专业英语课程考试试卷

专业英语课程考试试卷 A卷
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READING MATERIAL A:
Plastic are remarkably useful 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 seem to last forever. One way
to keep discarded plastics from overwhelming us is to recycle them, to use them over and over
again. But there are different kinds of plastics, and one sort doesn't mix well with another. To
reuse plastics we have to separate 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 use 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 use 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, dense 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 these extraordinary substances
of our everyday world, lies not only one of the shining achievements of modern chemistry but, as
we'll see at the end of this chapter, perhaps even the secret of life itself. 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 itself 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 cellulose 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
compose these substances, polymer. A polymer is a molecule of very high molecular weight,
composed 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 thousands 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 composed 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 processes that produce our everyday polymers. Regardless of
what kinds of chemical reactions actually produce them, these polysaccharides and proteins do
provide us with fine illustrations of the structures of condensation polymers.
In a condensation reaction two molecules combine with the formation and loss of another,
smaller molecule, usually water or a simple alcohol. (The general term condensation reaction
probably originated as early chemists observed water or similar liquids forming droplets of
condensate on the sides of flasks during this sort of reaction.) Each of the condensing molecules
contributes some portion of the smaller molecule being eliminated.
The first and probably the best known of all the synthetic condensation polymers is nylon, a
plastic developed by the Dupont Corporation. In 1928 Wallace H. Carothers (1896-1937), an
Iowa-born chemist, left his post as instructor in organic chemistry at Harvard University to lead a
research group in Dupont's Wilmington, Delaware, laboratories. There he began a program of
fundamental research into polymers, studying how they form and what factors affect their
properties. Within a few years he and his co-workers found that by polymerizing a mixture of
adipic acid and 1,6-diaminohexane, they could produce a plastic (nylon) that can be drawn out
into strong, silky fibers.
Nylon's first practical application to a consumer product came in 1938, when the new