专业英语课程考试试卷 A卷
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READING MATERIAL A:
Plastic are remarkably uful materials, but one of their greatest advantages over other materials, their durability, is also one of their greatest handicaps. They don’t disintegrate. Once we dump them into the environment, they just stay there. Some plastic em to last forever. One way to keep discarded plastics from overwhelming us is to recycle them, to u them over and over again. But there are different kinds of plastics, and one sort doesn't mix well with another. To reu plastics we have to parate one kind from another so that we can combine all compatible plastics into one group and reprocess them as a whole. One way to sort them is by taking advantage of differences in the densities of different kinds of plastics.
If an era is known by the kinds of materials its people u to build the world they live in, then the Stone Age, the Bronze Age, and the Iron Age have given way to our own Plastic Age. Plastics form much of our packing and wrapping materials, many of our bottles and containers, textiles, plumbing and building materials, furniture and flooring, paints, glues and adhesives, electrical insulation, automobile parts and bodies, television, stereo and computer cabinets, medical equipment, video and audio tapes, records and compact disks, personal items including pens, razors, toothbrushes, and hairsprays, and even the plastic trash bags we u to discard our plastic trash, Except for our food, air, and water, almost every ordinary thing we come in contact with each day contains some kind of a plastic somewhere in, on, or around it. Or it comes to us wrapped in plastic. So many of our throwaway goods are made of plastic that, despite its lightness, the material currently makes up an estimated 7% of the total weight of all solid municipal wastes and is expected to grow to 10% by the year 2000. What's more, plastics are a highly visible part of what we discard, making up roughly a quarter of the entire volume of our trash.
Durable or fragile, rigid or flexible, sturdy or flimsy, den or light, strong or weak, plastics
provide us with inexpensive materials of virtually unlimited properties. With chemical ingenuity we can transform them into almost whatever shapes we wish with almost whatever properties we desire. And at their root, in the polymeric molecules that make up the extraordinary substances of our everyday world, lies not only one of the shining achievements of modern chemistry but, as we'll e at the end of this chapter, perhaps even the cret of life itlf. We'll begin, though, by examining the difference between the plastics of our world and the polymers that form them.
Plastics and Polymers
Plastics, especially the plastics of our most common commercial products, are extraordinary kinds of materials that we can shape into virtually any form we want. The word itlf comes from the Greek plastikos, ''suitable for molding or shaping.'' We can form them into round, hard, resilient bowling balls, draw them out into the thin, flexible threads of synthetic fibers, mold them into intricately designed, long-running machine parts, or flatten them into flimsy but tough sheets of clinging kitchen film. Today, the word
plastic refers mostly to a property of a material: its ability to be shaped into the myriad forms of today's commercial and consumer products.
When we speak of a polymer, though, we return to the molecular level of matter. All the plastics of our everyday lives, as well as all the proteins and the starch and cellulo of our foods, the cotton, silk, and wool of our textiles, and even the DNA that carries the genetic code within the nucleus of the cell are formed of enormously large polymeric molecules. The combination of the Greek words poly, meaning ''many,'' and meros, ''parts,'' gives us the word for the molecules that compo the substances, polymer. A polymer is a molecule of very high molecular weight, compod of many - a great many much smaller parts joined together through chemical bonds. .
As the word implies, polymers are extremely large molecules, sometimes called macromolecules to emphasize their very large size. The individual parts that combine to form them, monomers from the Greek mono, ''one,'' join to each other in enormously large numbers to produce polymers with molecular weights ranging from the tens of thou
sands to millions of atomic mass units. Often the monomers unite to form an enormously long, linear molecular thread, very much like a long chain we might find in a hardware store. In other polymers the chains may be branched to various degrees, or they may be interconnected at occasional junctions, or so frequently that they form a web or even a rigid, three-dimensional lattice. In any event, a polymer is a substance compod of huge molecules, sometimes in the form of very long chains, sometimes as sheets, sometimes as intricate, three dimensional lattices. A plastic, on the other hand, is a material that can be molded readily into a variety of shapes. All of today's commercial plastics are polymers, even though some of our most important polymers are not at all plastic.
Condensation Polymerization
The actual linking of the monomers through covalent bonds occurs during polymerization, a chemical process easily divided into two broad categories: condensation polymerization and addition polymerization. The products are condensation polymers and addition polymers, respectively.
We'll look first at a few condensation polymers, then we'll return to addition polymers. The naturally occurring polysaccharides and proteins provide us with good examples of condensation polymers, even though they form through complex enzymatic reactions, far removed from the relatively straightforward industrial process that produce our everyday polymers. Regardless of what kinds of chemical reactions actually produce them, the polysaccharides and proteins do provide us with fine illustrations of the structures of condensation polymers.
