In 1933, Morris Kharasch and his student, Frank Mayo, invoked a free-radical mechanism and discovered "the peroxide effect," which they subsequently applied to other chemical systems. Their work, coupled with that of Gomberg and others, paved the way to development of synthetic materials ranging from rubber to plastics.

Conventional polymerization continued to be used to produce nylon and other products. But free radical polymerization had advantages such as high tolerance of chemical impurities and extreme temperatures, and the ability to be used with a wide range of monomers (organic molecules). Today, free radicals are used to produce nearly half the polymers we use — materials used in everything from food wrapping to paint, adhesives, film, carpeting, piping, and more.

In medicine, understanding free radicals, particularly those formed by oxygen, has illuminated the nature of oxidative stress — damage that results when free radicals form faster than the body removes them. This, in turn, has revealed ways human health can be improved — for example, by using antioxidants. We now recognize that many free radicals are essential components of enzymes in the body, while others can damage DNA, leading to cancer or other diseases. We know, for example, that free radicals formed by excessive exposure to the sun's ultraviolet light can lead to cataracts.

Many free radical processes involve chain reactions that begin when an unpaired electron fails to find another unpaired electron with which it can easily bond. The free radical removes an atom (usually hydrogen) from another molecule, turning itself into a stable molecule; the molecule it attacked becomes a free radical. Such chain reactions are used to make environmentally friendly products such as recyclable automobile tires and soaps free of salts.