Cationic polymerizations of 1,3-pentadiene (PD) with AlCl3 in n-hexane was carried out in the absence and presence of trimethylsilyl chloride (TMSCl). The polymer yield is greatly increased by the addition of TMSCl, indicating that the TMSCl/AICl(3) combination is an efficient initiating system for PD cationic polymerization. The polymerization rate induced by TMSCl/AlCl3 is 10 times greater than that by AlCl3 alone. The introduction of TMSCl does not exert an effect on the crosslinking. reaction. The molecular weight of the polymer decreases with the addition of TMSCl to a extent and then remains constant. Structural evidences demonstrate that the polymerization is indeed initiated by AlCl3 in combination with HCl resulting from hydrolysis of TMSCl by adventitious water.