Mechanism of Isomerization in the Cationic Polymerization of Isobutylene

The complex mechanism of the carbocationic rearrangements leading to a variety of olefin (exo-, endo-, tri-, and tetra-substituted) end groups in the cationic polymerization of isobutylene (IB) catalyzed by ethylaluminum dichloride (EtAlCl2) in nonpolar solvents in the temperature range of -40 to 25 degrees C was studied by model ionization experiments employing poly(isobutylene chloride) (PIB-Cl) of low molecular weight (M-n similar to 1000-2000) obtained by living cationic polymerization. Ionizations were performed with EtAlCl2 in hydrocarbon media to mimic a conventional cationic polymerization of IB, but in the presence of a proton trap to suppress reprotonation of the first formed olefin and subsequent decomposition of the resulting PIB cation. In the absence of a proton trap, ionization of PIB-Cl resulted in about 70% of tri-substituted olefin end groups in the studied temperature range. The exo and endo olefin end-group content was negligible. MALDI and APPI TOP MS indicated that the tri-substituted PIB olefins contained irregular carbon numbers, suggesting chain scission. Ionizations carried out in the presence of the proton trap 2,6-di-tert-butylpyridine (DTBP) at 0 and -40 degrees C also gave mainly the tri-substituted olefin; however, at 25 degrees C only exo-, endo-, and tetra-substituted olefins were formed with regular carbon numbers. Ionization of PIBd8(IB)(n)-Cl with n = 1-6 showed that on average 4 IB units are cleaved at -40 degrees C, which suggests that the tri-substituted olefins are formed by backbiting via hydride transfer followed by chain scission. A mechanism is proposed to account for the olefin structures involving a sterically hindered cation arising via hydride and methyde shifts, which either eliminates a proton to yield the tetra-substituted olefin or undergoes a distant hydride shift by backbiting followed by a methyde shift and chain scission to yield the tri-substituted olefin of irregular carbon number.