Cationic polymerizations of 1,3-pentadiene (PD) with AlCl3 in n-hexane were carried out in the absence and presence of trimethylsilyl bromide (Me3SiBr or TMSBr). Structural evidence and proton trap experiment demonstrate that in the TMSBr/AlCl3 induced polymerization the real initiating system is the combination of AlCl3 with HBr resulting from hydrolysis of TMSBr by adventitious water existing in the system. The conversion of the polymerization is greatly increased by adding TMSBr, but the introduction of TMSBr does not exert an effect on the formation of gel (crosslinked product). The molecular weight of the polymer prepared by TMSBr/AlCl3 system shows a maximum in the range of [TMSBr]/[AlCl3] = 1, and excessive TMSBr gives rise to a drop of the polymer molecular weight. Kinetic data show that AlCl3 and TMSBr/AlCl3 induced polymerizations have the comparable polymerization rates.