8.6 Copolymers

A copolymer is a polymer made from two or more different monomers. For example，if styrene and acrylonitrile are allowed to polymerize in the same reaction vessel，a copolymer will be formed which contains both styrene and acrylonitrile residues. Many commercial synthetic polymers are copolymers. It should be noted that the sequence of monomer units along a copolymer chain can vary according to the method and mechanism of synthesis. Three different types of sequencing arrangements are commonly found.

（1）Random copolymers. In random copolymers，no definite sequence of monomer units exists. A copolymer of monomers A and B might be depicted by the arrangement shown in （6）. Random copolymers are often formed when olefin-type monomers copolymerize by free-radical-type processes. The properties of random copolymers are usually quite different from those of the related homopolymers.

（2）Alternating copolymers. As the name implies，alternating copolymers contain a regular alternating sequence of two monomer units （7）. Olefin polymerizations that take place through ionic-type mechanisms can yield copolymers of this type. Again，the properties of the copolymer usually differ markedly from those of the two related homopolymers.

（3）Block copolymers. Block copolymers contain a block of one monomer connected to a block of another，as illustrated in sequence （8）. Block copolymers are often formed by ionic polymerization processes. Unlike other copolymers，they retain many of the physical characteristics of the two related homopolymers.