The selective cationic polymerization of isobutylene(IB)initiated by a BF_3·cyclohexanol(CL)complex was carried out from the mixed C_4 fraction feed containing the 4C saturated and unsaturated hydrocarbons at-20℃.The effects of CL concentration,BF_3 concentration,solvent for preparing BF_3·CL complex and polymerization time on the chemical structure of end groups,number-average molecular weight(M_n)and molecular weight distribution(MWD,M_w/M_n)of the resulting polymers were investigated.The experimental results indicate that the BF_3·CL complex initiating system exhibited an extremely high selectivity toward the cationic polymerization of IB in the mixed C_4 fraction feed and low molecular weight(M_n=900-3600)polyisobutylenes(PIBs)with large proportion of exo-double bond end groups were obtained.The exo-double bond content in PIB chain ends increased by increasing CL concentration or by decreasing solvent polarity in initiating system,BF_3 concentration and polymerization time.The M_n and MWD of the resulting PIBs were dependent on the concentrations of CL and BF_3.Highly reactive PIBs with around 90 mol%of exo-double bonds were successfully synthesized by the selective polymerization of IB from the mixed C_4 fraction feed,providing a potentially practical process for its simplicity and low costs. The selective cationic polymerization of isobutylene (IB) initiated by a BF_3 · cyclohexanol (CL) complex was carried out from the mixed C_4 fraction feed containing the 4C saturated and unsaturated hydrocarbons at-20 ° C. The effects of CL concentration, BF_3 concentration, solvent for preparing BF_3 · CL complex and polymerization time on the chemical structure of end groups, number-average molecular weight (M_n) and molecular weight distribution (MWD, M_w / M_n) of the resulting polymers were investigated.The experimental results indicate that the BF_3 CL complex initiating system exhibited an extremely high selectivity toward the cationic polymerization of IB in the mixed C_4 fraction feed and low molecular weight (M_n = 900-3600) polyisobutylenes (PIBs) with large proportion of exo-double bond end groups were obtained. The exo-double bond content in PIB chain ends increased by increasing CL concentration or by solvent solvent polarity in initiating system, BF_3 concentration and polymerization time. M_n and MWD of the resulting PIBs were dependent on the concentrations of CL and BF_3.Highly reactive PIBs with around 90 mol% of exo-double bonds were successfully synthesized by the selective polymerization of IB from the mixed C_4 fraction feed, providing a potentially practical process for its simplicity and low costs.