As a sequel to our studies on isobutylene (IB) homopolymerizations, we have investigated the copolymerization of IB/isoprene (IP) mixtures containing up to similar to 20 mole% IP in the feed by the use of the in situ (CH3)(3)Si[B(C6F5)(4)] initiating system in close-to-neat monomers in the temperature range from -35 to -8 (reflux)degrees C. The effects of temperature and IB/IP feed ratio on copolymer molecular weights were determined and compared with those produced by AlCl3. The molecular weights of butyl rubbers obtained by the novel initiating system under a variety of conditions are invariably and significantly higher than those made with AlCl3. High molecular weight gel-free rubbers containing up to similar to 5 mol% unsaturation can be obtained at relatively high temperatures. Copolymer compositions can be controlled by the IB/IP ratio in the feed. Product molecular weights decrease with increasing IP content. To gain insight into the copolymerization mechanism, the activation enthalpy of molecular weights (Delta H-double dagger = -5.9 kcal/mol, -24.7 kJ/mol) and the reactivity ratios (r(IB) = 1.8, r(IP) = 1.5) have been determined.