the present invention provides a novel halogen - free solvent system which can produce a good isobutylene polymer and can be easily reused . a novel process for the production of an isobutylene polymer is provided which comprises using a hydrocarbon solvent having a boiling point of not lower than 105 ° c . and a melting point of not higher than - 90 ° c . heretofore , a solvent system containing a halogenated hydrocarbon such as methylene chloride has been used for the production of an isobutylene polymer . however , such a solvent system has a great adverse effect on the environment . therefore , a non - halogenated solvent is desirable . the present invention is characterized by the use of a hydrocarbon solvent as a reaction solvent . the resulting polymer has good properties . further , compounds which are produced as by - products during the reaction can be easily removed , enabling the recycling of the solvent used . thus , the production cost can be reduced .