2D-INADEQUATE Structural Assignment of Polybutene Oligomers

The structure of polybutene oligomers has been unambiguously determined using NMR techniques. We used 2D-INADEQUATE to show that the major polybutene isomer synthesized via BF(3) catalysis has the expected structure with a tert-butyl group at the beginning of the chain and the expected methylvinylidene double bond at the other end. Surprisingly, we found that the main isomer of polybutene synthesized from AlCl(3) catalysis has an isopropyl group at the beginning of the chain. A trisubstituted double bond structure has been assigned at the other end of the chain. Structural information about several minor polybutene isomers has also been determined. Formation of an isopropyl group can be rationalized by assuming a protonated cyclopropane intermediate in the propagation step. Previously, the mechanism of formation of polybutene via AlCl(3) catalysis was thought to involve exclusively the tert-butyl carbenium ion. It now appears that the mechanism is more complicated than previously thought.