The polymerization of olefines induced by free radicals

The rate of photolysis of the simple aldehydes at 300° is in general reduced by ethylene, propylene or iso -butylene, and many molecules of the olefine may undergo an induced polymerization for each quantum of light absorbed by the aldehyde. A similar polymerization is induced by photolysis of ketones. The kinetics of these interdependent processes have been investigated by combining pressure measurements with chemical analysis at each stage, the rates of olefine polymerization and of aldehyde photolysis being independently determined. The experimental results accord with a mechanism in which large radicals are built up by the successive addition of olefine molecules to the primary radicals from the photolysis of the aldehyde or ketone. When the polymer radicals contain approximately three olefine molecules they undergo one of two alternative processes, either breaking down by regeneration of a methyl radical which begins a new polymerization chain, or giving inactive products. The former ‘transfer process’ explains how the chain length of the reaction may be large, yet the molecular weight of the product comparatively small. Certain differences between acetaldehyde and propionic aldehyde are explained by the fact that the ethyl radicals from the latter may regenerate in the course of the reaction cycle m ethyl radicals o f slightly greater reactivity. A quantitative comparison o f the various reactions shows that on ascending the series of alkyl radicals their reactivity towards both aldehydes and olefines diminishes only slowly. On ascending the olefine series, how ­ ever, the rate of polymerization rapidly decreases. This depends not upon a lowered efficiency of reaction of the primary radicals with the olefines, but rather upon a greater tendency of the more complex polymer radicals to be transformed into products which do not continue the reaction cycle.