is a substance formed by the linkage (polymerization) of two or more simple, unsaturated molecules (see unsaturated hydrocarbon), called monomers, to form a single heavier molecule having the same elements in the same proportions as the original monomers; i.e. each monomer retains its structural identity. A polymer may be liquid or solid; solid polymers may consist of millions of repeated linked units. A polymer made from two or more dissimilar monomers is called a copolymer; a copolymer composed of three different types of monomers is a terpolymer. Natural rubber and synthetic rubbers are examples of polymers Polymers are commonly used as viscosity index improvers in multi-grade oils.