Mechanism of Isomerization in the Cationic Polymerization of Isobutylene

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• 作者：Philip Dimitrov ; Jack Emert ; Jun Hua ; Sandor Keki ; Rudolf Faust
• 刊名：Macromolecules
• 出版年：2011
• 出版时间：April 12, 2011
• 年：2011
• 卷：44
• 期：7
• 页码：1831-1840
• 全文大小：1059K
• 年卷期：v.44,no.7(April 12, 2011)
• ISSN：1520-5835

文摘

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 鈭?0 to 25 掳C was studied by model ionization experiments employing poly(isobutylene chloride) (PIB-Cl) of low molecular weight (M_n 1000鈭?000) 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 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 鈭?0 掳C also gave mainly the tri-substituted olefin; however, at 25 掳C only exo-, endo-, and tetra-substituted olefins were formed with regular carbon numbers. Ionization of PIB_d8(IB)_n-Cl with n = 1鈭? showed that on average 4 IB units are cleaved at 鈭?0 掳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.