1.1.9　Copolymers

A copolymer（共聚物）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（容器），a copolymer will be formed which contains both styrene monomer units and acrylonitrile monomer units.

Many commercial（商品化的）synthetic polymers are copolymers，because they can improve some properties of the materials and enhance the ability for adapting the environment.The common copolymers are in the following four categories，as shown in Figure 1.6.It is attracted to wide attention that the sequence of monomer units along a copolymer chain could vary according to the method and mechanism of synthesis.Four different types of sequencing arrangements are commonly found in Figure 1.6.

Figure 1.6　Structure of copolymers

1.1.9.1　Random copolymers（无规共聚物）

In random copolymers，the arrangement of the monomer units has not definite sequence，as showed in Figure 1.6（a）.A copolymer of monomers A and B might be depicted（描述）by the arrangement shown in Figure 1.6（a）.Random copolymers are often formed when olefin-type monomers（烯烃类单体）copolymerize（共聚）by free-radical-type processes.The properties of random copolymers are usually quite different from those of the corresponding homopolymers（均聚）.

1.1.9.2　 Alternating copolymers（交替共聚物）

As the name implies（顾名思义），alternating copolymers contain a regular alternating sequence of two monomer units，as showed in Figure 1.6（b）.Again，the properties of the copolymer are usually markedly different from those of the two corresponding homopolymers.

1.1.9.3　Block copolymers（嵌段共聚物）

Block copolymers contain a block of one monomer connected to a block of another，as showed in Figure 1.6（c）.Block copolymers are often formed by ionic polymerization（离子聚合反应）processes.Unlike other copolymers，they retain many of the physical characteristics of the two homopolymers.

1.1.9.4　Graft copolymers（接枝共聚物）

A graft copolymer is defined as a polymer that the atoms or groups of backbone chain（主链）are attached to the branched chains（支链）in which the monomeric unit contains different atoms or groups，as shown in Figure 1.6（d）.In general，the backbone chain is composed of one A monomeric unit and the branched chain is composed of another B monomeric unit.The main chain also may be a copolymer or may be derived from a single monomer.Graft copolymers can be obtained in two ways.First，monomer B can be polymerized from sites along the length of polymer A.Second，two preformed polymers derived from A and B can be induced to react with each other to form a graft structure.For example，if polymer B has end-groups that will react with the side groups on polymer A，a graft copolymer will be formed.Graft copolymers are sometimes produced when two polymers are mixed and either irradiated（辐射）with X-rays or gamma-rays，or are subjected to mechanical shearing（机械剪切）.The chains of polymer B may be broken and the active chain ends will then be grafted onto polymer A.Graft copolymers often display the properties that are derived from the two homopolymers.

（参见本章二维码：1-3 共聚物分类及标记）