Methods are provided to efficiently produce butyl rubber via a slurry polymerization process. The process comprises providing at least two monomers, wherein at least one monomer is an isoolefin and at least one monomer is a multiolefin. The monomers are combined with an initiator and an organic diluent comprising 40-60 volume % of methyl chloride and 40-60 volume % of 1,1,1, 2-tetrafluoroethane and polymerized.