聚合物和添加剂

8. POLYMERS AND

THEIR ADDITIVES

The term macromolecule, or polymer, is

applied to substances of high molecular

weight that are composed of a large number (usually at least 100) of units of low molecular weight joined by covalent bonds. If the low molecular weight units making up the macromolecule are bonded end-to-end in a long chain and no covalent chemical bonds exist between the chains, the macromolecules are called linear polymers. Such polymers, unless of extremely high molecular weight (1,000,000), can usually be dissolved and, when heated, they soften or melt so that they can be extruded into fibers or moulded into desired shapes. These polymers are said to be thermoplastic. On the other hand, if the polymer chains are linked together at numerous points, the polymer is one large three-dimensional molecule, infusible and insoluble. Such polymers are called cross-links. Certain linear polymers, referred to as thermosetting, contain groups which, when heated, react to give cross-linked polymers.1

The process by which small molecules undergo multiple combination to form macromolecules is polymerization. Small molecules from which a

macromolecule or polymer can be made are called

monomers. Two types of polymerization are

recognized: (1)condensation polymerization and (2) addition polymerization.

A polymer-forming reaction involving elimination of a small molecule such as water or alcohol between monomer units is described as condensation polymerization.2 In addition polymerization, unsaturated or cyclic molecules add to each other without elimination of any portion of the monomer molecule. The empirical formula of the polymer is then, of course, the same as that of the monomer.

Reactions capable of forming macromolecules by either addition or condensation polymerization must be functionally capable of proceeding indefinitely. Whenever two monomer molecules react, the product must contain a functional group capable of reacting with another molecule of monomer. In condensation polymerization, each monomer unit must have at least two functional groups. In addition polymerization, the monomer need have only one functional group - the presence of two or more functional groups usually leads to the production of cross-linked addition polymers.

The modulus and other physical properties of polymer can be improved by the addition of fillers and other additives, such as plasticizers, antioxidants, heat stabilizers, ultraviolet stabilizers, and flame retardants. Additives are essential functional ingredients of polymers, and whenever possible, each should be used in optimum amounts for the attainment of high-quality products.