2D-INADEQUATE STRUCTURAL ASSIGNMENT OF POLYBUTENE OLIGOMERS

The structure of polybutene oligomers has been unambiguously determine d using NMR techniques. We used 2D-INADEQUATE to show that the major p olybutene isomer synthesized via BF3 catalysis has the expected struct ure with a tert-butyl group at the beginning of the chain and the expe cted methylvinylidene double bond at the other end. Surprisingly, we f ound that the main isomer of polybutene synthesized from AlCl3 catalys is has an isopropyl group at the beginning of the chain. A trisubstitu ted double bond structure has been assigned at the other end of the ch ain. Structural information about several minor polybutene isomers has also been determined. Formation of an isopropyl group can be rational ized by assuming a protonated cyclopropane intermediate in the propaga tion step. Previously, the mechanism of formation of polybutene via Al Cl3 catalysis was thought to involve exclusively the tert-butyl carben ium ion. It now appears that the mechanism is more complicated than pr eviously thought.