The present invention relates to improved catalyst compositions used in the formation of substantially gel-free copolymers and terpolymers of isobutylene and one or more dienes having an Mn of about less than 500,000, a mole % unsaturation of about 0.1 to about 45, and a polydispersity value of about 2.5 to about 30, wherein the mixture of isobutylene and diene monomers and a cosolvent are contacted with the catalyst composition at polymerization conditions (0.degree. C. to -100.degree. C.) wherein the improvement includes the hydrocarbon soluble catalyst composition being formed from the reaction product of a material of the formula:R.sub.m AlX.sub.3-mwherein m is an integer of from 1 to 3 inclusive, R is the same or different alkyl radical of straight or branched chain structure of from 1 to 7 carbons and X is the same or different halogen selected from the group consisting of chlorine and bromine, and a halogen containing material which is Y moles of halogen, halogen acid, or mixed halogen per mole of aluminum compound wherein the halogen is selected from the group consisting of chlorine and bromine wherein the molar ratio of the formed catalyst composition to the monomer is critical by selecting at about 0.00001 to about 0.001 in order to obtain the desired elastomeric co- and terpolymers. The molar ratio of the halogen containing material to the R.sub.m Al.sub.3-m used in forming the catalyst is critically selected at about 0.014 to about 0.15. This improved catalyst composition permits the formation of high quality elastomeric products at improved polydispersity levels and higher monomer conversions wherein the catalyst composition is soluble in the inert solvent.