Polyester

A polyester is a condensation polymer whose chains are held together by ester links. It is made from two monomers, a diol (two –OH groups) and a dicarboxylic acid (two –COOH groups), which join and release water. Malay: poliester · Chinese: 聚酯.

Because each monomer has two functional groups, one at each end, the chain can grow in both directions. Every time an –OH of the diol reacts with a –COOH of the acid, an ester link (–COO–) forms and a molecule of water is eliminated. This is the same ester link found in a simple ester, but here it is repeated many times along a long chain, which is why the material is called a poly-ester. Terylene, used to make clothing fibres and drink bottles, is the polyester the syllabus names.

Example. Terylene is manufactured by the condensation polymerisation of a diol with a dicarboxylic acid. The two react through their functional groups to build a long chain joined by ester links, giving off water at each join, and the fibre is then spun into fabric.

Confusion to avoid. Do not confuse a polyester with a polyamide. Both are condensation polymers, but a polyester is joined by ester links (–COO–) made from a diol and a diacid, while a polyamide is joined by amide links (–CONH–) made from a diamine and a diacid. The name tells you the link: ester for polyester, amide for polyamide.

A quick way to spot a polyester link in a structure is to look for the ester group –COO– repeating along the backbone; each repeat marks where a diol and a diacid joined. In an exam you may be shown the two monomers and asked to circle the link formed and name the small molecule removed, which is always water for a polyester.

Polyester shows how the single esterification reaction from Carbon Compounds scales up into a manufactured material, tying that earlier chapter to Polymers and to questions on condensation in SPM Chemistry.