Polymers are unlike other types of compounds because of their extremely large size. Molecular weight is the value used to express the size of a molecule. Water, for example, has a molecular weight of 18 atomic mass units. Polymers are much larger, with molecular weights ranging from tens of thousands up to several million atomic mass units. The sheer size of polymers has a great impact on their unique properties.
One important implication of polymer size is how atoms behave in different phase states. Smaller-molecule compounds have three states of matter – solids, liquids, and gases. The particles of a solid are packed closely together and have very little motion. The particles of a liquid, by comparison, are more loosely packed and slide more easily past one another. And the particles of a gas are very loosely packed and move about with great energy.