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.