Mechanism of Isomerization in the Cationic Polymerization of Isobutylene

Abstract

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 (EtAlCl 2 ) in nonpolar solvents in the temperature range of −40 to 25 °C was studied by model ionization experiments employing poly(isobutylene chloride) (PIB-Cl) of low molecular weight ( M n ∼ 1000−2000) obtained by living cationic polymerization. Ionizations were performed with EtAlCl 2 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 TOF 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 °C also gave mainly the tri-substituted olefin; however, at 25 °C only exo-, endo-, and tetra-substituted olefins were formed with regular carbon numbers. Ionization of PIB d8 (IB) n -Cl with n = 1−6 showed that on average 4 IB units are cleaved at −40 °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.