Kevlar is a condensation polymer used to make bulletproof vests and reinforced tyres. Its repeat unit contains an amide linkage between two benzene rings. Explain why Kevlar is classified as a condensation polymer rather than an addition polymer, and describe the type of monomer or monomers required to form Kevlar.

Kevlar is a condensation polymer because each time a new –CO–NH– bond is formed a small molecule, in this case water, is eliminated from the reacting monomers. In a condensation reaction the loss of a small molecule is the defining feature, whereas addition polymers are formed by the successive addition of monomers without any small‑molecule loss. Kevlar is therefore not an addition polymer.

The repeat unit of Kevlar contains an amide linkage between two benzene rings, so the polymer must be built from two different bifunctional monomers. One monomer is a diamine, 1,4‑diaminobenzene (p‑phenylenediamine), which has an –NH₂ group at each end of the benzene ring. The other monomer is a diacid chloride, 4,4′‑difluorobenzophenone dichloride (or a similar diacid chloride), which has a –COCl group at each end. Each monomer is bifunctional, providing one reactive group at each end, allowing the –CO–NH– peptide linkage to form between the two monomers during the condensation reaction.

Thus Kevlar is a condensation polymer because water is eliminated during chain growth, and it requires two different bifunctional monomers – a diamine and a diacid chloride – to form the –CO–NH– amide bonds that give the polymer its strength.