A method was studied to obtain butyl rubber in hexane at -75 and -60degreesC using a mixture of a major amount of diethylaluminum chloride and a minor amount of ethylaluminum dichloride in the initiating system. The Jaacks method was used to determine the monomer reactivity ratio of isobutylene in copolymerization with isoprene in hexane at -75degreesC with the above initiating system; gamma(IB) = 1.54 +/- 0.06. This value was different from a literature value r(IB) 1.08 when just EtAlCl2 was used in hexane at -80degreesC. The mixture of both alkylaluminum chlorides in the initiating solution was subsequently directly activated with minute amounts of water or methylaluminoxane (MAO). The use of the modified initiators resulted in higher molecular weights, higher degrees of conversion, and higher isoprene contents in the butyl rubber. At the same time, temperature control was good during polymerizations. With modified alkylaluminum halides, the monomer reactivity ratio of isobutylene in copolymerization with isoprene in hexane at -75degreesC was close to unity, similar to the reported case with just EtAlCl2, The system activated with MAO made it possible to synthesize butyl rubber in hexane at -60degreesC having main characteristics typical of commercial butyl rubber manufactured in methyl chloride at -95degreesC. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2360 -2364, 2003.