Cationic copolymerization of 1,3-pentadiene (PD) with styrene (St) using the triethylamine hydrochloride-aluminium chloride (Et3NHCl-AlCl3) room temperature ionic liquid as an initiator in toluene has been investigated. The polymerization proceeds to high conversions, indicating high initiating reactivity of Et3NHCl-AlCl3 in these copolymerization systems, although molecular weights of the polymers are limited which are similar to polymerization initiated by Lewis acids such as TiCl4, BF3, BF3 center dot OEt2. The polymers were analyzed using IR spectra in conjunction with gel permeation chromatography (GPC).