Decomposition and Polymerization of the Olefinic Hydrocarbons

Abstract

This survey covers the changes occurring when ' .uns are subjected to heat or pressure, with or without the aid of catalysts, to a number of chemical reagents, to photolysis, to electric sparks or discharge, and to alpha particles. While these agencies are not alike in their effects on hydrocarbons of the ethylene series there are similarities in their action which justify their discussion together. The reactions occurring are of three types. There may be breakdown of the molecule consisting in some cases merely of hydrogen loss, and in others of more deep-seated change, which go so far as to produce carbon and hydrogen as sole products. As an alternative, the change can be one of synthesis giving rise to hydrocarbons with more atoms in their molecules than the starting substance. Or merely a molecular rearrangement takes place-usually a shift in the position of the double bond-without variation in molecular size. Under some conditions change of but one of these types occurs. But often changes of these three classes are brought about by the same means, either concurrently or simultaneously, the type of reaction changing with experimental conditions.The action of heat, for example, may produce changes of the three types With mild heating, rearrangement or polymerization reactions predominate, and may be the only ones occurring. They produce olefin or naphthene hydrocarbons of molecular weight greater than that of the starting substance. Change of this type is aided by superatmospheric pressure. At more elevated temperatures, while polymerization continues, there form simultaneously carbon, hydrogen, acetylene, and low molecular weight olefins and paraffins as a result of disruptive changes. At elevated temperatures the higher olefins produced by polymerization tend to become in part converted into aromatic hydrocarbons. At very high temperatures practically no polymerization takes place, and the primary products obtained from olefins are carbon and hydrogen. Catalysts notably lower the temperature at which changes in olefins take place. Some catalysts favor disruptive changes, while others aid polymerization.The electric spark or arc causes changes in olefins similar to those occasioned by high temperature-a small amount of polymerization occurs, but, in the main, there is decomposition to carbon, hydrogen, and methane. The reactions caused by the silent discharge, on the other hand, are indicated to be almost exclusively polymerization.The action of the chemical reagents discussed in this paper is largely polymerization, except that at elevated temperatures disruptive reactions occur.

Thermal Reactions of Terpene Hydrocarbons.

“…ISOPRENE Only a little work has been done on the thermal decomposition of isoprene. On the other hand, polymerization of isoprene, to which brief reference was made in a paper on olefins by Egloff, Schaad, and Lowry (44), has been the subject of quite extensive research. Indeed, since 1860 when Williams (192) established the fact that isoprene is not only a product of pyrolytic decomposition of rubber, but has the same empirical composition as rubber, a great amount of interest has been shown in the polymerization of isoprene to produce a commercially valuable synthetic rubber.…”

The Kinetics of Elementary Reactions of the Simple Hydrocarbons.

“…We shall consider the low-temperature reactions first, since they are the simpler. Insofar as the older work and theories of polymerization are concerned, reference may be made to the review of Egloff, Schaad, and Lowry (46), and to the comprehensive monograph of Egloff (40).…”

“…They review the proposed mechanisms in some detail, and suggest that the results can best be interpreted by assuming that polymerization and decomposition both occur, i.e. Egloff, Schaad, and Lowry (46) suggest that the primary reaction is C2H4 -> 2CH2…”

“…In this review an attempt has been made to include all data which seem to be pertinent to the kinetic analysis of the simple thermal and photochemical reactions of the lower paraffins, olefins, and acetylenic hydrocarbons. No attempt has been made to include references to purely pyrolytic investigations, or to catalytic processes, since a number of comprehensive reviews of this kind already exist (35,36,41,42,43,45,46,48,57,77, 181, and especially 40). In spite of their importance, the oxidation and halogenation reactions of the hydrocarbons have not been included, since these do not seem to have advanced as yet to the stage where we can disentangle the processes into their component parts with any degree of certainty.…”

Reactions of Organic Molecules in the Gaseous State.

“…OF UNSATURATED HYDROCARBONS A comprehensive review of the literature covering the pyrolysis of olefinic hydrocarbons has recently been published (28). Even in the early stages, the decomposition of these hydrocarbons is invariably accompanied by a polymerization process, which is especially marked at lower temperatures.…”