradable, and that same unreactivity is why disposal is a problem: hundreds of years in landfill, harmful products when burnt, and reuse or recycling as the alternatives.