Ethane-1,2-diol (ethylene glycol) has a hydroxy group at each end, and benzene-1,4-dicarboxylic acid (terephthalic acid) has a carboxy group at each end.
A hydroxy group and a carboxy group react to form an ester bond, giving off a water molecule. Because each monomer has a group at both ends, the reaction repeats at both ends and the molecules join into a long chain.
The polymer obtained, poly(ethylene terephthalate) or PET, has many ester bonds along its chain, so it is a polyester. It is used for drinks bottles and, as polyester fibre, for clothing.
Hexane-1,6-diamine (hexamethylenediamine) has an amino group at each end, and hexanedioic acid (adipic acid) has a carboxy group at each end.
An amino group and a carboxy group react to form an amide bond, \mathrm{{-}NH{-}CO{-}}, giving off water. Repeated at both ends, this joins the monomers into a long chain of nylon 6,6.
A polymer with many amide bonds is called a polyamide. The numbers 6,6 show that both the diamine and the dicarboxylic acid have six carbon atoms. Nylon 6,6 was first made by Wallace Carothers in 1935. It was the first fully synthetic fibre, and it is strong and elastic.
Nylon 6,6 can be made in the laboratory by carefully pouring a solution of hexanedioyl dichloride (adipoyl chloride) in hexane onto a solution of hexane-1,6-diamine and sodium hydroxide in water.
The two liquids do not mix, and nylon forms as a film where they meet. When the film is picked up with tweezers and pulled, new nylon keeps forming, and it can be drawn out as a continuous thread.
An acid chloride reacts much faster than a carboxylic acid, so the reaction goes at room temperature. Each link gives off hydrogen chloride instead of water, and the sodium hydroxide neutralizes it.
Nylon 6 is made from a single monomer, azepan-2-one (ε-caprolactam), whose molecule is a ring containing an amide bond.
When it is heated with a little water, the ring opens at the amide bond, and the molecules join into a chain with the repeating unit \mathrm{{-}NH(CH_2)_5CO{-}}.
No small molecule is given off, so this is not condensation polymerization but ring-opening polymerization. The 6 in the name shows that the repeating unit has six carbon atoms.
In a polyamide, the \mathrm{C{=}O} and \mathrm{N{-}H} groups of neighbouring chains form hydrogen bonds. These bonds hold the chains together, which is one reason why nylon fibres are strong.
In aramid fibres such as Kevlar, the amide groups are joined to benzene rings, so the chains are straight and rigid. They line up closely and form many hydrogen bonds, which makes the fibres extremely strong and resistant to heat.
Aramid fibres are used in bulletproof vests, protective gloves and firefighters' clothing.