The materials commonly called plastics are all synthetic polymers. Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. In many polymers, only one monomer is used.In others, two or three different monomers may be combined.OLYURETHANE. This important class of polymers is formed by the addition polymerization of an diisocyanate (whose molecules contain two NCO groups) and a dialcohol (two OH groups). The polymer chain is linked by urethane groups (OCONH).