Polymers

Condensation Polymerisation

Condensation polymerisation is the joining of two monomer units with the elimination of a small molecule such as water or $HCl$. Condensation polymerisation requires monomers with two different functional groups. A functional group on one of the monomer units bonds with a different functional group on the other unit.

Polyesters

A polyester is a polymer where the monomers are held together through ester linkages. In a polyester, monomers must have a carboxyl group ($-COOH$) and a hydroxyl group ($-OH$). There are two types of polyester.

Different Types of Monomer Units

With this method, one monomer is a dicarboxylic acid with two $-COOH$ groups while the other is a diol with two $-OH$ groups. The general formation is shown below:

General formation of polyester from different monomer units.

An example of this condensation reaction is the formation of Terylene which is made from the monomers ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

General formation of polyester from different monomer units.

One type of Monomer Unit

Polyesters can also be formed from hydroxycarboxylic acids, which contain both the carboxyl group, $-COOH$ and a hydroxyl group, $-OH$. The general formation is shown below:

General formation of polyester from the same monomer unit.

An example of this type of condensation reaction is the formation of polylactic acid (PLA) from the monomer unit 2-hydroxypropanoic acid. During this polymerisation, an ester bond forms between the carboxyl group on one unit and a hydroxyl group on another unit. In forming this ester linkage, water is eliminated.

Formation of PLA from the same monomer unit.

Polyamides

Like polyesters, polyamides can be formed from two types of monomer units or just one type of monomer unit. In forming a polyamide, the monomer units are joined through amide linkages, with the elimination of water. In a polyamide, the monomers must have a carboxyl group ($-COOH$) and an amine group ($-NH_{2}$).

Different Types of Monomer Units

Polyamides can be made by reacting together one monomer which is a dicarboxylic acid, with another monomer which is a diamine. The general formation is shown below:

General formation of polyester from the same monomer unit.

An example of this type of condensation polymerisation is the formation of Nylon-6,6. This is made from the monomers 1,6-diaminohexane and hexanedioic acid.

Formation of nylon-6,6 from the same monomer unit.

Nylon-6,6 is often used in clothing. Kevlar is another material used in clothing which is synthesised through condensation polymerisation. Kevlar is a material used for protective clothing such as bulletproof vests and helmets, due to its high strength and fire resistance.

Kevlar is formed from the polymerisation of benzene-1,4-dioic acid and benzene-1,4-diamine.

Formation of kevlar from the same monomer unit.

One type of Monomer Unit

Polyamides can also be formed from amino acids, which contain both the amine group ($-NH_{2}$) and a carboxyl group ($-COOH$). Polypeptides and proteins are polyamides formed from one type of monomer unit. The general formation is shown below:

Formation of polyamide from the same monomer unit.

Addition Polymerisation

Addition polymerisation is the process in which alkene monomers undergo addition polymerisation to produce a saturated polymer chain. Addition polymers are made from only one type of monomer. The polymer is the only product.

Formation of polyamide from the same monomer unit.

The table below shows a comparison between addition and condensation polymers.

Product Polyalkene Polyester Polyamide
Polymerisation Addition Condensation Condensation
Functional groups $C=C$ $-COOH$ and $-OH$ $-COOH$ and $-NH_{2}$
Type of monomer 1 1 or 2 1 or 2

Hydrolysis of Polyesters

Polyesters can be hydrolysed under acidic or basic conditions.

Base Hydrolysis

Polyesters are readily hydrolysed by hot aqueous alkalis, such as sodium hydroxide. Each ester linkage is hydrolysed to a carboxylate salt ($-COO^{-}Na^{+}$) and a hydroxyl group ($-OH$).

Hydrolysis of polyester by base hydrolysis.

Acid Hydrolysis

In a much slower reaction, polyesters can be hydrolysed by hot aqueous acids, such as aqueous hydrochloric acid. Each ester linkage is hydrolysed to a carboxyl group ($-COOH$) and a hydroxyl group ($-OH$).

Hydrolysis of polyester by base hydrolysis.

Hydrolysis of Polyamides

Polyamides can be hydrolysed under acidic or basic conditions.

Base Hydrolysis

Polyamides can be hydrolysed with hot aqueous alkalis, such as sodium hydroxide. With base hydrolysis, the sodium salt of the dicarboxylic acid and a diamine are both formed.

Hydrolysis of polyester by base hydrolysis.

Acid Hydrolysis

Polyamides can also be hydrolysed with hot aqueous acids, such as aqueous hydrochloric acid. With acid hydrolysis, a dicarboxylic acid and an ammonium salt of the diamine are produced.

Hydrolysis of polyamine by acid hydrolysis.

Degradable Polymers

Growing environmental concerns, customer demand and the increasing cost of fossil fuels had led to the development of renewable and sustainable polymers for consumer use. Such products include biodegradable carrier bags and packaging.

Biodegradable Polymers

Biodegradable polymers have bonds which will readily undergo hydrolysis allowing for easier breakdown of the polymer. Examples include:

Photodegradable Polymers

When exposed to sunlight, photodegradable polymers are designed to become weak and brittle. They are made by blending a polymer with light sensitive additives which catalyse the breakdown. Alternatively, carbonyl bonds ($C=O$) can be incorporated into the polymer which absorb energy and break down in prolonged sunlight. These polymers convert into waxy compounds when exposed to light before turning into carbon dioxide and water.

© Andrew Deniszczyc, 2018