By the close of the 19<sup>th</sup> Century numerous observations had been made related to the ability of unsaturated (vinyl and vinylidene) compounds to form thick oils or resinous substances with the same elemental composition as the starting material when stored and especially when heated or exposed to light. For instance, in 1835, Victor Regnault reported the polymerization of vinyl chloride and three years later, he described that vinylidene chloride, when stored in sealed ampoules, deposits a white non-crystalline substance, which he considered an isomeric form. While studying the properties of acrolein and acrylic acid in 1843, Josef Redtenbacher noticed that the former formed resin, named “disacryl”, when heated. In the same year, the formation of glass-like material from styrene (which refracted light very strongly and was “not improbable that it might be applied to several optical purposes”) was described by John Blyth and August Wilhelm von Hoffman. In fact, the “steady conversion of the oil [styrene] by air, light, and heat to a rubberlike substance” was communicated in 1839 by E. Simon who assumed the compound was styrene oxide. The nature of these and many other similar transformations was unclear and was the subject of speculations and (occasionally, lucky) guesses. There were indications that radicals were involved in the processes. For example, as early as 1924, Charles Moureu and Charles Dufraisse showed that hydroquinone, which inhibits the oxidation of acrolein (a chain reaction), also inhibits the formation of resin from it. In 1928, George Stafford Whitby and Morris Katz assumed that the chain growth in the thermal polymerization of indene (and presumably other unsaturated compounds) involved hydrogen migration. However, only within several years of these studies, the mechanism of radical polymerizations was already well understood and it was established that the reactions were comprised of three distinct steps (now termed initiation, propagation, and termination). Papers published in 1934 by William Chalmers, in 1935 by H. Dostal and Herman Mark, and by G. V. Schulz, and in 1937 by Paul Flory described the kinetics of the polymerizations as well as the molecular weight distribution functions of the polymers. The mentioned early studies of radical polymerization will be presented and discussed.

Early observations and studies of radical polymerization

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