Polymer Chemistry

Imagine that one evening you decide to go bowling. Wearing a pair of jeans, a bowling shirt, and a light-weight jacket you walk into the local bowling alley. After you change into an appropriate pair of shoes, you pick up a ball, stride to the line, smoothly release the ball, watch as it rolls down the lane until it collides with a set of wooden pins, and then use a pencil to record your score on a sheet of paper.

The fabric of your cotton jeans, your polyester shirt, your Nylon jacket, and your leather shoes have something in common with the rubber bowling ball, the polyurethane coating on the bowling lane, the wooden pins, the mixture of graphite and clay in the "lead" pencil, and the sheet of paper on which you wrote your score. Each of these substances is a polymer.

In 1833, Jons Jacob Berzelius suggested that compounds with the same molecular formula but different structures should be called isomers (literally, "equal parts"). He then proposed the term polymer (literally, "many parts") to describe compounds that had the same empirical formula but different molecular weights. Ethylene (C₂H₄) and butene (C₄H₈) are compounds that Berzelius would classify as polymers. Each compound has the same empirical formula (CH₂) but they have different molecular weights. Acetylene (C₂H₂) and benzene (C₆H₆) are another example of compounds Berzelius would call polymers.

The term polymer eventually came to mean compounds such as cellulose and natural rubber that have unusually large molecular weights, in the range of 10,000 to 100,000 or more grams per mole. These molecules are so large they are often called macromolecules literally, molecules large enough to be seen with the naked eye. A perfect diamond, for example, can be thought of as a single molecule containing an array of C-C bonds arranged toward the corners of a tetrahedron around each carbon atom in the crystal. The plastic case that enclosed one of the radios that your grandparents listened to can be thought of as a single molecule. So can a 14-pound bowling ball.

The first explanation for why polymer molecules are so heavy was offered by Herman Staudinger in 1920. Staudinger argued that polymers contain long chains of relatively simple repeating units, or monomers. Natural rubber, for example, is a polymer that contains large numbers of -CH₂C(CH₃)=CHCH₂- units. The number of monomers in a polymer can differ from one chain to the next. Rubber, for example, is a mixture of polymer chains whose mass differs by a factor of 10 or more, but whose average molecular weight is 100,000 grams per mole.

The term rubber was first used in 1770 by Joseph Priestley to describe the gum from a South American tree that could be used to "rub out" pencil marks. Because natural rubber is tacky, strong-smelling, perishable, too soft when warm, and too hard when cold, it had only limited uses. Nathaniel Hayward was the first to note that rubber loses some of its sticky properties when treated with sulfur. It was Charles Goodyear, however, who accidentally dropped a mixture of rubber and sulfur onto a hot stove and discovered "vulcanized" rubber, which is stable over a wide range of temperatures and far more durable than natural rubber.

Cellulose is another example of a polymer that contains many copies of a simple repeating unit: C₆H₁₀O₅. Wood is about 50% cellulose by weight; cotton is almost 90% cellulose. Cellulose has been used for centuries to make paper from wood pulp and cloth from cotton. In the last hundred years, it has also served as the starting material for the synthesis of the first plasticscellulose nitrate and celluloid and the first synthetic fibers Chardonnet silk, or Rayon.

Each -C₆H₁₀O₅- repeating unit in cellulose contains three -OH groups that can react with nitric acid to form nitrate esters known as cellulose nitrate. In 1869 John Wesley Hyatt found that mixtures of cellulose nitrate and camphor dissolve in alcohol to produce a plastic substance he named celluloid. Cellulose nitrate, or celluloid, was used as a substitute for ivory in the manufacture of a variety of items ranging from billiard balls to movie film. Because it is extremely flammable, cellulose nitrate has been replaced by other plastics for almost all uses except ping-pong balls. No other plastic has been found that has quite the same "bounce" as celluloid.

The cellulose from wood pulp contains too many impurities to be used to make fibers. It can be purified, however, by dissolving the polymer in a mixture of NaOH and carbon disulfide (CS₂). When this viscous solution is forced through tiny holes in a nozzle into an acid bath, the cellulose fiber is regenerated. When this process was introduced in 1885, the product was the first synthetic fiber. It was originally called Chardonnet silk, but soon become known as Rayon. A similar process is still used to make a thin film of regenerated cellulose known as Cellophane.