1.2　Definition-Ⅰ

Many of the terms and definitions used in polymer chemistry are not encountered in conventional chemical textbook. Some of these definitions will seem fairly obvious， but others will need explanation.

1.2.1　Monomers

A monomer is any substance that can be converted into a polymer. For example， ethylene is a monomer that can be polymerized to polyethylene，reaction（1.1）. An amino acid is a monomer which， by loss of water， can polymerize to give poly-peptides，reaction （1.2）. The term monomer is used very loosely，sometimes it applies to dimers or trimmers if they themselves can undergo further polymerization.

CH2CH2 —CH2—CH2—CH2—CH2—　　（1.1）

（1.2）

1.2.2　Dimers， Trimers， and Oligomers

The polymerization of a monomer often occurs in a sequential manner. In other words， two monomer molecules first react together to form a dimer. The dimer may then react with a third monomer to yield a trimer， and so on， can be linear or cyclic. low-molecule-weight polymerization products， for example， dimers， trimmers， tetramers， pentamers， etc.cyclic or linear is known as oligomers. Some care should be taken to avoid the use of the term “polymer” to describe materials that are really oligomers， because these two types of products have very different properties.

1.2.3　Polymers

The term polymer is used to describe high-molecular-weight substances. However， this is a very broad definition and in practice it is convenient to divide polymers into subcategories according to their molecular weight and structure. Although there is no general agreement on this point， in this book we will consider low polymers to have molecular weights below 10000～20000 and high polymers to have molecular between 20000 and several million. Obviously， this is a rather arbitrary dividing line， and a better definition might be based on the number of repeating units in the structure. For example， since polymer properties become almost independent of molecular weight when more than 1000～2000 repeating units are present， this point could also constitute a satisfactory dividing line between low and high polymers.

1.2.4　Linear Polymers

A linear polymer consists of a long chain of skeletal atoms to which are attached the substituent groups. Polyethylene is one of the simplest examples. Linear polymers are usually soluble in some solvent，in the solid state and at normal temperature they exist as elastomers， flexible materials， or glasslike thermoplastics. In addition to polyethylene， typical linear-type polymers include poly（vinyl chloride） or PVC， poly（methyl methacrylate）（also known as PMMA）， polyacrylonitrile and nylon 66.

1.2.5　Branched Polymers

A branched polymer can be visualized as a linear polymer with branches of the same basic structure as the main chain. A branched polymer structure is illustrated in Figure 1.1. Branched polymers are often soluble in the same solvents as the corresponding linear polymer. In fact， they resemble linear polymers in many of their properties. However， they can sometimes be distinguished from linear polymers by their lower tendency to crystalize or by their different solution viscosity or light-scattering behavior. Heavily branched polymers may swell in certain liquids without dissolving completely.

Figure 1.1　Branched polymer

1.2.6　Crosslinked Polymers

A crosslinked or network polymers is one in which chemical linkages exist between the chains， as illustrated in Figure 1.2. Such materials are usually swelled by“solvents”， but they do not dissolve. In fact， this insolubility can be used as cautious criterion of a crosslinked structure. Actually， the amount by which the polymer was swelled depends in the density of crosslinking： the more crosslink present， the smaller is the amount of swelling. If the degree of crosslinking is high enough， the polymer may be a rigid， unswellable solid. Light crosslinking of chains favor the formation of rubbery elastomeric properties.

Figure 1.2　Crosslinked macromolecule