Polymers

Alkenes can be used to make polymers such as poly(ethene) and poly(propene) by addition polymerisation.

In addition polymerisation reactions, many small molecules (monomers) join together to form very large molecules (polymers).

In addition polymers the repeating unit has the same atoms as the monomer, because no other molecule is formed in the reaction. That is what makes it an addition reaction: nothing is lost, so the repeating unit and the monomer have exactly the same formula.

You should be able to:

• recognise addition polymers and monomers from diagrams in the forms shown, and from the presence of the functional group C=C in the monomers • draw diagrams to represent the formation of a polymer from a given alkene monomer • relate the repeating unit to the monomer.

Drawing the polymer from the monomer is a fixed procedure. Redraw the monomer with the double bond replaced by a single bond. Extend a bond out of each of those two carbon atoms to the left and right edges of the unit. Enclose it all in square brackets with a subscript n outside. Every side group stays exactly where it was.

Condensation polymerisation involves monomers with two functional groups. When these types of monomers react they join together, usually losing small molecules such as water, and so the reactions are called condensation reactions.

The simplest polymers are produced from two different monomers with two of the same functional groups on each monomer. For example, ethanediol — which has two –OH groups — and hexanedioic acid — which has two –COOH groups — polymerise to produce a polyester.

You should be able to explain the basic principles of condensation polymerisation by reference to the functional groups in the monomers and the repeating units in the polymers.

The reason each monomer needs two functional groups is worth seeing clearly: a monomer with only one group could join at one end and then the chain would stop. With a group at each end, every monomer can bond on both sides, so the chain grows indefinitely.

Counting the small molecules lost is a standard question. Joining n molecules of one monomer to n of the other forms 2n links along the chain, so 2n molecules of water are released.

Amino acids have two different functional groups in a molecule — an amino group and a carboxylic acid group. Amino acids react by condensation polymerisation to produce polypeptides.

For example, glycine is H₂NCH₂COOH and polymerises to produce a polypeptide, losing a molecule of water at each link. Different amino acids can be combined in the same chain to produce proteins.

This is the same chemistry as the polyester above, with one difference: here the two functional groups are different, and both are on the same molecule, so a single amino acid can polymerise with itself.

DNA — deoxyribonucleic acid — is a large molecule essential for life. DNA encodes genetic instructions for the development and functioning of living organisms and viruses.

Most DNA molecules are two polymer chains, made from four different monomers called nucleotides, in the form of a double helix.

Other naturally occurring polymers important for life include proteins, starch and cellulose.

You should be able to name the types of monomers from which these naturally occurring polymers are made — which is exactly the table above. Starch and cellulose are both made from glucose; the difference between them lies in how those glucose units are joined, not in what they are made of.

The properties that make addition polymers useful are the same ones that make them a problem. They are unreactive, so they do not rot, do not corrode and do not react with what is stored in them — and so they persist in landfill and in the sea for hundreds of years, and animals swallow or become tangled in them.

Three problems are named and worth learning as a set:

• the starting materials come from crude oil, which is finite and non-renewable • the polymers are not biodegradable, because microorganisms have no enzymes that will break those unreactive carbon chains down • disposal is difficult: landfill takes up land and the plastic stays there, and burning releases carbon dioxide, and toxic gases from some polymers.

Recycling addresses all three, and an evaluation needs both sides.

In its favour: it saves crude oil, uses less energy than making new polymer, reduces the amount going to landfill, and creates jobs.

Against: the different polymers have to be separated first, which is slow and expensive because they look alike; collecting and transporting the waste costs energy and money; polymers that have been contaminated by food or by other plastics cannot always be used; and a recycled polymer is often of lower quality than the original, so it cannot be used for the same purpose.