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Polyesters are polymers of a group of organic compounds called esters. Saturated polyesters (those with no double molecular bonds), are thermoplastics made by a condensation reaction, and are used to make synthetic fibres such as Dacron and Terylene. Unsaturated polyesters (which do have double bonds) are used as resins to make thermosetting plastics, and are utilised in the manufacture of glass-fibre products.