The invention relates to a new catalyst system that improves the heat transfer capability of a butyl reactor slurry process system in the production of random copolymers of one or more isoolefin monomers and one or more conjugated diene monomers in continuous slurry polymerization processes. The process is carried out in an anhydrous polymerization system containing a mixture of the monomers in a polar diluent along with a Lewis acid and a C₅ or greater initiator having a tertiary halide.