In this work, we synthesized ionic liquid (IL) catalysts based on 1-butyl-3-methylimidazolium chloride (BMI center dot Cl), incorporating various types of metal chlorides (as the anions) and in different molar ratios, for application in the styrene bulk cationic polymerization. The polymerizations proceeded with high reproducibility, and by employing some IL catalysts, high conversions were achieved in short times. The reactivity of these catalysts depended on the concentration of the metal chloride in the IL. The synthesized polymers presented different molar mass ranges (2.8 kg.mol(-1)<= (M) over bar (n) <= 556 kg.mol(-1)), molar-mass dispersities, and glass transition temperatures (T-g), depending on the catalytic system employed. Structural analysis by the H-1 NMR revealed that the end group of the polymer chain is a methyne group attached to a chlorine and some chains have the terminal indanyl group. The Schulz-Flory distributions indicated the existence of three to four catalytic species in the most active catalysts for the styrene polymerization. The polymerization was investigated by ESI(+)-MS(/MS), which showed the IL effect in the formation and stabilization of cationic intermediates.