Addition and Condensation Polymer Structures (Paper 2)

Explained

Drawing polymer structures, and working backwards to the monomer

Two skills are tested here and they run in opposite directions. Given a monomer, draw the repeat unit. Given a repeat unit, deduce the monomer. Both come down to knowing what happens to the carbon to carbon double bond.

Addition polymers

An addition polymer forms when unsaturated monomers join without losing anything. The double bond opens, and the two spare bonds link to the neighbouring units.

Ethene gives poly(ethene). Propene gives poly(propene). Chloroethene gives poly(chloroethene), better known as PVC.

To draw the repeat unit: take the monomer, change the double bond to a single bond, and add a bond extending from each end. Those extending bonds are not decoration. They show the chain continues, and a repeat unit drawn without them is a molecule rather than a repeat unit.

Going backwards

Given a repeat unit, do the reverse. Remove the two extending bonds and put the double bond back between the two carbons in the backbone.

Everything hanging off those carbons stays exactly where it is. If the repeat unit has a chlorine on one carbon, the monomer has a chlorine on that same carbon.

Condensation polymers

Condensation polymerisation is different in two ways. It usually uses two monomers rather than one, and it releases a small molecule, normally water, at every link.

A polyester forms from a dicarboxylic acid and a diol. Each monomer has a functional group at both ends, which is what allows the chain to keep growing. An acid group reacts with an alcohol group to make an ester link, releasing water.

When you draw it, show one acid unit joined to one alcohol unit through the ester link, with bonds extending from both ends.

Why some polyesters break down and poly(ethene) does not

Addition polymers have a backbone of carbon to carbon bonds, which are strong and unreactive. There is nothing for water or enzymes to attack, so poly(ethene) persists for a very long time.

A polyester backbone contains ester links, and ester links can be hydrolysed, which means broken apart by reaction with water. Biopolyesters are designed so that this happens at a useful rate, often helped by microorganisms, so the polymer is biodegradable.

If a question asks why one polymer is biodegradable and another is not, the answer is in the backbone, not in the side groups.