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.