The simple picture of molecule as a tiny particle that interacts with its neighbors while being buffeted by thermal motion has proved valuable. However, when molecules become very large, they gain certain properties distinct from their smaller cousins that make them an interesting group to look at in their own right. “Very large” is intentionally a vague definition but if we take it to mean molecular weights in the tens of thousands and greater, then all of the molecules in this group are polymers. Polymers are formed from the combination of a series of smaller molecules (i.e., monomers) to form a chain. Thus, the fundamental requirement for a molecule to be able to polymerize is that it needs at least two reactive groups. If a molecule has one reactive group, then it can react with a second molecule to form a dimer but this blocks the reactive sites on both molecules and prevents further polymerization. If a molecule with two reactive groups forms a dimer, it blocks one reactive site but still has another available to continue the reaction and lengthen the chain. If a monomer has more than two reactive groups, it can form a branched chain.