A polymer is a large molecule (macromolecule) composed of repeating structural units. These subunits are typically connected by covalent chemical bonds. Although the term polymer is sometimes taken to refer to plastics, it actually encompasses a large class of natural and synthetic materials with a wide variety of properties. Because of the extraordinary range of properties of polymeric materials, they play an essential and ubiquitous role in everyday life. This role ranges from familiar synthetic plastics and elastomers to natural biopolymers such as nucleic acids and proteins that are essential for life.
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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 -NHCOCH 3 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 exist 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 Î± -helix and the Î² -pleated sheet. An Î± -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 Î± -helix is the basic structural unit of hair and wool, which are bundles of polypeptides called Î± -keratins. The helical structure imparts some Chitin, the earth’s second most abundant polysaccharide, is the fundamental substance in the exoskeletons of crustaceans. The polypeptides in silk, on the other hand, are Î² -keratins with the Î² -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 CH 2 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.
Finally, I will talk about some important applications of natural polymers. Xanthan gum is produced by a pure-culture fermentation of a carbohydrate with Xanthomonas capestris and purified. It is also known as corn sugar gum. It is the sodium, potassium or calcium salt of a high molecular weight polysaccharide containing D-glucose, D-mannose and D-glucuronic acid. Xanthan gum is used as a stabilizer, thickener and emulsifier extensively in pharmaceutical, cosmetic industries and in food industry for dairy products. In addition to xanthan gum, another important application is agar. It is the dried hydrophilic and phycocolloidal concentrate from a decoction of various marine red algae. The dried agar usually occurs in bundles comprising thin, membranous strips; or in cut, flaked or granulated forms. It has a yellowish color and odorless with mucilaginous taste. Agar contains two different polysaccharides named as agrose and agropectin. Agrose is responsible for gel strength of agar and composed of D-galactose and 3,6-anhydro-L-galactose units. It contains cellulose and nitrogen containing substances. Agaropectin is responsible for the viscosity of agar solutions. Agar is used as suspending, stabilizing, thickening or gelling agent and bulk laxative. It is also used in the preparation of jellies, confectionery items, tissue culture studies and in microbiology. The last important application I will talk about is gelatin. Gelatin is a product obtained by partial hydrolysis of collagen derived from skin, white connective tissue and bones of animals. The process converts insoluble collagens to soluble gelatin, the solution of which is then purified and concentrated to a solid form. It is soluble in a hot mixture of glycerol and water, whereas it is practically insoluble in alcohol, chloroform and oil. Gelatin is used in the preparation of pastes, pastilles, suppositories, coating of tablets and manufacturing of hard and soft capsule shells. It is also used for the microencapsulation of drugs and other industry materials.
Unlike natural polymers, synthetic polymers require human intervention. Polystyrene was first known in the 1800’s as a laboratory curiosity. It first found limited use because of brittleness. It was later found that if the formula weight was kept to about 106 amu, polystyrene became more flexible. It has hardness, brilliance and complete resistance against water and weak acids and bases. In the form of a hard solid, it is used to make glasses, containers and appliance parts. If the more flexible solid is injected with gases while in the liquid state it will entrap the gas bubbles and produce the insulating foam that is used for foodware, coolers and packing material. The reaction used to prepare polystyrene is called an addition polymerization reaction. Addition polymers are formed when the monomer starting materials are bonded together without the loss of any of the atoms of the monomer. Polystyrene is prepared by the addition polymerization of styrene. Polymers are generally named by placing a “poly” in front of the monomer name.
High density polyethylene (HDPE) is mostly linear and is stronger, stiffer, more heat resistant, more flexible at lower temperature and more chemical and UV resistant than los density polyethylene (LDPE). HDPE has a melting point of 130oC and is used to manufacture kayaks, toys, gasoline tanks, electronic equipment cases and food containers. The higher melting point allows items made from HDPE to be washed in a dishwasher, which can melt items made from LDPE. A special fiber made of HDPE is called Spectra. It is used to make surgical gloves because it is very resistant to cutting.
Teflon was discovered by accident in 1938 by Plunkett. He was preparing new freons to be used as refrigerants. When he was using a cylinder of one of these freons that he had prepared, he noticed that the flow stopped. Upon cutting open the cylinder he discovered a white powder. This white powder was found to be very inert to chemical attack and is now used to manufacture nonstick surfaces for cooking utensils. Gore-Tex is made of nylon material sandwiched over a stretched sheet of Teflon. The stretching creates pores in the sheet that will pass water vapor but not water droplets. Teflon is formed by an addition polymerization of tetrafluoroethene.
Polymethyl -cyanoacrylate (Eastman 910…Super Glue) undergoes polymerization due to oxygen acting as a free radical initiator. Strength of the glue is attributed to both ester and cyano interactions with the material that is being glued. Polymethyl -cyanoacrylate is formed by an addition polymerization reaction of methyl -cyanoacrylate.
Nylon is the accepted generic term for synthetic aliphatic amides. An amide is the same grouping of atoms that was called a peptide bond in protein. Nylon 66 was first prepared by Carothers while working for DuPont in 1931. Nylon is exceptionally strong, elastic, abrasion resistant, lustrous, resistant to chemicals and oil, can be dyed or precolored and it has low water absorbency. Nylon 66 is the predominate nylon manufactured in the U.S. and is used in clothing, carpet, fishing leader, surgical sutures, parachutes and molded items. Nylon is a condensation polymer. Condensation polymers are formed when monomer units are bonded together with the loss of a small molecule such as water.
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The polyester poly(ethylene terphthalate) (PETE) was first produced by DuPont in 1953. As a fiber it is called Dacron and as a film it is called Mylar. Aluminized Mylar was used to make the Echo satellite, wine bags and balloons. When coated with magnetic particles, it is used to make audio and video tapes. As the fiber it is strong, resistant to stretching and shrinking, resistant to most chemicals, quick drying, wrinkle resistant and abrasion resistant. PETE is the plastic that is used for soft drink bottles. PETE is formed by a condensation polymerization of ethylene glycol and terphthalic acid.
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