The present invention relates to a process for producing polymers comprising repeating units derived from at least one isoolefin monomer, at least one multiolefin monomer and optionally further copolymerization monomers in the presence of at least one organic nitro compound and AlCl3/water wherein the process is conducted in the absence of compounds selected from the group consisting of vanadium compounds, zirconium halogenid, and hafnium hallogenides.