Unsaturated polyesters arecondensation polymersformed by the reaction ofpolyols(also known aspolyhydric alcohols), organic compoundswith multiple alcohol or hydroxy functional groups, with saturated or unsaturated dibasic acids. Typical polyols used areglycolssuch asethylene glycol; acids used arephthalic acid, isophthalic acidandmaleic acid. Water, a by-product ofesterificationreactions, is continuously removed, driving the reaction to completion. The use of unsaturated polyesters and additives such asstyrenelowers the viscosity of the resin. The initially liquid resin is converted to a solid bycross-linkingchains. This is done by creatingfree radicalsat unsaturated bonds, which propagate in achain reactionto other unsaturated bonds in adjacent molecules, linking them in the process. The initial free radicals are induced by adding a compound that easily decomposes into free radicals. This compound is usually and incorrectly known as thecatalyst.Initiator is the more correct term. Substances used are generally organic peroxides such asbenzoyl peroxideormethyl ethyl ketone peroxide.