Cellulose acetate and nitrate

The base material for these polymers is the natural polymer cellulose, which consists of long molecular chains. It is the structural mate­rial of plants; the major component of wood pulp, paper, and cotton; and one of the cheap­est and most abundant organic substances available. Cellulose acetate is made by adding acetic acid to cellulose derived either from wood pulp or cotton. It can be spun into yarn to give the widely-used textile fabric rayon ac­etate. It is also employed as a film base and as a plastic.

Cellulose nitrate, or nitrocellulose, was one of the earliest polymers known. It is made by treating cellulose with a mixture of nitric and sulfuric acids under carefully controlled con­ditions. Guncotton, an explosive, is highly ni­trated. The low-nitrated form, pyroxylin, is used in fast-drying lacquers and in table-tennis balls. Its use for motion-picture film has been discontinued because of its high inflammabil­ity.

Resins

Natural resins are soft and pliable when warm and hard and brittle when cool. Synthetic poly­mers, such as polystyrene, rarely display these types of characteristics. To call them resins is somewhat misleading. Synthetic polymers often exhibit good chemical and electrical re­sistance. Normally, they are insoluble in water. In addition, they are easy to process by mold­ing, machining, or special shaping. Syntheti­cally formed epoxy resins have a broad range of uses, the major one being as a strong adhe­sive. They also form tough protective coatings

Organic chemistry: Polymers 103

and are employed in corrosion-resistant paints for use on metals.

Silicones

Useful polymers that exist as rubbers, resins, and oils, silicones can be synthetically (artifi­cially) formed. Their value lies in their inert­ness. They remain uninfluenced by water, chemicals, high and low temperatures, and electricity. The rubbers can be used as seals and gaskets at extreme temperatures, the res­ins as electrical insulators and water-repellent paints, and the oils as lubricants in extreme conditions of temperature.

Low-density polyethylene filmcan be made by blow­ing hot air through a poly­ethylene tube. The heat soft­ens the plastic and the air pressure inflates the tube, stretching it like a balloon to form a thin film. Polyeth­ylene is used mainly for making plastic bags and wrapping material.

Fact entries

Polyalkenesare the major group of synthetic poly­mers. They include polyeth­ylene (a very durable ther­moplastic used for containers, insulation, and tubing) and polypropylene (a harder thermoplastic used for insulating material, baby bottles, and piping).

Other polyalkenes are closely related chemically. They include polyvinyl chlo­ride (a hard resin used in fabrics, pipes, and floor cov­erings), polytetrafluoroeth-ylene (PTFE, a slippery and acid-resistant resin used as a dry lubricant, best known as Teflon), and polystyrene

(a transparent plastic used in making toys, reeds for musical instruments, and many implements). Most synthetic rubbers are poly­mers based on butadiene and similar gases.

Polyamidesare best known in the various types of nylon.

Polyestersinclude polye-thenyl acetate (used in emulsion paints and adhe-sives), polymethyl methacry-late (used in lenses), and polyethylene terephthalate (used for fibers and films).

Biochemistry

Biochemistry is concerned with the chemistry of living things. During the twentieth century and particularly in the postwar period, scien­tific knowledge about the working of living systems at the molecular level has increased dramatically. This field studying the structure and functioning of molecules is now generally treated separately from the rest of chemistry. A part of the subject is concerned with the way in which genetic information is coded and used by organisms. Genetic information deals with "blueprints" within genes, which deter­mine the origin and natural growth of organ­isms. All living cells contain these genes. This branch of biochemistry has had such an explo­sive growth in recent decades that it is often regarded as a separate science—molecular bi­ology.

Biochemistry also studies how such mole­cules are produced; what changes they may undergo in living cells; how they interact with different parts of an organism; what chemical events underlie the effects that they conse-

Digestive actions (hydrolysis)

Mouth

Polysaccharides to disaccharides

e.g.

(C₆H₁₀O₅)_n - C,₂H₂₂O_r

Stomach

Proteins to polypeptides

e.g.

H(NHRCHCOi ,,OH - H(NHRCHCO)„OH

Small intestine

Fats to fatty acids and glycerol e.g.

[R(CH₂)_nCOOCH]₃H₂ -* R(CH₂)_nCOOH + (CH₂OH)₂CHOH

Polypeptides to amino acids

e.g.

H(NHRCHCO) _nOH -+ NH₂RCHCOOH

Disaccharides (e.g. maltose) to

monosaccharides (e.g. glucose)

e.g.

C₁₂H₂₂0„ -* C₆H₁₂0₆

Also

Polysaccharides to disaccharides Proteins to polypeptides

quently bring about; and what subsequently happens to them. Thus, the process of diges­tion is a proper study for biochemistry. It studies the kinds of molecules taken in by an organism in its food, how it breaks them down, and what it does with the end products.

Biochemistry is also concerned, in this ex­ample, with an important intermediate product—energy. The organism obtains en­ergy from food in order to sustain itself. How it extracts this energy from the food substances and how this is used in the energy-requiring processes of life are among the questions that biochemists have tackled. Another fundamen­tal question is how the energy of sunlight is trapped by green plants in the creation of food substances by the process of photosynthesis.

Date: 2015-12-11; view: 1025