The cationic polymerizations of isobutylene(IB) coinitiated by AlCl_3 were carried out in solvent mixture of n-hexane/methylene dichloride (n-hex/CH_2Cl_2) of 60/40 V/V in the presence of ethyl benzoate(EB)at various temperatures range from -80℃ to -30℃.The effccts of EB concentration([EB])and polymerization temperature on monomer conversion,weight-average molecular weight(M_w)and molecular weight distribution(MWD,M_w/M_n) of polyisobutylene (PIB)products were investigated.The rate of polymerization decreased while M_w of PIB products increased with increasing [EB].The polymers with high molecular weight could be prepared in the presence of a suitable amount of EB.Significantly,the polymers with high M_w of 80.2×10~4 and 65.4×10~4 could be produced at -80℃ and -70℃ at[EB]=0.24×10~(-3) mol/L respectively,which were much higher than that(M_w=57.9×10~4)of PIB prepared at -100℃ in the absence of EB.A simple but effective method for preparing the high molecular weight polyisobutylenes was developed in this work.It has been also found that the activation energy for propagation(E_p)depended on the polymerization temperature range in the presence of EB.An obvious inflection of the linear plots of lnXn versus 1/T_p occurred at the temperature range from -60℃ to -50℃ at four different concentrations of EB from 0.19×10~(-3) mol/L to 0.33×10~(-3) mol/L,and thus the inflection temperature(T_(inf))was in the range of -60℃ to -50℃.When[EB]was in the range of 0.24×10~(-3) mol/L to 0.33×10~(-3) mol/L,E_p was determined to be around-12 kJ/mol when the polymerization was carried out at temperatures from -80℃ to T_(inf) and to be around -28 kJ/mol at temperatures from T_(inf) to -15℃ respectively.