Intramolecular hydrogen transfer in thermal degradation: comments on the size distribution of the backbite rings

摘要

Consideration is given to the probable sizes of the ring transition states corresponding to intramolecular hydrogen transfer reactions occurring during thermal degradation, since these influence the yields, sizes, and structures of the oligomeric products formed by this mechanism. Published data from intramolecular hydrogen transfer reactions in gas kinetics, and also from short- chain branching in high-pressure polyethylene, are first examined for evidence of relative ring sizes in these analogous processes. Although these analogies clearly indicate that 6-membered backbite rings are the most probable, there are considerable uncertain- ties about other ring sizes. Attempts are therefore made to deduce ring closure probabilities from calculations of relative rate con- stants for the backbiting processes for which the ring transition state applies. Data on ring strain energies, and also from values of thermodynamic functions for ring closure, have been used, with certain assumptions, for this purpose. On this basis, the general conclusion is reached that 6-membered ring backbites are at least an order of magnitude more probable than any other process, and of the other possibilities, only 5- and 7-membered rings have sufficient probability to make any significant contribution to the overall product formation. The more reliable calculations, based on the thermodynamic functions, indicate that the relative prob- abilities for the 5- and 7-membered rings are in the approximate ratio 5:1.