Copolymer are formed by polymerizing C4 to C7 isoolefin monomers and alkyl-styrene monomers. The method comprises first providing feed streams into a reactor. The various feed streams provide monomers, a polar diluent or polar diluent mixture, and a catalyst system into the reactor. In the reactor, the feed streams contact one another so that the monomers form a polymer in a stable slurry, wherein the amount of polymer in the slurry yields a slurry concentration greater than 22 wt %. The ratio of moles of polymer formed per mole initiator is in the range of 0.25 to 4.0 moles polymer per mole of initiator.