Most chemists will not have heard of storax, yet it plays an important role in chemical history. It isn’t a specific compound, it is a resin produced by a number of tropical trees of the family Styracaceae in response to an injury to their bark. It has a long history of use in perfumes because of its lingering fragrance and its ability to slow the evaporation of other compounds that contribute to the overall scent. This “fixative” effect allows a perfume to keep the original fragrance for a longer time. Like other fixatives, such as sandalwood, patchouli, frankincense, benzoin, cedarwood, ambergris, musk and castor oil, storax is a complex mixture of compounds. Cinnamic acid, alpha-pinene, ethyl cinnamate and vanillin are just some that have been isolated. But in terms of historical impact, perhaps the most interesting compound found in storax is styrene.
In 1839, Eduard Simon, a German apothecary was attempting to separate the components of storax obtained from the Liquiambar orientalis tree by distillation. One of the fractions he collected was an oily substance that seemed to be a single compound. Simon named it styrol and stored it in a bottle. Much to his surprise, a few days later he noted that his styrol had changed from an oil into a hard translucent mass. Since he hadn’t added anything to the sample, he figured that it must have reacted with oxygen, and dubbed the new material “styrol oxide.” As it turned out, he was wrong. August Wilhelm Hofmann, one of the leading lights of German chemistry, showed that the styrol transformation also occurred in the absence of oxygen.
A solution to the mystery was proposed in 1866 by the brilliant French chemist, Marcelin Berthelot. The molecules of styrol, Berthelot suggested, must have joined together to form a long chain. Just three years earlier, he had delivered a landmark lecture to the Chemical Society of Paris in which he introduced the novel idea that small molecules could link together to form giant molecules, or polymers. But Berthelot did more than theorize. He carried out experiments to show that ethylene molecules could be joined together to form a new substance he called “polyethylene,” surely the first time that term was ever used. The history of polymer chemistry can be said to have begun with Berthelot’s pioneering work.
Simon’s “styrol” was eventually identified as styrene, or “vinylbenzene,” if we want to use proper chemical terminology, and his “styrol oxide” was actually polystyrene. Neither Simon nor Berthelot found a practical use for polystyrene, but by the 1930s German chemists did. They discovered that polystyrene could be cast into virtually any shape by pouring the molten substance into moulds and that it could also be extruded into sheets or films. Today, polystyrene is used to make a myriad items ranging from clear plastic glasses, laboratory equipment and cases for compact discs, to smoke detectors and disposable razors.
The most common use for polystyrene, however, is to produce “expanded polystyrene.” That’s the foamy stuff of coffee cups, insulation materials and those packing peanuts used to cushion electronic equipment. The “expansion” is produced by blowing pentane or carbon dioxide into the melted plastic. Pentane is a liquid, but quickly evaporates to yield a vapour and carbon dioxide of course is a gas. These gases attempt to escape from the molten polystyrene but they get trapped as the material cools, forming pockets. The result is polystyrene foam. “Styrofoam” is the Dow Chemical Company’s trade name for its version of foamed polystyrene. Of course, the styrene needed to make polystyrene is no longer obtained from the distillation of storax. It is now produced on a gigantic scale from benzene and ethylene derived from petroleum.
While polystyrene is a very useful substance, it is overused. We need computer housings and insulation material, but do we need all those disposable razors, cutlery and coffee cups? Polystyrene can be recycled but it is not always economical to do so, and it often ends up in landfills or being incinerated. That needs to change. We have to conserve our petroleum resources and our environment.