A heteropolymer, also called a copolymer, is a polymer formed when two different types of monomer are linked in the same polymer chain.

A special type of copolymer is called a "block copolymer". Block copolymers are made up of blocks of different polymerized monomers. For example, PS-b-PMMA is short for polystyrene-b-poly(methyl methacrylate) and is made by first polymerizing styrene, and then subsequently polymerizing MMA. This polymer is a "diblock copolymer" because it contains two different chemical blocks. You can also make triblocks, tetrablocks, pentablocks, etc. Diblock copolymers are made using "living polymerization" techniques, such as atom transfer free radical polymerization (ATRP), reversible addition fragmentation chain transfer (RAFT), living cationic or living anionic polymerizations.

Block copolymers are interesting because they can "microphase separate" to form nanoscopic sized structures.

Think of oil and water. They don't mix together- they MACROphase separate. Block copolymers behave in a similar way called MICROphase separation. You have an "oil-like" first block and a "water-like" second block. They want to get as far from each other as possible, but they are covalently bonded! So they're not going to get very far. This causes "microphase separation" in which the "oil" and "water" phases separate to form nanometer-sized structures. These structures can look like sphere of PMMA in a matrix of PS or visa versa, or they could be stripes or cylinders. Structures on the nanoscale can be used for creating even smaller devices that could potentially be used in computer memory, nanoscale-templating and nanoscale separations.

In more official terms, polymer scientists wouldn't use "oil-like" and "water-like" to describe the polymer block's interactions. Polymer scientists use thermodynamics to describe how the different blocks interact. The "interaction parameter", also called "chi" gives an indication on how different chemically the two blocks are and if they will microphase separate. If chi is large, the blocks will microphase separate. If chi is too small, the different blocks "like" each other enough to mix.