Entry by Haifei Zhang, AP 225, Fall 2009
What is a polymer
The most simple definition of a polymer is something made of many units. The units or “monomers” are small molecules that usually contain ten or less atoms in a row. Carbon and hydrogen are the most common atoms in monomers, but oxygen, nitrogen, chlorine, fluorine, silicon and sulfur may also be present. Think of a polymer as a chain in which the monomers are linked (polymerized) together to make a chain with at least 1000 atoms in a row. It is this feature of large size that gives polymers their special properties. Polymerization can be demonstrated by linking countless strips of construction paper together to make paper garlands or hooking together hundreds of paper clips or gum wrappers together to form extended chains.
Where are polymers
Macromolecules or polymers are found in the human body, animals, plants, minerals and manufactured products. Substances like the following contain polymers: diamond, concrete, quartz, glass, nylon, plastics, DNA, tires, cotton, hair, bread, and paint. The macromolecule can have different end units, branches in the chain, variations in the sequence of the monomers, and different monomers repeated in the same chain which leads to the large number of manufactured polymers as well as all of the natural polymers. The table above shows just a few manufactured polymers that are made from the monomer on the right. The double bond in the monomer is broken or water (or some other molecule that can be boiled off) is eliminated in the polymerization process.
Natural polymers are in living animals and plants as building materials, storage substances and playing a role in biochemical reactions. Cellulose and lignin give structure to plants. Cellulose (starch or polysaccharide) is a macromolecule composed of individual sugar molecules (glucose) that are bonded together to give molecular weights in the millions. Cellulose is the basis for cotton and rayon fibers. Starch in plants stores glucose and, therefore, energy. Starch consists largely of two forms, one linear and one branched, both of which differ from cellulose because of the different way the glucose units are connected. Chitin is a nitrogen-containing polysaccharide found in shells, wings, and claws of animals. Proteins are polymers that are responsible for animal hair and fibers such as wool and silk. DNA is a polymer necessary for life processes in plants and animals. Natural rubber, from a tree, has isoprene (2-methyl-1,3-butadiene) as the monomer producing a very elastic product. Artificial rubbers are made from butadiene and other monomers and have many uses. Additives are necessary to change the physical properties for the manufacture of automobile tires. Inorganic polymers (not based on the carbon atom) include glass with its silicon-oxygen framework and other silicates as in granite and agate.
Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain. During the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization of PET polyester. The monomers are terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2-CH2-OH) but the repeating unit is -OC-C6H4-COO-CH2-CH2-O-, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue.
Polymerization reactions can be classified into two or three basic types. Carothers, a great and tragic figure in the history of polymer science, suggested that most polymers could be classified into two broad categories, condensation or addition. For reasons that will become obvious, the terms step-growth and chain polymerization's provide a more accurate and complete description.
The essential difference between step-growth polymerization and chain-growth polymerization is that in chain growth polymerization, monomers are added to the chain one at a time only, whereas in step-growth polymerization chains of monomers may combine with one another directly. However, some newer methods such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without a catalyst. Efforts towards rational synthesis of biopolymers via laboratory synthetic methods, especially artificial synthesis of proteins, is an area of intense research.
Applications of polymers
It is difficult to find an aspect of our lives that is not affected by polymers. Just 50 years ago, materials we now take for granted were non-existent. With further advances in the understanding of polymers, and with new applications being researched, there is no reason to believe that the revolution will stop any time soon.
Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. This material, obtained from the bark of the rubber tree, has been used by humans for many centuries. It was not until 1823, however, that rubber became the valuable material we know today. In that year, Charles Goodyear succeeded in "vulcanizing" natural rubber by heating it with sulfur. In this process, sulfur chain fragments attack the polymer chains and lead to cross-linking. The term vulcanization is often used now to describe the cross-linking of all elastomers. Much of the rubber used in the United States today is a synthetic variety called styrene-butadiene rubber (SBR). During World War II, hundreds of thousands of tons of synthetic rubber were produced in government controlled factories. After the war, private industry took over and changed the name to styrene-butadiene rubber. Today, the United States consumes on the order of a million tons of SBR each year. Natural and other synthetic rubber materials are quite important.
Americans consume approximately 60 billion pounds of plastics each year. The two main types of plastics are thermoplastics and thermosets. Thermoplastics soften on heating and harden on cooling while thermosets, on heating, flow and cross-link to form rigid material which does not soften on future heating. Thermoplastics account for the majority of commercial usage. Among the most important and versatile of the hundreds of commercial plastics is polyethylene. Polyethylene is used in a wide variety of applications because, based on its structure, it can be produced in many different forms. The first type to be commercially exploited was called low density polyethylene (LDPE) or branched polyethylene. This polymer is characterized by a large degree of branching, forcing the molecules to be packed rather loosely forming a low density material. LDPE is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials.
Fibers represent a very important application of polymeric materials, including many examples from the categories of plastics and elastomers. Natural fibers such as cotton, wool, and silk have been used by humans for many centuries. In 1885, artificial silk was patented and launched the modern fiber industry. Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combination of strength, weight, and durability have made these materials very important in modern industry. Generally speaking, fibers are at least 100 times longer than they are wide. Typical natural and artificial fibers can have axial ratios (ratio of length to diameter) of 3000 or more. Synthetic polymers have been developed that posess desirable characteristics, such as a high softening point to allow for ironing, high tensile strength, adequate stiffness, and desirable fabric qualities. These polymers are then formed into fibers with various characteristics. Nylon (a generic term for polyamides) was developed in the 1930's and used for parachutes in World War II. This synthetic fiber, known for its strength, elasticity, toughness, and resistance to abrasion, has commercial applications including clothing and carpeting. As the technology of fiber processing expands, new generations of strong and light weight materials will be produced.
 Polymer science learning center. http://pslc.ws/