Condensation polymers-polyester

Addition polymers such as polythene and PVC use small unsaturated monomers called alkenes which then link or add together to form long chain polymers. During addition polymerisation all the monomer is used up to form a single polymer and no waste products are produced. The polymers produced by addition polymerisation also have a backbone which consists entirely of carbon atoms. However this is not the only way in which polymers can be made. Condensation polymerisation is another method used to make large polymer molecules, this method of polymerisation does not use unsaturated monomers (monomers with carbon carbon double bonds) but instead it uses monomers with reactive end groups.

Note - before you read this section it might be a good idea to revise functional groups. Make sure you know what carboxylic acids, alcohols and esters are.

Polyester

alcohol	displayed formula	molecular formula
methanol		CH3OH
ethanol		C2H5OH

However it would not be possible to make a polymer using any of these alcohols because the reactive hydroxyl group (-OH) is only on one end of the molecule. What we need is another homologous series with the reactive alcohol hydroxyl group on both ends of the molecule. Diols (remember di =2, -ol is ending for the naming alcohols) are a homologous series with the reactive hydroxyl functional group on both ends of the molecule.

The first member of this diol homologous series is called ethane-1,2-diol or simply ethanediol. The numbers in the name of the molecule simply tell you that the reactive hydroxyl functional group is on carbon atoms 1 and 2. The structure of the first two members of the diol homologous series are ethane-1,2-diol and propane-1,3-diol and they are shown below.

diol	displayed formula	formula
ethane-1,2-diol		HOCH2CH2OH
propane-1,3-diol		HOCH2CH2CH2OH

carboxylic acid	displayed formula	formula
methanoic acid		CHCOOH
ethanoic acid		CH3COOH
propanoic acid		C2H5COOH

However none of these carboxylic acids can be used as monomers in a condensation polymerisation reaction for the same reason we met for the alcohols above; the reactive functional group (the carboxyl group) needs to be on both ends of the molecule. This leads us to another homologous series of carboxylic acids; the dicarboxylic acids or diacids. These are similar to the carboxylic acids you have previously met but they have the carboxyl group functional group on both ends of the molecule. The first two members of this homologous series of dicarboxylic acids are shown below.

dicarboxylic acid	displayed formula	formula
ethane-1,2-dioic acid		HOOCCOOH
propane-1,3-dioic acid		HOOCCH2COOH

Now recall that Esters can be made in a condensation reaction between an alcohol and a carboxylic acid, as shown below. Here the alcohol ethanol undergoes a condensation reaction with the carboxylic acid ethanoic acid to make the ester ethyl ethanoate and water:

During this esterification or condensation reaction an -OH group is lost from the carboxylic acid and a -H atom is lost from the alcohol, these two groups combine to form a molecule of water and the remainder of the carboxylic acid and alcohol molecules join together to form the ester molecule ethyl ethanoate, as shown below:

However no polymerisation reaction is possible using these reactants because once the ester is formed the reaction is effectively finished as the alcohol and the carboxylic acid cannot react further. However if we replace the alcohol with a diol and the carboxylic acid with a dicarboxylic acid then once the ester linkage is formed the reaction can still carry on as the diol and dicarboxylic acid still have reactive functional groups on the end of the molecules; this is outlined below:

This reaction can effectively carry on as the ester molecule has a reactive hydroxyl group on one end and a reactive carboxyl group on the other. Each of these can react with other monomers to build up a large polymer molecule made up of lots of ester linkages; that is a polyester!
We can summarise this as:

Here n represents the number of diol and dicarboxylic molecules that react to form the polyester.

Poly(lactic acid)

The examples of condensation polymerisation given above use two different monomers with each monomer having the same two functional groups on the ends of the monomer molecule. This is the usual case but it by no means the only option; it is possible to have monomers with two different functional groups on the ends of the monomer molecule or even have one single monomer with different functional groups on its ends. Lactic acid is one such monomer. The structure of lactic acid (2-hydroxypropanoic acid) is shown below. It contains the acidic carboxyl group (-COOH) on one end of the molecule and a hydroxyl group (-OH) on the other end. Lactic acid molecules can undergo a condensation polymerisation reaction to form the polyester poly(lactic acid) or PLA as shown below.

If many more lactic acid molecules were to react and form more ester linkages the the structure shown below would form, it is clear that there is a single repeating unit (shown below) that repeats to build up the polyester poly(lactic acid).

Uses of poly(lactic acid)

• Used in 3d printers to produce a large number of varied products.
• Food packaging.
• Disposable cups and cutlery.
• Compostable bags.
• Feminine hygiene products and baby's nappies.
• Agricultural mulches to suppress weeds and agricultural netting for crops.
• Car parts such as mats and plastic panels.

Key Points

• Condensation polymers are formed using monomers with reactive functional groups on BOTH ends of the monomers.
• Addition polymers have a backbone of carbon atoms. Condensation polymers do not.
• No waste products are produced during addition polymerisation; however during condensation polymerisation a small molecule is released.
• Polyesters are an example of a condensation polymer made by reacting a dicarboxylic acid with a diol.

Check your understanding - Questions on condensation polymers

Check your understanding - Additional questions on condensation polymers

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