In the chapter in polymers, it is explained that polymers are compounds of very high molecular masses formed by the combination of a large number of simple molecules. Simple example of a polymer is polyethelene. This polymer is formed by the compound formation between molecules of ethelene. The molecules of ethylene in the case of formation of polymer are referred to as monomers.
But what is the difference between a metal and polymer?
At a molecular level, the basic units that make up a metallic structure are relatively small and uniform in size. Consequently, they readily arrange into a structure that is very regular and predictable. Material scientists call these structures as crystalline solids.
The smallest unit in a polymeric material is a very large molecule with an extended chain shape. (Remember linear chains, branched chains and cross linked chains). Even the smallest polymer molecule will be about 50 times more massive than the heaviest naturally occurring unit in a metallic structure. These polymer molecules can twist, turn, fold, and entangle in almost unlimited variations (Remember the point regarding straightening of chains due to stretching). In addition, not all of these molecules are of the same size. The smallest molecule in any random sample of plastic is often a thousand times smaller than the largest, which increases the possibilities for local variation in the structure of a material.
In a metal there ionic forces between free flowing electrons and the positive charged nucleuses that hold the metal together. In a polymer there are covalent bonds between molecules of various sizes, with each molecule being polymer of number of monomers.