Polyesters and Polyamides

Polyesters and polyamides are both made by condensation polymerisation, a process in which monomers join together and a small molecule, almost always water, is released at every new bond formed. This is very different from addition polymerisation, where monomers with a carbon to carbon double bond simply add together with no atoms lost. Because condensation polymers keep a reactive linkage in the backbone, they can often be broken back down again, which explains a lot about how they behave in the environment and in the body.

To build a long chain by condensation polymerisation, each monomer needs at least two reactive functional groups so it can bond on both sides. The two families you need for this topic are:

• Diols and dicarboxylic acids (or their diacyl chloride equivalents), which react to form polyesters.
• Diamines and dicarboxylic acids (or diacyl chlorides), which react to form polyamides.

The reaction between a carboxylic acid group and an alcohol group, or between a carboxylic acid group and an amine group, is the same esterification and amidation chemistry you meet in carboxylic acids, acyl chlorides and esters and in amines, amides and amino acids, just repeated over and over along a chain instead of happening once between two small molecules.

A polyester chain contains the ester linkage, written as -CO-O-, repeated along the backbone. It forms when the -OH group of an alcohol reacts with the -COOH group of a carboxylic acid, releasing a molecule of water each time:

\( n\,\mathrm{HO{-}R{-}OH} + n\,\mathrm{HOOC{-}R'{-}COOH} \)→\( \mathrm{[{-}O{-}R{-}O{-}CO{-}R'{-}CO{-}]}_n + 2n\,\mathrm{H_2O} \)

The classic example is poly(ethylene terephthalate), better known as PET or Terylene, made from ethane-1,2-diol (a diol) and benzene-1,4-dicarboxylic acid (terephthalic acid, a dicarboxylic acid). Because the acid monomer contains a benzene ring, the chain has some rigidity, which is one reason PET is strong enough to be drawn into fibres for clothing and moulded into drinks bottles.

A polyamide chain contains the amide linkage, written as -CO-NH-, repeated along the backbone. It forms when an amine group reacts with a carboxylic acid group, again releasing water:

\( n\,\mathrm{H_2N{-}R{-}NH_2} + n\,\mathrm{HOOC{-}R'{-}COOH} \)→\( \mathrm{[{-}NH{-}R{-}NH{-}CO{-}R'{-}CO{-}]}_n + 2n\,\mathrm{H_2O} \)

Nylon 6,6 is made this way, from hexanedioic acid and 1,6-diaminohexane. If a diacyl chloride is used instead of the carboxylic acid, the reaction is faster and gives off hydrogen chloride rather than water, which is useful for making nylon in the laboratory. Kevlar is a related aromatic polyamide built from monomers containing benzene rings, which you can revisit in structure, bonding and reactions of benzene; the rigid, flat rings let the chains pack tightly and hydrogen bond strongly to each other, giving Kevlar its exceptional strength.

Both linkages contain a carbonyl group, so a quick way to tell polyesters and polyamides apart on a structure diagram is to look at what sits next to the carbonyl carbon: an oxygen atom for an ester, or a nitrogen atom for an amide.

Because the ester and amide linkages were formed by loss of water, they can also be broken by adding water back, a reaction called hydrolysis. Acidic or alkaline conditions speed this up considerably:

• Acid hydrolysis of a polyester regenerates the diol and dicarboxylic acid; acid hydrolysis of a polyamide regenerates the diamine and dicarboxylic acid.
• Alkaline hydrolysis instead produces the diol (or diamine) together with the carboxylate salt of the dicarboxylic acid.

This reversibility is exactly why polyesters and polyamides are described as biodegradable in principle, though in practice many, such as PET bottles, resist hydrolysis under normal environmental conditions and persist for a long time. It also explains why proteins, which are natural polyamides built from amino acids, are broken down by hydrolysis of their amide bonds during digestion; you can see how amino acids form these same amide links in amines, amides and amino acids.

Draw the repeat unit formed from hexanedioic acid, \( \mathrm{HOOC(CH_2)_4COOH} \), and 1,6-diaminohexane, \( \mathrm{H_2N(CH_2)_6NH_2} \).

Each -COOH reacts with an -NH₂ group, losing a molecule of water and forming an amide linkage. The repeat unit is \( \mathrm{[{-}NH(CH_2)_6NH{-}CO(CH_2)_4CO{-}]} \), and this is nylon 6,6, named after the six carbon atoms contributed by each monomer.

Related lessons

• Uk A Level Chemistry