Stopped-flow investigation of trifluoromethanesulfonic acid initiated cationic oligomerization of trans-1,3-diphenyl-1-butene .1. Analysis of products and UV-visible spectroscopic study

Cationic oligomerization of the trans ethylenic dimer of styrene (1,3-diphenyl-1-butene, D) initiated with trifluoromethanesulfonic acid was investigated using the high-purity stopped-flow technique coupled with W-visible spectroscopy. This dimer was protonated into the distyryl cation (1,3-diphenyl-1-butylium, D+) which absorbs at 340 nm, as expected from styrene polymerization results. This species appeared quickly and reached its maximum within about 1 s at low temperature (<-64 degrees C) and then decreased slowly during about 1 min at this temperature. The higher the temperature, the lower the intensity of this peak and the shorter the time to reach its maximum. The cation D+ either cyclizes into 1-methyl-3-phenylindan or reacts with D to produce oligomers, and these two reactions lead to a complete consumption of the double bond evidenced by a decrease of the 296 nm optical density. It has been shown that the main final products of the reaction were always indanic styrene ''tetramers'' (dimers of 1,3-diphenyl-1-butene) and that the proportion of 1-methyl-3-phenylindan was higher when the temperature was increased. No styrene trimers or pentamers have been detected although they are formed at temperatures higher than +50 degrees C. Two other absorptions appearing immediately after mixing and increasing more slowly than the 340 nm peak were observed at 349 and 505 nm: they reached a very stable plateau at T lower than -30 degrees C, but at higher temperatures they passed through a maximum and were replaced by two other peaks at 316 and 415 nm. The 349 and 505 nm peaks were attributed to the same cationic species, plausibly an allylic cation (1,3-diphenyl-1-buten-3-ylium) produced by hydride abstraction from trans 1,3-diphenyl-1-butene. At temperatures above -30 degrees C, the absorptions at 316 and 415 nm were assigned to indanylium cations resulting from different cyclic species produced during the course of the reaction. The process described above can be considered as a model system for the behavior of the unsaturated chain ends in the cationic polymerization of styrene.