condenastion polymerisation

Chemistry higher tier


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

Polyester is perhaps the best known condensation polymer. But what exactly is a condensation polymer? Condensation polymerisation or step-growth polymerisation as it is sometimes known is the reaction of two different monomers each of which has reactive end groups (functional groups) on the end of the monomer molecules. During a condensation reaction a small molecules (or monomers) add together to form a larger molecule and a small molecule such as water (H2O), or hydrogen chloride gas (HCl) or even the alcohol methanol (CH3OH) is given off.

The monomers used in a condensation reaction have reactive ends; that is functional groups on both ends of the monomers e.g. you are probably already familiar with the structure of alcohols; recall that alcohols all contain the hydroxyl group functional group, that is an -OH group bonded to a carbon atom (C-OH) e.g. the first two members of the alcohol homologous series are shown in the table below.
alcohol displayed formula molecular formula
methanol methanol displayed formula CH3OH
ethanol Ethanol displayed formula 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.

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

Carboxylic acids

Carboxylic acids are a family of weak acids that all contain the carboxyl functional group (-COOH), you are probably already familiar with this homologous series of acids. The first three carboxylic acids are shown in the table below.

carboxylic acid displayed formula formula
methanoic acid Displayed formula of methanoic acid CHCOOH
ethanoic acid Displayed formula of ethananoic acid. CH3COOH
propanoic acid Displayed formula of 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.

carboxylic acid displayed formula formula
ethane-1,2-dioic acid Displayed formula of ethane-1,2-dioic acid HOOCCCOOH
propane-1,3-dioic acid Displayed formula of propane-1,3-dioic acid HOOCCH2COOH

Making esters

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:

equation for the reaction of ethanoic acid and ethanol to form 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: esterification reaction, the formation of ethyl ethanoate from ethanol and ethanoic acid.

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:

esterification reaction, the formation of ethyl ethanoate from ethanol and ethanoic acid.

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: polyester mechanism

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.

Formation of poly(lactic acid) from lactic acid molecules.

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).

Formation of poly(lactic acid) from lactic acid molecules, the structure of the repeat unit.

Uses of poly(lactic acid)

Use of poly(lactic acid) PLA as a mulch in the agricultural industry. Poly(lactic acid) is a biodegradable polyester since lactic acid is a natural occurring substance which is usually obtained from either maize or corn; this has lead to an increase in possible applications for its use which may result in a reduction in the large amount of non-biodegradable plastics that go to landfill. The main uses for PLA include:

Key Points


Practice questions

Check your understanding - Questions on condensation polymers

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