Nylon and polyesters

Section 3 of 4

Nylon and polyester are made another way. Each monomer has a functional group at both ends, and two different monomers take turns along the chain.

You have already met condensation: two molecules join and give off a small molecule, usually water. A condensation polymer forms when this happens at every link in the chain, so one water molecule is given off for every link.

Nylon: amine meets acid

Nylon-6,6 is made from two monomers. Hexane-1,6-diamine is a chain of 6 carbons with an amine group, -⁠NH₂, at each end. Hexanedioic acid is a chain of 6 carbons with a carboxylic acid group, -⁠COOH, at each end. You have met both groups before. The two 6s in the name count the carbons in each monomer.

Hexamethylenediamine (hexane-1,6-diamine), H₂N(CH₂)₆NH₂A chain of six carbon atoms with an amine group, -NH₂, on each end carbon.NH2NH2
Hexane-1,6-diamine: an -⁠NH₂ group at each end of a 6-carbon chain.
Adipic acid (hexanedioic acid), HOOC(CH₂)₄COOHA chain of six carbon atoms with a carboxylic acid group, -COOH, on each end carbon.OHOHOO
Hexanedioic acid: a -⁠COOH group at each end of a 6-carbon chain.

When an acid end meets an amine end, the -⁠OH from the acid and one H from the amine leave together as a water molecule. The carbon and the nitrogen bond to each other, making an amide link, -⁠CONH⁠-.

An acid end and an amine end join with an amide link and give off water Top: the end of one molecule with a carboxylic acid group, C double-bonded to O and single-bonded to O-H, next to the end of another molecule with an amine group, N with two hydrogen atoms. The O-H of the acid and one H of the amine are shaded and labelled leave as water. An arrow points down to the product: the carbon of the acid group is now bonded straight to the nitrogen, the C-N bond in amber, labelled amide link. A water molecule, H2O, is given off. COOHHNHCONHH2O + + leave as water amide link acid end amine end
An acid end and an amine end join. The shaded -⁠OH and H leave as water, and the new C–⁠N bond (amber) is the amide link.

Each monomer has two ends, so it joins on both sides: diamine, acid, diamine, acid, thousands of times over. Your body uses the same link to join amino acids into proteins, where it is called a peptide bond, as the next lesson shows.

The repeat unit of nylon-6,6 Square brackets with a small n hold one repeat unit of nylon-6,6. From left: an N-H group, a chain of six CH2 groups, written (CH2)6, another N-H group, then a carbon double-bonded to oxygen, a chain of four CH2 groups, and a second carbon double-bonded to oxygen. The N-H and C=O pair in the middle is shaded and labelled amide link. A brace under the first part says from the diamine, and one under the second part says from the acid. The bonds leaving the brackets, and the C-N bond of the amide link, are amber. N(CH2)6NC(CH2)4CHHOO n amide link from the diamine from the acid
The repeat unit of nylon-6,6 holds one of each monomer. The amide link in the middle is shaded; the amber bonds at the brackets make the next amide links, with the neighbouring units.

Polyester: alcohol meets acid

A polyester joins its monomers with the familiar ester link. One monomer is a diol, an alcohol with an -⁠OH group at each end, such as ethane-1,2-diol. The other is an acid with a -⁠COOH group at each end. Each -⁠OH meets a -⁠COOH, water is given off, and an ester link, -⁠COO⁠-, joins them.

The best-known polyester is PET, made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid, whose two -⁠COOH groups sit on opposite corners of a benzene ring. PET makes plastic drinks bottles, and spun into fibres it makes the polyester in many clothes.

The repeat unit of PET, a polyester Square brackets with a small n hold one repeat unit of PET. From left: an oxygen atom, two CH2 groups and a second oxygen atom, then a carbon double-bonded to oxygen, a benzene ring drawn as a hexagon, and another carbon double-bonded to oxygen. The oxygen and the C=O next to it are shaded and labelled ester link. A brace under the first part says from the diol, and one under the second part says from the acid. The bonds leaving the brackets, and the bond that makes the ester link, are amber. OCH2CH2OCOCO n ester link from the diol from the acid
The repeat unit of PET. The ester link is shaded. The hexagon is a benzene ring drawn as a skeletal formula: a carbon at each corner, with its hydrogens left out.

Quick check

A polymer is made from ethane-1,2-diol, which has an -⁠OH group at each end, and an acid with a -⁠COOH group at each end. How do the monomers join?