Abstract: A polymerization process is provided for producing a butyl rubber with a bimodal molecular weight distribution which comprises introducing isobutylene, a conjugated diolefin monomer having from 4 to 8 carbon atoms, from about 1.times.10.sup.-5 to about 1.times.10.sup.-3 moles of an initiator component per mole of isobutylene, said initiator component being selected from the group consisting of an initiator component having the formula: ##STR1## in which R.sub.1, R.sub.2 and R.sub.3 are selected from the group consisting of alkyl, aryl and aralkyl groups and can be the same or different and i is a positive whole number from 2 to 6, and an initiator component having an adamantyl nucleus bearing from 2 to 4 hydroxyl groups an inert organic solvent and from about 1.times.10.sup.-4 to about 1.times.10.sup.-3 moles of water per liter of inert organic solvent into a suitable reaction vessel at a temperature of from about -120.degree. C. to about -50.degree. C., adding a solution of from about 5.times.10.sup.