Effect of Light on TiCl4-Induced Cationic Model Reactions

On exposure to UV or visible light, TiCl4 readily decomposes homolytically and yields TiCl3 plus chlorine. Similarly, irradiation of TiCl4,-hydrocarbon systems produce chlorinated hydrocarbon and HC1. 2,4,4- Trimethyl- 1-pentene, a nonpolymerizable model for isobutylene, rapidly dimerizes in the presence of TiCl4 during irradiation with UV and visible light and yields a mixture of conventional head-to-tail C16 -olefins. While conversions are higher in the presence of light than in darkness, the structures of the dimers formed under these conditions are indistinguishable. The structures of these dimers of 2,4,4-trimethyl-1-pentene are incompatible with the cation-radical initiation mechanism of isobutylene polymerization effected by TiCl4 under irradiation proposed by Czechoslovak investigators. Polymerization by condensation experiments have been carried out in the dark and in visible light, and an interpretation of this phenomenon that complements the one proposed by French authors has been developed.