In chemistry, a polymer is a chain of molecules made by linking many small molecules together. Polymers come in different varieties, one kind being the copolymer. A copolymer is made when two different types of molecules are joined in the same polymer chain.
Each of the two types of molecules is called a monomer, and the arrangement of the monomers leads to different categories and subcategories of copolymers. For example, an alternating copolymer consists of regularly alternating monomers A and B, so the copolymer chain goes: A-B-A-B-A-B-A-B. A random copolymer is one with random sequences of A and B, such as B-A-A-A-B-A-B-A-A-B-B. Periodic copolymers are yet more complicated, since they consist of A and B monomers arranged in a repeating sequence. For instance, if the A and B sequence described in the second example above were to repeat without changing, even though the sequence itself seems random, this would be a periodic copolymer.
Block copolymers have perhaps the simplest structure, yet also the strangest attributes. These copolymers are made up of “blocks” where all of one type of monomer are grouped together, with all those of the other kind being grouped together. It is as if two ordinary polymers were joined together at the ends. In these compounds, the two monomers behave in a manner that scientists call phase separation.
Phase separation is the same thing that happens when oil and water -- two incompatible substances -- separate in a container. However, the compounds in block copolymers are atomically bonded together, so instead of visually separating like oil and water, they separate on a microscopic scale. This separation in the blocks forms very small, but very rigid structures. These substances are strong enough that some types of block copolymers are used in shoe soles and automobile tires. It is also interesting to note that block copolymers can be made up not just of two monomers, but in some cases up to five or more.
Another way of categorizing copolymers is by the shape of the structures formed by the polymer chains. Linear copolymers are just one single chain, whereas branched copolymers have a main chain with other chains branching off to the side at intervals. A common type of branched copolymer is the graft copolymer, where the main chain consists just of monomer A, and the branches consist only of monomer B.
In this case, both monomers can lend properties to the finished graft copolymer. One example would be high-impact polystyrene. It has a polystyrene main chain which gives the material its strength. Grafted onto the polystyrene backbone are chains of the compound polybutadiene, which gives the material a resilience that ordinary polystyrene lacks.
