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
Cellulose(C) is the chief structural material of plants. It is a natural polymer of the sugar glucose (A). The formula for glucose is diagrammed in the green box at far left. Glucose is obtained from cotton or wood pulp. It can be converted chemically into nitrocellu-ose (B), used as a propel-Iant explosive, or cellulose acetate (D), for textiles and films.
CH20H
Glucose foi
Nitrocellulose (polymer
Cellulose (polyme
OH
|\
H
I >
\l C
/I
/
OH
HH
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
Digestionbreaks down the food we eat into very small pieces. These can then be absorbed through the walls of the alimentary tract. The main sites of digestion in humans are the mouth, stomach, and small intestine. The table below gives the principal digestive reactions that occur at each site.
Stomach-Duodenum
sltt^
Small intestine
Main digestive enzymes
MouthAmylase (polysaccharides)
StomachPepsin (proteins)
Small intestineLipase (fats) Peptidase (polypeptides) Maltase (disaccharide) Amylase, lactase and sucrase
(polysaccharides) Trypsin (proteins)
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,2H22Or
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.