A process for producing an unsaturated isoolefin copolymer involves: sonicating a solution of an initiator system in an organic solvent, the initiator system comprising a Lewis acid catalyst and a proton source, to produce a sonicated initiator solution, the sonicating performed at an energy input of 100 J/mL or greater, based on volume of the initiator solution; and then, contacting the sonicated initiator solution with a reaction mixture of at least one isoolefin monomer and at least one copolymerizable unsaturated monomer in an organic diluent to produce the unsaturated isoolefin copolymer. Sonication of the initiator solution improves catalyst activity, thereby improving conversion of the monomers during production of the unsaturated isoolefin copolymer.