Block copolymers are a special type of polymer in which each molecule consists of two or more segments of simple polymers (blocks) joined in some arrangement. More precisely, polymers can be divided in homopolymers, in which each molecule is composed of the same type of monomer, and copolymers, in which each molecule is composed of more than one type of monomer. Copolymers are divided into random and block types. In block copolymers, the different monomers are organized into distinct segments, or blocks. Block copolymers are further classified by the number of blocks each molecule contains, and how they are arranged. Block copolymers with two, three, and more blocks are called diblocks, triblocks, and multiblocks. Some arrangements are linear, in which the blocks are connected end-to-end, and star, in which all of the blocks are connected via one of their ends at a single junction. Of course more complicated arrangements are possible. The number of monomer types in a block copolymer may be less than or equal to the number of blocks. Thus, an ABC linear triblock consists of three monomer types, whereas an ABA linear triblock consists of two monomer types.

The following diagram shows the arrangement of blocks in an AB diblock (1), an ABA triblock (2), an ABC triblock (3), and a starblock (4).

Examples of the monomers which can be used to build block copolymers are isoprene and styrene.