Condensation Polymers

Polyesters

An ester is formed and a small molecule lost when a carboxylic acid or acyl chloride reacts with an alcohol. For example:

ethanoyl chloride + ethanol ? ethyl ethanoate + HCl

This is an example of a condensation reaction: two or more molecules combine and a small molecule is eliminated.

Therefore, it follows that when a dicarboxylic acid anda diol react, the -COOH group at both ends of the dicarboxylic acid join to an -OH group and, at both ends of the diol, the -OH group joins to a -COOH group. This means that it should be possible for all the molecules to link up and create a polymer.

For example: benzene-1,4-dicarboxylic acid and ethan-1,2-diol. When these compounds combine to create a polymer, water is the small molecule lost. This creates the following repeating unit:

Each monomer unit links to the next by an ester group:

Therefore, a polymer which consists of an ester linkage is called a polyester.

The polymer used in the above example is called terylene and is used in fire-resistant clothing.

It is possible to form the same polyester by combining a diacyl chloride and a diol. For example, benzene-1,4-diacyl chloride and ethan-1,2-diol. However, HCl instead of H₂O is produced.

The yield through polymerisation of diacyl chlorides as opposed to dicarboxylic acids is much higher. This is because the former is much more reactive and because, as HCl is a gas and therefore escapes, the reaction is harder to reverse.

Therefore, before polymerisation is carried out it is common for dicarboxylic acids to be converted into diacyl chlorides by adding PCl₅.

Polyamides

An N-substituted amide is formed when a carboxylic acid or acyl chloride reacts with a primary amine. For example:

propanoic acid + ethylamine ? N-ethylpropanamide + H₂O

Therefore, it follows that when a dicarboxylic acid and a diamine react, the -COOH group at both ends of the dicarboxylic acid combine with an -NH₂ group and the -NH₂ group at either end of the diamine combine with a -COOH group. This means that it should be possible for all the molecules to link up and create a polymer.

For example: hexanedioic acid and 1,6-diaminohexane. When these compounds combine to make a polymer, water is the small molecule lost. This creates the following repeating unit:

Each monomer unit links to the next by an amide or peptide link:

Therefore, a polymer which consists of an amide or peptide linkage is called a polyamide.

The polyamide used in the example above is called nylon 66 and is used in clothing

It is possible to form the same polyester by combining a diacyl chloride and the diamine. However, HCl instead of H₂O is produced.

As before, the yield through polymerisation of diacyl chlorides as opposed to dicarboxylic acids is much higher. This is because the former is much more reactive and because, as HCl is a gas and therefore escapes the reaction, the reaction is harder to reverse.

Therefore, before polymerisation is carried out it is common for dicarboxylic acids to be converted into diacyl chlorides by adding PCl₅.

Properties and uses of condensation polymers

Polyester and polyamides are known collectively as condensation polymers because they are formed by a condensation reaction.

Condensation polymers are generally formed from straight chains which consist of few branches. This is due to the fact that they are created by reactions with heterolytic mechanisms as opposed to homolytic mechanisms (which are more random). On the other hand, addition polymers are created through free radical addition mechanisms which form a variety of products and, therefore, more branching.

The lack of branching means that condensation polymers tend to be linear and the chains are able to pack closely together. Therefore, these polymers are more rigid and have a higher tensile strength than addition polymers. Polyamides also contain hydrogen bonding which increases their strength further. Those that occur naturally can even contain intramolecular hydrogen bonding which causes the molecule to curl up into a helical structure.

Both polyamides and polyesters are used popularly in high-strength synthetic fibres:

• Polyesters are used to substitute wool and cotton in clothing, carpets and rugs. They are also present in bullet-proof vests and some forms of fire-resistant clothing.
• Polyamides have more elastic properties and are used in items like fishing nets and underwear as well as other synthetic fibres.

Possibly the most important difference between condensation and addition polymers is that the former is composed of chains which contain polar bonds. In other words, the polymer units are linked with C-N and C-O bonds. As nucleophiles can readily attack these polar carbon atoms, condensation polymers can be readily broken up and the monomers reformed. This means that they are biodegradable making them much less of an environmental hazard than addition polymers.

Condensation polymers can be broken up in aqueous solution, a reaction which is classed as a hydrolysis reaction.

• Polyesters hydrolyse best in strongly alkaline conditions. Here they undergo saponification.
• Polyamides hydrolyse best in strongly acid conditions.

However, their biodegradability means that they are less durable. It is important, therefore, that a balance is struck between practical durability and biodegradability.

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Disposal and Recycling of Polymers

Disposing of non-biodegradable polymers poses a lot of problems. There are three main options available:

Landfill sites: although burying non-biodegradable polymers in landfills is a very popular option throughout developed countries it is not one which is sustainable: eventually they will all fill up. In addition, they are unsightly and not hygienic.

• Burning: this is another common practice but comes with its own problems. Burning polymers produces greenhouse gases like carbon dioxide as well as toxic gases (depending on which polymer is being burned).
• Recycling: this is the most environmentally friendly option but not the easiest. Plastics need to be collected then separated and cleaned before being melted and recast. This method tends to cost more than manufacturing the plastic straight from the crude oil. In addition, it is not possible to melt all plastics as some harden or burn instead. This means that they can only be recycled in the shape they were cast originally.

Biodegradable polymers, on the other hand, are able to decomposed naturally so burying them poses less environmental concerns. They can also be recycled but need to be collected, separated and cleaned before they can be reused like non-biodegradable polymers.