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A heteropolymer, also called a copolymer, is a polymer formed when two (or more) different types of monomer are linked in the same polymer chain, as opposed to a homopolymer where only one monomer is used. If exactly three monomers are used, it is called a terpolymer.
The assembly of the monomers in the copolymers can be head-to-tail, head-to-head, or tail-to-tail.
Block Copolymers
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 periodic nanostructures.
Microphase separation is a situation similar to that of oil and water. Oil and water don't mix together- they macrophase separate. If you have an "oil-like" first block and a "water-like" second block, the block copolymers undergo microphase separation. The blocks want to get as far from each other as possible, but they are covalently bonded, so they're not going to get very far. In "microphase separation" the "oil" and "water" blocks form nanometer-sized structures. These structures can look like spheres of PMMA in a matrix of PS or vice versa, or they could be stripes (often called lamellae in the technical literature) or cylinders. The nanoscale structures created from block copolymers could potentially be used for creating devices for use in computer memory, nanoscale-templating and nanoscale separations.
In official terms, polymer scientists wouldn't use "oil-like" and "water-like" to describe the polymer blocks' interactions. Polymer scientists use thermodynamics to describe how the different blocks interact. The "interaction parameter", also called "chi" gives an indication of how different, chemically, the two blocks are and whether or not they will microphase separate. If the product of chi and the molecular weight is large (greater than 10.5), the blocks will microphase separate. If the product of chi and the molecular weight is too small (less than 10.5), the different blocks are able to mix.
Types of copolymers
Since a copolymer consists of at least two types of repeating units (not structural units), copolymers can be classified based on how these units are arranged along the chain. These include
- Random copolymer: -A-A-B-B-A-A-A-B-A-A-B-B-A-B-A-A-A-B-A-A-A-B-B-
- Alternating copolymer: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)n-
- Block copolymer: -A-A-A-A-A-A-A-B-B-B-B-B-B-B-A-A-A-A-A-A-A-B-B-...
- Graft copolymer:
- -A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-
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- B-B-B-B-B-B-B
- Star copolymers
See also
"Heteropolymer"