Condensation Polymers & Polyesters

Explained

Condensation polymers and polyesters

There are two ways to build a polymer, and the difference between them is whether anything is left over. Addition polymerisation uses one monomer, an alkene, and produces nothing but the polymer. Condensation polymerisation usually uses two different monomers and releases a small molecule, normally water, at every link.

Why two monomers are needed

Each monomer in condensation polymerisation carries two functional groups, one at each end. A dicarboxylic acid has a carboxylic acid group at both ends. A diol has an alcohol group at both ends.

An acid group reacts with an alcohol group to form an ester link, and water is released. Because each molecule has a spare group at its other end, the chain can keep growing in both directions, alternating acid, alcohol, acid, alcohol.

If a monomer had only one functional group, it could join once and then stop. Having two at each end is what makes a long chain possible.

Polyesters

A polyester is the polymer formed when a dicarboxylic acid reacts with a diol. The name says what it is: many ester links. Terylene, used in clothing, is the common example.

Because the linkage is an ester, it can be broken by hydrolysis, which is the reverse reaction with water. That has a practical consequence worth knowing.

Comparing the two types

• Addition uses one monomer with a carbon to carbon double bond. Condensation usually uses two monomers, each with two functional groups.
• Addition produces only the polymer. Condensation also produces a small molecule, usually water.
• Addition polymers have an unreactive carbon backbone and are hard to break down. Condensation polymers contain ester links, which can be hydrolysed, so they are more easily broken down.

That last contrast is the one questions about disposal and the environment rely on. Addition polymers such as poly(ethene) are not biodegradable, which is why they persist. Polyesters can be broken down more readily because of the linkage in the chain.

Drawing the repeat unit

Show one acid monomer joined to one alcohol monomer through an ester link, with a bond extending from each end of the repeat unit to show that the chain continues. Those extending bonds matter. A repeat unit drawn as a closed molecule is a molecule, not a repeat unit, and does not gain the mark.

If asked for the other product, name water, and say that one water molecule is released for each link formed.