Disproportionation and Recombination of Cyanisopropyl Radicals: Rearrangement of Dimethylketenecyanisopropylimine to Tetramethylsuccinodinitrile

Abstract

Evidence is presented that the ketenimine intermediate formed in the thermal decomposition of azo-bis-isobutyronitrile (AIBN) rearranges q~lantitatively to tetramethyls~lccinodinitrile (TMSDN), mainly by a molecular mechanism. Interpretation of experimental data 011 the basis of a reaction scheme consistent with the nature of the main products and the features of the kinetics of the decomposition permits estimation, by means of a simple diagram, of the extent of disproportionation, as well as that of the normal and abnormal recombination of cyanisopropyl radicals. The result obtained for the latter is in good agreement with that determined previously by an alternative method. The analysis does not esclude absolutely the possibility of side reactions in which the azo-compound partly decomposes by a molecular process, and the intermediate decomposes into free radicals. Monrever, it is concluded that these side reactions, if any, must be of minor importance.A recent work: by the present authors (7) has show11 that as much as 55% of the initial recombination of cya~lisopropyl radicals is to dirnethylketenecyanisopropylimine, an unstable intermediate detected by . On the other hand, Bickel and Waters (3) have isolated amounts of the normal dirner, TivISDN, which correspond to a n 84% radical recombination. These two results would be coinpatible only if part of the dinitrile is a transformation product of the intermediate. There are other observatioils which support this conclusion: The oily fraction, isolated by Tal2t-Erben and Bywater (6), which contained the kete~~imine in a high concentration, slowly deposited T M S D N crystals when left standing, and ill an attempt to purify the oil by chromatography, using alu~niiluin oxide as absorbent, these authors obtained a high yield of dinitrile, and none of ketenimine. Moreover, the slope of the experimental curve reported by Bevington (1) for the formation of T M S D N as a functio~l of conversio~l is not constant during decomposition as would be expected for a product forming solely a t the expense of the free radicals; instead, the curve presents a nzarked convexity towards the conversion axis, which lllust indicate that a iloticeable fraction of the dinitrile is produced from a precursor other than the free radicals." Therefore, it would be of interest to reconsider the reaction schelne proposed in reference (6), in which the stable products of decompositioll of the intermediate would now have to be specified as TMSDN. T h e reaction schelne under consideration is the following:

Effect of moisture on the kinetics of the free radical bulk copolymerization of styrene and acrylonitrile

“…However, the significant reduction in the initiator efficiency in the presence of moisture could be explained by the thermal decomposition mechanism of AIBN. The products of the decomposition and the kinetics of the reaction were studied in detail by Talat-Erben and Bywater (9). The ketenimine derivatives [dimethyl-N(2cyano-2-isopropyl)ketenimine, DKI] isolated by them corresponded to about 33% total formation over the whole reaction.…”

Die Startgeschwindigkeit des Systems Styrol/Maleinsäureanhydrid

“…Wird Azo-bis-isobutyronitril (AIBN) als Initiator eingesetzt, kann der beim Zerfall freigesetzte Stickstoff volumetrisch [l-71 oder manometrisch [8] erfaBt werden. Die Anwendung der UV-Spektroskopie fur die Untersuchung des AIBN-Zerfalls geht auf TALAT-ERBEN und BYWATER zuruck [9]. Weiterhin 1aBt sich die Polarographie zur Bestimmung heranziehen [7, lo].…”