STOPPED-FLOW INVESTIGATION OF TRIFLUOROMETHANESULFONIC ACID INITIATEDCATIONIC 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 wit h W-visible spectroscopy. This dimer was protonated into the distyryl cation (1,3-diphenyl-1-butylium, D+) which absorbs at 340 nm, as expec ted from styrene polymerization results. This species appeared quickly and reached its maximum within about 1 s at low temperature (<-64 deg rees C) and then decreased slowly during about 1 min at this temperatu re. The higher the temperature, the lower the intensity of this peak a nd the shorter the time to reach its maximum. The cation D+ either cyc lizes into 1-methyl-3-phenylindan or reacts with D to produce oligomer s, and these two reactions lead to a complete consumption of the doubl e bond evidenced by a decrease of the 296 nm optical density. It has b een shown that the main final products of the reaction were always ind anic styrene ''tetramers'' (dimers of 1,3-diphenyl-1-butene) and that the proportion of 1-methyl-3-phenylindan was higher when the temperatu re was increased. No styrene trimers or pentamers have been detected a lthough they are formed at temperatures higher than +50 degrees C. Two other absorptions appearing immediately after mixing and increasing m ore 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 hi gher temperatures they passed through a maximum and were replaced by t wo other peaks at 316 and 415 nm. The 349 and 505 nm peaks were attrib uted to the same cationic species, plausibly an allylic cation (1,3-di phenyl-1-buten-3-ylium) produced by hydride abstraction from trans 1,3 -diphenyl-1-butene. At temperatures above -30 degrees C, the absorptio ns at 316 and 415 nm were assigned to indanylium cations resulting fro m different cyclic species produced during the course of the reaction. The process described above can be considered as a model system for t he behavior of the unsaturated chain ends in the cationic polymerizati on of styrene.