英语翻译The term macromolecule,or polymer,is applied to substanc

英语翻译
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 (1000000),can usually be dissolved and ,when heated,they soften or melt so that they can be extruded into fibers or molded 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-linked polymers,and the bonds connecting the chains are cross-links.Certain linear polymers,referred to as thermosetting,contain groups which ,when heated,react to give cross-linked polymers.