Polymers, Natural

Polymers, Natural

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The word "polymer" means "many parts" (from the Greek poly, meaning "many," and meros, meaning "parts"). Polymers are giant molecules with molar masses ranging from thousands to millions. Approximately 80 percent of the organic chemical industry is devoted to the production of synthetic polymers, such as plastics, textiles fibers, and synthetic rubbers. A polymer is synthesized by chemically joining together many small molecules into one giant molecule. The small molecules used to synthesize polymers are called monomers. Synthetic polymers can be classified as addition polymers, formed from monomer units directly joined together, or condensation polymers, formed from monomer units combining such that a small molecule, usually water, is produced during each reaction.

Polymers are widely found in nature. The human body contains many natural polymers, such as proteins and nucleic acids. Cellulose, another natural polymer, is the main structural component of plants. Most natural polymers are condensation polymers, and in their formation from monomers water is a by-product.

Starch is a condensation polymer made up of hundreds of glucose monomers, which split out water molecules as they chemically combine. Starch is a member of the basic food group carbohydrates and is found in cereal grains and potatoes. It is also referred to as a polysaccharide, because it is a polymer of the monosaccharide glucose. Starch molecules include two types of glucose polymers, amylose and amylopectin, the latter being the major starch component in most plants, making up about three-fourths of the total starch in wheat flour. Amylose is a straight chain polymer with an average of about 200 glucose units per molecule.

A typical amylopectin molecule has about 1,000 glucose molecules arranged into branched chains with a branch occurring every 24 to 30 glucose units. Complete hydrolysis of amylopectin yields glucose; partial hydrolysis produces mixtures called dextrins, which are used as food additives and in mucilage, paste, and finishes for paper and fabrics.

Glycogen is an energy reserve in animals, just as starch is in plants. Glycogen is similar in structure to amylopectin, but in a glycogen molecule a branch is found every 12 glucose units. Glycogen is stored in the liver and skeletal muscle tissues.

Cellulose is the most abundant organic compound on Earth, and its purest natural form is cotton. The woody parts of trees, the paper we make from them, and the supporting material in plants and leaves are also mainly cellulose. Like amylose, it is a polymer made from glucose monomers. The difference between cellulose and amylose lies in the bonding between the glucose units. The bonding angles around the oxygen atoms connecting the glucose rings are each 180° in cellulose, and 120° in amylose. This subtle structural difference is the reason we cannot digest cellulose. Human beings do not have the necessary enzymes to break down cellulose to glucose. On the other hand, termites, a few species of cockroaches, and ruminant mammals such as cows, sheep, goats, and camels, are able to digest cellulose.

Chitin, a polysaccharide similar to cellulose, is Earth's second most abundant polysaccharide (after cellulose). It is present in the cell walls of fungi and is the fundamental substance in the exoskeletons of crustaceans, insects, and spiders. The structure of chitin is identical to that of cellulose, except for the replacement of the OH group on the C-2 carbon of each of the glucose units with an –NHCOCH₃ group. The principal source of chitin is shellfish waste. Commercial uses of chitin waste include the making of edible plastic food wrap and cleaning up of industrial wastewater.

All proteins are condensation polymers of amino acids. An immense number of proteins exists in nature. For example, the human body is estimated to have 100,000 different proteins. What is amazing is that all of these proteins are derived from only twenty amino acids. In the condensation reaction whereby two amino acids become linked, one molecule of water forming from the carboxylic acid of one amino acid and the amine group of the other is eliminated. The result is a peptide bond; hence, proteins are polypeptides containing from approximately fifty to thousands of amino acid residues.

The primary structure of a protein is the sequence of the amino acid units in the protein. The secondary structure is the shape that the backbone of the molecule (the chain containing peptide bonds) assumes. The two most common secondary structures are the α -helix and the β -pleated sheet. An α -helix is held together by the intramolecular hydrogen bonds that form between the N-H group of one amino acid and the oxygen atom in the third amino acid down the chain from it.

The α -helix is the basic structural unit of hair and wool, which are bundles of polypeptides called α -keratins. The helical structure imparts some

elasticity to hair and wool. The polypeptides in silk, on the other hand, are β -keratins with the β -sheet structure, in which several protein chains are joined side-to-side by intermolecular hydrogen bonds. The resulting structure is not elastic.

Nucleic acids are condensation polymers. Each monomer unit in these polymers is composed of one of two simple sugars, one phosphoric acid group, and one of a group of heterocyclic nitrogen compounds that behave chemically as bases. Nucleic acids are of two types: deoxyribonucleic acid (DNA ), the storehouse of genetic information, and ribonucleic acid (RNA), which transfers genetic information from cell DNA to cytoplasm, where protein synthesis takes place. The monomers used to make DNA and RNA are called nucleotides. DNA nucleotides are made up of a phosphate group, a deoxyribose sugar, and one of four different bases: adenine , cytosine , guanine , or thymine . The nucleotides that polymerize to produce RNA differ from DNA nucleotides in two ways: they contain ribose sugar in place of deoxyribose sugar and uracil instead of thymine.

Natural rubber is an addition polymer made up of thousands of isoprene monomer repeating units. It is obtained from the Hevea brasiliensis tree in the form of latex. The difference between natural rubber and another natural polymer, gutta-percha (the material used to cover golf balls), is the geometric form of the polyisoprene molecules. The CH₂ groups joined by double bonds in natural rubber are all on the same sides of the double bonds (the cis configuration), whereas those in gutta-percha are on opposite sides of the double bonds (the trans configuration). This single structural difference changes the elasticity of natural rubber to the brittle hardness of gutta-percha.

see also Deoxyribonucleic Acid; Nucleic Acids; Polymers, Synthetic; Proteins.

Melvin D. Joesten

Bibliography

Atkins, Peter W. (1987). Molecules. New York: W. H. Freeman.

Joesten, Melvin D., and Wood, James L. (1996). The World of Chemistry, 2nd edition. Fort Worth, TX: Saunders College.