Synthetic polymers are formed by addition or condensation polymerization. The length of the polymer chains, usually measured by molecular weight, has a
very significant effect on the performance properties and a profound effect on processibility.
Thermosetting resins can be used without any reinforcement as bulk plastics or foams.
Fibre-reinforced plastics normally use a thermosetting matrix to support the fibres and allow load-transfer. These can be structural or semi-structural parts. A further use for composites is as electronic circuit board substrate materials,
Polymer resins used are mainly epoxies, cyanates,
phenolics, polyesters, bismaleimides, polyimides, silicones, diallylphthalate and diphenyloxide. Polyimides are really thermoplastic ladder polymers, but are included here by analogy.