AbstractMixtures of RZnCl and tBuCl (R = Et) initiate the polymerization of isobutene in the temperature range of (−35)—(+35) °C. The order of addition of the reagents is important for effective polymerization. Increasing the solubility of the zinc reagent (R = n‐octyl) and use of cumyl chloride as co‐initiator provides a system that is active from (−90)—(+35) °C. The molecular weights show a remarkably small temperature dependence, EDP = −3.3 kJ · mol−1, compared to EDP = −23 kJ · mol−1 in classical systems (AlCl3, EtAlCl2). The molecular weights increase with decrease in zinc concentration and are consistently higher for n‐octyl zinc chloride, as expected for a bulkier, less coordinating counteranion. The polymerizations are not retarded by isoprene addition; homopolymers and copolymers show essentially identical conversions and molecular weights.magnified image